GEOLOGICAL SERIES

OF

FIELD MUSEUM OF NATURAL HISTORY

Volume VII Chicago, May 26, 1937 No. 2

THE HISTORY AND PETROGRAPHY
OF FROBISHER'S "GOLD ORE"

By Sharat K. Roy
Curator of Geology

Results of the Rawson-MacMillan Subarctic Expedition of 1927-28

The story of Frobisher's "gold ore," which filled the coffers of
unscrupulous alchemists and supplied one of the major sensations
in the history of arctic explorations, dates back to 1576. In that
year Captain Martin Frobisher (fig. 10), an Elizabethan seaman and
adventurer, set out to explore the Arctic in quest of a northwest
passage to Cathay, the modern China.

In 1497, John and Sebastian Cabot — father and son — sailing
under the English flag, skirted the shores of Newfoundland. At
that time the prevailing view of geographers was that North America
was a collection of islands, through which a commercial passage
to the Orient could be found. Twenty-three years later, in 1520,
Magellan, a Portuguese in the service of Spain, discovered the
southwest passage, through Capo d'Buono, round Patagonia. With
the news of this discovery, the old conception of the configuration
of North America underwent rapid modifications. The contempo-
rary cartographers then concluded that North America was not a
collection of islands but that it tapered to the north and that
there existed a northwest passage around Labrador, leading directly
' "i from the Atlantic to the Pacific. This new conception was probably

the source of Frobisher's belief in the existence of a sea route to the
Orient by way of the northwest, rather than the Zeno chart, from
which it has sometimes been alleged that Frobisher received his
inspiration. Whatever the source of Frobisher's conviction may
have been, the British public was intensely interested in any effort
that might lead to the discovery of a northern passage to the shores
of the Orient, for such a passage, if practicable, would shorten the
sea route to the Orient by at least one-half and would provide a

No. 384 21

Library

22 Field Museum of Natural History— Geology, Vol. VII

solution to the vital national problem of matching the astonishing
riches of the Spanish Main, which was closed to England.

The wealth which the Spaniards wrenched from all who fell
under their sway and which poured into Europe was steadily lowering
the purchasing power of the pound, and the increased cost of living
was driving the English to distraction. Sporadic plundering of a
few thousand ducats' worth of cargo by English buccaneers was not
doing any great harm to Spain nor doing any great good to England.
Something more substantial had to be done and done soon. Fro-
bisher's proposed exploration in search of a route to Cathay in
northern latitudes, therefore, was discussed in England with much
the same enthusiasm and hope as the exploration of a southwest
passage had been discussed in Spain and Italy a century before.

Born in Altofts, Yorkshire, in 1539, Martin Frobisher, when
barely ten, was sent to London to his uncle, Sir John Yorke, for
the advantage of a city education. Young Frobisher, however,
showed little love for schooling. He whistled his time away at the
Thames Estuary and was vastly more interested in ships and ship-
men than in sitting on a counting-house stool. Sir John perceived
the inclinations of his ward and shipped him on a vessel of the
merchant fleet, bound for West Africa. Ten years later, when next
heard of, Frobisher, now Captain Frobisher, and only twenty-four,
had already been engaging in piratical ventures. Piracy, however,
in those days, was a gentleman's game. It was not a felony nor were
pirates looked upon with disfavor. Elizabeth occasionally signed
orders to arrest them merely as a move to pacify the King of Spain.

With but a few brief respites, Captain Frobisher continued to
rove the seas until 1572. In that year, ostensibly to advance good
will at Madrid, the Queen outlawed piracy and set a limitation
upon letters of marque. This sudden change in the governmental
policy robbed Frobisher of his favorite occupation, but it gave him
an abundance of time to foster actively his earlier ambition to find
a sea route to the East beyond the Spanish rule by doubling the
American continent to the northward.

Voyages of discovery, however, are costly undertakings. In
spite of the Earl of Warwick's patronage, in spite of public interest,
the paid-up capital for the polar expedition was only £1,600, not
sufficient to finance it. In fact, it is questionable if the enter-
prise would ever have materialized had not one of the London
merchants, Michael Lock, finally come forward and offered to be
responsible for the anticipated deficit on the voyage.

v.T-

Fig. 10. Sir Martin Frobisher. From Captain Richard Perry, Tkt Jrnwriir. Negative
No. 79188.

23

24 Field Museum of Natural History— Geology, Vol. VII

The expedition, consisting of two small barques, the Gabriel and
the Michael, and a pinnace, with a crew of thirty-five men (fig. 11),
sailed from Black wall on June 7, 1576 — eighty-four years after
Columbus led the way to the New World and forty-four years before
the Pilgrim Fathers ventured from their island home to found
Plymouth Colony.

The start was ominous. Hardly had the ships neared the east
coast of Greenland before the pinnace keeled over in a gale and
went down with all hands. Close on this loss, the Michael, Owen
Gryffyn commanding, deserted and sailed safely back home.
Severely handicapped, but still determined, Frobisher went on alone.
Rounding Cape Farewell he sailed northwestward until he reached
an island at 62° 32' N. Lat. and 64° W. Long., which formed
the north foreland of a great waterway. Frobisher called this fore-
land Hall's Island. After a brief stay he entered the adjoining
waterway and succeeded in sailing over it for about one hundred and
thirty miles to what is now known as "Frobisher's Farthest." He
was frankly uncertain as to whether the passage was an arm of the
Atlantic or a strait leading to the Pacific but he named it "Fro-
bisher's Strait," and gave his crew to understand that the land to
the right was the coast of Asia, that on the left the continent
of America.

While in the "Strait" five of Frobisher's crew were lured away
and captured by the Eskimos. All efforts for the release of the men
failed and Frobisher was compelled to return home without them.
He, however, succeeded in seizing one of the natives, whom he
carried to England.

Back in London, Frobisher was pleasantly surprised to learn
that he had not only sailed sixty leagues over the passage to Cathay
but had discovered vast deposits of gold. It seems that among
the souvenirs collected by the crew was a piece of black stone,
"much like sea cole in colour." To test its ignitable virtues, the
wife of the sailor who brought it home threw it into fire, where,
instead of burning like coal, it "glistened with a bright marquesset
of golde." The story of the incident soon spread and pieces of
the stone were turned over to three different London goldsmiths,
all of whom reported no gold in it. But the investors refused to
admit that the stone was worthless. So they took it to an Italian
assayist, Giovanni Baptista Agnello, then a resident of London,
who, after "due tests," declared that the sample did contain gold
and produced a few grains purporting to be from it. This aroused

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25

26 Field Museum of Natural History -Geology, Vol. VII

considerable controversy, and the wily Agnello, when asked how
he happened to find gold where all his brother assayists had failed,
replied in Italian, "Bisogna sapere adulare la natura." (It is neces-
sary to know how to coax nature.)

Michael Lock, who had much real gold tied up in this enter-
prise and who saw the way the tide was moving, was the first to
seize the opportunity. Hastily he formed the Cathay Company and
appointed Captain Frobisher High Admiral of Cathay and of the
passage leading thereto. New shareholders were charged a stiff
levy to pay for the eight hundred pound deficit on the first voyage.
The total cost of outfitting a second expedition was four thousand
pounds, of which the Queen subscribed one-fourth. She also ap-
proved the loan of the Navy's two hundred ton ship, the Aid.
The Gabriel and the Michael, which had proved seaworthy on the
last expedition, made up the rest of the fleet, and Captain Frobisher,
a bit bewildered at the lucky turn of fortune the little black stone
had brought upon him, once again weighed anchor from Blackwall
with 120 men on board. It was Whit-Sunday, May 26, 1577.

Aided by following winds, the fleet arrived in Frobisher's "Strait"
on the eighteenth of July, and eleven weeks later, after a tempestu-
ous passage, during which the Master of the Gabriel was washed
overboard and lost, returned to England with two hundred tons of
"ore" in the ships' holds. The ships were not filled to capacity but,
to quote George Best, the historian of the voyage, "It was good
time to leave; for as the men were well wearied, so their shoes and
clothes were well worn out, their tools broken, and the ships reason-
ably well filled. Some with overstraining themselves received hurts
not a little dangerous, some having their bellies broken and others
their legs made lame." Long drifts in the hillsides, evidences of
frantic digging, may still be seen today across Countess of Warwick
Island (fig. 12), some thirty miles northwest of Hall's Island (see
Map), where the black stone of the first voyage was supposed to
have been picked up.

A special commission of reputed "experts" in mining and metal-
lurgy was appointed immediately by the Court to look thoroughly
into the nature of the ore. The evidence on which the opinion of
these gentlemen rested was not made known, but they reported
that the ore was very rich, worth forty pounds sterling to the ton,
or eight thousand pounds sterling in all, a highly profitable return on
the investment. Following this official report the "ore" was lugged
to the dungeons of Bristol Castle and locked under four great locks!

SKETCH MAP OF FROBISHER BAY, BAFFIN LAND. ARROW NEAR ANCHORAGE I I POINTS TO APPROXIMATE POSITION OF COUNTESS
OF WARWICK ISLAND. KNOWN TO ESKIMOS AS KODLUNARN OR WHITE MAN'S ISLAND

Frobisher's "Gold Ore" 27

The prestige of Cathay Company was enormously increased.
It was no longer dealing with a dubious speculation beyond the
unknown sea! Many who had not invested previously were now
eager to subscribe. Michael Lock, the astute promoter, disappointed
none. Twelve thousand pounds were promptly raised and fifteen
ships, varying from twenty to two hundred tons, were placed at
Frobisher's command for a third voyage. The personnel of the
expedition, besides the regular officers and crew, consisted of one
hundred picked men — miners and assayers — who were to establish
a colony and build a fort to guard the gold deposits against the
natives.

The fleet, largest in the annals of explorations, sailed from
Harwich on May 31 and arrived at the mouth of Frobisher's "Strait"
on July 1. Christopher Hall, chief pilot, sailing ahead, reported
that the passage was blocked by an unbroken sheet of ice, but, in
spite of the adverse report, Frobisher signalled the fleet to sail in.
During the struggle to work their way through the ice, the Dennis,
Captain Kendall in command, crashed against a berg and sank.
Following this, the crew of the Thomas of Ipswich, its ardor chilled,
mutinied, and forced the Master to set a course for home. Worse
still, heavy fog rolled down the coast blotting out the view of land
and sea for twenty days, and when, at length, it lifted, Frobisher
found himself sailing up a hitherto undiscovered waterway. The
fleet had drifted southward with the ice and was swept westward
by the tide and current into what is now known as Hudson Strait.
Had Frobisher been on a voyage of discovery he would have been
glad to forget all about the gold and to continue sailing westward
until he had seen the end of this new and promising channel; how-
ever, the search for gold being the object, he turned back.

On the last day of July, the fleet again entered Frobisher's
"Strait" and assembled alongside Countess of Warwick Island, near
which the "ore" — thirteen hundred tons of it — was mined and loaded
by the end of the month. The question of founding a winter colony
came up, but on examining the bills of lading it was discovered
that the timber for the west and north sections of the proposed
fort had gone down in the Dennis. There was also insufficient
drink and fuel for a hundred men. This unfortunate combination
of circumstances prompted Frobisher to abandon plans for leaving
a colony there and strike for home. Apparently, Frobisher expected
to be back again, for, before sailing, he built a small house in order
to see what effect the climate would have on it by the ensuing year.

28 Field Museum of Natural History — Geology, Vol. VII

No cheering crowd gathered to celebrate the return of the fleet.
The bubble had burst. The "precious ore" brought home on the
second voyage had proved during the summer to be worthless stone.
Frobisher was in the port of Dartford, now a part of the city of
London, when the news reached him, but there was nothing he
could do to remedy the situation. He dumped the rocks alongside
the quay and went to London, where he was at once besieged by
angry investors, who demanded an explanation. The man who was
the first to venture into the American Arctic, who bore Elizabeth's
pennant northward before Davis and westward before Hudson,
and who attempted to plant the first colony in the icy wastes of
the north, was now accused of every form of incompetence, of the

Fig. 12. Countess of Warwick Island, known to Eskimos as Kodlunarn Island. Negative No. 79190.

conduct of a mad visionary, and the crookedness of a swindler.
Poor Michael Lock also was heavily compromised. Being the
guarantor of the enterprise, he was called upon to meet the deficit,
amounting to some six thousand pounds. He went about to collect
the unpaid dues from the shareholders, but it was like adding insult
to the injury. The final result was bankruptcy, followed by his
imprisonment. He was eventually released but was ruined irremedi-
ably. Frobisher escaped a similar ignominy only through the
intervention of the Queen. Always an admirer of brave men, she
absolved him of all charges on the ground that there was no record
that Frobisher ever claimed that he had found gold, and to further
redeem his name from any obloquy that might otherwise have
attached to it she gave him, in 1585, the command of the Triumph

Frobisher's "Gold Ore" 29

against the Armada. With astounding audacity, Frobisher defended
his ship against four galleons of vastly superior strength — a fight
unsurpassed in the whole course of the war with Spain. For this,
he was knighted by the Lord High Admiral on board his own ship
at sea. A long and stormy career had received its final accolade.
On November 7, 1594, while in action against the Spaniards off
Brest, Sir Martin received a ball-wound in the hip, which proved
mortal. Thus died one of the ablest and most daring seamen of any age.

Fig. 13. Digging for relics in remains of Frobisher's house. Negative No. 79206.

During the 359 years that have elapsed since Frobisher returned
from his third and last voyage of discovery with his supposed
riches, the site of his mining operation on Countess of Warwick Island,
known to the Eskimos as Kodlunarn or WThite Man's Island, has
been visited and reported upon by only two expeditions. The first
of these, in 1861, was composed of a single American, Captain
Charles Francis Hall, and his Eskimo guides. Hall, seeking possible
survivors of Sir John Franklin's ill-fated expedition, visited Fro-
bisher's "Strait," reached its headwaters and, for the first time,

30 Field Museum of Natural History— Geology, Vol. VII

brought to light the fact that the "strait"— the supposed passage
to Cathay— was only a long bay. On his return journey from the
head of the bay, he accidentally stumbled upon evidence of Fro-
bisher's third voyage on Countess of Warwick Island. At that
time, Hall was not familiar with Frobisher's voyages but he made
a thorough search of the ruins, which he later found were linked
inseparably with the activities of Frobisher. He also heard from
his Eskimo companions the fate of Frobisher's five lost men. The
story was that a great many years ago white men with big ships
came there and that five of the crew, deserting their commander,
became willing captives of the natives and roamed with their migra-
tory captors. Later they returned to Countess of Warwick Island,
where they built and rigged a boat with timber left by Frobisher's
party and set off for home. Here the curtain falls, for neither the
boat nor the men were ever seen again.

In 1927, the Rawson-MacMillan Subarctic Expedition to
Frobisher Bay, of which I was a member, again visited Countess
of Warwick Island. Our ship, the Boivdoin, Captain J. T. Crowell
in command in the absence of Captain MacMillan, anchored along-
side Countess of Warwick Island. We spent an afternoon and the
following morning checking up Hall's description of Frobisher's
ruins, but so completely and accurately had he made his observations
that our visit could add little more than confirmation of all that
he had reported.

Kodlunarn Island is situated at 62° 47' 30" N. Lat., 65° 10' W.
Long., some thirty-five miles northwest of the entrance to Fro-
bisher Bay. It is a low, rocky island, about 1,200 feet long and
900 feet wide, is without water, and supports no vegetation other
than trailing ground willows, lichens, and reindeer mosses. It is
so incredibly desolate and offers so little protection from the in-
clemencies of weather that a more forbidding place for setting up
a colony is hard to imagine.

On the north shore of the island, which has a steep bank of
about thirty feet, we found a deep canal some 110 feet long, 15 feet
wide and 25 feet deep (fig. 15). Here Hall reports the finding of
a large piece of iron, probably a bloom, pinned between excavated
rocks. The canal may have been one of Frobisher's mining trenches,
but it probably served as a dry dock for repairing small vessels.
The shape and the uniformity of width and depth of the trench
point strongly to the latter supposition. Southeast of this trench
we saw another similar but smaller excavation. Hall calls this a

I •£**■+

m

Fig. 14. Relics of Frobisher's house on Countess of Warwick Island, a. Chips of bricks; b, coal;
r, flint; d, mortar; and e, tile. Negative No. 79194.

81

32 Field Museum of Natural History— Geology, Vol. VII

reservoir for fresh water and such it may well have been. Between
the "ship's trench" and the "reservoir" there stand the ruins of
two furnaces, with evidence of old fires and clinkers in them. Judging
from their small size it is safe to say that assaying on any large
scale could not have been carried on. The foundation of the experi-
mental house, in spite of the fact that the natives had been picking

■ a* *<c^

Fig. 15. A long excavation on the north shore of Countess of Warwick Island,
served as a dry dock for repairing small vessels. Negative No. 79205.

It probably

up whatever took their fancy or met their needs, was still visible.
Digging around it (fig. 13) we found pieces of plaster, brick, por-
celain, lime, coal, and a number of flint stones (fig. 14). The latter
were, doubtless, used for "strike-a-lights." Dr. William Duncan
Strong, my former colleague, now Senior Ethnologist at the Smith-
sonian Institution, Washington, D.C., has in his possession a frag-
ment of a dull black glazed porcelain with metallic slag adhering
outside the rim, indicating that the vessel of which it formed a part
had served as a crucible. On the east side of the island, facing the
sea, we found a sort of crude stone breastwork such as the Eskimos
erect for shelter when hunting. It may have been built to serve as
a fortification. Near this stone structure, and most interesting
of all, we noticed a small heap of rocks (fig. 16), some of which
fitted the description, "much like sea cole in colour." These
rocks are apparently foreign to the island, and, as evidenced by the

Frobisher's "Gold Ore"

33

growth of mosses and lichens over them, they have not been disturbed
since Frobisher's time. The "sea cole colour" of some of these rocks,
their transportation from elsewhere, and their accumulation on the
edge of the island whence they could be readily loaded into the
ships' holds, strongly suggest that they were the few remainders
of the thirteen hundred tons of "gold ore," which Frobisher and his
men dug up and carried across three thousand miles of storm-tossed
seas to swell the rock piles of England.

Curious as to the source of these black rocks, I combed the
adjacent areas in search for it, but without success. The only rock
that I could find in the vicinity and in situ, which to the layman
might seem to contain gold, was a granular, greenish rock flecked

Fig. 16. A pile of lichen-covered rocks, mostly dark-colored — the supposed gold ore. Found
on Countess of Warwick Island. Negative No. 79205.

with mica (fig. 18, a). A hand specimen of this rock, as well as one
of the black rocks found on Countess of Warwick Island, is now in
the lithological collection of Field Museum. The two are briefly
described on pages 34-37.

The subsequent history of the two hundred tons of rocks brought
home by Frobisher on the second voyage and locked in Bristol

34 Field Museum of Natural History — Geology, Vol. VII

Castle under quadruple locks is not available. But the cargo of
the third voyage, consisting of no less than thirteen hundred tons
of rocks, which Frobisher hurriedly dumped alongside the quay
at Dartford, has recently been discovered in the course of drainage
works there. Samples of these rocks were sent to the University
of Cambridge, and Dr. R. S. Priestly, geologist of the First Shackle-
ton Expedition, now Vice-Chancellor of the University of Melbourne,
on examining a specimen, found it to be Amphibolite,* consisting
mainly of hornblende with a little pyrite and brassy yellow mica.

Another sample from the same source was examined more
recently by Dr. C. E. Tilley of the University of Cambridge. He
writes,2 "We have here a large specimen showing a weathered
surface. It is a dark granular rock — here I may interpolate that
the phrase, 'much like sea cole in colour,' is not altogether inappro-
priate, as its appearance to the layman is somewhat that of coal —
composed essentially of monoclinic pyroxene (augite) and horn-
blende. The petrographic name would be Hornblende Pyroxenite.
In places and conspicuously on part of the weathered surface of
our specimen, occur small plates of brown mica (biotite) which has
a bronzy color characteristic of weathered biotite. It appears that
this bronzy lustred mica was the cause of the 'gold rush.' This
mineral and not pyrites then is the evidence of our specimen."

Of the two Field Museum specimens, the one that was found on
Countess of Warwick Island is Amphibolite; the other, collected
from a nearby outcrop, is Pyroxenite. The physical characters and
mineral constituents of these two rocks are as follows:

(1) Amphibolite. F.M. No. Li 3102, fig. 17, a, b. Obtained
from a rock pile (fig. 16) on Countess of Warwick Island, Frobisher
Bay, Baffin Land.

Megascopic characters. — The rock is black, somewhat resembling
"sea cole in colour"; greenish black when freshly fractured; and
greenish brown in thin section. Luster vitreous. Texture coarse,
granular. Composed mainly of amphibole and pyroxene. Some
of the amphibole crystals are interlocked ; others show slight parallel
arrangement. Specks of biotite may be seen with the aid of a
hand lens.

Microscopic characters. — The amphibole is hornblende; greenish
brown and faintly pleochroic. It forms about 80 per cent of the

1 From information received by Dr. Vilhjalmur Stefansson.

2 Letter to Dr. Stefansson. Quoted by permission of Dr. Tilley.

Fin. 17.

Amphibolite. a, Specimen taken from the pile of rocks shown in Fig. 1G; b, thin section
of above amphibolite. Negative Nos.: a, 79192; b, 79186.

35

Fia. 18. Pyroxenite. a, Specimen collected from an outcrop near Countess of Warwick Island-
b, thin section of above pyroxenite. Negative Nos.: o, 79191; b, 79185.

36

Frobisher's "Gold Ore" 37

rock and may have been altered from omphacite. The pyroxene,
which constitutes 10 per cent, is both augite and omphacite (diopside-
like). Other minerals, composing the remaining 10 per cent, are
calcite, sodic plagioclase, biotite, magnetite, ilmenite, and titanite,
with a sprinkling of quartz, apatite, garnet, and zircon. The calcite
fills the interspaces and is altered from garnet.

The nature of alterations of the minerals observed in this amphi-
bolite strongly suggests that the rock is a metamorphic eclogite.

(2) Pyroxenite. F.M. No. Li 3108, fig. 18, a, b, collected from
an outcrop, about two hundred yards from Countess of Warwick
Island, Frobisher Bay, Baffin Land.

Megascopic characters. — The rock is dark green, coarsely granular
and friable. Composed chiefly of green pyroxene, amphibole, and
bronze-colored mica. It is a rock of distinct individuality and may
be readily distinguished from any other rocks of the adjacent region.

Microscopic characters. — The pyroxenite is essentially an aggre-
gate of augite (50 per cent) with high interference colors, prismoids
of hornblende (15 per cent), and strongly pleochroic, idiomorphic
crystals of biotite. Other minerals are calcic plagioclase, apatite,
zircon, and leucoxene; the latter two occur as sporadic grains.

The rock is liberally spotted with biotite, which imparts to it
a distinctive sparkling surface.

It will be noticed that the Field Museum specimens are es-
sentially the same as those identified by Dr. Priestly and Dr. Tilley.
I am inclined to believe that the rocks brought back by Frobisher
were chiefly basic, but that they represented more than one species
and were both eruptive and metamorphic in origin. I have not
found pyrite, which has probably precipitated more "gold rushes"
than any other mineral, in the sections I have examined of the
two Field Museum specimens, but it is likely that the mineral is
sparsely disseminated and traces of it can be found in both speci-
mens. I, however, favor Dr. Tilley 's conclusion that it was the
bronzy lustered mica and not the pyrite which was the real
basis on which Signor Agnello and the so-called "experts" in mining
and metallurgy succeeded in perpetrating one of the most amazing
hoaxes in the history of maritime adventuring.

The total cost of Frobisher's three voyages, including the deficit,
was £24,000, equal to about £600,000 ($3,000,000) of the present
coinage. This was a staggering sum, particularly when compared
with the cost of Columbus' voyage of 1492, which was less than

38 Field Museum of Natural History — Geology, Vol. VII

£450. Furthermore, the immediate results of Frobisher's voyages,
unlike Columbus', were unproductive. They enriched commerce
not at all, and geographical knowledge only a little. Yet, it must
be admitted that his voyages were not without significance. They
did serve to fire contemporaries with emulative zeal. The grim
saga of the American Arctic was begun when the doughty captain
sailed out of Blackwall in quest of a northerly passage to Cathay.

UNIVERSITY OF ILLINOIS-URBANA

3 0112 084203279

550.5
Fl

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SURVEY LIB/t'

GEOLOGICAL SERIES *—*■*— .

OF
FIELD MUSEUM OF NATURAL HISTORY

Volume VII Chicago, December 28, 1937 No. 3

ASTERISM IN GARNET, SPINEL, QUARTZ,
TOEUBRWWOFTHE AND SAPPHIRE

JUL 2 3 1938 by Albert J. Walcott

,UJtl0,S ASTERISM IN GARNET

Two spheres of asteriated almandite garnet, respectively 1.86
inches and 1.47 inches in diameter, are on exhibition in H. N. Higin-
botham Hall (Hall 31) of Field Museum of Natural History. The
asterism of each sphere consists of four great circle, narrow, well
denned chatoyant bands with definite angular relations to symmetry
directions, and therefore with a definite angular relation to each other.
The result is that at twelve identical crystallographic points on the
sphere two bands intersect forming at each point a "four rayed star,"
thus forming twelve "four rayed stars" each with two angles of 70° 32'
and two angles of 109° 28'.

A microscopic examination of the spheres under strong illumina-
tion reveals an inclusion system of closely crowded, very fine,
elongated crystals, much finer than the silk fibers of a cobweb.
There are two sets of included crystals about each of the six axes
at each of whose poles there is an intersection of two chatoyant
bands. The elongated directions of the included crystals intersect
to form two angles of 70° 32' and two angles of 109°28'. These angles
were determined by the usual method of measuring angles with a
polarizing microscope.

The crystallographic relations of the included crystals and the
chatoyant bands resulting from them may be visualized if the garnet
sphere is considered as a projection of a rhombic dodecahedron.

Two sets of elongated crystals are parallel to each pair of parallel
dodecahedron faces. These elongated crystals are also parallel
respectively to two trigonal axes of symmetry which lie in a plane
of symmetry designated by some authors as a diagonal plane of
symmetry. The trigonal axes of symmetry intersect each other to
form two angles of 70° 32' and two angles of 109° 28'. Hence, the

No. 397 39

40 Field Museum of Natural History — Geology, Vol. VII

chatoyant bands which are produced by the included elongated
crystals and whose planes are perpendicular to the elongated direc-
tions of the included crystals intersect at angles equal to the angles
formed by the intersection of two trigonal axes of symmetry.

The plane of each great circle chatoyant band is perpendicular
to a trigonal axis of symmetry. If such a band is traced on a crystal
model of a rhombic dodecahedron, it will be drawn across six faces
of the zone whose faces and edges are parallel to a trigonal axis of
symmetry. If a similar band is traced for each trigonal axis of
symmetry there will be four such bands, and two bands will be found
to intersect on each rhombic dodecahedron face at the termination
of a binary axis of symmetry. The two 70° 32' angles formed by
this intersection are opposite the two 109° 28' angles of the crystal

□ = tetragonal axis of symmetry. A =■ trigonal axis of symmet ry . O = binary axis of symmetry

Fig. 19. Asterism of garnet sphere, Field Museum of Natural History, o shows four great circle
chatoyant bands and four "four rayed stars." b three "four rayed stars."

face, and the two 109° 28' angles formed by the intersection of two
chatoyant bands are opposite the two 70° 32' angles of the crystal face.

summary: The asterism on the sphere consists of four great
circle chatoyant bands. The plane of each band is perpendicular
to a trigonal axis of symmetry. Two bands intersect at angles equal
to the angle of intersection of the two trigonal axes of symmetry,
to which the planes of the bands are respectively perpendicular.
These intersections occur at each pole of the six binary axes of
symmetry, thus forming twelve "four rayed stars" on the entire
sphere. Each "star" forms two angles of 70° 32' and two angles of
109° 28'.

The asterism manifested by the garnet spheres may be explained
as mainly a diffraction effect, produced by the system of regularly

FI3

V.7"5

Goo. 2*

ASTERISM

41

included elongated crystals, of the light reflected from the interior
of the spheres. A similar effect may be seen by looking through a
fine mesh at a light having a dark background, such as a distant
street light at night-time or the headlights of an automobile at a
_* c distance of one hundred yards or more.

A single cabochon cut garnet showing asterism was loaned for
.' stu^y by Mr. Henry Franklin of Louis Franklin Company, New
uMERSiV York City. The asterism of this stone is identical with the asterism

Itt

1 orK City

of the garnet spheres of Field Museum. It shows five "four rayed
stars" on the upper surface which are identical with the "stars" on
Field Museum spheres.

<\

r1

h-

-«.

Fig. 20. Two views of model of a rhombic dodecahedron showing relations of chatoyant bands
of a and b (Fig. 19) to crystal faces and to the trigonal and the binary axes of symmetry.

Another garnet, also cut single cabochon, was loaned by Mr.
Theodore Pontius of the Chicago Lapidary. This stone is of par-
ticular interest because it shows on the upper surface seven chatoyant
bands, seven "six rayed stars" and three "four rayed stars."

The asterism of this stone, like the asterism of the spheres, is
caused by a system of included, very fine, elongated crystals. The
relations of the included crystals and the chatoyant bands to the
symmetry directions can be explained to advantage by considering
how the phenomenon would appear on a sphere and how it would
be related to a rhombic dodecahedron crystal of which the sphere
jnay be considered a spherical projection.

In this stone there are three sets of included crystals about each
of the six binary axes of symmetry. Each binary axis of symmetry

r*

42 Field Museum of Natural History — Geology, Vol. VII

is perpendicular to a diagonal plane of symmetry. In each of the
six diagonal planes of symmetry there are two trigonal axes of
symmetry, one tetragonal axis of symmetry, and one binary axis of
symmetry. The elongations of the three sets of inclusions are respec-
tively parallel to the two trigonal axes of symmetry and the
tetragonal axis of symmetry which lie in the diagonal plane of
symmetry.

In a rhombic dodecahedron two opposite dodecahedron faces are
parallel to each diagonal plane of symmetry. In such a crystal,
having the inclusion system referred to above, there are three sets
of included crystals parallel to each opposite pair of dodecahedron
faces. The symmetry relations of the great circle chatoyant bands

b

□ = tetragonal axis of symmetry. A = trigonal axis of symmetry. O = binary axis of symmetry.

Fig. 21. Asterism of a sphere of garnet having two sets of inclusions, one set parallel to the
trigonal axes of symmetry and one set parallel to the tetragonal axes of symmetry, a shows seven
great circle chatoyant bands, five "six rayed stars," two "four rayed stars." 6 shows one "four rayed
star" and four "six rayed stars."

which would be produced on a sphere may be visualized if they are
traced on a model of a dodecahedron. Four bands would have the
same symmetry relations as the bands on the spheres of Field Mu-
seum. The plane of each band is perpendicular to a trigonal axis
of symmetry, and two bands intersect at each pole of the six binary
axes, forming two angles of 70° 32' and two angles of 109° 28'.

The included crystals whose elongations are parallel to a tetrag-
onal axis of symmetry would, on a sphere, produce a great circle
chatoyant band whose plane is parallel to an axial plane of symmetry
and perpendicular to a tetragonal axis of symmetry. The great
circle chatoyant band passes through the poles of each of the two
tetragonal axes of symmetry and the two binary axes of symmetry
which lie in an axial plane of symmetry. Since there are three

ASTERISM

43

tetragonal axes of symmetry all at 90° to each other, it is self-evident
that the sets of inclusions whose elongations are parallel to these
axes produce three great circle chatoyant bands whose planes are
at 90° to each other. Two bands intersect at 90° at the poles of each
of the three tetragonal axes and therefore form on a sphere six "four
rayed stars" with angles of 90°. Since each band passes through the
poles of the two binary axes of symmetry which lie in an axial plane
of symmetry, it forms with the two bands which intersect at these
points four "six rayed stars." The axial plane of symmetry bisects
the two opposite angles of 109° 28' which are formed by the intersec-
tion of the two chatoyant bands. These bands are produced by the
inclusions whose elongations are parallel to the trigonal axes of

Fig. 22. Two views of model of a rhombic dodecahedron showing relation of chatoyant bands
of a and b (Fig. 21) to crystal faces and to the axes of symmetry.

symmetry. The chatoyant band, therefore, whose plane is parallel
to an axial plane of symmetry also bisects these angles, forming a
"six rayed star" with four angles of 54° 44' and two angles of 70° 32'.
The asterism of a sphere of garnet with a system of inclusions
like the single cabochon stone of the Chicago Lapidary would there-
fore consist of seven (4+3) great circle chatoyant bands, twelve
"six rayed stars" each with four angles of 54° 44' and two angles of
70° 32', and six "four rayed stars" with angles of 90°. The planes of
four of the great circle bands are each perpendicular to a trigonal
axis of symmetry. The planes of three of the great circle chatoyant
bands are each parallel to an axial plane of symmetry and perpen-
dicular to a tetragonal axis of symmetry. The direction of a tetrag-
onal axis of symmetry is also the direction of a crystallographic axis.

44 Field Museum of Natural History — Geology, Vol. VII

ASTERISM IN SPINEL

A gem stone, cabochon cut, showing well defined asterism was
submitted for identification by B. J. Hersch of Frederick J. Essig
& Company, Chicago.

From its optical and physical properties the stone was identified
as a spinel. It is dark red in color and weighs a trifle over three
carats. Near the center of the upper surface is a "six rayed star"
with 60° angles, which may be clearly seen in a strong source of
artifical light but shows up to the best advantage in direct sunlight.

The asterism of this stone is of special interest because there are
three "six rayed stars" on the upper surface immediately above
the girdle. Also, there are three "four rayed stars" a trifle higher

□ = tetragonal axis of symmetry. A = trigonal axis of symmetry. O = binary axis of symmetry.

Fig. 23. Asterism of a spinel sphere. Inclusions are parallel to binary axes of symmetry, a shows
six great circle chatoyant bands, four "six rayed stars," and three "four rayed stars." b shows one
"four rayed star" and four "six rayed stars."

above the girdle than the "six rayed stars." This whole group of
four "six rayed stars" and three "four rayed stars" is produced
by the intersections of six narrow, sharply defined chatoyant bands.
An examination of the spinel stone with a microscope under
strong illumination reveals an inclusion system of a large number
of very fine elongated crystals. Around the center of each "six
rayed star," over an appreciable area, there are three sets of included
crystals whose elongations extend in three directions at angles of
60° to each other. The value of this angle was determined with a
polarizing microscope, using the method for determining crystal
angles or cleavage angles. It is quite evident that the system of
included crystals produces the asterism in the spinel gem.

ASTERISM

45

The relations of the chatoyant bands to symmetry directions
may be visualized by assuming a spherical projection of an octa-
hedron oriented so that a trigonal axis of symmetry is in a vertical
position. Three binary axes of symmetry, intersecting at angles of
60°, lie in a plane perpendicular to the trigonal axis of symmetry.
Three sets of included crystals are parallel to the pair of octahedron
faces which intercept the vertical trigonal axis at 90°, and are
respectively parallel to the three binary axes referred to above.
This accounts for the 60° angles formed by the included crystals.
Three chatoyant bands produced by the included crystals intersect
at 60° on the octahedron faces at the terminations of the trigonal
axis of symmetry.

The same condition with its resulting phenomenon prevails for

Fig. 24. Two views of a crystal model showing relation of chatoyant bands of a and 6 (Fig. 23)
to crystal faces and to the axes of symmetry.

each pair of parallel octahedron faces. If a chatoyant band is traced
on an octahedron, it will be found that the plane of this band is
perpendicular to a binary axis of symmetry and is parallel to a
diagonal plane of symmetry. The plane of the band therefore,
includes one tetragonal axis of symmetry, one binary axis of sym-
metry, and two trigonal axes of symmetry. It will readily be seen
that for the whole crystal there are six chatoyant bands.

The interesting asterism phenomenon described above may be
visualized more clearly if the octahedron is projected on a sphere.
A sphere of spinel with inclusions like the stone here investigated
will show six great circle chatoyant bands. Three bands will inter-
sect, forming angles of 60°, at each of the poles of the four trigonal
axes of symmetry thus producing eight "six rayed stars." Two

46 Field Museum of Natural History — Geology, Vol. VII

bands will intersect at 90° at each of the poles of the three tetragonal
axes of symmetry and therefore produce six "four rayed stars."

ASTERISM IN CUBIC MINERALS OF THE
HEXOCTAHEDRAL CLASS

The asterism manifested by the two garnet spheres, the spinel
cabochon, and the garnet cabochon stones is of special interest
because it corrects the erroneous idea which prevails generally
that the only asterism possible in minerals of the cubic system is
a "four rayed star" with angles of 90°. They illustrate four well
denned types of asterism each of which is the result of a system of
very fine elongated crystals occurring as inclusions whose elongated
directions are parallel to axes of symmetry. The examples described
show two types of "four rayed stars," one type with two angles of
70° 32' and two angles of 109° 28' and one type with four angles of 90°.
The examples also show two types of "six rayed stars," one type
with four angles of 54° 44' and two angles of 70° 32' and one type
with six angles of 60°. In the garnet spheres the elongated directions
of the included crystals are parallel to the trigonal axes of symmetry.
In the cabochon spinel the elongations are parallel to the binary
axes of symmetry, and in the garnet cabochon stone the system of
inclusions consists of elongations that are parallel to the tetragonal
axes of symmetry and also elongations that are parallel to the binary
axes of symmetry.

A further search for asterism in minerals of the cubic system
may result in discovering other manifestations of the phenomenon.
Many other types are possible in minerals of the hexoctahedral
class of the cubic system in which asterism is produced by inclusions
of fine elongated crystals, as is the case in the examples available
for study.

The possible arrangements of elongated included crystals in
cubic minerals of the hexoctahedral class may be considered under
the following three groups:

Group I. Inclusions parallel to the trigonal axes of symmetry.
Group II. Inclusions parallel to the binary axes of symmetry.
Group III. Inclusions parallel to the tetragonal axes of symmetry.

The garnet spheres of Field Museum represent a possibility of
group I consisting of the highest number of sets of inclusions possible
in this group.

The spinel cabochon represents a possibility of group II consisting
of the highest number of sets of inclusions possible in this group.

ASTERISM 47

The cabochon garnet of the Chicago Lapidary represents a
combination of the highest number of sets of inclusions possible in
group I and group III.

It is possible for a cubic mineral of the hexoctahedral class to
contain a system of inclusions of group III, in which there would
be three sets of inclusions each parallel to a tetragonal axis of sym-
metry. The asterism that would result from this system of inclusions
has been described (p. 42) in connection with the cabochon garnet
in which this system of inclusions occurs in combination with the
system of inclusions of group I.

A cubic mineral with a system of inclusions consisting of a com-
bination of the highest number of sets possible in group II and the
highest number in group III would have nine (6+3) great circle
chatoyant bands on a sphere. The bands would intersect to produce
at the poles of the three tetragonal axes of symmetry six "eight
rayed stars" with angles of 45°, twelve "four rayed stars" with angles
of 90° at the poles of the six binary axes of symmetry, and eight
"six rayed stars" with angles of 60° at the poles of the four trigonal
axes of symmetry.

If a cubic mineral of the hexoctahedral class should contain a
system of inclusions consisting of a combination of the highest
number of sets possible in group I and the highest number of sets
possible in group II, ten (6+4) great circle chatoyant bands would
appear on a sphere. The bands would intersect to produce twelve
"six rayed stars" at the poles of the six binary axes of symmetry.
Each star would have four angles of 35° 16' and two angles= 109° 28'.
Six "four rayed stars" with angles of 90° would be formed at the poles
of the three tetragonal axes of symmetry.

Another system of inclusions is possible consisting of a combina-
tion of the highest number of sets of inclusions possible for each of
the three groups. In a crystal consisting of such a system there
would be a set of inclusions parallel to each of the four trigonal axes
of symmetry, to each of the six binary axes of symmetry, and to each
of the three tetragonal axes of symmetry. On a sphere consisting of
this system of inclusions there would be four great circle chatoyant
bands whose planes would be, respectively, perpendicular to the
four trigonal axes of symmetry. There would be six great circle
chatoyant bands whose planes would be, respectively, perpendicular
to the six binary axes of symmetry and parallel to the six diagonal
planes of symmetry. There would be three great circle chatoyant
bands whose planes would be, respectively, perpendicular to the

48 Field Museum of Natural History— Geology, Vol. VII

three tetragonal axes of symmetry and parallel to the three axial
planes of symmetry. On such a sphere there would be thirteen
(4+6+3) great circle chatoyant bands. These would intersect to
form twelve "eight rayed stars" at the poles of the six binary axes
of symmetry. Each "star" would consist of four angles of 35° 16'
and four angles of 54° 44'. At each of the six poles of the three tetrag-
onal axes of symmetry there would be an "eight rayed star" with all
angles of 45°. There would be eight "six rayed stars" formed at the
eight poles of the four trigonal axes of symmetry, with all angles of 60°.
To summarize the above possibility: A sphere with the inclusions
considered here would show thirteen (4+6+3) great circle chatoyant
bands, twelve "eight rayed stars" each with four angles of 54° 44'

a b

□ — tetragonal axis of symmetry. A = trigonal axis of symmetry. O = binary axis of symmetry.
Fig. 26. Asterism as it would appear on a sphere having three sets of inclusions, o shows
seven "eight rayed stars" and two "six rayed stars." b shows five "eight rayed stars" and four "six
rayed stars."

and four angles of 36° 16', six "eight rayed stars" each with eight
angles of 45°, and eight "six rayed stars" each with six angles of 60°.

This is the most complex manifestation of asterism that can
result from a system of inclusions. The hexoctahedral class is the
most complex class of symmetry of crystals, and all the axes of
symmetry of this class have here been considered.

The examples and possibilities given here of asterism resulting
from inclusions of very fine elongated crystals in cubic minerals of
the hexoctahedral class are the result of a system consisting of the
highest number of sets or a combination of the highest number of
sets for the axes of symmetry to which the elongations of the included
crystals are parallel. It seems reasonable to consider that asterism
may occur in cubic minerals in which one or more of the sets possible
may be lacking. For example, in group I there are four sets of

ASTERISM 49

inclusions if inclusions occur with elongations parallel to each of the
four trigonal axes of symmetry. There are, however, three other
possibilities in group I: (a) One set of inclusions with elongations
parallel to one of the four trigonal axes of symmetry, (b) Two sets
of inclusions with elongations parallel to each of two of the trigonal
axes of symmetry, (c) Three sets of inclusions with elongations
parallel to each of three of the trigonal axes of symmetry. The
fourth possibility has been fully described. It consists of four sets
of inclusions with elongations parallel to each of the four trigonal
axes of symmetry.

The following is a list of the possibilities of asterism within each
group, assuming that it is possible for included crystals to occur
parallel to any number other than the complete set of the axes of
symmetry of any one group. At the time of this writing, examples
of such occurrences have not come to my attention.

Group I

(1) One set of inclusions parallel to a trigonal axis of symmetry.
Result: One great circle chatoyant band. The plane of this

band is perpendicular to the trigonal axis of symmetry. On a
rhombic dodecahedron the band may be traced on the six faces of
the zone parallel to a trigonal axis of symmetry.

(2) Two sets of inclusions, each parallel to a trigonal axis of
symmetry.

Result: Two great circle chatoyant bands and two "four rayed
stars" formed by the intersection of these two bands on opposite
poles of a binary axis of symmetry. Each "star" with two angles of
70° 32' and two angles of 109° 28'.

(3) Three sets of inclusions, each parallel to a trigonal axis of
symmetry.

Result: Three great circle chatoyant bands and six "four rayed
stars." Each "star" with two angles of 70° 32' and two angles of
109° 28'. The stars are at the poles of three binary axes of symmetry.

(4) Four sets of inclusions, each set parallel to a trigonal axis
of symmetry.

Result: Four great circle chatoyant bands, twelve "four rayed
stars" formed at the poles of the six binary axes of symmetry.

Example: Two garnet spheres of Field Museum.

In each of the four possibilities mentioned above, the plane of
each great circle chatoyant band is perpendicular to a trigonal axis
of symmetry.

50 Field Museum of Natural History — Geology, Vol. VII

Group II

(1) One set of inclusions parallel to a binary axis of symmetry.
Result: One great circle chatoyant band. The plane of this band

is perpendicular to a binary axis of symmetry and is parallel to a
diagonal plane of symmetry.

(2) Two sets of inclusions, each parallel to a binary axis of sym-
metry. The binary axes of symmetry lie in the same axial plane
of symmetry and are at right angles to each other.

Result: Two great circle chatoyant bands, each plane perpendic-
ular to a binary axis of symmetry and parallel to a diagonal plane
of symmetry. The planes of the two bands are at right angles to
each other. The bands intersect at the poles of the tetragonal axis
which they have in common, to form two "four rayed stars" with
angles of 90°.

(3) Three sets of inclusions, each parallel to a binary axis of
symmetry.

Result: Three great circle chatoyant bands, each plane perpen-
dicular to a binary axis of symmetry. The plane of each is parallel
to a diagonal plane of symmetry. Two of the planes of the great
circle chatoyant bands are at 90° to each other. Two "four rayed
stars," angles of 90°, are found at the poles of a tetragonal axis of
symmetry which two of the bands have in common as in (2). Four
"four rayed stars" are formed at the poles of two trigonal axes of
symmetry. Two angles of 60°, two angles of 120°.

(4) Four sets of inclusions, each parallel to a binary axis of
symmetry.

Result: Four great circle chatoyant bands, each plane perpen-
dicular to a binary axis of symmetry, and the plane of each band
is parallel to a diagonal plane of symmetry. Two "four rayed stars"
formed at the poles of a tetragonal axis of symmetry which the two
planes of the chatoyant bands which are at right angles to each other
have in common. The angles of these "stars" are 90°. Two "four
rayed stars" formed at the poles of a trigonal axis of symmetry with
two angles of 60° and two angles of 120°. Two "six rayed stars"
formed at the poles of a trigonal axis of symmetry with angles of 60.

(5) Five sets of inclusions, each parallel to a binary axis of
symmetry.

Result: Five great circle chatoyant bands, each plane perpen-
dicular to a binary axis of symmetry. The plane of each is parallel
to a diagonal plane of symmetry. Four "four rayed stars" formed

ASTERISM 51

at the poles of two tetragonal axes of symmetry with angles of 90°.
Four "six rayed stars" with angles of 60° formed at the poles of
two trigonal axes of symmetry. Two "four rayed stars" with two
angles of 60° and two angles of 120°, formed at the poles of a
trigonal axis of symmetry.

(6) Six sets of inclusions, each parallel to a binary axis of symmetry.
Result: Six great circle chatoyant bands, each perpendicular to

a binary axis of symmetry. Eight "six rayed stars" with angles of 60°
formed at the poles of the four trigonal axes of symmetry. Six
"four rayed stars" with angles of 90°, formed at the poles of the three
tetragonal axes of symmetry.

This type of asterism is represented by the cabochon spinel of
Essig & Company.

There are two other possibilities with inclusions parallel to the
binary axes of symmetry:

(7) Two sets of inclusions, each parallel to a binary axis of sym-
metry.

The two planes of the great circle chatoyant bands may be at
angles of 60° instead of 90° as in (9). In this case there are two
"four rayed stars" at the poles of a trigonal axis of symmetry, with
two angles of 60° and two angles of 120°.

(8) Three sets of inclusions, each parallel to a binary axis of
symmetry. The three binary axes lie in the same plane perpendicular
to a trigonal axis of symmetry. The result is three great circle
chatoyant bands at angles of 60°. One trigonal axis in common.

From such an arrangement of inclusions two "six rayed stars"
are produced at the poles of the trigonal axis of symmetry which the
three chatoyant bands have in common. The asterism thus produced
is identical with that produced in minerals of the hexagonal system.

Asterism that is produced by parallel arrangement of inclusions
to binary axes of symmetry offers eight possibilities: Inclusions
parallel to from one to six axes. There are two possible arrangements
of inclusions parallel to two binary axes and two possible arrange-
ments of inclusions parallel to three binary axes of symmetry.

Group III

(1) One set of inclusions parallel to a tetragonal axis of symmetry.

Result: One great circle chatoyant band. The plane of this band
is perpendicular to a tetragonal axis of symmetry and is parallel
to an axial plane of symmetry.

52 Field Museum of Natural History — Geology, Vol. VII

(2) Two sets of inclusions, each set parallel to a tetragonal axis
of symmetry.

Result: Two great circle chatoyant bands, each plane perpen-
dicular to a tetragonal axis of symmetry and parallel to an axial
plane of symmetry. The planes of the two bands are at right angles
to each other, and the bands intersect at the poles of the tetragonal
axis of symmetry which the two planes have in common, to form two
"four rayed stars" with angles of 90°.

(3) Three sets of inclusions, each set parallel to a tetragonal axis
of symmetry.

Result: Three great circle chatoyant bands, each plane perpen-
dicular to a tetragonal axis of symmetry and parallel to an axial
plane of symmetry. Their respective planes are perpendicular to
each other. Two bands intersect at each pole of the three tetragonal
axes of symmetry to form six "four rayed stars" with angles of 90°.

Possibilities resulting from combinations:

Each possibility that may occur with reference to any one of
the axes of symmetry may occur in combination with any one of
the possibilities for another class of axes of symmetry.

For example, the first and simplest possibility listed for group I
may occur in combination with any one of the possibilities of group
II. This is also true for any one of the other three possibilities of
group I.

For group I and group II a minimum of 32 combinations are
possible.

For group I and group III there are a minimum of 12 possibilities.

The four possibilities of group I may occur with each of the
possibilities resulting from combinations of groups II and III.
Since there are 24 possible combinations of groups II and III, the
minimum number of combinations of the four possibilities of group
I with the 24 possible combinations of groups II and III would be 96.

Total minimum number of possibilities, 179.

Summary of Asterism possibilities in the hexoctahedral class:
Group I — Number of possibilities 4; group II — 8; group III — 3.

Possibilities from combinations: Groups I and II — Number of
possibilities 12; groups I and III — 32; groups II and III — 24;
groups I, II, and III— 96. Total 179.

Examples are available for three of the above possibilities.

ASTERISM 53

For completeness and continuity each example is here briefly
reviewed.

(a) Garnet spheres of Field Museum represent possibility 4 of
group I with four sets of inclusions each parallel to a trigonal axis
of symmetry.

(b) Garnet cabochon of Chicago Lapidary represents a combina-
tion of two possibilities, 4 of group I and possibility 3 of group II.

(c) The cabochon spinel of Essig & Company represents possi-
bility 6 of group III.

ASTERISM IN QUARTZ AND SAPPHIRE

A specimen of clear, colorless, transparent quartz showing well
defined asterism is on exhibition in H. N. Higinbotham Hall of
Field Museum of Natural History. The stone is cut double cabochon,
the upper part somewhat higher than a hemisphere, girdle circular,
and is 14 mm. in diameter through the girdle.

A sharply defined "six rayed star" appears at the upper part
in direct sunlight. It is of interest to note that a star is also formed
on the upper part of the stone by transmitted light, if the lower
part is held toward the direct sunlight. The crystallographic c axis
extends from approximately the center of the base to very nearly
the center of the top.

At the time this was available for study an asteriated rose quartz,
single cabochon cut, 18mm. x 13mm. was made available through the
kindness of Mr. Theodore Pontius of the Chicago Lapidary.

The asterism of these stones like the asterism of star sapphires
and star rubies is an intersection of three chatoyant bands forming
angles of 60° at the pole of the crystallographic c axis. Each band
extends over the entire upper surface. A sphere of asteriated quartz
would show three meridional chatoyant bands and two "six rayed
stars." Several spheres were made of asteriated sapphires to verify
this in the case of the star sapphire.

The chief interest in the asterism of the quartz specimens is to
determine the cause of the phenomenon. Nothing can be seen with
a microscope under ordinary illumination that might be interpreted
as causing asterism. However, if the substage diaphragm is adjusted
to admit a very narrow beam of light using artificial light and working
in a room free from reflections and direct light from windows, with
a magnification of lOOx, a network of very fine striae may be seen.
When these striae are observed, looking in the direction parallel

54 Field Museum of Natural History — Geology, Vol. VII

to the crystallographic c axis, it will be noted that they cross in
three directions at angles of 60°.

The conclusion is that the system of striae is a system of included
edges. These correspond to the horizontal striations on the prism
faces formed there by an oscillatory development of the rhombohedron
faces and the prism faces. When the habit of a crystal is a com-
bination of a prism of the first order, a first order unit rhombohedron,
and several modifications of the first order rhombohedron, a large
number of very fine striations may be formed. Many of the stria-
tions are microscopic. When these striations become included in
the crystal, they produce the phenomenon of asterism through a
diffraction effect on light.

Many specimens of star sapphire have been examined to deter-
mine the cause of asterism in corundum. The entire collection of
Field Museum was made available. In each of the sapphires ex-
amined there was found a system of a large number of very fine
striae. If a polished convex surface is produced at one end of a
crystal having well defined bipyramid faces, the chatoyant bands
which intersect to form a "star" extend across the top in the planes
of the crystallographic a axes. In other words they are perpendicular
to the second order prism faces.

Asterism in sapphires as in quartz, spinel, and garnet is clearly
a diffraction effect. In the specimen of clear quartz of Field Museum
the star produced by transmitted light substantiates this. If any-
one of the bands of a star sapphire is examined in direct sunlight
with a binocular microscope, a repetition of the spectrum colors red,
orange, yellow, green, blue may be distinctly seen. This is identical
with the effect obtained when a strong light against a dark back-
ground is observed through a very fine mesh.

In connection with asteriated corundum it is well known that
star sapphires occur in far greater abundance than star rubies.
To this it should be added that in general the prevailing crystal
habit of star sapphires is a combination of several modified hexag-
onal bipyramids of the second order and sometimes a second order
prism and a basal pinacoid, while the prevailing habit of the ruby
is a combination of a hexagonal prism of the second order, rhombo-
hedron of the first order and basal pinacoid. The oscillatory growth
of the bipyramids produces on many crystals an abundance of
striations. Such striations occur sparingly on the prevailing habit
of ruby crystals because rubies with the prevailing habit of sapphire
are found sparingly. The chance for crystals with an abundance

ASTERISM 55

of included striations is therefore much more favorable for sapphire
than for rubies.

A microscopic examination of the striae in the direct sunlight
lying in the path of a chatoyant band reveals characteristics that
are identical with the striations that occur on the crystal faces.
The striations occurring on the crystals of quartz and sapphire are
slightly wavy and jagged and in many instances interrupted, instead
of continous clean straight lines like the elongated included crystals
of garnet and spinel. These same characteristics may be observed
on the striae in asteriated crystals of quartz and sapphire.

In many star sapphires striae extend across the entire diameter
of the stone. When a large percentage of the striae for each of the
three directions extend in this manner, a fine network is formed by
their crossing. In some sapphires such a network of striae crossing
in three directions exists over the entire diameter of the stones. In
some stones the network occurs in patches. There is generally a
set of fine striae at 90° to each of the crystallographic a axes. Many
of these striae do not extend beyond the width of the crystal face
on which they were originally formed. However, some extend in
length and the result is that in a few small areas striae cross in two
directions and in other places they cross in three directions. In
some stones most of the striae extend only a few millimeters beyond
their original length, and in such stones the striae form a hexagonal
pattern.

Striae are flat and some appear slightly angular. Striae in the
shape of rounded tubes have not been observed in this study. The
width is a very small dimension, in many cases microscopic. The
terminations of the striae which do not extend greatly beyond their
original length commonly appear like short extensions of microscopi-
cally narrow cracks varying in length and generally show interference
colors. These observations suggest that when the striae extend in
length considerably beyond the striae originally formed on the surface
of the sapphire crystal, the extensions are minute cracks. These
extension cracks of the striae, which in many sapphires result in a
network of crossing striae, crossing at angles of 60°, may have been
produced under the severe metamorphic conditions to which these
sapphires were subjected.

Striae vary considerably in size and number in different stones.
Sharply defined narrow bands are produced by stones in which a
hexagonal pattern is formed by fine striae and in which there is
no appreciable amount of network. The chatoyant bands formed

56 Field Museum of Natural History— Geology, Vol. VII

in a stone in which there is a network of striae are generally feathery
and are not sharply defined. While each of the three chatoyant
bands is a diffraction effect of the striae which it crosses at right
angles, some influence is produced by the other two sets of striae
extensions, resulting in a feathery band. Coarse striae produce
wide, irregular bands which are pronouncedly feathery. There are
other factors which influence the quality of the chatoyant bands.
Pronounced zonal development, twinning striae, and various inclu-
sions absorb and scatter the light that enters the stone, and the
chatoyant bands produced in such stones appear faded and dull.
In some stones striae occur mainly along one of the crystallo-
graphic a axes; such a system of included striae produces a single
chatoyant band, and if properly cut will yield a sapphire cat's eye.

SUMMARY

Asterism may be produced in garnet and in spinel by an inclusion
system of very fine elongated crystals, the elongated directions being
parallel to axes of symmetry.

There are two types of "four rayed stars" and two types of "six
rayed stars" on the garnet and spinel stones available for study.
"Four rayed stars" and "six rayed stars" occur on each of a cabochon
garnet and a cabochon spinel.

A third type of "six rayed star" is possible on a stone whose system
of inclusions consists of elongations parallel to each of the four
trigonal axes of symmetry and to each of the six binary axes of
symmetry.

Two types of "eight rayed stars" would result from a system of
inclusions whose elongations are parallel to each of the thirteen axes
of symmetry.

Asterism is produced in quartz and in corundum by a system
of fine striae which appear to be included striations formed by an
oscillatory growth of two or more crystal forms on one crystal.
The striae are parallel to the prism of the first order in quartz and
to the prism of the second order in corundum.

In many sapphires the striae extend in length beyond that of the
striations originally formed on the crystal. These extensions appear
to be minute cracks which may have been produced under the in-
fluence of metamorphism.

Asterism is a diffraction effect caused by a system of very fine
included crystals in garnet and in spinel and by a system of fine
striae in quartz and in corundum.

ASTERISM 57

BIBLIOGRAPHY

Brewster

Asterism. Phil. Mag. Jan. 1853.

Haushofer, Dr. Karl

1865. Asterismus und die Brewsterschen Lichtfiguren. Miinchen.

Kadokura, Mitsuyoshi

1915. Asterism in Quartz (rosy) from Godo near Taira, Iwake Province.
Beitr. Min. Japan, No. 5, pp. 269-274. Min. Abstracts, Min. Mag., Vol.
19, 1920-22, p. 132. L. J. Spencer.

Kalkowsky, Ernst
1915. Opaleszierender Quartz. Zeits. Kryst. Min. Vol. 55, pp. 23-50. Min.
Abstracts, Min. Mag., Vol. 19, 1920-22, pp. 295-296. L. J. Spencer.

Kobell Von

Asterismus und die Brewsterschen Lichtfiguren. Vol. G-65-Pamphlet series
F. C. M. Library.

Miers, Henry A.
Asterism in Calcite. Mineralogy, p. 399.

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