GEOLOGY

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1976
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FIELDIANA • GEOLOGY

Published by
FIELD MUSEUM OF NATURAL HISTORY

Volume 16 December 19, 1969 No. 15

Mineral Assemblages and the Chemical History
of Chondritic Meteorites

Robert F. Mueller

National Aeronautics and Space Administration
GoDDARD Space Flight Center, Green Belt, Maryland

and

Edward J. Olsen

i Curator, Mineralogy, Field Museum of Natural History

« ABSTRACT

An attempt has been made to correlate the mineralogical and bulk chemical
characteristics and the textural and structural features of chondritic meteorites.

The thermodynamic basis for the observed mineral assemblages of the ordinary
chondrites is discussed in some detail with respect to the temperature of crystalli-
zation, state of oxidation and the liquidus relations. The approximate oxidation
fields of the ordinary, enstatite and carbonaceous chondrites are delineated and
compared with a gas of solar composition. It is shown that the field of the ordi-
nary chondrites is quite well defined and that these chondrites are more oxidized
than the solar gas so that direct condensation from this medium is excluded. The
evidence for initial high temperature liquidus crystallization for most ordinary
chondrites is discussed, and it is concluded that the chondrites of uniform compo-
sition probably represent rapid crystallization through liquidus temperatures, with
perhaps some annealing to a near equilibrium distribution of Mg and Fe^~^ just
below the liquidus. The nonuniform, disequilibrium chondrites appear to repre-
sent, in part, a lower rate of cooling and crystallization and show evidence that
they originated under conditions of varying oxidation.

The observed textural features of the metal grains of most ordinary chondrites
with the coexisting silicates are interpreted as having resulted, not from any mag-
matic or mechanical processes but from transport and deposition from the vapor
phase. This accounts well for nearly all the observed textural and structural
features.

The analytical relations are derived relating the normative olivine contents of
the chondrites to the ratio of oxidized to reduced iron and consequently to the
degree of oxidation. This derived relation is then compared with that observed
for 43 chondrites and a good correspondence is found. This again tends to support

Library of Congress Catalog Card Number: 78-106197

No. 1084 377 ^^ ^^^^^ o' the

Q^OLOQy MAY 1 5 1972

^^^f^HX

University of Illinois
at Urbana-Chsmpaign

378 FIELDIANA: GEOLOGY, VOLUME 16

the approximate validity of Prior's rules when account is taken of analytical errors
and a certain amount of variation in the bulk composition.

A critique is also presented of certain views on the origin of chondrites which
depend primarily on mineral chemistry and little agreement with these is found.

INTRODUCTION

In the past many hypotheses have been advanced for the origin
of various classes of meteorites with the chondrites receiving the most
attention. Concentration in these studies has been largely on the
bulk chemical properties, but recently a keener appreciation for the
importance of mineral chemistry and phase equilibria has been appar-
ent. It has been more widely recognized that the relations between
coexisting mineral phases are detailed expressions of the environment
of crystallization and that it is more informative to work with min-
erals and their inter-relations than with bulk meteorites.

Recent detailed studies of the chondrites have revealed that they
may be divided into two types depending on whether or not the sili-
cates of a given meteorite are of almost constant composition through-
out the meteorite and whether or not magnesium and ferrous iron are
distributed in an orderly way between the silicates. The critical
questions involved here are the mode of crystallization that can ac-
count for the textural and chemical characteristics in each case,
whether or not crystallization was from a melt, the rapidity of this
crystallization and the extent to which equilibrium was attained.
Since it has also been frequently proposed that some chondrites have
undergone metamorphism or recrystallization, it is desirable to see
to what extent this hypothesis is compatible with the observed tex-
tures and predicted chemical relations. One of the most critical sets
of data in this study is that relating to the temperature coefficient
of the distribution of Mgand Fe"^"^ among the coexisting olivines and
pyroxenes since it may tell us the temperature of crystallization
and whether or not a given meteorite has been recrystallized. Un-
fortunately, the experimental and observational data bearing on this
problem are as yet inconclusive because of the large experimental
uncertainties (Mueller, 1964). However, in the past the high tem-
perature experimental data have been almost completely ignored
by the advocates of metamorphism. We believe that these data
require consideration in any interpretation of the temperature of
crystallization.

Another important issue concerns the disposition of the metal
grains in chondrites, the mechanism by which these grains arrived
at their present positions and their chemical relations to the silicates.

MUELLER AND OLSEN: CHONDRITIC METEORITES 379

Then, also, the sequence of meteoritic types discovered by Prior
(1916a) which presumably reflects different degrees of oxidation, im-
plies a systematic and predictable variation in the norms of the min-
erals according to the reaction, pyroxene + metal + 02^ olivine.
It is interesting to compare these theoretical norms with those
calculated from the analyses, since this provides a further test of
Prior's rules.

In this paper an attempt is made to answer some of these ques-
tions or at least to set limits to which hypotheses must conform
regardless of specific modes of origin. While no attempt is made to
review' again all the hypotheses proposed previously, a critique of
certain views which depend primarily on mineral chemistry is pre-
sented and these are compared with observational data and thermo-
dynamic and kinetic considerations.

PHASE RELATIONS
Equilibrium Between Crystals and Gases

The major phase relations among the crystalline phases and with
the associated gas may be expressed in terms of the following reac-
tions (Mueller, 1963) :

i,\ olivine pyroxene olivine pyroxene

^MgjSiOn + FeSiOj ?=^ ^Fe2Si04 + MgSiOj

pyroxene
(2) 2MgSi03+2Fe + O2 ^=^ Fe2Si04 + MgeSiO^

pyroxene metal gas

oliv

2MgSi03 + 2Fe + O2

■> C/-,

. \-^z

pyroxene metal gas

olivine

FeSiOj + Fe + ^Oe.

^ Fe2Si04

(3)

Only two of these reactions are independent, so that any two
may be used in the analysis of the phase relations. However, because
of the lack of reliable thermal data for FeSiOa (f errosilite) , it is con-
venient to use (1) and (2) rather than (3). Then the corresponding
equations of equilibrium may be written as

K, -~

Y (\ - y '

^ Mg \ ' ^Mg '

yOl / . _ yPX .

'^Mg V ' /^Mg/

* A comprehensive review of the various hypotheses on the origin of the various
meteorite classes, including the chondrites, has recently been presented by Anders
(1964).

380 FIELDIANA: GEOLOGY, VOLUME 16

(5) K

x;)^ ( I - K/

2 , ,,px ,2 /^M \ 2

x:r (a:.v p.

' Mg ' \ ^-'Fe

'2

In these expressions X(ol/Mg) and X(px/Mg) represent the atomic
fraction Mg/(Mg + Fe^''") in olivine and pyroxene respectively;
a (M/Fe), the relative activity of iron in the metal; and P(02), the
fugacity of oxygen. In the derivation of these equations, it has been
assumed, on the basis of certain criteria (Mueller, 1964), that both
olivines and pyroxenes behave essentially as ideal solutions.

If equation (4) holds approximately, then, as was shown by
Ramberg and DeVore (1951), the coexisting olivines and pyroxenes
provide us with a simple thermometer for the environment of crys-
tallization. It was previously shown (Mueller, 1964), on the basis
of optical data obtained by Ringwood (1961) that for the chondritic
meteorites Ki approximates 1.13, and it was argued that this coeffi-
cient decreases with increasing temperature.'

Recently, a valuable body of new data on the compositions of
coexisting olivines and pyroxenes from 95 chondrites was presented
by Keil and Fredriksson (1964). These data were obtained on the
the electron microprobe and appear to be of a high order of precision.
Of the total, 86 specimens showed almost constant or uniform com-
position of the silicates throughout the individual meteorite speci-
men. Keil and Fredriksson termed these "ordinary chondrites";
however, we shall refer to them as "uniform chondrites" since the
term "ordinary" is usually applied to a broader class. The values
of Fe^'^/(Fe^"'' +Mg) for the 86 uniform chondrites have been plotted
on the conventional Roozeboom diagram in Figure 1. These data
agree to a surprising degree with the optical data of Ringwood (1961)
but show a more distinct division into three groups which coincide
with high (H) and low (L) iron groups of Urey and Craig (1953) and
a group designated "LL" by Keil and Fredriksson. This latter
group has been included with the L group here.

The data representing the latest experimental work on this sys-
tem are also shown in Figure 1. It may be significant that of the
points representing intermediate values^ of Fe^'*"/(Fe^"''+Mg), those

* The opposite result was obtained by Craig (1964). However, his conclusion
is unwarranted when account is taken of uncertainties in the data used.

^ It may be shown that values of Ki calculated from extreme values of Fe2+/
(Fe2++Mg) have greater uncertainties than those calculated from intermediate
values.

MUELLER AND OLSEN: CHONDRITIC METEORITES

381

of Ernst lie farthest from the 45° line. Since these represent the lowest
temperatures of synthesis (^-'1200°K), they are consistent with a
high temperature of crystallization for the chondrite mineral pairs.

0.4

0.3 -

LiJ

>

_J
O

f 02

C\J(D

+^

CVJ<U

0.1 -

— \ \ 1 1 \ \ \ \ S"

A MUAN AND OSBORN [\^^'o)-CLINOPYROXENE-OLIVINE -

o ERNST (I960) \ orthopyroxene-

+ KEIL AND FREDRIKSSON (1964) J OLIVINE

Fe"!/(Fl^+Mg) PYROXENE

Fig. 1. Compositions in terms of atomic fractions of coexisting olivines and
pyroxenes of 84 uniform chondrites as compared with experimental data. The
lines which pass through the three groups of meteorite points approximate distri-
bution curves based on ideal solutions. These data are compatible with a distribu-
tion constant which decreases with increasing temperature. Some points, which
fell too close to others, were not plotted in this figure.

If we examine the distribution of points representing the chon-
drite minerals in Figure 1, we see that the three groups lie at succes-
sively greater distances from the 45° line, with the highest iron group
closest to the line. Also, it is apparent that each distribution is some-
what linear and oriented approximately parallel to the curves repre-
senting the types of distributions defined by equation (4), although

I

I

382 FIELDIANA: GEOLOGY, VOLUME 16 /

it is patently impossible to tell from the data whether this relation is
obeyed over greater ranges of composition. Three such curves have
been drawn to pass through the three groups. Thus, if, as we have
indicated, Ki decreases with increasing temperature, then the high
iron group represents the highest temperature of crystallization. We
shall see that there is consistency between this result and the ex-
pected partitioning under conditions of rapid crystallization at liqui-
dus temperatures.

The link between the solid solutions and the associated gases
is provided by equations (4) and (5) solved to eliminate either
X(ol/Mg) or X(px/Mg). It is convenient to discuss the equations
first in terms of the pure system Fe-MgO-Si02-02 with a (M/Fe) = l.
We may then evaluate the effect of variable nickel content of the
metal phase separately. Under these circumstances P(02) is a
univariant function of the temperature if one of the parameters
X(ol/Mg) or X(px/Mg) is arbitrarily fixed.' The results of such
calculations for three silicate compositions are plotted in Figure 2
where they have been superimposed on the stability fields of the pure
iron oxides. However, the field of wustite has been omitted since it
does not appear as a phase in these high silica systems.

In Figure 2 we have roughly delimited the fields of the various
types of chondritic meteorites as they appear from observational
data and theoretical considerations. We have also drawn a curve in
Figure 2 that represents the degree of oxidation, over the entire tem-
perature range, of a gas of solar composition as given for the solar
abundances tabulated by Aller (1961). This calculation is based on
the reaction

gas gas gas

(6) He + IO2 F=^ H2O

The equation of equilibrium for this reaction may be written as fol-
lows according to the method of Urey (1952):

(7) log P02 -- 2 log(2Ao/AH) - 2 log K^

Here A(0) and A(H) are the total relative solar abundances of oxy-
gen and hydrogen respectively. Over most of the temperature range
of interest, the oxygen will be tied up predominantly in H2O and CO

1 We assume here that the total gas pressure is small enough to have negligible
effect on the solids.

i

MUELLER AND OLSEN: CHONDRITIC METEORITES 383

since CO2 becomes an important species only below 400°Ki and no
other element is abundant enough to preempt much oxygen. This
may be seen as follows: If we arbitrarily set A(H) = 10^-, then A(0) =
10*-^^ and A(C), the abundance of total carbon, is 10^'^^ The next
most abundant constituents Si and Mg have abundances of only
10''-'° and lO'^-"" respectively.

For the higher hydrogen pressures methane is the only important
carbon species but at lower pressures CO dominates. For example,
at 800°K P(C0=P(CH4) at P(H2) ~ 10-- atm. In calculating the
curve of Figure 2, it was assumed that the total pressure was high
enough to keep all the carbon as methane. However, if we had as-
sumed that all the carbon occurred as CO (corresponding to low
pressures), the equation (7) would be rewritten as

(8) log P02 - 2 log (0.85 Aq/ Ah) - 2 (log Kg)

It is apparent that on the scale of Figure 2, this shifts the curve only
slightly.

The field of the ordinary chondrites is fairly well defined in Fig-
ure 2. In terms of log P(02), it is limited approximately by the values
corresponding to 0.22 and 10^ for X (ol /Mg) , since ordinary chondrites
have silicates only slightly more iron rich than this and none is more
magnesium rich. We have already indicated that Ki, as determined
from Figure 1 is consistent with high temperatures of crystallization.
Miyashiro (1962a) also found that those meteorites commonly con-
tain high temperature plagioclase. Further evidence that most of
these meteorites represent crystallization at or near liquidus tem-
peratures will be discussed later. Thus, it is natural that we choose
these liquidus temperatures of approximately 1800°K as defining the
upper temperature limit of the field. However, the lower tempera-
ture limit is more difficult to fix and we have somewhat arbitrarily
set it at 1000°K.

It has already been shown (Mueller, 1964) that some of the most
common gases in equilibrium with material of chondritic composition
are H2 and CO rather than H2O and CO2, which are so important in
geologic processes. Thus, the behavior of systems such as melts,
which absorb these gases, may be quite different from their terrestrial
counterparts. Also, we must remember that the equilibrium comple-
ment of gases is unlikely to be retained by the meteorites without
contamination or alteration so that it is not generally safe to draw

' This and other conclusions regarding the solar gas are based on unpublished
calculations by one of us (Mueller, MS).

384

FIELDIANA: GEOLOGY, VOLUME 16

conclusions from gas analyses without determining their compatibil-
ity with the state of oxidation of the meteorite.^

Although the ordinary chondrites are quite reduced by terrestrial
standards, they are far less so than the enstatite chondrites. It is
obvious from the mineralogy of the latter meteorites (Mason, 1962)
that their field of crystallization lies at considerably lower values of
P02 than ordinary chondrites (Olsen and Fuchs, 1967). This is
shown, for example, by the analysis of such reactions as the following

(9)

crystal

metal
- Si

gas
0,

I I I I I I I I

Fig. 2. Approximate fields, in terms of oxidation, of the three major chondrite
groups. The following fields are designated: 0C = ordinary chondrites, EC = ensta-
tite chondrites and CC = carbonaceous chondrites. The light numbered curves
indicate olivine compositions for the assemblage olivine-pyroxene-metal-gas. The
dot-dash curve indicates a gas of solar composition with hydrogen pressures ade-
quate to maintain most carbon as methane. Of the three groups, only the field of
the ordinary chondrites is well defined.

' We should suspect gas analyses which contain high CO2/CO ratios such as
the New Concord chondrite analyzed by Wright (1876) and quoted by Mason and
Wiik (1961). It seems virtually certain that the high CO2 values result from either
late oxidation, adsorption of CO2 or simply from analytical errors.

MUELLER AND OLSEN: CHONDRITIC METEORITES 385

Preliminary results (McCallum, 1965) indicate that log P(0)2 curve
for (9) lies near the center of the area indicated for the enstatite chon-
drites in Figure 2 when account is taken of the silicon dissolved in
the metallic phase.

The limits of crystallization of the carbonaceous chondrites are
particularly ill defined. Many of these meteorites contain two frac-
tions which appear to have been formed under greatly different con-
ditions (Dufresne and Anders, 1962). The matrix, which tends to
dominate, frequently consists of an intimate mixture of carbonaceous
materials such as complex hydrocarbons, magnetite and serpentine-
like silicates, whereas the chondrules consist of olivine and pyroxene
which appear to have crystallized under higher temperatures and
more reducing conditions. This is especially marked in the Murray
carbonaceous chondrite in which some olivine chondrules include
spheres of nickel-iron that appear to have been molten (Fredriksson
and Keil, 1964).

The hatched area by which we designate the field of the carbona-
ceous chondrites in Figure 2 thus applies largely to the matrix mate-
rial. Although most of this area falls in the field of magnetite, it also
is astride the magnetite-metal boundary to take into account the
presence of some metal. It is interesting that the oxidation curve for
the solar gas crosses into the magnetite field in this region close to
400°K, and it may be that this has some significance in the history
of these chondrites.

LiQUiDus Relations Among the Silicates

The crystallization history of the ordinary chondrites can be un-
derstood only in terms of the liquidus relations in the subsystem
MgO-FeO-Si02 or in the somewhat more complex system of the mete-
orites themselves. The major features of the subsystem were worked
out by Bowen and Schairer (1935).i Their results are shown in
Figure 3 in terms of the mole fractions of the components. This
method of plotting is preferred over the more familiar weight-percent
plot since it exhibits the inherent symmetry of the system and is at
the same time directly comparable with other molar plots, such as
Figure 1.

In Figure 3 we have plotted the mean tie lines and normalized
mean bulk composition points for the high and low iron groups.

' Actually Bowen and Schairer worked in the system Fe-MgO-SiOi-Oj. How-
ever, the extra component is compensated by the extra phase represented by the
iron crucibles in which the experiments were conducted.

386

FIELDIANA: GEOLOGY, VOLUME 16

S1O2

MgO

MgSiOj 0.5

0.5 FeSiO,

Mg2Si04
0.3

Fe2Si O4
0.3

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

FeO

Fig. 3. Liquidus relations in the system MgO-FeO-Si02 after Bowen and
Schairer (1935). The diagram is plotted in terms of mole fractions of the com-
ponents. The two points in the olivine field refer to the arithmetic means of the
bulk compositions of 16 low (L) and 16 high (H) iron chondrites whose minerals
were also analyzed by Keil and Fredriksson (1964) (see Table 1). The two min-
eral tie lines shown refer to arithmetic means of the corresponding coexisting
mineral compositions. A liquid-crystal tie line for the low iron group is also shown.
In addition, a curved path of equilibrium crystallization is shown diagrammatically.

The data on which these values are based are given in Table 1. The
normalized points for the bulk compositions refer to mole fractions
of MgO, FeO and Si02 after the subtraction of quantities of these
oxides corresponding to normative feldspars, wollastonite, etc.

We have also drawn as an illustration in Figure 3, the approxi-
mate tie line, as determined by Bowen and Schairer, between the
silicate minerals of the low (L) iron group and the equilibrated liquid.
The composition so defined represents the last small quantity of
liquid in equilibrium with the crystals at the end of the path of equi-
librium crystallization in the field of olivine.

If the bulk chemical analyses agreed perfectly with the mineral
compositions as determined from the electron probe analyses, the

MUELLER AND OLSEN: CHONDRITIC METEORITES 387

points would fall directly on the corresponding tie lines. Although
these points are not far off, they are sufficiently off to show that
errors are involved. The direction of displacement of the points in-
dicates that too much FeO appears in the bulk analysis. This would
be expected if oxidation of some metal had occurred or if metal were
present as inclusions within the silicates. Since both of these sources of
error should occur quite frequently, this deviation is understandable.

In the use of Figure 3, it is important to keep in mind what occurs
during the course of various types of crystallization. For bulk com-
positions like the chondrites, the first crystal to separate is a Mg-rich
olivine if there is sufficient time for this to occur. The composition
of the liquid then follows the curved path to the phase boundary with
pyroxene, and both minerals crystallize simultaneously. The com-
position of the liquid moves along the boundary, and if equilibrium
is maintained by constant adjustment of crystal' composition to
liquid, crystallization ends when the olivine-pyroxene tie line inter-
sects the bulk composition point.

If, on the other hand, Mg-rich crystals are continuously with-
drawn by some such mechanism as crystal settling in a gravitational
field or armoring, then the composition of the liquid may move far-
ther along the pyroxene-olivine boundary. Such fractional crystal-
lization is equivalent to changing the bulk composition and thus
moving the total composition point. This may also be accomplished
by changing the oxidation state during crystallization. For example,
if P(0)2 is arbitrarily decreased, the bulk composition point will move
directly away from the FeO corner toward the MgSi03-Mg2Si04
join. It should be observed that such movement is nearly parallel to
the isotherms of Figure 3 and thus does not involve much of an in-
crease in liquidus temperatures. We shall see that such a mechanism
of changing P(0)2 is capable of explaining certain peculiar composi-
tional relations in some chondrites.

Because the bulk compositional points in Figure 3 represent the
average high and low iron groups after the deduction of certain nor-
mative minerals such as the feldspars, these compositions cannot be
directly related to the plotted isotherms of the subsystem MgO-FeO-
Si02. The temperatures of the more complex liquidus of meteorites

• This seldom occurs in statically crystallizing silicate melts because activation
energies are too high to permit the required exchange difTusion from the interior
of the crystal. Thus, equilibrium crystallization of this type is only a formal
idealization.

388 FIELDIANA: GEOLOGY, VOLUME 16

may lie roughly 100°K lower.' Thus, crystallization of the high (H)
iron group should begin at about 1850°K whereas that of the low
(L) group should begin at about 1825°K.

The relations just discussed show that there is no simple con-
nection between the composition of coexisting chondritic olivines and
pyroxenes and any hypothetical liquid which has not reached the
olivine-pyroxene boundary in equilibrium crystallization. This point
was misinterpreted by Keil and Fredriksson (1964) who sought to
relate the observed condritic olivine compositions to the composition
of liquids in the olivine field as determined by Bowen and Schairer
(1935). Their conclusion that more metal was oxidized at the time
of crystallization than is observed at present is not demonstrable by
such argument. In fact, the relation between the tie lines and the
bulk composition points is not a matter of phase equilibria at all but
is merely a condition of stoichiometry.

If an equilibrium path of crystallization had been followed, the
final equilibration between crystals and liquid would have occurred
at a lower temperature for the low (L) group than for the high (H)
group with a difference of perhaps 25-50°K, if we assume that such
substances as the feldspars lower the liquidus uniformly. Thus, there
is a qualitative agreement between the different values of Ki deduced
from Figure 1 and the expected relation under equilibrium crystalli-
zation. However, we shall see that it is unlikely that crystallization
occurred in this manner.

In addition to the equilibrium and fractional modes of crystalliza-
tion, which occur under conditions of relatively slow cooling, it is
necessary to consider a possible mechanism under conditions of rapid
cooling. The evidence for rapid cooling of the silicates which com-
prise most of the chondrule types is of different kinds and has been
discussed by numerous authors. This evidence takes the form of
such features as microlitic, skeletal and radiating crystals combined
with material that has the appearance of devitrified glass. The latter
material sometimes shows typical glass fracture patterns. If crystal-
lization-cooling is rapid enough, the rate of crystallization and the
composition of the crystals and associated supercooled liquids or
glasses will be restricted by the rate of diffusion. Thus, we can con-

1 Alexeyeva (1958) describes melting experiments on whole meteorites in which
melting began at 1453°K and was complete at 1623°K. Unfortunately, such data
cannot be applied to the determination of liquidus temperatures for the silicate
component of chondrites since oxidation of the metal, and the ferrous component
in the silicates, interferes with this. Also, there is no evidence that the minimum
requirement of a controlled atmosphere was used in the experiments.

MUELLER AND OLSEN: CHONDRITIC METEORITES 389

ceive of a theoretical limiting case in which little or no change in the
ratio of Mg Fe"*""^ occurs on crystallization or congealing of the melt.
However, it is unlikely that this case can be realized if any crystalli-
zation occurs since the time scales of crystallization and the distribu-
tion of Mg and Fe''"''' should both be governed by the time scale of
diffusion. Thus, wherever olivine crystallizes there must be an out-
ward diffusion of Si"^ and an inward diffusion of Mg++ and Fe"^"*",
and, therefore, a distribution of Mg"*"*" and Fe"'""'' consonant with an
approach to equilibrium should also occur.

If, however, we assume the ideal limiting case, this may be illus-
trated as in Figure 4, which is a magnified part of Figure 3. If no
distribution of Fe"^"^ and Mg"^"*" occurred on quenching of the chon-
drules, then all possible proportions of olivine and pyroxene lie on
the dashed line ol' - px", which extends between the olivine and py-
roxene joins and passes through the meteorite bulk composition
point P. If any adjustment toward equilibrium, such as crystalliza-
tion or recrystallization, followed the quenching process, then the
limiting tie lines representing chondrules poor in pyroxene and chon-
drules poor in olivine would be ol' - px' and ol" - px", respectively
while chondrules of unchanged composition would yield the tie line
or - px°.

Vaporization of Metal

At the high temperatures of the silicate liquidus, and, in fact,
down to temperatures of 1500°K, the vapor pressure of iron and
nickel is appreciable. Consequently, transport and deposition of
metals through the vapor phase could play an important role in the
distribution of these substances in the chondrites. At 1700°K, the
vapor pressure of iron is lO"^-^^ atm., while at 1500°K it is 10"^-^^
atm (Kubaschewski and Evans, 1958). The vapor pressure of nickel
is similar but somewhat less. For example, at 1500°K it is 10"^-^^
atm. There will be a mutual lowering of these pressures in the me-
tallic solid and liquid solutions, but this should be so small that it is
probably less than the errors involved in the thermochemical data
for the vapor pressures of the pure substances.

FvELATIONS BETWEEN THE NORMATIVE PHASE
CONTENT AND THE BULK COMPOSITIONS

As originally stated, Prior's rules (Prior, 1916a) recognize the
qualitative relations that exist between the silicate and metal com-

390

FIELDIANA: GEOLOGY, VOLUME 16

Si02

Mg2SiO.

>FeSi03

> Fe2Si04

Fig. 4. Enlarged portion of Figure 3 to illustrate the idealized limiting case of
rapid cooling and crystallization. The point P represents the initial bulk composi-
tion. The dashed lines ol'-P-Px", which also passes through the Si02 apex, repre-
sents cooled mixtures of crystals or glass of varying olivine/pyroxene ratios and
constant Mg/Fe2+. The lines ol'-px', ol" px° and ol"-px" represent near-equilib-
rium tie lines at temperatures which are probably just below the liquidus.

positions and between the quantities of reduced and oxidized iron.*
The first quantitative expression of Prior's rules as concerns the
amount of oxidized and reduced iron was provided by Urey and
Craig (1953) who showed that the relation between these quantities
must be linear with a negative slope if the rules are strictly applicable.
The actual relation found by them from the meteorite analyses was
only a crude approximation to that expression and showed clustering
of the analyses into the familiar high and low iron groups.

The other part of Prior's rules, that relating the intensive com-
positional parameters of the silicates and metal was first discovered
empirically by Ringwood (1961), but it was later shown (Mueller,
1963) that the observational curve agreed in form with that of the
equation of condition for the oxidation of iron at constant Mg, Si,

' Prior believed that the sequence of types could be traced back to a parent
magma of varying oxidation state. However, his observations do not require such
a magma since all the relations could be derived equally by subsolidus oxidation
or reduction.

MUELLER AND OLSEN: CHONDRITIC METEORITES 391

Ni and total iron. The form of the curve generated is highly dis-
tinctive, and although Prior's rules may be satisfied in a variety of
ways in a qualitative sense, only one general shape of curve is pos-
sible under the quantitative constraints.

Prior's oxidation sequence may be formally regarded as having
originated by the displacement to the right of reactions (2) and (3).'
If we begin with a meteorite of a certain fixed quantity of Mg, Si, Ni
and total Fe, then the quantity of olivine increases from some initial
value as pyroxene and metal disappear. Also, according to equation
(5), P(0)2 is approximately proportional to X(ol/Fe)/a(M/Fe)^ if
X(ol/Fe)~X(px/Fe) is as shown by Figure 1. Thus, X(ol/Fe)^
X(px/Fe)~X(S/Fe), where X(S/Fe) refers to the silicates as a
whole. Also, a(M/'Fe) is approximately proportional to X(M/Fe) =
Fe/(Fe+Ni) in the metal. The ratio X(S/Fe)/X(M/Fe), which
we designate as R, is then a measure, in terms of intensive parame-
ters, of the degree of oxidation. Thus, the olivine content is some
increasing function of R. If we designate the molecular normative
olivine content as L, we have the following definitions:

R =

Xl

X"e

moles(Mg,Fe)2Si04

moles (Mg,Fe)2Si04+ moles (Mg,Fe)Si03+moles metallic Fe

The analytical relation between L and R may be found by com-
bining reactions (2) and (3) since the olivine content will depend
only on the ratio (Mg, Fe)0/Si02 but not on MgO/FeO. Thus, we
write

(10) (Mg,Fe)Si03 + Fe +^02-^ (Mg,Fe)2Si04

At any stage of oxidation we designate the "extent of the reac-
tion" in moles of reacted pyroxene and iron as a. Thus, each reacted
mole of (Mg, Fe)Si03 and Fe produces one mole of (Mg, Fe) 28104.
We designate the number of moles of pyroxene and metallic iron ini-
tially present as px° and Fe°, respectively. Also, if there is any olivine
present at the beginning of the process we designate this as 0!°.
Since initially all the iron is reduced, 0!° refers to Mg2Si04 only.

' This formalism does not imply that the members of the sequence were neces-
sarily derived from each other by either oxidation or reduction.

392 FIELDIANA: GEOLOGY, VOLUME 16

Thus, for any stage

(11) R -- Q:[(Fe°-a)^Ni]

(Fe°-a:)(Px°+2oi°-^fl:)

(12) L =

0:^01°

px° + or+ Fe°-CL

It is now possible to compare the theoretical relation between L
and R with observed values of these quantities.

Observed values of L and R for 43 chondrites of uniform com-
position are given in Table 1 and are plotted in Figure 5. Most of
these chondrites represent bulk analyses selected by Urey and Craig
(1953) as "superior" and whose coexisting olivines and pyroxenes
were analyzed by Keil and Fredriksson (1964). To this group have
also been added certain specimens for which recent bulk analyses and
mineral determinations were available and which served to extend
the compositional range. In addition, one enstatite chondrite analysis,
that of Daniels Kuil (Prior, 1916b), is also presented for comparison.

The arithmetic means of the various quantities of the high and
low iron groups are also given in Table 1. The points representing
these mean values are also shown in Figures 3 and 5. In Figure 5 a
straight line was extended through these values for comparison with
the distribution. It is apparent that this line must fall near any
straight line approximation curve for the distribution, except that the
latter would lie a little to the left of the plotted line. Nonetheless,
the slope would be nearly the same. However, any such approxima-
curve would be meaningless since in theory the curve is not a straight
line and, as we shall see, could not in any event accommodate the
inhomogeneous data of Table 1.

In Figure 6 are shown examples of the family of theoretical curves
obtained by the elimination of a from equations (11) and (12) and
with the parameters Px°, Fe° and ol° in the range of meteorite com-
positions. We have also included the line of mean values from Fig-
ure 5, for comparison. Curve 1 of Figure 6 represents the hypotheti-
cal "pure enstatite chondrite" with 01°= 0 and all initial Mg and Si
occurring in pyroxene. Curves (2) and (3) represent more realistic
compositions with initial olivine, which is equivalent to MgOSi02.
These excess olivine contents may be compared directly with the
mean values for the high and low iron groups of Figure 3 by project-
ing through their points from the FeO corner to the Mg2Si04-
MgSiOa join. Such a projection shows that L°, the initial value of

MUELLER AND OLSEN: CHONDRITIC METEORITES 393

L, is approximately 0.1 for both groups but that it is a little smaller
for the low iron group.

Although there is much scatter in the points of Figure 5, the dis-
tribution is obviously an approximation to the theoretical curves in
the middle and upper ranges of values where the slopes are in essen-
tial agreement. The marked deviation in the lower range indicates
that the bulk compositions are not constant and that crossing of the
theoretical curves results. It has not been determined whether this
is due to errors in the analyses or whether it results from the excess
total iron content of the high iron group relative to the low iron group.

Figure 6 is an alternative quantitative expression of Prior's rules
to the relations discussed earlier. The limits imposed on the obser-
vational evidence for Prior's rules have been discussed by a number
of authors. For example, Craig (1964) is of the opinion that the
chondrites as a whole approximate Prior's rules, but that the indi-
vidual groups do not. However, it is difficult to see how this can be
true since there are only two major groups and these groups are well
distiguished by only certain types of data. For example, they are
not well distinguished in Figure 5 which shows a continuous distri-
bution of points. Keil and Fredriksson (1964) also attempted to
show that the part of the rules relating to the quantities of oxidized
and reduced iron did not hold. However, they plotted values for
olivine and pyroxene separately (Keil and Fredriksson, 1964, fig. 11)
without taking account of the total quantity of FeO as governed also
by the amounts of these phases. This type of plot certainly cannot
provide a test for the rules. Still, as was pointed out by these same
authors, there are some large discrepancies between the ratio Fe^"*"/
Fe^'^+Mg) as calculated from the bulk compositions and the same
quantity as limited by the observed mineral compositions. Some of
these discrepancies show up clearly in Figure 5. For example, the two
points representing Collespicoli and Djati-Pengilon, which lie farthest
from the line in the lower part of the diagram, show very large dis-
crepancies of this type (Keil and Fredricksson, 1964, table 6). Yet
some other specimens such as Tomhannock Creek and Forksville,
which also show discrepancies, do not plot unusually far from the line.
In such cases the scatter attributable to analytical error is probably
compensated by some other factor. This is quite possible since, even
if all chemical analytical errors were absent, great scatter could still
occur because of the expected independent variations in MgO, Si02
and total Fe, which to a large extent are also responsible for devia-
tions from Prior's rules.

1.0

0.9

\ 1 1 r

o "HIGH iron" group
- n "LOW IRON" GROUP

L

0.8
0.7 h
0.6
0.5
0.4
0.3
0.2
0.1

0

J L

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

R

Fig. 5. Molecular normative olivine content L, of 43 uniform chondrites
plotted against the ratio of oxidized to reduced iron R (see text). The straight
line joins the arithmetic means of the high and low iron groups.

394

MUELLER AND OLSEN: CHONDRITIC METEORITES

395

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 I.I 1.2

R

Fig. 6. Theoretical plot of the molecular normative olivine content L as a
function of R for various Mg/Fe^+ ratios expressed or "initial olivine" 01°. The
line (0) of mean values is the same as that shown in Figure 5. Note the similarity
of slopes of (0) and curve (3) in the upper and middle ranges and the divergence of
the two slopes in the lower range of (0).

RELATIONS BETWEEN MINERAL CHEMISTRY
AND TEXTURES

Chondrites of Uniform Composition

Comprehensive knowledge of the history of the chondritic mete-
orites can come only through the interpretation of their textures and
the correlation of grain and chondrule types with the mineral chem-
istry. This is more difficult than the deduction of the physico-
chemical environment from phase relations since we are here faced
with identifying the mechanisms which comprise what may have been
a complex series of interwoven chemical and mechanical events.
Given this state of affairs, it seems desirable to copy to a certain
degree our procedure in phase equilibrium studies and discuss only

396 FIELDIANA: GEOLOGY, VOLUME 16

those mechanisms of sufficient generality to encompass the most
observations.

The division of the chondrites into a uniform or "equihbrium"
type, on one hand, and variable or nonequilibrium type, on the other,
is convenient, but it is probable that a gradation between the two
groups exists. The nature and orderliness of the distributions of Mg
and Fe^"*" shown by Figure 1 points to the attainment of a measure
of chemical equilibrium; however, it seems improbable that perfect
equilibrium was ever attained even within this group. This may be
seen by considering the mixture of phases of different densities which
comprise these bodies. If even a weak gravitational field existed at
the place of formation, it is certain that the bodies were not in me-
chanical equilibrium (and hence also not in perfect chemical equi-
librium), since there should then have been a segregation of silicate
and metal phases. Of course, this state of mechanical disequilibrium
is common enough in terrestrial metamorphic rocks which exhibit a
fairly close approach to local chemical equilibrium.' The explana-
tion is to be found in the high viscosities of solids which prevent their
attaining the level appropriate to their densities. However, if the
metal and silicate components of meteorites had been associated
when either or both were molten, separation should have occurred
in a gravitational field.

Any cursory examination of large slabs of chondrites (such as
those in the Field Museum of Natural History) reveals their striking
uniformity in the distribution of metal grains. However, many of
these same specimens, some of which range up to 0.5 meter in dimen-
sion, also contain segregations of metal in the forms of veins which
appear to mark healed fractures. Specific examples of such mete-
orites are the chondrites Farmington, Cullison, Bluff, Arapahoe and
Rose City. Also, it is well known that the metal grains are usually
highly irregular and of an interstitial character. A typical relation
between the metal and silicates is shown in Figure 7 from the chon-
drite Allegan. The metal here is closely molded to the chondrules
but is not restricted to any one chondrule. Metal apophyses extend
into the surrounding matrix of silicates.

These facts regarding the form and disposition of metal grains
within the body of the meteorite and the segregation into veins seem
to argue for a high degree of mobility of the metal after or at the time
of consolidation of the bodies. They thus appear to rule out both

' This local equilibrium is usually restricted to volumes less than one cubic
centimeter (Mueller, 1960).

MUELLER AND OLSEN: CHONDRITIC METEORITES

397

i mm

Fig. 7. Drawing of thin section of the chondrite Allegan illustrating typical
silicate-metal relations.

magmatic and mechanical processes as important in the formation
of the existing textural relations.

A detailed study of the structural and textural relations of metal
particles in chondrites was undertaken by Urey and Mayeda (1959).
Many of the grains observed by them seemed to exhibit complicated
mechanical and thermal histories. Some grains were found to con-
sist of plessite while others combined large crystals of pure kamacite
and taenite. In a few cases truncated diffusion borders indicated
mechanical disruption. From this Urey and Mayeda concluded that
the grains had been derived from a pre-existing "primary object"
which had been broken up and reconstituted to form the meteorites.

398 FIELDIANA: GEOLOGY, VOLUME 16

However, R. E. Marringer (Mason and Wiik, 1961) has discussed
this problem in terms of the Mocs chondrite and concluded that
the disruption of metal grains appears to have occurred essentially
in place. As we shall see, it is also easy to account for such features
as individual crystals of kamacite and taenite which were not de-
rived by unmixing.

One of the best known mechanisms of crystallization and mineral-
forming processes is deposition from a fluid or vapor phase. We have
already called attention to the high vapor pressures of the metals
near the silicate liquidus and indicated that this favored rapid trans-
port and deposition of metal. There is also experimental evidence
which supports this contention. Thus, Morelock (1962) described the
growth of iron whiskers from vapor at temperatures as low as 1333°K
with 200m whiskers formed in 16 hours.

The deposition of iron and nickel from the vapor phase goes a
long way in explaining the observed textural features. It accounts
especially well for the interstitial and irregular character of the metal
particles, and the common occurrence of veins indicates that trans-
port and deposition continued after the meteorites consolidated
enough to fracture. This type of deposition is also capable of ac-
counting for the variety of occurrences of kamacite and taenite
observed by Urey and Mayeda (1959). Thus, for example, isolated
grains of either of these might grow from the vapor below the trans-
formation curve without the appearance of exsolution textures. How-
ever, such low temperature growth should be much slower than that
just below the silicate liquidus.

If growth occurred from the vapor phase, one might expect some
relation between the distribution of metal grains and the available
interstitial space. It seems conceivable that the excess iron of the
high iron group might therefore be attributable to a higher porosity
at the time of consolidation. If consolidation of the chondrites oc-
curred in some type of parent body with a metal core, it might have
happened that iron vapor transported from the hot, metal-rich inte-
rior deposited between the silicate grains, filling most of the avail-
able space.

There can be very little doubt that the unique textural features
of the chondrites result from some mode of crystallization on the
liquidus. This crystallization mechanism has time after time given
rise to a variety of chondrule forms which fall roughly into the fol-
lowing major groups: 1) barred olivine with interstitial rest crystals
or glass, 2) porphyritic olivine with a microcrystalline ground mass

MUELLER AND OLSEN: CHONDRITIC METEORITES 399

or glass, 3) granular olivine with a microcrystalline ground mass or
glass and 4) nearly pure pyroxene chondrules, usually with an eccen-
tric radiating structure. Frequently all four forms of these chon-
drules occur within the same thin section although they may vary
greatly in abundance. Furthermore, they seem to occur without any
obvious relation to the ordinary chondrite subdivisions. They are
all common in both the high and low iron groups and within the uni-
form and variable chondrites as well. For example, such high iron
group uniform chondrites as Allegan, Barbotan and Kesen all contain
a variety of well-defined chondrules as do also the low iron group
representatives Albareto, AMnanello, Aussun and Bjurbole.' In
most of these chondrites the glassy-appearing material has appar-
ently devitrified or consisted of microcrystals from the start. How-
ever, fairly convincing evidence for true glass in the Breitscheid
chondrite was presented by Hentschel (1959). Yet recently much
emphasis has been given by Wood (1963a) to a few low-iron group
chondrites, such as Lissa and Bath Furnace, that show only a few
well-developed chondrules, and these have been taken to represent
the end products of a metamorphic sequence.

In the uniform chondrites we are then faced with the difficulty of
reconciling the coexistence in a single meteorite of widely different
chondrule types with the apparent uniformity of mineral composi-
tion throughout the specimen and with the additional evidence of
rapid quenching from the liquidus. Thus, if we accept the results
of the electron probe, optical and X-ray investigations, this uniform-
ity applies to chondrules of nearly pure olivine and to those of nearly
pure pyroxene. If we assume that each chondrule represents a speci-
men of pure liquid or of a liquid-crystal suspension, then systems of
varying composition always gave rise to the same mineral composi-
tions. This was refered to as "Fredriksson's paradox" by Suess
(1964) and is an impossible state of affairs under either equilibrium
or fractional crystallization.

Keil and Fredriksson (1964) found it difficult to believe that
equilibrium could be attained under conditions of rapid cooling from
the liquidus and attempted to show by references to the Bowen and
Schairer (1935) phase diagram that the olivines contained too much
magnesium relative to the pyroxenes. While this conclusion does
not seem acceptable in terms of later experimental work (fig. 1), the
degree to which equilibrium was approached and the temperatures

1 Based on the Field Museum of Natural History collection.

400 FIELDIANA: GEOLOGY, VOLUME 16

represented by the observed meteorite distributions are far from clear.
An interesting suggestion was made by Fredriksson (1963) that rap-
idly cooled chondrules would give rise to olivine grains of almost
constant composition in a glassy matrix and that this matrix would
later devitrify to pyroxene and plagioclase. Presumably, the glass
would also have a nearly constant Mg and Fe^""" content. We have
already discussed an extension of this idea in relation to Figure 4 in
which the ideal limiting case of constant Mg/Fe^"*" ratio for chondrules
of varying olivine and pyroxene content was considered. We still
need, however, to consider a mechanism by which the different chon-
drules can be derived.

It is possible to outline a scheme for obtaining the four major
chondrule types through some unspecified process which brings about
disruption of a liquid-crystal suspension so that separation of these
components can occur. The general scheme is shown in Figure 8.
Obviously, such a mechanism must be highly dependent on the am-
bient temperature and the kinetic properties of crystals and liquid.
According to this scheme, the barred olivine and radiating enstatite
chondrules come closest to representing possible liquids. Only the
barred type would have a high probability of representing the liquid
of the bulk meteorite composition, although it could also represent
liquids of varying degrees of differentiation. In contrast, the radiat-
ing pyroxene type could, assuming it consists of almost pure pyrox-
ene, represent only liquids near the pyroxene join.

The porphyritic condrules, at least where well developed, should
indicate a period of relatively slow cooling and crystallization in the
olivine field before the disruptive process. Consequently, their crys-
tals should be more Mg-rich than those formed by quenching.

Presumably, the granular olivine chondrules are nearly pure oli-
vine with most of the rest liquid excluded. They may in some cases
be gradational into the porphyritic type.

An examination of thin sections shows that the various chondrules
are frequently composites. Thus, for example, the composite porphy-
ritic and barred olivine type observed in Allegan and the combined
porphyritic and radiating types in Aussun may indicate a period of
slow cooling before the ultimate quench. The porphyritic chondrules
are usually not as abundant as other types so that any Mg-rich crys-
tals they contain might go undetected.

If the unspecified disruptive process brings about a separation of
crystals and liquid, the allowable variation is between the tie lines
ol'-px' and ol"-px" of Figure 4 as already discussed. This allowable

MUELLER AND OLSEN: CHONDRITIC METEORITES

401

LIQUID OF \
CHONDRITIC ^
COMPOSITION

barred

porphyritic

OLIVINE

a

LIQUID

granular

radiating
Pyroxene

CHONDRULE
TYPE

Fig. 8. General scheme for the derivation of various chondrule types by an
unspecified, rapid disruptive process which brings about separation of crystals
and liquid.

variation is not large and might be reduced by the chemical analyti-
cal limitations. However, we have already concluded that the ideal
limiting case is probably not realized, so that we must either call
upon a modified mechanism in which a distribution of Mg and Fe^+
occurs during quenching or one in which variations are annealed out
below the liquidus. The nature of the distribution shown by Fig-
ure 1 indicates that if the former takes place, the distribution cannot
have occurred between olivine and liquid or a liquid-like glass since
the liquid is always richer in Fe^"^ than either crystal. It may be that
the extreme cases of Mg-rich olivine and Fe^''"-rich liquid are not
realizable in a rapid quench but that a continuous rapid adjustment
of olivine crystal to microcrystalline pyroxene or pyroxene-like glass
represented by the tie lines is realizable. The only alternative to

402 FIELDIANA: GEOLOGY, VOLUME 16

this seems to be the annealing adjustment to an approximate equi-
librium distribution between the olivine and devitrifying glass just
below liquidus. The latter would be considered a form of meta-
morphism by some.

It seems likely that there is a close relation between the chondrule
structures and the limitations imposed by crystallization kinetics.
As an illustration, the widths of the bars in barred olivine chondrules
are usually in the range of 10-- mm. and this may represent the dif-
fusion length of Si'*"'' at the temperature at which crystallization
occurs. It would seem that such correlations would open a fruitful
line of investigation in the future.

Chondrites of Variable Composition

These chondrites are characterized by a variability of composi-
tion of their olivines and pyroxenes both with respect to position
within the specimen and by a lack of orderly distribution relations
between the silicates. Significant variations of this type were found
in eight chondrites by Keil and Fredriksson (1964). An extensive
investigation of variability in the carbonaceous chondrite Murray
was also made by Fredriksson and Keil (1964) and of Chainpur by
Keil et al. (1964). Recently variation has also been found in 11 more
chondrites by Dodd and Van Schmus (1965).

In the Chainpur chondrite (Keil et al., 1964) both pyroxene and
olivine from the same chondrule show values of Fe^''"/(Fe^"'' + Mg)
of about 0.05 whereas in other chondrules the minerals are far more
iron rich. Also, generally, the olivine shows a larger range of compo-
sitions than the associated pyroxene grains, which is compatible with
a wider expected range of crystallization for the former mineral. In
addition, some grains show marked compositional zoning with the
iron content increasing outward. As was pointed out by Keil and
Fredriksson (1964), this zonal structure and variability seems more
compatible with a slower rate of cooling than one can imagine for the
uniform chondrites.

In order to explain the coexistence of Mg-rich pyroxenes and oli-
vines, we must call on a different mechanism. A suitable one is that
of reduction of some of the meteoritic material which then becomes
mixed with that which is more oxidized so that the average for the
meteorite is attained. This is equivalent to moving the bulk com-
position point of the reduced material toward the Mg2-Si04-MgSi03
join, as has already been stated.

MUELLER AND OLSEN: CHONDRITIC METEORITES 403

It seems that mixtures of materials of varying oxidation state is
a characteristic also of many of the carbonaceous chondrites which
as a class appear somewhat similar to the variable ordinary chon-
drites under discussion here. Dodd and Van Schmus (1965) inter-
preted the latter group as representing a recrystallization sequence
based on correlation of such features as degree of variation of the
iron content of olivines with the presence of glass and the extent of
chondrule-matrix intergrowth. However, as these authors themselves
pointed out, these correlations are very difficult and uncertain. In
the opinion of the writers, even their interpretation of the Bruder-
heim chondrite as "strongly recrystallized" and as representing the
end of the sequence is open to serious doubt. It is difficult, for ex-
ample, to see how this meteorite could have retained its observed
isotopic fractionation between chondrules and matrix if this were
true (Merrihue, 1963). The observed properties of Bruderheim agree
more closely with the scheme of rapid crystallization outlined here
for the uniform chondrites.

CRITIQUE OF SOME PREVIOUS CONCLUSIONS
ON THE CHONDRITES

Of the modern ideas on the origin of the chondrites and chon-
drules only those of Mason (1960) and Wood (1963a,b) rest primarily
on a mineralogical base, so that they become valid subjects of this type
of critique. Their authors have committed themselves to definite
positions which are open to test by presently-available information.

Mason arrived at the opinion that chondrules were derived from
pre-existing hydrous silicates by crystallization in the solid state,
that they were analogous to porphyroblasts and resulted from ther-
mal metamorphism. In view of the wealth of evidence presented
here and elsewhere (Keil and Fredriksson, 1964) for a high tempera-
ture liquidus history, it would seem that this and analogous views
are no longer tenable.

Wood argued that the ordinary chondrites were derived from par-
ent material represented by the single specimen Renazzo by a type
of "metamorphic" recrystallization. The Fe^+-poor chondrules of
this meteorite, which lie in a more oxidized matrix consisting of
Fe^'''-rich ferrite and silicates (Mason and Wiik, 1962), were shown
by him to be compatible with the oxidation state of a gas of solar
composition at high temperatures (~2000°K). The matrix on the
other hand presumably could be in equilibrium with the solar gas
only at much lower temperatures, as is shown also by Figure 2. The
mechanical mixture of these high and low temperature products

404 FIELDIANA: GEOLOGY, VOLUME 16 ^

would, if heated, be expected to strive for an intermediate state. Ac-
cording to Wood (1963a), this was attained when "Fe^"*" from the fine
Fe304 grains diffused into the chondrule minerals." Wood (1963b)
also presented a detailed set of time scales for this process operating
at different temperatures. His calculations were presumably based
on diffusion coefficients deduced by Naughton and Fujikawa (1959)
(Wood, 1963a) . Wood does not seem to have concerned himself with
the necessity of exchanging Mg^+ ions in the same process, although
this could in principle be the rate controlling factor. Also, the work
of Naughton and Fujikawa refers to intergranular diffusion which, as
is well known, is almost always much faster than lattice diffusion.
Certainly the magnitude of the change required in Wood's hypothesis
requires lattice diffusion to be the major type operative so that the
time scales presented by him have no validity whatever.

The validity of Wood's scheme has also been challenged on other
grounds. Keil and Fredriksson (1964) noted that contrary to his
hypothesis there appeared to be no connection between the iron con-
tent of the silicates and the distinctness of chrondritic texture. Even
a very cursory examination shows that some of the most iron-rich
silicates occur in meteorites with well-defined chondrule structure,
microlitic texture and glass. Wood himself (1963a, p. 169) notes
features of several chondrites which are not possible to reconcile with
his hypothesis, and says: ". . .in some respects the concept of thermal
metamorphism taxes credibility." He concludes, nevertheless: "In
spite of these difficulties, the evidence seems very strong to me that
most of the ordinary chondrites have been metamorphosed."

In contrast to the contradictory character of Wood's scheme for
chondrite metamorphism, his attempts to correlate the mineral chem-
istry with the physicochemical environment of the hypothetical solar
nebula appears to form a valid approach. His application of the
methods first introduced by Urey (1952) have the advantage of mak-
ing use of stable mineral compounds rather than such substances as
MgO, FeO, etc. However, we have seen that few meteorites are re-
duced enough to have been equilibrated with the solar gas at the high
temperatures implied by their mineralogy. As Wood himself indi-
cated, it is possible that condensation occurred in regions which had
been relatively depleted in hydrogen so that more iron-rich silicates
were possible. Perhaps this is a natural and predictable consequence
of the processes which gave rise to the condensed bodies of our solar
system.

I

MUELLER AND OLSEN: CHONDRITIC METEORITES 405

ACKNOWLEDGMENTS

The writers would like to express their thanks to Dr. H. B. Wiik
for making available to them several unpublished meteorite analyses,
and to Mrs. Dorothy Walker for typing the manuscript. The writers
are grateful to the National Science Foundation which provided sup-
port for this work under grants GP-3688 and GA-307.

406

FIELDIANA: GEOLOGY, VOLUME 16

Table 1. — Normalized mole fractions of bulk compositions, normative olivine (L)
and ratios of oxidized to reduced iron (R) for 43 ordinary chondrites. The
normalized mole fractions result from the subtraction from the original analy-
ses of quantities of oxides corresponding to normative albite, anorthite, ortho-
clase, wollastonite, chromite, ilmenite and Ca3P208. In the source column for
the bulk analyses U & C refers to Urey and Craig (1953). Additional sources
of analyses are as follows: Duke et al. (1961): Bruderheim; Mason (1963):
Yonozu; Mason and Wiik (1961, 1963): Ottawa, Chateau Renard, Mocs, New
Concord, Richardson, Estacado, and Knyahinya; Miyashiro (1962b,c): Kes-
sen, Mino; Miyashiro et al. (1963a,b,c): Sone, Tomita, and Kasamatu; Mura-
yama et al. (1962): Sasagase; Prior (1916b): Daniel's Kuil; Vilcsek (1959):
Breitscheid; Wiik (1956, 1965): Farmington, Hamlet, Hokmark, Holbrook,
Kyushu, Ochansk, Rose City, Tomhannock Creek, and Varvik. *Refers to
chondrites for which mineral analyses exist (Keil and Fredriksson, 1964) and
whose average values appear in the last two rows. These averages appear as
points in Figure 3 with the corresponding tie lines. The values of L and R
are plotted in Figure 5.

Bulk

Source of

Bulk
Analysis

Mole Fraction

Name

Type

MgO

FeO

Si02

L

R

Albareto*

L

U&C

0.4716

0.1625

0.3659

0.631

0.296

Allegan*

H

U&C

0.4848

0.1009

0.4143

0.231

0.186

Beaver Creek*

H

U&C

0.4677

0.1103

0.4219

0.248

0.206

Bjurbole

L

U&C

0.4798

0.1329

0.3873

0.476

0.243

Breitscheid*

H

Vilcsek

0.5154

0.0863

0.3984

0.318

0.1.54

Buderheim*

L

Duke et al.

0.4619

0.1421

0.3960

0.427

0.272

Chantonnay*

L

U&C

0.4790

0.1554

0.3656

0.595

0.285

Chateau Renard*

L

Mason &

Wiik

0.4749

0.1289

0.3962

0.417

0.243

Collescipoli*

H

U&C

0.5348

0.0704

0.3948

0.308

0.125

Coon Butte*

L

U&C

0.4369

0.1406

0.4225

0.302

0.279

Daniel's Kuil

EC

Prior

0.4961

0.0001

0.5038

0

0

Djati-Pengilon*

H

U&C

0.4302

0.1566

0.4132

0.251

0.298

Ekeby*

H

U&C

0.4874

0.1249

0.3877

0.351

0.224

Estacado*

H

Mason &

Wiik

0.4583

0.1354

0.4062

0.306

0.250

Farmington*

L

Wiik

0.4498

0.1496

0.4007

0.416

0.289

Forest City*

H

U&C

0.5013

0.0947

0.4040

0.286

0.169

Forksville

L

U&C

0.4515

0.1789

0.3696

0.577

0.321

Hamlet

L

Wiik

0.4462

0.1690

0.3848

0.554

0.357

Hessle*

H

U&C

0.4764

0.1250

0.3986

0.320

0.233

Hokmark*

L

Wiik

0.4449

0.1658

0.3893

0.361

0.239

Holbrook*

L

Wiik

0.4684

0.1218

0.4097

0.364

0.236

Homestead*

L

U&C

0.4698

0.1300

0.4002

0.376

0.242

Kasamatu

H

Miyashiro

et al.

0.5048

0.0964

0.3988

0.303

0.175

Kesen*

H

Miyashiro

0.4916

0.1093

0.3991

0.292

0.197

Knyahinya*

L

Mason &

Wiik

0.4498

0.1612

0.3889

0.521

0.341

Kyushu*

L

Wiik

0.4438

0.1520

0.4042

0.402

0.307

Merua*

H

U&C

0.4894

0.0832

0.4273

0.211

0.159

Mino

L

Miyashiro

0.4747

0.1352

0.3901

0.444

0.253

Mocs*

L

Mason &

Wiik

0.4700

0.1194

0.4106

0.346

0.230

New Concord*

L

Mason &

Wiik

0.4731

0.1203

0.4065

0.368

0.232

MUELLER AND OLSEN: CHONDRITIC METEORITES

407

Table 1 (Continued)

Bulk

Source of

Mole Fracti

on

Bulk

^

Name

Type

Analysis

MgO

FeO

SiO.

L

R

Oakley*

H

U&C

0.4857

0.1143

0.4000

0.326

0.212

Ochansk*

H

Wiik

0.5052

0.0835

0.4113

0.253

0.153

Ottawa

L

Mason &

Wiik

0.4448

0.1763

0.3790

0.593

0.384

Richardton*

H

Mason &

Wiik

0.4874

0.1173

0.3953

0.312

0.210

Rose City

H

Wiik

0.5136

0.0715

0.4148

0.191

0.132

Sasagase

H

Murayama

et al.

0.4862

0.1073

0.4064

0.292

0.197

Soko-Banja*
Sone

L
H

U&C
Miyashiro

0.4395

0.1681

0.3924

0.500

0.364

et al.

0.4891

0.1153

0.3956

0.329

0.210

Stalldalen*

H

U&C

0.4877

0.0935

0.4188

0.235

0.175

St. Michel

L

U&C

0.4776

0.1434

0.3790

0.504

0.263

Tomhannock

Creek*

H

Wiik

0.4506

0.1559

0.3935

0.394

0.293

Tomita

L

Miyashiro

etal.

0.4705

0.1333

0.3962

0.399

0.251

Varvik*

L

Wiik

0.4380

0.1765

0.3855

0.576

0.472

Yonozu

H

Mason

0.4878

0.1114

0.4008

0.304

0.204

Av H (Total)

0.4878

0.1075

0.4048

0.289

0.199

Av L (Total)

0.4598

0.1482

0.3919

0.462

0.290

Av H (K&F)

0.4846

0.1101

0.4053

Av L (K&F)

0.4594

0.1454

0.3951

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1