Aragonite in California Glaucophane Schists, and the
Kinetics of the Aragonite—Calcite Transformation
by W. H. BROWN, W. S. FYFE, and F. J. TURNER
Department of Geology, University of California, Berkeley, U.S.A.
WITH THREE PLATES
INTRODUCTION
COLEMAN & Lee (1961) made the interesting discovery that aragonite—the
high-pressure polymorph of CaCO3—occurs widely as a metamorphic mineral
in continuous mappable units of glaucophane and lawsonite schists in the
Franciscan of California. Though occurring mainly as veins and lenses, it is
intimately associated with typical minerals of the glaucophane-schist facies—
glaucophane, lawsonite, pumpellyite, stilpnomelane, and others. Aragonite is
considered by Coleman & Lee to be a characteristic mineral of the glaucophaneschist facies and an index of metamorphism at high pressures and relatively low
temperatures.
In this paper we present additional information regarding the occurrence of
aragonite in Californian glaucophane schists as contrasted with calcite in the
greenschist facies of New Zealand and Japan. Optical methods have been
developed for distinguishing the two polymorphs microscopically. The kinetics
of the aragonite-calcite transition at geologically low temperatures have been
explored experimentally with a view to setting limits upon the temperature range
of glaucophane schists containing aragonite.
(Journal of Petrology, Vol. 3, Part 3 , pp. 566-82, 1962)
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ABSTRACT
Coleman & Lee's discovery that aragonite is a widespread metamorphic mineral in California glaucophane schists is confirmed and amplified. Methods of microscopic distinction
between aragonite, calcite, and dolomite, including a universal-stage technique, are described.
Further data are recorded regarding the paragenesis of aragonite-bearing gJaucophane and
lawsonite schists. Carbonates in the greenschist facies are found to be exclusively calcite and
dolomite. In many Californian metamorphic aragonites partial inversion to calcite has been
observed. This appears to be an equal-volume replacement in which an a axis and an edge
[/:/] of_ calcite—both directions of Closest spacing of Ca"1"1" ions—commonly are parallel to
Jwoof the crystal axes a, b, and c of aragonite.
The problem of survival of metamorphic aragonite through a necessarily long period of
post-metamorphic unloading is approached by experimental exploration of the kinetics of
the aragonite -> calcite transformation. It is found that Californian aragonite could survive
unloading from a depth of 20 km if the linear temperature gradient were 10° C per km, but
not appreciably higher. Available experimental data are consistent with crystallization of
aragonite, jadeite, and lawsonite at depths of 20-30 km if a gradient of 10° C per km is
assumed. The corresponding conditions of the glaucophane-schist facies (T = 200°-300° C,
P = 6000-9000 bars) are attributed essentially to deep burial in regions (subsiding geosynclines) of unusually low temperature gradient (10° C per km).
ARAGON1TE IN CALIFORNIA GLAUCOPHANE SCHISTS
567
MICROSCOPIC DISTINCTION BETWEEN ARAGONITE, CALCITE, AND
DOLOMITE
FIG. 1. A, poles of cleavages r, twin lamellae c and /, and crystal a,, a,, a,, and c in rhombohedral
carbonates. Principal zone circles, broken arcs. Equal-area projection, upper hemisphere, B, poles of
twin lamellae m, crystal axes a, b, c ( = indicatrix axes Y, Z, X) and optic axes Au A,, in aragonite
(circled points). Crosses (K', m'lt Sec.) show corresponding points in a lamella twinned on 110. Broken
diameter, TP, is trace of twin plane. Equal-area projection.
Identification with the flat stage. In metamorphic rocks all three carbonates
commonly display lamellar twinning of a kind diagnostic for each species.
In calcite, twinning is very widespread, though unfortunately not universal.
The twin plane is {0lT2} = e (cf. Fig. 1A). Where two sets of twins are present
the angle of intersection, ex A e2 is 45°. Especially characteristic, in sections showing clear-cut e lamellae and marked change of relief on rotation in polarized
light, is the extinction angle X' A e > 55°.
In dolomite many grains may show no twinning, and in twinned grains lamellae
tend to be few. The twin plane is {0221} = / , and the angle / i A / 2 is 80° (cf. Fig.
1A). In sections showing clear-cut / lamellae and marked change of relief, the
extinction angle, X' A / = 25°-4O°, is diagnostic.
Aragonite in glaucophane schists is almost universally twinned on m = 110
(cf. Fig. 1B). Any section cut nearly normal to [001] ( = c = Bxa = X) shows
two sets of lamellae intersecting at 64°; and if the lamellae are broad they may
6233.3
OO2
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The three common carbonates of metamorphic rocks—aragonite, calcite, and
dolomite—are readily distinguished one from another in crushed grains separated from the mother rock. However, to investigate textural relationships
between carbonates or between a carbonate and a silicate, individual grains as
seen in an ordinary thin section must be specifically identified. The mineralogical
identity of twinned carbonate grains is readily determined in the course of
routine examination with a flat stage; and even untwinned grains may be quickly
identified by a universal-stage technique to be described below.
568
W. H. BROWN, W. S. FYFE, AND F. J. T U R N E R
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themselves contain visible fine lamellae twinned on {lTO}'. Sections of this
orientation may be identified by low-order polarization colors, uniformly high
relief on rotation, and a characteristic bisectrix interference figure indicating
2V = about 20°. The biaxial condition is not of itself distinctive, however, for
metamorphic calcite, too, is commonly biaxial. Because both sets of lamellae
have [001] = X in common and 2V is
small, twinning in aragonite is not optically
recognizable except in sections nearly normal
to [001]. Most grains of aragonite therefore
show no visible twinning. Nevertheless, with
a little experience the probable identity of
non-twinned aragonite may be suspected by
virtue of the appearance of the grain as it is
rotated in polarized light with the analyser
removed. Whereas the a indices of calcite
(1-486) and dolomite (1-501) are considerably lower than the index of standard
mounting media (1-54), a for aragonite is
FIG. 2. Circle of measured slow rays Z'
in relation to X of the indicatrix in a 1 -530. Highly birefringent grains of aragonite
carbonate crystal. Equal-area projection, therefore assume a smooth appearance and
lower hemisphere. Readings of Z' are: almost disappear when the fast direction is
55°-+20°; 62° -* 10"; 69° -.-O";
brought parallel to the analyser; whereas
80°— 20°; 89°-* 30°.
the apparent relief of a calcite or a dolomite
grain changes from positive to negative on rotation but remains distinct in all
positions.
Identification with the universal stage. We recommend use of a universal stage
for certain identification of aragonite, and especially for eliminating the possibility that the mineral tentatively identified in the course of routine examination
as aragonite may actually be non-twinned calcite or dolomite.
The c axis, X, is first located as accurately as possible. In sections inclined to
A'at high angles this can be done by the conoscopic method. In sections markedly
oblique to X, between three and six Z directions (cu in rhombohedral carbonates;
Z' in aragonite) are measured and plotted, and will be found to lie on a great
circle normal to X (Fig. 2; cf. Turner, 1949, pp. 598-600). The axial angle of
aragonite {IV'= 18°) is small enough to allow aragonite to be treated as uniaxial in sections acutely inclined to X. When the X ( = c) axis of a grain has
been located, the section is now scanned for twin lamellae or—if the section is
very thin—for rhombohedral cleavages. These are plotted and, using the crystallographic angles of Table 1 and zonal relations of Fig. 1, identified as e,f, r, or
m as the case may be. The mineral is thus identified as aragonite, calcite, or
dolomite.
A grain lacking visible twin lamellae or cleavages but in contact with the
mounting medium, quartz, or albite, may be identified with certainty by matching
ARAGONITE IN CALIFORNIA GLAUCOPHANE SCHISTS
569
TABLE 1
Diagnostic interfacial angles in carbonates
Angle
Aragonite
[0001] toj_{10ll}; c t o r
[0001] to_L{0U2};c t o e
[O001]to±{022l};cto/
r, to r,
«! to e,
el to r,
Calcite
Dolomite
441°
26°
44°
621°
74°
75°
45°
38°
[001] t o J_{110}; c t o m
64°
90°
its refractive index with that of the adjacent material. The method used is an
extension of that described by Emmons (1943, p. 189) and by Gilbert & Turner
(1949, pp. 23, 24) for distinguishing calcite from dolomite. With both tilting
axes of the stage at zero, the section is rotated on the innermost axis to bring the
c axis (X) onto the NS cross hair (assuming that the vibration direction of the
polarizer is NS). With illumination adjusted by substage and objective iris
diaphragms, the section is tilted on EW until the refractive index of the carbonate
exactly matches that of the adjoining material. If X is inclined at a low angle to
the plane of the section—especially in the case of aragonite—it is possible to
determine two matching positions equidistant from c. The angle between the c
axis and the matching position (matching angle) is diagnostic of each mineral
species (Table 2).
TABLE 2
Matching angles* for refractive indices of carbonates and adjacent media
Adjacent medium
Albite
Canada balsam
Quartz
R.I.
Aragonite
Calcite
Dolomite
a = 1-528
y = 1-538
1-540"
a = 1-544
y = 1-554
3°
14°
15°
18°
25°
31°
35°
36°
38°
41°
24°
28°
29°
31°
35°
•Determined from PI. 10 in Emmoru (1943).
Where the adjoining mineral is quartz or albite a value of refractive index
intermediate between a. and y can be estimated roughly from the angles between
the indicatrix axes (of quartz or albite) and the matching ray. Allowing for this
uncertainty, for fluctuations in the refractive index of the mounting medium, and
for inaccuracy in locating X in the carbonate, the error in the matching angle is
usually less than 3°. For example, in many measurements involving matching
of aragonite with the mounting medium the range of values of matching angle
was 11° to 17° as compared with the theoretical value (for /x = 1-540) of 15°.
Distinction from calcite (matching angle = 36°) is unambiguous.
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80°
/ito/,
±{110} ioS_{\\Q};m1 torn,
570
W. H. BROWN, W. S. FYFE, AND F. J. TURNER
CARBONATES IN CALIFORNIAN GLAUCOPHANE SCHISTS:
PETROGRAPHY
1
Numbers refer to thin sections in collections of the Department of Geology, University of
California, Berkeley.
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The conclusion of Coleman & Lee that aragonite is the typical carbonate of
the glaucophane-schist facies in California has been confirmed by microscopic
identification of carbonates in about 50 thin sections selected from several
hundred glaucophane- and lawsonite-bearing rocks from northern California.
The following notes amplify the petrographic and paragenetic data recorded by
Coleman & Lee:
(1) In two of the most extensive areas of glaucophane schist in northern
California—San Benito County (Alfors, 1959) and Healdsburg Quadrangle
(Borg, 1956)—the carbonate of the metamorphic rocks is almost exclusively
aragonite. It commonly occurs as a minor member of metamorphic assemblages
including glaucophane (Plate 1,fig.A), lawsonite, pumpellyite, aegirine, jadeite,
muscovite, chlorite, and rarely epidote. Where relatively plentiful it tends to be
closely associated with lawsonite (379-2, 379-3, 331-M38).1 In the same areas
aragonite may form veins cutting glaucophanic rocks, and containing, in
addition to aragonite, such silicates as glaucophane (Plate 1, fig. B) and pumpellyite (379^6). Rare occurrences of calcite in metamorphic rocks of these
areas are limited to minor cross-cutting veinlets.
(2) In three altered eclogites from widely separated localities (38-M89, Valley
Ford; 38-M84, Tiburon Peninsula; 331-MH11, near Healdsburg) aragonite is
one of the minerals that rims and veins relict crystals of garnet (Plate 2); with
it may be associated chlorite, muscovite, or lawsonite, and it may show incipient
patchy replacement by calcite. These rocks represent different stages in replacement of the eclogite assemblage, garnet-pyroxene-rutile, by the glaucophaneschist assemblage, glaucophane-chlorite-muscovite-lawsonite-sphene (Borg,
1956).
(3) Among the blocks of glaucophane and lawsonite schist that are profusely
scattered as tectonic inclusions in serpentinite in the hills of north Berkeley
(Brothers, 1954) are some that contain carbonate. This is invariably calcite
(e.g. 21M-1 J, 4, 47A). It may vein and replace lawsonite porphyroblasts (21M49). Calcite is also the last of a sequence of vein minerals (lawsonite, pumpellyite, calcite) formed along fractures in a massive lawsonite vein at one locality
(Davis, 1960).
(4) Carbonate-bearing veinlets are not uncommon in Franciscan rocks of
glaucophane-schist areas in northern California. Calcite is the most widely distributed of the vein carbonates, but aragonite is by no means rare. Some typical
vein parageneses are as follows:
(a) Calcite-quartz veins cutting sheared graywacke (379-94, 173G, adjacent
to glaucophane schist, Panoche Valley, San Benito County) or jadeitebearing metagraywacke (46-X9, Russian River).
ARAGONITE IN CALIFORNIA GLAUCOPHANE SCHISTS
571
CARBONATES OF THE GREENSCHIST FACIES
Apart from the calcite of marbles and calc-schists, calcite is widely distributed
as a minor constituent of greenschists and mica schists of the greenschist facies.
In fact, carbonates seem to be much commoner here than in chemically similar
rocks of the glaucophane-schist facies. The carbonates of greenschists and mica
schists have been universally identified as calcite or as dolomite; and our experience confirms this. Calcite has been identified by us in many such rocks from the
chlorite zone of Otago, New Zealand, and from the Bessi series of Japan. It is
unlikely that aragonite has been mistaken for calcite in rocks of the greenschist
facies, for the carbonate almost invariably shows readily recognizable {0lT2}
lamellar twinning.
PETROGRAPHIC OBSERVATIONS ON REPLACEMENT OF ARAGONITE
BY CALCITE
Not uncommonly the aragonite of glaucophane schists, especially along the
margins of veinlets, is partially replaced by calcite. The replacing calcite crystals
are extremely irregular in outline and tend to develop as dendritic patches within
the host grains. The following details of replacement were observed in an aragonite vein 4 mm in width cutting glaucophane-pumpellyite-sphene schist (46225) from Russian River.
In some aragonite grains calcite crystals of different orientation have developed
from several centers. In others, dendritic patches of calcite, all having precisely
the same orientation, have spread irregularly through a single aragonite; this
must be an equal-volume replacement, implying removal of CaCO3. Measurement of c axes and twin lamellae in both carbonates brings out several alternative regular crystallographic relationships between aragonite and replacing
calcite, as summarized in Table 3. Since aragonite is almost invariably twinned
on at least one of the prisms (110) and (lTO), there are three possible orientations
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(b) Calcite-albite veins cutting metamorphic rocks tectonically included in
serpentinite (21M-27A, 28 C, north Berkeley; 462-87, Russian River).
(c) Calcite veins and vesicles in basalts, spilites and keratophyres (88-5, 8,
9, Mt. Boardman Quadrangle; 462-88, Russian River).
(d) Aragonite veins in spilite and metabasalt (379-133, 136, Panoche Valley;
462-63, Russian River).
(e) Aragonite veins cutting glaucophane-pumpellyite schists and other metamorphic rocks tectonically enclosed in serpentinite (21M-40, north
Berkeley; 462-25, Russian River). The aragonite is partially replaced by
calcite, especially along the margins of the veinlets. In one rock (462-25)
a veinlet of lawsonite passes into a lawsonite-aragonite veinlet in which
the two minerals seem to have crystallized together.
(5) Dolomite has been observed only in serpentinites or in rodingitic lenses
(containing hydrogrossular: 379-101) in serpentinite.
W. H. BROWN, W. S. FYFE, AND F. J. TURNER
B
[010-Z-ij
.010«Z
1OO-T-1i50-g
>010=Z
100-Y
Fia. 3. Orientations of calcite replacing aragonite (cf. Table 3). Equal-area projection, upper hemisphere. CalcJte; solid circles (a, m, Sec.), broken arcs. Aragonite; circled crosses (100, 110, &c.),
full lines.
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ioo.r-ii5o-a
100.Y-ll2O-a
ARAGONITE IN CALIFORNIA GLAUCOPHANE SCHISTS
573
TABLE 3
Orientation of replacing calcite in relation to aragonite
Coinciding planes and directions
Orientation
I. Fig.
n. Fig.
3A
3B
Calcite
[100] =±(100)
±(021)
a =±(1120)
[100] =±(100)
[001] =±(001)
[010] =±(010)
±r=±(10lV
a =±(1120)
III. Fig. 3c
±r=±(10lV
/i = (0221)
mt = (1010)
[ri-r,]
m, = (01 TO)
a =±(1120)
III
3E
±r=±(10lV
II II II
V. Fig.
3D
[100] =±(100)
[010] =±(010)
[001] =±(001)
(101)
III
a =±(1120)
TV. Fig.
Aragonite
a =±(1120)
c = [0001]
(100)
±(051)*
(203)
±(322)»
* Coincidence within 2°, i.e. within limits of measurement on a universal stage.
THE GEOLOGICAL PROBLEM OF THE RATE OF THE
ARAGONITE -HI-CALCITE TRANSFORMATION
The work of Coleman & Lee, supplemented by our observations recorded
above, shows that aragonite commonly forms synchronously with mineral
assemblages of the glaucophane-schist facies. It is assumed in what follows that
in this facies aragonite is the stable polymorph of CaCO 3 : and this assumption
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of the aragonite lattice from which to choose. All three have X = [001] in
common; but the [100] or [010] axes of the three orientations are mutually
inclined at 64°, 64°, and 52°. The intersection of the respective great circles of
the prism zones of calcite is either [100] or [010] of one of the three aragonite
lattices (usually the dominant one) or (110) of the principal twin; it is normal to
either (112"0) or (10T0) of calcite.
Of the orientations summarized in Table 3,1 is possibly not real, for it differs
by only 9° from orientation II. Although in both carbonate lattices the (CO3)°
groups are aligned in planes normal to the c axis, there is invariably a wide
divergence between the respective c axes of associated calcite and aragonite.
Mutual orientation seems to be controlled mainly by the directions of closest
spacing of Ca ++ ions—in calcite parallel to the three edges of the type [fi-ft]
— [fi:r3i a n d the three a axes (cf. Turner, Griggs, & Heard, 1954, p. 887). In
each of the orientations II, III, and IV of Table 3 an a axis and an [/:/] edge of
calcite are parallel to two of the crystal axes {100}, [010], and [001] of aragonite.
574
W. H. BROWN, W. S. FYFE, AND F. J. TURNER
EXPERIMENTS ON THE RATE OF THE TRANSFORMATION
ARAGONITE ->-CALCITE
Three series of experiments have been conducted. In one series, samples open
to the atmosphere were heated for varying times at different temperatures. In
another series the samples, sealed in contact with water, were heated to various
temperatures at respective saturation vapor pressures. Finally, samples sealed in
benzene were used to explore the pressure coefficient of the transformation rate.
After each experiment the percentage of aragonite that had been converted to
calcite was determined by X-ray diffraction, using as indicators the relative peak
heights on the diffraction curve. The method was checked against various prepared mixtures of calcite and Franciscan aragonite. With careful attention to
reproducible preparation of samples, it was found that at low concentrations of
calcite—for which the method has maximum sensitivity—the relative amounts of
the two phases could be estimated to within ± 2 per cent. Chaudron (1954) used
volume changes as an index of the extent of transformation. His method is
capable of high precision, but the necessarily large size of the sample protracts
the time required to heat the sample to constant temperature and so introduces
additional errors.
A. Experiments at 1 bar. In each experiment small samples, spread on thin
sheets of aluminium foil placed on a large metal block, were heated in a furnace
to the desired temperature. The aim of this technique was to minimize the time
required to heat the sample to constant temperature. Measurements were made
at 20° intervals between 380° C and 440° C. Below 380° C the rate of transformation was too slow for our purposes, while above 440° C reaction was so rapid
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is in harmony with the contrasted widespread appearance of calcite and universal absence of aragonite in the greenschist facies.
Metastable aragonite of sedimentary rocks inverts rapidly to calcite in the
course of diagenesis (e.g. Pettijohn, 1949, pp. 108, 109). How, then, can aragonite, formed at high pressures of glaucophane-schist metamorphism, persist
unchanged during the necessarily long period of unloading to atmospheric
temperature and pressure? The problem is emphasized by our record of partially
transformed aragonite in some schists, and a tendency for calcite to become
relatively concentrated close to the weathered surfaces of aragonite-bearing
schists.
To answer the question, we require experimental data on the rate of transformation of aragonite to calcite under conditions comparable with those of the
geological environments of unloading. The rate of the reaction has already been
reported by Chaudron (1954), but as the rate was found by him to vary from
one sample to another, we have further investigated the transformation rate
using a typical Franciscan metamorphic aragonite from Cazadero kindly
supplied by Dr. R. G. Coleman (Coleman & Lee, sample 51-CZ-59).
ARAGONITE IN CALIFORNIA GLAUCOPHANE SCHISTS
575
that serious uncertainty was introduced by the time required to bring the sample
to constant temperature. The percentage of calcite formed in any experiment
was found to be directly proportional to time:
dy
where y is the percentage of calcite, t the time, and K a rate constant. This rate
equation differs from that proposed by Chaudron (1954):
Kr
FIG. 4. Logarithm of time t required for 10 per cent conversion of aragonite to
calcite, plotted against the reciprocal absolute temperature: A, at 1 bar, in
contact with air. B, in contact with liquid water at saturation vapor pressure.
where x is the amount of aragonite present at time t. Chaudron found a distinct
induction period followed by an essentially linear rate of transformation. The
fact that no induction period could be recognized in our experiments may reflect
the presence of minor amounts of calcite in the Franciscan aragonite. Moreover,
it seems likely that in our experiments the 'heating up' time is shorter than in
those of Chaudron. The linear relationship of transformation to time suggests
that reaction is favored on certain lattice planes, in accord with observations
mentioned above on the orientation of calcite or aragonite.
In the temperature range studied, the logarithm of the time for conversion of
a fixed amount of aragonite is a linear function of the reciprocal absolute temperature, indicating a constant value of the activation energy. In Fig. 4, line A,
the time for 10 per cent conversion is plotted. In Fig. 5 the nature of the conversion rate as a function of time is shown for various temperatures. Relevant
experimental data are given in Table 4. The simplicity of these relations allows
easy extrapolation to any time and temperature.
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dx
W. H. BROWN, W. S. FYFE, AND F. J. TURNER
576
50
70
Fio. 5. Percentage conversion of aragonite to calcite plotted against time /.
Specimens at 1 bar, in contact with air.
TABLE 4
Experimental data on aragonite transformation at one atmosphere pressure
T
Time
(°Q
460
(hours)
017
017
0-25
0-25
0-33
0-33
0-25
0-42
0-66
0-83
1-0
20
1-0
20
30
50
»»
M
»f
440
f9
»»
»»
„
420
»»
".
% Calcite formed
29
37
55
58
75
67
8
13
42
54
70
90 (approx.)
3 (approx.)
6
12
24
T
Time
(°Q
420
(hours)
60
8-0
400
3!
6-3
180
20-3
24-6
43-5
66-0
480
910
96-0
115
120
139
144
168
% Calcite formed
37
47
9
16
17
25
38
70
2-5
3
4
4-5
4
4
4-5
6
B. Experiments in liquid water. In liquid water at its saturation vapor pressure,
rates of reaction could be observed conveniently between 220-295° C. Again,
the logarithm of time for conversion of a fixed amount was found to be a linear
function of reciprocal absolute temperature; and data for 10 per cent conversion
are shown in Fig. 4, line B. Experimental data are given in Table 5. In this range,
pressures vary from 23 to 80 bars. It is clear that an aqueous solvent exerts a
strong catalytic influence and lowers the energy of activation of the transition.
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40
time (hourt)
ARAGONITE IN CALIFORNIA GLAUCOPHANE SCHISTS
577
TABLE 5
Experimental data on aragonite transformation in liquid water at its
saturation vapor pressure
Time
CO
(hours)
304
300
295
270
253
224
215
198
48
17
2
20
%
162
72
144
% Calcite formed
100
100
10
70
100
70
6
10
C. Experiments at moderate pressures. To apply the rate data to a geological
situation, it is necessary to know the pressure coefficient of the reaction rate.
The observed rate can be considered to involve the difference between two competing reactions:
Vl
aragonite <
>
calcite.
When vx and v2 are equal the system is in equilibrium, so that as the equilibrium
pressure is approached at any temperature the rate at which aragonite is transformed to calcite must diminish. The purpose of this third series of experiments
was to explore this rate at some convenient temperature.
Ten experiments were conducted near 400° C at pressures up to 5000 bars
(the transition pressure at 400° C is 9000 bars) in cold-seal rod bombs. To avoid
shear stress in aragonite grains, the samples were sealed in capsules partially
filled with benzene, and then pressure was applied to the exterior of the capsule
by raising the pressure in an enclosing fluid (water) to the desired value. Experiments at a few hundred bars showed that benzene was thermally stable and that
transformation times were almost identical with those found at 1 bar.
Evidently the pressure coefficient of reaction is small. For example, the time
required for 40 per cent conversion at 3000 bars was only 20-30 per cent longer
than at one bar. The results indicate that even rather close to the equilibrium
pressure, the rates of reaction will not differ drastically from those shown in
Fig. 4. This conclusion is supported by data of Clark (1957) who obtained reversible transitions in the temperature range 400-450° C in times of an hour within
a thousand bars or less of the transition pressure.
Examination of Fig. 4 makes it quite clear that good temperature control is
essential for these measurements. In rod bombs, there is some uncertainty as to
the gradient over the sample length, and differences in the amount of conversion
were noted in different parts of the sample. To eliminate this uncertainty, seven
experiments in the range 1-3500 bars were conducted in a small hot-seal vessel.
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T
578
W. H. BROWN, W. S. FYFE, AND F. J. TURNER
ax
it=
j
j
*
*
where kx is the rate constant for the aragonite to calcite reaction and k% for the
calcite to aragonite reaction. It could first be assumed that kx and k2 have very
different orders of magnitude at low pressures and that AF* is small and AF|
much larger (a requirement if the rate constants are to be equal at the equilibrium
pressure). This is considered unlikely, as it is difficult to envisage a great difference in AVX for either process. It could also be assumed that as the AV of reaction
is small, then both AV\ and AV$ will also be small and the rate constants quite
similar. This situation, shown in Fig. 6A, would lead to an effective rate of
transformation indicated in Fig. 6B. If this is the case, significant slowing down
will not occur until pressures are close to the equilibrium pressure, a suggestion
supported by Clark's data. In addition to these factors, some processes are
speeded by pressure, due to an increased contact between regions of reactant
and product. It should be stressed, however, that a detailed study of velocities
of forward and back reactions through the transition pressure is desirable.
Miscellaneous rate observations. Chaudron (1954) noted that different aragonite samples reacted at significantly different velocities, similar velocities being
observed over a range of 20° or more. He also noted that addition of calcite or
iso-structural sodium nitrate accelerated the transition. We have also noticed
differences between samples. Large single crystals of natural aragonite reacted
more slowly than material from Cazadero; while fine needles of aragonite
synthesized near 50° C reacted only very slowly. This anomaly regarding grainsize can probably be related to relative thermal strains introduced during rapid
heating to the desired temperature. Dry samples were also studied under vacuum
and with several bars pressure of carbon dioxide but no significant differences
in rate were observed.
GEOTHERMAL GRADIENT INFERRED FROM EXPERIMENTAL DATA
We now return to the problem of the survival of metamorphic aragonite during
unloading. From the experimental data on the rate of the aragonite -*• calcite
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This vessel was about 3 in. long in a 24-in. furnace, pressure being fed in via the
usual capillary tubing. Temperature uncertainty is probably no more than 1° C.
The benzene was pumped directly onto the sample, 45-CZ-59, supplied by Coleman. The results showed consistently that the time for 70± 10 per cent conversion
was independent of pressure. The slight depression observed in rod bombs could
easily be explained by an increased thermal gradient as the fluid density increases.
The pressure experiments indicate that there is no drastic variation in rate
over a moderate pressure range and that the slowing factor used in the discussion
which follows is probably reasonable. The lack of this slowing down requires
some comment. The results could be explained in two possible ways. If it is
assumed that both reactions are of zero order, then the rate of transformation of
aragonite to calcite is:
,
A R A G O N I T E IN CALIFORNIA GLAUCOPHANE SCHISTS
579
inversion it is possible to set limits on the geothermal gradient in a region where
aragonite has crystallized as a metamorphic mineral and survived subsequent
erosion of the overlying rock column. It is reasonable to assume that during
unloading, depth-temperature relations were essentially linear.
FIG. 6. A, suggested relations between the rate constants kx
(aragonite -v calcite) and kx (calcite—>-aragonite) with
pressure. kx and kt are assumed to be of similar magnitude.
B, times for afixe^Jamount of conversion, as a function of
pressure, which would result from the constants in A.
The experimental data indicate that in wet rocks with an open-pore system
aragonite could survive near the surface for 100,000 years at 50° C, and only
a few million years at 10° C. This conclusion agrees with the observed general
destruction of aragonite during protracted diagenesis of sediments. It suggests,
too, that from the close of metamorphism onward, aragonite-bearing schists
have remained essentially non-porous and contained no significant amount of a
free aqueous phase.
The geothermal environment of dry aragonite-bearing rocks is illustrated in
Fig. 7. Here the P-T curve of the aragonite ^ calcite inversion (Jamieson, 1953;
Clark, 1957) is compared with linear geothermal gradients for 10°, 12°, and 15° C
per km. For the 10° gradient, aragonite at 200° C would be only 1000 bars
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log K
580
W. H. BROWN, W. S. FYFE, AND F. J. TURNER
below its stability field; in the dry state it would take 1012 years to invert completely to calcite, and clearly would survive post-metamorphic unloading. This
figure is obtained from curve A, Fig. 4 by taking the time for 10 per cent conversion at 200° C and increasing by a factor of 10 for 100 per cent conversion,
and again by a factor of 10 for the pressure correction. There must be considerable uncertainty in such an estimation, but on account of the logarithmic
variation with temperature, the uncertainties are not critical. In general, if
uncertainties were as great as two powers of 10, our conclusions would not
o
100
200
Temperoture °C
300
400
Fro. 7. P-Tcurve for calcite ^ aragonite (after Jamieson, 1953; Clark, 1957) compared with ideal
geothermal gradients (broken lines).
differ. For the 12° gradient, the temperature at which aragonite would be 1000
bars below the inversion pressure is 350° C; complete transformation to calcite
at this temperature requires only 10* hours (about 100 years)—an impossibly
short period for geological unloading. For the 15° gradient, survival of aragonite
is even more obviously impossible. We infer that in regions where metamorphic
aragonite occurs in surface rocks the thermal gradient during metamorphism
and subsequent unloading cannot greatly have exceeded 10° C per km. This is
by no means an improbable gradient for a rapidly subsiding geosyncline (cf.
Birch, 1955, p. 113). We find it unnecessary, therefore, to appeal to tectonic
stress as more than a subsidiary factor in contributing to the high pressures
which, combined with relatively low temperatures, provide a geologically possible environment for crystallization and protracted survival of aragonite.
TEMPERATURE AND PRESSURE OF THE GLAUCOPHANE-SCHIST FACIES
The gradient of 10° C per km inferred from the kinetics of the aragonite -*•
calcite transformation is within the lower limit of the possible gradients discussed
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10
ARAGONITE IN CALIFORNIA GLAUCOPHANE SCHISTS
581
ACKNOWLEDGEMENTS
We are indebted, for financial support, to the National Science Foundation
(grants 16316 and 11589) and the Petroleum Research Fund of the American
Chemical Society. We gratefully acknowledge the courtesy of R. G. Coleman and D. E. Lee for advance access to their manuscript on 'Metamorphic
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by Birch (1955, p. 113). It agrees with that tentatively proposed by Fyfe, Turner,
& Verhoogen (1958, pp. 237-8, figs. 107, 108) to account for regional metamorphism leading to the glaucophane-schist facies. Significant reactions that
have been explored experimentally to varying degrees include calcite ^ aragonite (Jamieson, 1953; Clark, 1957), albite ^ jadeite+quartz (see Fyfe &
Valpy, 1959), and anorthite+water ^ lawsonite (Fyfe, unpublished data). All
the experimental data are consistent with the thesis that, given a thermal gradient
of 10° C per km, aragonite, jadeite+quartz, and lawsonite would be stable at
depths of the order of 20-30 km (T = 2OO°-3OO° C; P = 6000-9000 bars).
As with all facies boundaries on a P-T diagram, that between the glaucophaneschist and the greenschist facies—marked by the appearance of glaucophane,
acmitic jadeite (+quartz), lawsonite, and aragonite—must be a band, not a
sharp line. Glaucophane certainly appears in many greenschists transitional to
the glaucophane-schist facies (Turner & Verhoogen, 1960, p. 543). Lawsonite
has been recorded (Coombs, 1960, p. 454) in both calcite marbles and albitebearing metagraywackes. At a given temperature, the pressure for equilibrium
between jadeite+quartz and albite would be lowered by the iron content of the
jadeite. Taking such facts into account it is not possible—nor is it desirable—
to set sharper limits to the physical conditions of the glaucophane-schist facies
than pressures of the order of 6000 to 10,000 bars and temperatures of 200° to
300° C.
In the above discussion we have attributed temperatures and pressures of
metamorphism solely to depth of burial. Deformation enters into glaucophaneschist metamorphism; and tectonically induced non-hydrostatic stress could
locally increase effective pressure and so permit development of aragonite,
jadeite, or lawsonite in areas otherwise under conditions of the zeolite or the
greenschist facies. But the frequent occurrence of aragonite and lawsonite
filling veins and vesicles, and the general prevalence of unstrained crystals—
sometimes even in delicate radial growths—is not consistent with long-sustained
non-hydrostatic stress of high magnitude. Tectonic stress would seem to play
only an auxiliary role in augmenting the pressure of metamorphism. To this role
is added the generally accepted accelerating influence of stress upon metamorphic reactions, the activities of which are lowered by reduction of grain-size,
storing of strain energy in the reactant grains, and repeated renewal of surface
contacts. But in the main we attribute glaucophane-schist metamorphism—
including crystallization of aragonite—to deep burial in regions of low temperature gradient such as might develop in rapidly subsiding geosynclines.
582
W. H. BROWN, W. S. FYFE, AND F. J. TURNER
aragonite in the glaucophane schists of Cazadero, California', and of J. Alfors for
permission to examine material used by him in preparation of his doctoral
thesis.
FYFE, W. S., TURNER, F. J., & VERHOOGEN, J., 1958. Metamorphic reactions and metamorphic facies.
Ibid. 73, 259 pp.
& VALPY, G. W., 1959. The analcime-jadeite phase boundary: Some indirect deductions.
Amer. J. Sci. 257, 316-20.
GILBERT, C. M., & TURNER, F. J., 1949. Use of the universal stage in sedimentary petrography. Amer.
J. Sci. 247, 1-26.
JAMIESON, J. C , 1953. Phase equilibrium in the system calcite-aragonite. J. Chem. Phys. 21, 1385-90.
PETTUOHN, F. J., 1949. Sedimentary rocks. New York: Harper & Brothers, 526 pp.
TURNER, F. J., 1949. Preferred orientation of calcite in Yule marble. Amer. J. Sci. 1X1, 593-621.
GRIGGS, D. T., & HEARD, H. C , 1954. Experimental deformation of calcite. Bull. geol. Soc.
Amer. 65, 883-934.
& VERHOOGEN, J., 1960. Igneous and metamorphic petrology. 2nd ed. New York: McGrawHill, 694 pp.
EXPLANATION OF PLATES
PLATE 1
FIG. A. Glaucophane-lawsonite schist (379-2), San Benito County. Polarized light; x 65. Aragonite
(a) associated with glaucophane (gl), aegirine (p), and lawsonite (/).
FIG. B. Aragonite vein (379-132A) in glaucophane schist, San Benito County. Polarized light;
x 65. Radial prisms of glaucophanic amphibole (?/) in coarsely crystalline aragonite (a). Laterally,
the vein passes within 5 mm. into a lawsonite vein.
PLATE 2
FIG. A. Eclogite (38M-89) in process of retrogressive change to glaucophane schist, north of Valley
Ford. Polarized light; x65. Garnet porphyroblast (ga), partially replaced by aragonite (a), chlorite
(c), and lawsonite (/). Narrow veinlets in garnet are mainly aragonite.
FIG. B. Eclogite (38M-84) in process of retrogressive change to glaucophane schist, Tiburon Peninsula. Polarized light; x65. Garnet porphyroblast iga), partially replaced by aragonite (a).
PLATE 3
FIG. A. Polarized light; x 130. Crystallographically continuous calcite (c) replacing a crystal of
aragonite (a) in aragonite vein cutting glaucophane-pumpellyite-sphene schist (462-25), Russian River.
FIG. B. Same field, nicols crossed.
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REFERENCES
ALFORS, J., 1959. A structural and petrographic investigation of an area of glaucophane-bcaring rocks
in Panoche Valley, San Benito Quadrangle, California. Unpublished thesis, University of California, Berkeley.
BIRCH, F., 1955. Physics of the crust (in: Crust of the earth). Ceol. Soc. Amer., Special Paper No. 62.
BORG, I. Y., 1956. Glaucophane schists and eclogites near Healdsburg, California. Bull. geol. Soc.
Amer. 67, 1563-84.
BROTHERS, R. N., 1954. Glaucophane schists from north Berkeley hills, California. Amer. J. Sci. 252,
614-26.
CHAUDRON, G., 1952. Contribution a l'itude des reactions dans l'6tat solide cinetique de la transformation aragonite-calcite. International Symposium on the Reactivity of Solids, pp. 9-20. Gothenburg.
CLARK, S. P. J., 1957. A note on the calcite-aragonite equilibrium. Amer. Min. 42, 564-6.
COLEMAN, R. G., & LEE, D. E., 1961. Metamorphic aragonite in glaucophane schists of Cazadero,
California. Geol. Soc. Amer., Cordilleran Section Meeting, Abs., p. 26.
COOMBS, D. S., 1960. Lawsonite metagraywackes in New Zealand. Amer. Min. 45, 454-5.
DAVIS, G. A., 1960. Lawsonite and pumpellyite in glaucophane schist, north Berkeley hills, California. Amer. J. Sci. 258, 689-97.
EMMONS, R. C , 1943. The universal stage. Mem. geol. Soc. Amer. 8, 205 pp.
Vol. 3, Part 3
Journal of Petrology
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W. H. BROWN, W. S. FYFE, o/irfF. J. TURNER
Plate J
Vol. 3, Part 3
Journal of Petrology
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W. H. BROWN, W. S. FYFE, andF. J. TURNER
Plate 2
Vol. 3, Part 3
Journal of Petrology
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W. H. BROWN, W. S. FYFE, andF. J. TURNER
Plate 3