[AMEIuCAN JOURNAL .oF SCIENCE, VOL. 260, JANUAIlY 1962, PP. 44-56]

SOME RELATIONS BETWEEN THE PHASES ANOR~HITE, ZOISITE AND LAWSONITE

AT IDGH TEMPERATURES AND PRESSURES

CARL W. F. T. PISTORIUS, GEORGE C. KENNEDY, and S. SOURIRAJAN

,

[AMERICAN JOURNAL OF SCIENCE, VOL. 260, JANUARY 1962, P. 44·56]

SOME RELATIONS BETWEEN THE PHASES ANORTHITE, ZOISITE AND LAWSONITE

AT HIGH TEMPERATURES AND PRESSURES*

CARL W. F. T. PISTORIUS, ** GEORGE C. KENNEDY, and S. SOURIRAJANt

Institute of Geophysics, University of California, Los Angeles 24, California

ABSTRACT. Some compositions in the system CaO-Al.0 .... SiO .... H20 have been studied in the temperature range 400°·900°C and H20 pressures ranging from 5000 to 42,000 bars. The phase assemblage boundary of the reaction anorthite + corundum + water = zoisite + sillimanite has a slight negative slope.

The assemblage anorthite + quartz + water is stable at H.O pressure below 12,000 bars at 800°C and 14,000 bars at 400°C. Clinozoisite and zoisite are the stable phases at high pressures, but at H.O pressures above 23,000 bars at 400°·600°C and 20,000 bars at 850°,C, lawsonite is the stable phase.

INTRODUCTION

The a'lteration products of anorthite, CaO ' Al20 a . 2Si02 , under conditions of high water pressure and moderate to high temperatures, involve mineral phases such as zoisite, 4CaO' 3AL03' 6Si02 • H 2 0, and lawsonite, CaO· AlzOs' 2Si02 • 2H20. The stability of lawsonite is of special interest, as it occurs only in the glaucophane· schist facies. The remarkable fact that the molar volume of lawsonite, despite its two molecules of water, approximates that of anorthite is an indication that lawsonite might be stable only at high H 20 pressures. The stability of lawsonite should be scarcely affected by load pressure though very strongly influenced by water pressure. Fyfe and Turner (1958) pointed out that this makes lawsonite an excellent indicator of high water pressure. Epidote and clinozoisite are important minerals in the greenschist facies, and they are of interest for that reason.

EXPERIMENTAL DETAILS

Several hundred experiments at high temperature and pressure have been made in the above systems. The majority of the experiments were carried out in the "simple squeezer" high. pressure apparatus designed by Griggs and Ken­nedy (1956) in which two sintered carbide pistons of 6 mm face· diameter were pressed together by a commercial 20·ton hydraulic jack, supported by a framework of plates and tie rods. The finely ground sample, in a platinum foil capsule, was pressed between the piston faces and heated by an external fur­nace enclosing the sample and pistons. Temperature was measured by a chromel·alumel thermocouple placed in a hole in the top piston with the tip of the thermocouple approximately lti inch from the sample. The difference be­tween temperature at the sample position and temperature at the thermocouple junction was determined by calibration runs. Temperatures are believed to be accurate to -t- 8°C.

* Publication No. 170, Institute of Geophysics, University of California, Los Angeles. ** On leave from the National Physical Research Laboratory, South African Council for Scientific and Industrial Research, p. O. Box 395, Pretol'ia, Union of South Africa. t Present address: Division of Applied Chemistry, National Research Council, Ottawa,

Canada.

44

Carl W. F. T. Pistorilts, George C. Kennedy, and S. SOltrirajan 4.5

Sintered chromium carbide pistons were used in Blackalloy and in Udimet-500 piston holders. Slightly higher pressures couid be reached at high temperatures with the Udimet-500 holders.

The pressure was determined from the measured force applied to the pistons; the sample was assumed to be under hydrostatic pressure. The pres­sure measured in this way, under favoraible circumstances, is a<JCurate to ap· proximately 5 percent.

Two series of runs were made. The starting material in the first series was finely ground gla s, prepared by melting natural anorthite containing less than 2 percent Ab, mixed with ahout one-third as much Baker's analyzed aluminum hydroxide. The starting material in the second series was Iceland scolecite (CaO·AbOs·3Si02·3H20), spectroscopically found to have no major im­purities. Unfortunately chemical analyses were not available for the scolecite. J. Murdoch kindly supplied the scolecite and its identity was confirmed by C. Frondel. Scolecite was used rather than a mixture of the various oxides be­cause equilibrium can be more readily obtained by the breakdown of a hy­drated compound than by the diffusing together of a mixture of oxides. The powdered samples were mixed with distilled water to form a paste merely for convenient handling, as excess water does not affect the experimental results.

The pressure was first raised to the desired value and the sample then heated. Along hydration or dehydration boundaries the water vapor pressure was equal to the total pressure (Kennedy, 1959). Only a few seconds were re­quired to bring the sample to the desired pressure. The time required to bring the sample to the selected temperature ranged from 10 to 15 minutes. After the sample had been held at the required temperature and pressure for approxi­mately 40 minutes, it was quenched in an air jet without change of pressure. Temperature dropped from 800°C to 200°C in about 3 minutes. The products of the runs were identified by their X-ray powder diffraction patterns, obtained in a Norelco thigh angle recording diffractometer, and using CuKa radiation (,\ = 1.5418 A) and a Ni filter. The majority of the powder patterns were taken at a scanning speed of 1/8° (20) per minute in order to increase reso· lution. A quartz plate was used as a sample holder (Buerger and Kennedy, 1958) .

RESULTS WITH STARTING MATERIALS OF THE COMPOSITION

CaO' Ai20 S ' 2Si02-Al20s-H20 The pressure, temperature, duration of run, and resulting phases are given

in table 1 for all the runs made from a mixture of anorthite glass and alumi­num hydroxide. Products of these runs fall into two groups, one, including zoisite or clinozoisite, the other, lacking these minerals but including much anorthite. A boundary curve, shown in figure 1, separates these two groups. The reaction represented by this curve is expressed as:

4(CaO·AI20 3 ·2Si02) + Al20 S + H20+24CaO·3AJ20S·6Si02·H20 anorthite corundum zoisite or clinozoisite

+ 2(Al20s·Si02 ).

sillimanite or various other aluminum silicates The corundum is formed by the dehydration and subsequent recrystal­

lization of the aluminum hydroxide. A portion of the diaspore = corundum

46 Carl W. F. T. Pistorius, George C. Kennedy, and S . Sourirajan

TABLE 1

Results of runs made with anorthite glass + AI(OH) 3

Pressure PC Time Run kb ± 1 kb ± 10°C mills Results

481 30.0 500 60 Cl + Ka + D 489 30.0 600 60 Cl + Co + Ka 490 30.0 700 45 CZ +K 534 20.0 700 60 Z+K 535 10.0 700 60 A+ Co 536 20.0 600 60 Z+CZ+Co+P 571 15.0 800 60 Z+Cl+S 572 15.0 700 60 CZ + 5 574 10.0 BOO 60 A + Co 599 11.4 700 60 A + Co 613 13.4 BOO 60 Z+S 614 12.0 600 60 A + Co 615 15.0 700 60 Z+A + S 616 11.0 BOO 60 Z+Co+A+S 617 13.0 705 60 A+Z+Co+S 632 16.0 600 60 A+Z+P 633 10.0 900 60 A + Co 634 IB.O 600 60 A+CZ+P 637 6.0 900 60 A + Co 638 B.O 900 60 A +Co 647 16.0 500 60 CZ+A+D+K 648 IB.O 500 60 CZ+D+K

+ water boundary (Kennedy, 1959), and the sillimanite = kyanite equilib­rium curve (Griggs and Kennedy, 1956; Clark, Robertson, and Birch, 1957), are shown in figure L The kaolin and pyrophyllite boundaries were taken from unpublished work by Kennedy. No special attempt wa made in this investiga­tion to re-establish these boundaries. In general the present runs yielded the phases to be expected from the earlier work.

It was impossible to establish a phase boundary between zoisite and dino­zoisite. Runs yielding either of these phases are randomly pread across the pressure-temperature region where anorthite + corundum + water is un­stable. The relations between zoisite and dinozoisite are discussed in some de­tail later in this report. The word "zoisite" is used to mean either zoisite or clinozoisite in discussing results from the starting materials.

Anorthite, corundum, and water are stable at low water vapor pressure (below 10.5 kb at 800°C and below 14 kb at 550°C), wherea zoisite and sillimanite, kyanite, pyrophyllite, or kaolin are stable at higher water vapor pressures. The slope of the phase assemblage boundary, dp/ dt, is slightly negative. The reaction is not sensitive to temperature but to water vapor pres­sure.

In a number of runs made at low temperature (481, 647, 648, 649) diaspore was formed instead of corundum, in agreement with previous work (Kennedy, 1959).

o a..%"

Some Relations Between the Phases Anorthite, Zoisite and Lawsonite 4,7

10 Anor thire + corundum + woter 8L-__________ L-__________ L-__________ L-___ _____ ~L_ ________ ~

500 600 100 800 900 1000 TOr.

Fig. 1. Results with starting materials of the composition CaO · AbO.· SiO ..... AbOr­H.O.

This portion of the system CaO-Al20 a- Si02-H20 has three sextuple points and one quintuple point in the pressure· temperature region under in· vestigation. These were not all directly determined. By using information from the previously tudied system Al20 a- Si02- H2 0 , one can with reasonable con· fidence predict the following invariant points. Three sextuple points where six phases all coexist are present.

(a) 4 anorthite + diaspore + 3 water = zoisite + kaolin + diaspore = zoi ite + kaolin + corundum + water = 4 anorthite + corundum + 4 water.

(b) 4 anorthite + 3 water = zoisite + kaolin = zoisite + lh pyrophyllite + lh corundum + Ilh water, located a.t P H20 = 13.5 + 2 kb, T = 570 -+- 30°C;

(c) 4 anorthite + corundum + Ilh water = zoisite + lh pyrophyllite + Ilh corundum = zoisite + 2 sillimanite + water at P H20 = 13.0 -+- 2 kb, T = 620 -+- 20°C; One quintuple point, involving five stable phases and the following reac·

tion, is found. 112 pyrophyllite + Ilh corundum = 2 kyanite + lh water = 2 sillimanite + lh water, Ps20 = 13.5 -+- 2 kb, T = 625 -+- 20°C. The slope of the boundary between the stability field of anorthite +

corundum + water = zoisite + sillimanite is dp dt = - 10 -+- 7 bars/degree.

RESULTS WITH STARTING MATERIALS OF THE COMPOSITION

CaO· AI20 a• 2Si02- Si02-H20

Most of the runs in the system CaO· AI20 a · 2Si02-Si02-H20 were made with Iceland scolecite as the starting material. The pressure, temperature, duration of run, and resulting phases are given in table 2 for all the runs which were made with scolecite as the starting material. The boundary curves found are shown in figure 2. Four scolecite molecules react to yield 2Si02 and 8H20 ,

48 Carl W. F. T. Pistorius, George C. Kennedy, and S . Sourirajan

26r----------r---------.----------~--~----~--------~ I /,

24

22

20

.0 1B .><

o x ~ ~ oJ)

'" 0:: 16

14

12

10

Lawsonite + coesite

,,~:<1. "';-0...,)0

/.

Clinozoisite + pyrophyllite + water

(b)

Anorthite + a quartz + water

(0)

/.o;fi> /. \

/~l"~ /~<f-

Lawson ite + a quartz

:8' • 91IS ~: Clinozoisite + kyanite ./

l' + a quortz + water ./ N /. ~fo , ~ (f) / ,0; , fr /.IS'>

u Q ././~4"(\ :£/+ /

rr .~ (i'~.~ ~/ ~ -t-.'\/o<" Q.p~ /4-" {(' , //

/ / '// Y , ,

/ I ,

/ / ,

/ /

Clfnololsite + sillimanite 1

$ ' J/(e)

+ a quartz + wc ter

(C)

ClinOloislte+sillimonite ~ _

~ _______ +~p~q~u~or~t~z~+~w~o~t.~r __ ~ __

1

.:;.1 ,}/~ ~/li ~'<l

1 / Anorth ite + fJ quartz

/ + woter

/ (d) B~--------~--------~------____ ~ ______ L-~ ________ __J

400 500 600 Temperature

700 (0 C)

BOO 900

Fig. 2. Results with starting materials of the composition CaO·Al.O •. 2SiO .... SiO .... HoO.

Some Relations Between the Phases Anorthite, Zoisite and Lawsonite 49

more Vhan t:he maximum needed for any of the reactions, so 2Si02 and 8H20 have been subtracted in plotting figure 2, The following reactions take place:

4(CaO'Al2 0 s'3Si02 '3H2 0 = 4(CaO'Al20 s '2Si02 ) + 4Si02 + 12H20 (a) scolecite anorthite a-quartz

= 4CaO' 3Al2 0 S ' 6Si02 ' H20 + Ai20 S ' 4Si02 - H20 + 2Si02 + lOH20 (b) olinozoisite pyrophyllite a-quartz

= 4CaO'3Al20s'6Si02 ,H20 + Al20 s'Si02 + 5Si02 + llH20 (c) dinozoisite sillimanite a-quartz

= 4(CaO'Al20 s' 2Si02 ) + 4Si02 + 12H20 (d) anorthite ,8-quartz

= 4CaO -3Al20 S .. 6Si02 , H20 + M 20 S - Si02 + 5Si02 + llH20 (e) clinozoisite sillimanite ,8-quartz

= 4CaO'3Al20s'6Si02 -H20 + Al20s'Si02 + SSi02 + llH20 (f) clinozoisite kyanite a-quartz

= 4CaO'3Al20 s-6Si02 'H20 + Al20s'Si02 + 5Si02 + llH20 (g) zoisite kyanite a-quartz

= 4CaO'3M.20 s '6Si02 ' H20 + Al20s'4Si02 -H20 + 2Si02 + lOH20 (h) zoisite pyrophyllite a-quartz

= 4(CaO-Al20s'2Si02 -2H20) + 4Si02 + 4H20 (i) lawsonite coesite

= 4(CaO'M20 S'2Si02 '2H20) + 4Si02 + 4H20 (j) ,lawsonite a-quartz

J1he ,8-quartz (assemblages d and e) inverts to a-quartz on quenching (Yoder, 1950) ,

The boundaries between the assemblages (b) and (c), between (b) and (f), between (c) and (f), and between (i) and (j) were largely based on the pyrophyl~ite -decomposition ,boundaries (Griggs and Kennedy, 1956) and the a-quartz = coesite boundary (MacDonald, 1956) previously determined on the same equipment, In all cases of duplication of experiments the agreement was excellent,

Recenrly Boyd and England (1960a) redetermined the a-quartz = coesite phase boundary by means of an internally heated apparatus utilizing a solid pressure medium (Boyd and England, 1960b) _ J1heir phase boundary at 600°C is located at a pressure approximately 3 -+- 2 kb a:bove MacDonald's boundary (1956), They ascribed this discrepancy to pressure -gradients, possibly owing to the growth of ~arge coesite crystals, in runs made in the simple squeezer_ No large coesite crystals were found in tlle present runs_ The quartz = coesite break in the lawsonite stability Iboundary is between the positions predicted by MacDona'Id's (1956) curve and Boyd's and England's (1960a) curve, and within the limits of error of each,

A few runs were made with lawsonite + coesite as starting material, J1hese results, shown in ta!hle 3, indicate that the lawsonite + quartz (coesite) = cIinozoisite + pyrophyllite + water boundary is reversible within 1.5 kb,

50 Carl W. F. T. Pistorius, George C. Kennedy, and S. Sourirajan

TABLE 2

Results of runs made with scolecite

Pressure PC Time Run kb ± 1 kb ± 8°C mins Results

105 5.9 404 45 Sc 106 6.2 505 45 No cryst. materi,al 107 10.6 402 45 Sc 108 11.0 505 45 A+Q 111 21.0 403 30 Sc 113 31.7 403 30 Sc 114 32.2 507 30 L+C+Z 11S 41.0 405 30 L+C 116 41.7 509 30 L+C 117 5.9 605 30 A+Q 118 6.1 709 30 A+Q 119 5.9 808 30 A+Q 120 6.1 900 30 A+Q 121 10.0 612 35 A+Q 122 10.3 700 35 A+Q 124 19.7 604 40 CZ+P+Q 125 10.0 906 30 A+Q 126 10.5 798 30 A+Q 127 21.6 705 40 CZ+P+Q 128 21.7 760 40 Z+L+Q+P+K 129 24.5 744 45 Z+L+Q+P 130 25.0 611 30 L+Q + C 131 32.3 611 40 L+C 132 36.5 498 30 L+ ,C 133 12.9 798 15 A+CZ+S+Q 134 12.9 892 15 CZ+S+Q+A 139 5,3 457 65 Sc+A+Q 140 5.5 530 65 A+Q 143 11.9 798 30 A+Q 144 12.3 705 30 A+Cl+Q+P+S 145 22.4 703 45 Z+P+Q+L 145 23.3 516 45 CZ+P+L+Q+C 148 10.4 406 1140 A + Q + Sc 149 21.'2 609 45 CZ+P+Q 150 21.7 511 35 CZ+P+Q+C(?) 151 10.6 608 45 A+Q 152 10.9 506 45 A+Q 153 11.3 560 30 A+Q 154 11.6 516 30 A+Q 155 22.1 613 50 CZ+P+Q 156 22.8 527 50 CZ+P+Q+C 157 10.3 720 50 A+Q 158 10.6 506 50 A+Q 159 20.6 767 60 CZ+Q+K 160 21.3 611 60 CZ+P+Q

Some Relations Between the Phases Anorthite, Zoisite and Lawsonite 51

Pressure PC Time Run kb ± lkb ± 8°C mins Results

163 13.3 603 40 A+Q 164 13.8 512 30 CZ+A+Q+P 165 13.2 808 30 A+Q+CZ+S 166 13.8 698 30 CZ+P+Q 167 12.3 623 80 A+Q 168 12.7 403 80 A+Q 169 10.7 747 SO A+Q

170 11.2 798 30 A+Q 173 9.5 747 60 A+Q ])74 10.0 798 60 A+Q 175 13.4 613 40 A+CZ+P+Q 176 13.0 520 40 A+Q 177 12.7 746 40 CZ+A+S+Q 178 12.2 665 40 A+CZ+P+Q 179 22.9 604 40 CZ+P+Q ISO 21.9 705 40 Z+P+Q

181 13.4 664 45 CZ+A+P+Q 182 12.9 601 45 ·CZ+A+P+Q 183 11.9 798 30 A+Q+CZ+S 184 11.6 700 SO A+Q+CZ+P+S 185· 24.6 424 360 Sc+CZ+P+C 186* 23.8 425 360 CZ+P+C+A 187 24.8 411 1230 L+C+Sc 188 24.1 426 1230 L+C

189 111.8 746 415 A+Q+CZ+S 190 11.4 700 45 A+Q 191 14.0 454 195 CZ+A+P+Q 192 13.3 497 60 A+Q

193 1M 559 60 CZ+P+Q+A 194 23.1 623 40 CZ+P+Q 195 22.,6 462 55 CZ+P+C+Q 197 23.4 407 1680 CZ+P+ ,C+L+Q

198 22.5 405 1380 CZ+P+C+Q 199 14.0 415 1260 A+<CZ+Q+P

200 13.5 403 lQ60 A+Q+Sc 201 22.1 675 30 CZ+P+Q 202 21.5 730 30 Z+ 'CZ+P+Q 203 23.3 623 25 CZ+L+Q+P 204 22.5 682 25 Z+Q+P+L 205 21.9 726 30 Z+K+P+Q+L 206 21.2 762 30 Z+L+K+Q 207 23.5 465 1500 L+CZ+C+P+Q 208 22.9 423 1500 L+CZ+C+P+Q 209 11.7 849 30 A+Q 210 11.6 799 30 A+Q

52 Carl W. F. T. Pistorius, George C. Kennedy, and S. Sourirajan

TABLE 2 (Co ntinued )

P ressure TOC Run kb ± 1 kb ± 8°C

Time mins Resuhs

211 11.7 212 11.4 213 23.1 214 22.5

680 748 658 363

30 30

4500 4500

A+ Q A+Q CZ + C + A + unid. CZ + C + A + unid.

* Loss of H.O vapor possible. Sc = Scolecite A = Anorthite e: = Zoisite

Gl = Clinozoisite P = Pyrophyllite

Q=Quartz K=Kya'llite S = Sillimanite C= Coe~i te L= Lawsoni te

'J1he boundary separating the stability fields of alJ10rthite and clinozoisite has ah o been reversed. Two runs were made at 7900 ,C alJ1d 10.7 kb using mix· tures of clinozoisite plus sillimanite a'nd olinozoisite plus kyanite. 'J1hese mix· tures reacted to form anorthite. Attempts to reverse this boundary in the 500°· 600°C range were unsuccessful. The observed slopes of the phase boundaries are :

(a ) / (b ) between 400°C and 5500 ,C: ~ + 0.9

bars/ degree. - 2.3

(a) / (b) between 550°C and 6800 ,C: - 10.2 + 2.5

bars/ degree. - 1.9

(a) / (c) between 680°C and 8500 ,C: + 0.3 + 1.9

bars/ degree. - 1.8

(h ) / (i ) + 3.5 + 2.0

bars/ degree. -3.0

(h) / (j ) and (b ) / ( j) - 14.0 +3 bars/ degree. -4

(g) / (j) as well as (f ) / (j) - 25 +15 bars/ degree. -18

(b) / (h ) -18.5 + 5 bars/ degree. -5

Six invariant sextuple points were found: (1) anorthite + a ·quartz + water = clinozoisite + pyrophyllite

= clinozoisite + sillimanite + a -quar tz + water at PH 20 = 11.6 -+- 2 kb

T = 690 -+- 18°C; (2) clinozoisite + pyrophyllite + water = lawsonite + coesite = lawsonite

+ a -quartz at PH z O = 23.6 -+- 1.5 kb

T = 606 -+- 18°C; (3) cIinozoisite + pyrophyIIite + water = zoisite + pyrophyllite + water

= lawsonite + a-quartz at PH ZO 22 .6 -+- 1.5 kb

T 668 -+- 40°C;

Some Relations Between the Phases Anorthite, Zoisite and Lawsonite 53

(4) zoisite + kyanite + a-quartz + water = zoisite + pyrophyllite + water = lawsonite + a-quartz

at P H2 0 = 21-6 -+- 1-5 kb T = 736 -+- 25°C;

(5) lawsonite = ZOlslte + kyanite + a-quartz + water = clinozoisite + kyanite + a-quartz + water

at P H2

0 = 20.2 -+- 2 kb T = 790 -+- 40°C;

(6) anorthite + water = clinozoisite + sillimanite + a -quartz = clinozoisite + sillimanite + ,8-quartz

at P H20 = 11-7 -+- 2 kb T = 845 -+- 15°C.

Our synthetic zoisite and clinozoisite had X-ray powder patterns identical with the typical patterns shown by Seki (1959). The two phases, zoisite and clinozoisite, are very readily distinguished.

Small amounts of clinozoisite, in runs that yield much anorthite and quartz, are difficult to detect, as the strongest diffraction peak of clinozoisite (d = 2.89 A) coincides with a minor peak of anorthite. A relative intensity of the 2.89 A peak 50 percent higher than for pure anorthite was taken as an arbi­trary criterion for the presence of clinozoisite. Such a peak corresponds to a clinozoisite-anorthite ratio of approximately 1/10. Optical methods of observ­ing the clinozoisite could not be used because grain size was too small.

Scolecite is evidently unstable at 400°C. Scolecite was still present among the products of a few short time runs at 400° -450°C but was invariably lacking in longer runs.

In this system, the phase boundary between clinozoisite and zoisite is sharp, with little or no region of indifference. Runs 500 bars apart show a complete transition from clinozoisite to zoisite.

If Seki' s (1959) values for the unit-cell volumes of the clinozoisite-pistaci te family, with varying Fe20 a content, are linearly extrapolated, a value of 452.0 A3 is obtained for the pure end member, clinozoisite_ The carefully selected zoisite which was measured by Seki (1959) has a unit-cell volume of 904.0 Aa,

TABLE 3

Results of runs made with lawsonite + coesite and with a mixture of hydroxides of scolecite composition

Pressure Run kb ± 1 kb 215 22.2 216 21.9 '217 12.7 218 11.4 147 16.4 137 9.7 138 10.0 132 36.5

L - Lawsonite C = Coesite Q= Q1.tartz

526 704 549 706 393 383 510 498

Time mins

45 30 40 35

1080 105 105

30

Starting material Products

L+C L+C+CZ+P L+C L+Q+Z L+C CZ+P+Q+L+A L+C L+Q+ ,CZ+S Hydroxides Q + amorphous mat. Hydroxides Q + amorphous mat_ Hydroxi,des Q + amorphous mat. Hydroxides L + Q + C GZ _ Clino2lCl'is;ite

P = Pymphyllite A = AnCl'rthite Z = Zoisite

54 Carl W. F. T. Pistorius, George C. Kennedy, and S. Sourirajan

i.e. 2 x 452.0 A3. Consequently, the densities of pure zoisite and clinozoisite are identical within experimental error. The present clinozoisite-zoisite phase boundary has a slope dp/ dt = -18.5 -+- 5 bars/ degree, and application of the Clapeyron-Clausius law consequently demands that the difference in entropy between clinozoisite and zoisite must also be extremely small.

This result has several consequences. If, for example, pure clinozoisite is denser than pure zoisite, zoisite may be the denser phase if a very small amount of ferric iron is present. The sign of the slope of the clinozoisite-zoisite phase boundary will then change from positive to negative if we assume that As does not change its sign. The reverse situation could as easily be true. In either situation the slope and position of the clinozoisite-zoisite phase boundary will change by large amounts of impurities in the minerals. Furthermore, MacDonald (1957) pointed out that nonhydrostatic stress may have a large effect on transitions with small volume and entropy changes. However, since the volumes and entropies of pure zoisite and clinozoisite are so similar, the slope and position of a phase boundary involving zoisite or clinozoisite, to­

gether with other phases, will change very little if the zoisite is replaced by clinozoisite.

These considerations also suggest an explanation for the results with start­ing compositions of CaO· Al20 3· 2Si02-Ab03-H20. In this system no bound­ary could be drawn separating runs that yield zoisite from those that yield clinozoisite, using anorthite glass and bayerite as starting materials. Minor compositional inhomogeneities in the anorthite glass, which was prepared from natural anorthite, could well have shifted the position of the zoisite-clinozoisite phase boundary from run to run.

Some thermochemical properties of the minerals involved in the various reactions can be estimated from the slopes of the relevant phase boundaries. The molar volumes of the various minerals are known. No experimental data were available for the specific volume of water at the pressures and tempera­tures of these reactions. However, Knopoff, Holser and Kennedy (unpublished data) have prepared tables of the specific volume of water by use of an equa­tion of state of the Thomas-Fermi-Dirac type interpolated between the specific volumes of water at lower pressures, and the specific volumes of water at ex­treme pressures, as found by shock wave methods. The molar entropies of kyanite and sillimanite (MacDonald, 1954) are 20.0 and 22.96 cal/deg mole, respectively, the molar entropy of a-quartz is 10.0 cal/deg mole (Kelley, 1950), and the molar entropy of coesite is 8.6 -+- 0.7 cal/deg mole (Mac­Donald, 1956). The molar entropies of water at the temperatures and pressures of the reactions were taken from the tables by Pistorius and Sharp (1960). From these data the following approximate values can be deduced:

Slawsonite -14 Szoisite = 69 ± 20 cal/deg mole. Sanortb i te - 14 Szo i site = 0 -+- 20 call deg mole. Szoisite - Sclinozoisite = 0 -+- 2 cal/deg mole. SpyrOPhyllite = 4.5 ± 1.5 cal/deg atom at 600°C, 13 kh.

DISCUSSION

Lawsonite is found in glaucophane schists. The present stability curves

Some Relations Between the Phases Anorthite, Zoisite and Lawsonite 55

and estimated temperature and pressure gradients within the Earth (Birch, 1955) would seem to indicate that lawsonite should be unstable at depths less than about 60 kilometers within the Earth. Coes (1954) reported that he failed to synthesize lawsonite at pressures below 40 kb at 800°C. Fyfe and Turner (1958, p. 177) pointed out that suitable conditions for the formation of law­sonite may develop in deeply buried, water-saturated sediments in a region of exceptionally low thermal gradient. The occurrence of some Californian law­sonite-bearing rocks in shear zones suggest the possibility that pressures in such zones may locally have exceeded load pressures.

Epidote is a mineral of the greenschist facies, and somewhat lower pres­sures of formation may be expected than are reported here for zoisite and clinozoisite. Goldsmith (unpublished data) synthesized zoisite from a hy­droxide mixture of anorthite composition at water vapor pressures of 8 to 10 kb and temperatures between 550°C and 800°C. The usual presence of some Fe203 and FeO in natural zoisites and clinozoisites may cause a lower ap­pearance pressure. However, Goldsmith substituted as much as 40 percent Fe203 for Al20 3 in his experimental mixtures but found no noticeable change in the pressure required for the first appearance of epidote. At 10 to 15 kb Goldsmith obtained zoisite from runs made on iron-free starting material. Clinozoisite was obtained at similar pressures in the present series of experi­ments because of the small amounts of impurities in the scolecite used as the starting material. It is unlikely that this would cause an appreciable shift of the phase boundaries involving other phases as well as zoisite or clinozoisite.

Throughout this manuscript we have discussed the various mineral re­actions as though they were equilibrium reactions. Two phases may be in equilibrium in respect to each other but not in equilibrium in respect to a third phase. Nucleation of certain phases in the system CaO-AI20 3-Si02-H20 seems to be exceedingly difficult. W_ S. Fyfe (oral communication) has made some recent experiments in which he synthesized zoisite at low temperatures from mixtures of anorthite, wollastonite, and gehlinite_ We have discussed only specific reactions from specific starting ingredients and have no certainty that other reactions cannot take place, so that the various mineral phases may form at much lower temperatures and pressures than reported herein. Furtilermore, only a few of our boundaries have been "reversed". Solid state reactions in this system are exceedingly slow and difficult to bring about. The mere fact of reversing a boundary does not mean that the boundary is an equilibrium boundary or represents phases of the lowest chemical potential in the system. It is possible to reverse metastable extensions of boundaries. There is nothing in reversing a boundary that precludes the existence of other phases of lower chemical potential. Fyfe has suggested that the fields reported in this paper be called "synthesis fields" rather than "stability fields." In the light of the pres­ent very limited studies of solid state reactions in this system, Fyfe's suggestion is a good one.

In a recent paper, Pistorius and Kennedy (1960) discuss the relations of grossularite and hydrogrossularite, phases in the system CaO-AI203-SiO~-H20. Fyfe has pointed out (oral communication) that a field of prehnite apparently occupies part of the area where we find anorthite + wollastonite. This is en-

56 Carl W. F. T. Pistorius, George C. K.ennedy, and S. Sourirajan

tirely possible, as we have studied no reactions involving prehnite, and our failure to nucleate prehnite during the course of these experiments would give us phase assemblages stable in relation to each other but not stable in relation to a mineral of lower chemical potential, i.e. presumably prehnite or some other calcium aluminum silicate.

An error in our paper on grossularite and hydrogrossularite has been kindly pointed out by A. Pabst. We took the unit cell edge of hibschite to bt> 12.0 A. Pabst writes that the spacing at 12.0 A is presumably correct for the type hibschite which he examined but that this material was somewhat impure. Pure hibschite should be approximately 12.09 A. This means that our figure 1, in the paper on grossularite.hydrogrossularite, should be modified, and the water content of the hydrogrossularites we have shown is probably somewhat too high owing to the incorrect value we took for pure hibschite.

ACKNOWLEDGMENTS

Thanks are owing to T. Thomas, W. Hoffman, and S. Schutz for keeping experimental apparatus in good repair. We are much indebted for critical comments on this manuscript to G. W. Morey, W. S. Fyfe, J. Goldsmith, P. LaMori, R. ewton, G. D. Robinson, and L. Knight.

REFERENCES

:Birch, Francis, 1955, Phy,sics of the crust, in Poldervaart, Arne, ed., Crust of the Earth: Geol. Soc. America Spec. Paper 62, p. 101·117.

Boyd, F. R., and England, 1. L., 1960a, Quartz~coesite transition: Jour. Geophys. Re· search, v. 65, p. 749·756.

---- 1960b, Apparatus for phase equilihrium measurements at pressures up ,to 50 kilobars and temperatures up to 1750°C: Jour. Geophys. Research, v. 65, p. 741·748.

Buerger, M. J., 'and Kennedy, G. C., 1958, An improved specimen holder for the focusing. type x·ray spectrometer: Am. Mineralogist v. 43, p. 756·757.

Clark, S. P., Jr., Robertlion, E. C., and Birch, Francis, 1957, Experimental determination of kyanite-sillimanite equilibrium relations: Am. Jour. Sci., v. 255, p. 628·640.

Goes, L., J r., 1954, personal communication quoted by Fyfe and Turner (1958). Fyfe, W. 5., and Turner, F. J., 1958, Correlation of metamorphic facies with experimental

data, in Fyfe, W. 5., Turner, F. J., and Verhoogen, J., ed., MetamoI1phic reactions and metamorphic facies: Geol. Soc. America Mem. 73, p. 149·185.

Griggs, D. T., and Kennedy, G. c., 1956, A simple apparatus for high pressures and tern· 'peratures : Am. Jour. Sci., v. 254, p. 722·735.

Kelley, K. K., 1950, Contributions to tile data on <theoretical metallurgy XI. Entropies of inorganic subsMinces, Revision (.1948) of data ,and methods 'of oalculalIi()n: U. S. Bur. Mines Bull. 477, 147 p.

Kennedy, G. C., 1959, Phase relations ~n the system AbO..-'H.O at high temperatures and pressures: Am. Jour. Sci., v. 257, p. 563-573.

MacDonald, G. J. F., 1954, A critical review of geologically important thermochemical data: Ph.D. thesis, Harvard University.

---- 1956, Quartz-coesite stability relations at high temperatures and pressures: Am. Jour. Sci., v. 254, p. 713·721.

1957, Thermodynamics ()f soHds under non-hydrostatic stress with geologic applications: Am. Jour. Sci., v. Q55, p. 266-281.

Pistorius, C. W. F. T., and Kennedy, G. C., 1960, Stability relations of grossularite and hydrogrossularite at high temperatures and pressures: Am. Jour. Soi., v. 258, p. 247-257.

P,istorius, C. W. F. T., ffild Shal'p, W. E., 1960, Properties of water. P'art VI. Entropy and Gibbs free energy of water ,in the range 10-1000°C ,and 1-250,000 bars: Am. Jour. Sci .• v. 258, p. 757-768.

Seki, Yotaro, 1959, Relation between chemioal composition and lattice constants of epidote: Am. Mineralogist, v. 44, p. 720-730.

Yoder, H. 5., Jr., 1950, High-low quartz invellSion UJl to 10,000 bars: Am. Geophys. Union Trans., v. 31. p. 827·835.