The Stability of Grossular in H20-Co2 Mixtures

J. Japan. Assoc. Min. Pe tr. Econ. Geol. 72, 30-41, 1977

THE STABILITY OF GROSSULAR IN H20-CO2 MIXTURES

TETSUYA SHOJI

Department of mineral development engineering, University of Tokyo

The stability field of grossular in H20-C02 mxitures has been calculated on the basis of

previous work in the system CaO-A12O8 SiO2-H2O-CO2. The following reactions restricting the grossular field are, from high to low temperatures:

(1) grossular + C02 =calcite+ anorthite + wollastonite (2) grossular+ C02 =calcite+ anorthite +quarte (3) grossular + C02 + H20 =calcite+ zoisite +quartz (4) grossular+CO2+H2O=calcite+prehnite. These reactions give three isobaric invariant points, that is, point A (calcite-wollastonite-

grossular-anorthite-quartz): T=564•Ž and Xco2=18.8 mole % at 1000 bars, and T= 575•Ž and Xco2 = 13.9 mole % at 2000 bars; point B (calcite-grossular-zoisite-anorthite-

quartz): T=461•Ž and Xco2=5.18 mole % at 1000 bars, and T=503•Ž and Xcp2=6.30 mole % at 2000 bars; and point C (calcite-grossular-zoisite-prehnite-quartz) : T=406•Ž and Xco2=1.60 mole % at 1000 bars, and T=405•Ž and Xco2=0.919 mole % at 2000 bars. In the system containing iron, at the temperatures where the assemblage prehnite iron oxide is not stable, the reaction.

(3') grandite+C02+H2O=calcite+epidote+quartz+hematite (or magnetite), places the strict limit on the condition of formation of grandite garnet growing in equilibrium with a H2O-CO2 fluid. The upper and lower limits on the carbon dioxide conditions of formation of ore-bearing skarn consisting of calcite, grandite and quartz are approximately

placed by Reaction (3') and the reaction, calcite+quartz = wollastonite+CO2, respectively.

INTRODUCTION pressures, and by the thermodynamic calculations. Gordon and Greenwood

The common gangue minerals of skarn (1971) determined the stability of grossular type ore deposits are garnet of grossular in H2O-CO2 mixtures, and Skippen (1974)

andradite series and clinopyroxene of dio derived the phase relations in the system

pside-hedenbergite series. Therefore, for the CaO-MgO-SiO2-CO2-H2O. Generally , the discussion of the genesis of ore deposits of temperatures discussed by these investiga this type, it is necessary to understand the tions are higher than 500•Ž . On the other stabilities of these minerals . Especially, hand, it is considered that ore-bearing skarn it is invaluable to investigate their stabilities seems to have been formed below 400•Ž in H2O-CO2 mxitures , because the mixed (Shoji, 1975). Accordingly, it is necessary fluids must have played the most important for the interpretation of genesis of ore role for the formation of skarn. bearing skarn to make clear the phase rela Recently, the stability relations in H 2O tions at tempeartures below 500•Ž. For -CO2 mixtures have been studied by the this purpose, the writer tries to estimate the experiments carried out under controlled CO 2 stability field of grossular . in H2O-CO2

(Manuscript received August 27, 1976) The stability of grossular in H20-C02 mixtures 31

mixtures. +Reaction (7)} (12) =3 Reaction (2)+Reaction (8) (13) REACTIONS CONCERNING THE GROS Reaction (4)

SULAR FIELD = 1/5 Reaction (3)+Reaction (9). (14)

The reactions limiting the grossular As described latter, the equilibrium field are as follows, from high to low tem boundaries for Reactions (1), (2), (5), (6) ,

peratures: (7), (8) and (9) were determined by previous CaCO3+CaAl2Si2O8+CaSiO3 work. Accordingly, it is possible that the

=Ca3Al2Si3O12+CO2 (1) equilibrium boundaries for Reactions (3)

2CaCO3+CaAl2Si2O8+SiO2= and (4) are calculated on the basis of those

Ca3A12Si3O12+2CO2 (2) results. 5CaCO3+2Ca2Al3Si3O12(OH)+3SiO2 =3Ca3Al2Si3O12+5C02+H 2O (3) CALCULATION OF THE EQUILIBRIUM CaCO3+Ca2Al2Si3O10(OH)2 BOUNDARIES FOR REACTIONS =Ca3Al2Si3O12+CO2+H2O. (4) Grossular is stable on the carbon dioxide The procedure of calculation is the same as that by Shoji (1976c). The reaction poor (water-rich) side of the boundaries for these reactions (Gordon and Greenwood, isobar of van't Hoff is integrated to give

1971). The temperature ranges where these In Kp ‡™H=RT,+const, (15) reactions take place are limited by the following reactions: where Kp is equi birium constant, ‡™H is CaCO3+SiO2=CaSiO3+CO2 (5) enthalpy change, R gas constant, and T is Ca3Al2Si3O12+SiO2 temperature (•‹K). ‡™H varies with fluid =CaAl2Si2O8+2CaSiO3 (6) pressure, Pf, as follows : 4Ca2Al3Si3012(OH)+SiO2 ‡™H =‡™H•‹+‡™V5•‹(Pf-1), (16) =5CaAl2Si2O8+Ca3A12Si3O12 where ‡™H•‹is standard enthalpy change, and +2H2O (7) 4V•‹is volume change of solid phases. 2Ca2Al3Si3O12(OH)+CO2 Therefore, equation (15) is reduced to =CaCO3+3CaAl2Si2O8+H2O In Kp=‡™H•‹+‡™V•‹(Pf-1) (8) 5Ca2Al2Si3O1O(OH)2= +co nst., (17) 2Ca3Al2Si3O12+2Ca2Al3Si3O12(OH) where const. is ‡™S•‹/R, and ‡™S•‹is entropy +3SiO2+4H2O. (9) change. Lewis' rule (ideal mixing of fluid) That is, Reactions (2), (3) and (4) are obtained by the linear combinations among Table 1. List of minerals. Reactions (1), (5), (6), (7), (8) and (9) as follows : Reaction (2)= Reaction (1)+Reaction (5) (10) =2{Reaction (1) +Reaction (6)}(11) Reaction (3)=

1/2{5 Reaction * Calculated from the data compiled by Strunz (1970). 32, Tetsuya Shoji

Table 2. Volume changes, enthalpy changes and entropy changes of some stable reactions in the system CaO-Al2O3-SiO12H2O-CO2 (at 1 bar).

*B=Boettcher (1970) S=Shoji (in press), , S-N=StorreG=Greenwood and(1967). NitschG-G=Gordon (1972) andGreenwood (1971),H-T=Harker andTuttle (1956), L=Llou (1971), N-Newton (1966).

has been assumed and the fugacity data for out from 1500 bars to 8000 bars, and the

H2O and CO2 have been taken from latter between 560•Ž and 660•Ž at 4000

Burnham et al. (1969) and Mezbmm (1972), bars and 5300 bars. Both results coincide

respectively. The molar volumes of solid with each other on a p-T diagram as shown

phases (Table 1) have been calculated from in Fig. 1. The equation of the line fitted

the data compiled by Strunz (1970). Table in Fig. 1 is

2 shows the volume, enthalpy and entropy P=24.1•~T-18900. changes of Reactions (1)-(9), which have

been determined by the following procedure . Reactions (5), (6), (7), (8) and (9)

Harker and Tuttle (1956) and Green

wood (1967) determined the equilibrium

boundary for Reaction (5) in COs fluids and

H20-Col mixtures, respectively. Both

results are in good agreement. The stand

ard enthalpy change of Reaction (5) , ‡™H5•‹, is 23.1 kcal (Table 2; Shoji, 1976c) . From both results, the equilibrium constant was

reduced to Fig. 1. A p-T plot of the experimental data on lnKP=-23.1•~103-0.468•~(Pf-1)/RT Reaction (6) reported by Newton (1966) and Boettcher (1970). Open symbolsindi cate reaction to grossular-quartz; closed symbolsto wollastonite-anorthite +19.24 Newton (1966) and Boettcher (1970) . investigated the equilibrium for Reaction Using Clapeyron-Clausius equation* , the (6). The former experiments were carried enthalpy change of Reaction (6) is calculated

* On the assumption that ‡™H/‡™V5 , is constant, Clapeyron-Clausius equation is integrated to give P=‡™H/‡™V . ln T+const . In this case, ‡™H5 is 18.9 kcal . The stability of grossular in H20-COs mixtures 33 to be ‡™H6•‹=14.7 kcal from this equation

(Table 2). Newton (1966) and Boettcher (1970) investigated also Reaction (7). The former experiments were carried out between 500•Ž and 800•Ž from 1000 bars to 8000 bars, while the latter between 600•Ž and 800•‹

at 3000 bars and 6000 bars. Fig. 2 shows

a •o2 ln fH2O=‡™Vs•‹(pf-1)/RT} vs 1000/T

plot of both experimental data. They are in good agreement. The equation of the

line fitted in Fig. 2 is l

n Kp=-55.7•~103+1.333(Pf-1) Fig. 3. A (ln (fH2o/fco2)+‡™Vs•‹(Pf-1)/RT•pvs

1000/T plot of the experimental data on /RT +50.8. Reaction (8) reported by Storre and Nitsch

(1972).

Fig. 3 shows a (ln (faao/fcoa)+‡™Vs•‹(pf-1)/

RT•pvs 1000/T plot of their experimental

data. The data are so scattered that no

straight line can be fitted for them. For this

reason, it is impossible to calculate the

Fig. 2. A (2lnfHao+‡™Vs•‹(Pf-1)/RT•pvs 1000/T

plot of the experimental data on Reaction

(7) reported by Newton (1966) and Boett cher (1970). Open symbols indicate reac tion to zoisite-quartz; closed symbols to anorthite-grossular.

From this equation, the enthalpy change of Fig. 4. A (4lnfH2O+‡™Vs•‹(Pf-1)/RT•pvs 1000/ Reaction (7), 4H,°, is calculated to be 55.7 T plot of the experimental data on kcal (Table 2). Reaction (9) reported by Strens (1968) Storre and Nitsch (1972) investigated and Liou (1971). Open symbols indicate r eaction to prehnite; closed symbols to

Reaction (8). The experiments were grossular-zoisite-quartz. Abbreviations carried out from 2000 bars to 7000 bars. are as shown in Table 1. 34 Tetsuya Shoji

enthalpy change of Reaction (8), ‡™H8•‹ From this equation, the enthalpy change of

Strens (1968) and Lion (1971) in Reaction (2), ‡™H2•‹, is 32.4 kcal (Table 2).

estigated Reaction (9). The former experi The enthalpy change of Reaction (2) can be

ments were carried out from 1000 bars to calculated also from the following equa

2000 bars, while the latter from 3000 bars to tions :

5000 bars. Fig. 4 shows a •o4lnfH2O+‡™V•‹ ‡™ H2•‹=‡™H1•‹+‡™H5•‹

s(pf-1)/RT{vs 1000/T plot of their data. or Both results differ slightly from each other. ‡™ H2•‹=2¥‡™H1•‹+‡™H6•‹,

The line shown in Fig. 4 is the equilibrium which is derived from equation (10) or (12),

boundary given by Liou (1971). The equa respectively. They give ‡™H2•‹=31.5 kcal tion is l (Table 2). This value agrees with that calculated from the results of Gordon and n Kp••-117.5•~103-2.568•~(Pf-1)/ Greenwood (1971), given in Table 2.

RT +107.4. Reactions (3) and (4)

The enthalpy change of Reaction (9), ‡™H•‹9, Equation (12) or(13) means that the is 117.5 kcal (Table 2). enthalpy change of Reaction (3) can be

Reactions (1) and (2) calculated by the following equation,

Gordon and Greenwood (1971) in respectively.

vestigated the equilibria of Reactions (1) and ‡™H3•‹=1/2(5¥‡™H2•‹+‡™H7•‹) (2) in H20-CO, mixtures. According to or them, the equilibrium boundary for Reac ‡™H3•‹=3¥‡™H2•‹+‡™H8•‹

tion (1) at 2000 bars passes approximately The former equation gives ‡™H3•‹

through the points: Xco2••11.5 mole %, T 109.0 kcal (Table 2), but the latter =590•Ž; Xco2=20 mole % , T=665•Ž; and cannot give the dH2 value, because ‡™H8•‹ Xco2=30 mole %, T=790•Ž. The equili have not been determined as mentioned brium boundary for Reaction (2) passes previously.* Determining the slope of the approximately through the points: Xco2=10 equilbirium boundary for Reaction (3) from

mole %, T=550•Ž, and Xco2=8 mole % , ‡™H3•‹=109.0 kcal, and its constant term T=530•Ž. The equilibrium constant for from the fact that 'the line passes through

Reaction (1) is l the intersecting point between the equilib

rium boundaries for Reactions(2) and (7), n Kp=-8.4•~103-1.250•~(Pf-1)/RT the equation of equilibrium boundary for

+9.81. Reaction (3) is Accordingly, the enthalpy change of Reac ln Kp=-109.0•~103-3.632•~(Pf-1)/RT tion (1), ‡™H1•‹, is 8.4 kcal (Table 2) . Mean ‚— hile, the equilibrium constant for Reaction +99.71. (2) is The equilibrium boundary for Reac

l n Kp=-32.4•~103-1.718•~(Pf-1)/RT tion (4) is calculated also by the same procedure. The enthalpy change of Reac +29.72. tion (4) is given as

* ‡™H8•‹can be calculated revers ely from ‡™H2•‹32 .4 kcal and ‡™H3•‹=109.0 kcal. In this case , the enthalpychangeof Reaction (8) , ‡™H8•‹, is 11.6 kcal (Table 2) . The stability of grossular in H20-C02 mixtures 35

Table 3. Equations of the equilibrium boundaries for Reactions (1), (2), (3), (4), (5), (7) and (9) in H2O-CO2 mixtures.

‡™ H4•‹=1/5(‡™H3•‹+‡™H9•‹) grossular field in H2O-CO2 mixtures from high to low temperatures. Grossular is not from equation (14). The calculated enth stable in fluids which is richer in carbon alpy change of Reaction (4), ‡™H4•‹, is 45.3 dioxide than the equilibrium boundaries kcal (Table 2). The equation of the for the above reactions. Let us calculate equilibrium boundary for Reaction (4) is the T-Xco$ relations of equilibria of Reac ln Kp=- 45.3•~103-1.240•~(Pf-1) tions (1), (2), (3), (4), (5), (7) and (9) under fluid pressures of 1000 bars and 2000 bars, /RT +53.54. on the basis of the above-mentioned equa tions and some assumption (Shoji, 1976c). EQUILIBRIUM BOUNDARIES FOR REAC The calculated T-Xco2 relations of the TIONS IN H2O-C02 MIXTURES equilibrium boundaries for Reactions (1), Reactions (1), (2), (3) and (4) limit the (2), (3), (4), (5), (7) and (9) are listed in

Fig. 5. A calculated stability field of grossular in Fig. 6. A calculated stability field of grossular in H2O-CO2 mxitures under a fluid pressure H2O-CO2 mixtures under a fluid pressure of 2000 bars. Heavy lines represent the of 1000 bars. Heavy lines represent the limit of grossular field. Numerics corres limit of grossular field. Numerics corre spond to number of reaction. Abbrevia pond to number of reaction. Abbrevia tions are as shown in Table 1. tions are as shown in Table 1. 36 Tetsuya Shoji

Table 4. List of some isobaric invariant points in the system CaO-A12O2-SiO2-H2O-CO2.

Table 3, and are shown in Figs. 5 and 6. phase transition are rather small than those On the calculation, following two points by the chemical reaction. Compared the have been taken into account. (i) T-Xco2 errors of thermodynamicproperties estimat relations for Reactions (1) and (2) at 2000 ed by the above-mentioned procedure, the bars, and for Reaction (5) at 1000 bars and differences of thermodynamic properties 2000 bars are the same as those reported by between zoisite and clinozoisite seem to Gordon and Greenwood (1971) and Green small. For this reason, it is possible to wood (1967), respectively. (ii) The con consider that the thermodynamic properties stant terms of the equations of the equilib of clinozoisite are approximately equal to rium boundaries for Reactions (2), (3) those of zoisite within the limit of error and (4) have been determined as the lines caused by the thermodynamiccalculation. If pass through the intersecting points between so, the ine 3 shown in Figs. 5 and 6 indicates the equilibrium boundaries for Reactions the boundary where grossular decomposes (1) and (5), Reactions (2) and (7), and Reac to the assemblage calcite-zoisite (or clino tions (3) and (9), respectively. These zoisite)-quartz. Whether zoisite or clino intersecting points are invariant at a zoisite is formed depends on the other specified pressure. The mineral assemblages factors. and T-Xco2 values of the isobaric in Generallyspeaking, most of ore-bearing variant points are listed in Table 4. skarn consist of garent of the grossular andradite series and/or clinopyroxene of the DISCUSSION diopside-hedenbergiteseries. For this re ason, the stability relations of these minerals As shown in Figs. 5 and 6, grossular indicate the conditions of the formation of decomposes to calcite and one or two other ore-bearingskarn. According to Reactions minerals in carbon dioxide-rich fluids. The (1), (2), (3) and (4), grossular decomposes decomposed products are characterized by under high carbon dioxide pressures. There wollastonite, anorthite, zoisite and prehnite, fore, the stability field of grossular indicates from high to low temperatures. Among the upper limit of carbon dioxide pressures them, zoisite is not common in skarn , but where ore-bearing skarn can be formed. clinozoisite of epidote group occurs . Meanwhile,the lower limit of carbon dioxide Therefore, the calculation must be made of amount in skarn-forming fluids is estimated the case that grossular decomposes to form to be the stability field of assemblage clinozoisite. Generally, the changes of calcite-quartz (Shoji, 1975; 1976c) . Calcite thermodynamic properties caused by the and quartz combine to form xonotlite The stability of grossular in H1O-CO9 mixtures 37

or wollastonite under low carbon dioxide (1) and (5) is T=564•Ž and Xco2=18.8 pressures. The lower limit of the calcite mole % at 1000 bars, and T=575•Ž and quartz field coincides approximately with Xco2=13.9 mole % at 2000 bars. Mean the equilibrium boundary for Reaction while, Reaction (6) occurs at 549•Ž under

(5) above 200•Ž (Shoji, 1976c). That is, 1000 bars, and 590•Ž under 2000 bars the line 5 shown in Figs. 5 and 6 indicates independently of Xco, (Fig. 1). The diff

the lower limit of carbon dioxide contents erence between both temperatures is +15•Ž

for the formation of ore-bearing skarn. at 1000 bars, and -15•Ž at 2000 bars.

According to Figs. 5 and 6, in the The enthalpy change of Reaction (4),

calcite-quartz field below 370•Ž, grossular ‡™ H4•‹, is calculated from the equation,

is not stable but prehnite is formed. ‡™ H4•‹=1/5(‡™H5•‹+‡™H9•‹) Prehnite is rarely found in ore-bearing

skarn. These facts seem to suggest that =1/5{1/2(5¥‡™H2•‹+‡™H7•‹)+‡™H9•‹}.

ore-bearing skarn was formed above 370•Ž. This fact means that ‡™H4•‹inherits all errors

However, it is not acceptable to consider caused by the calculations of ‡™H2•‹, ‡™H7•‹

that most of ore-bearing skarn were formed and ‡™H9•‹. The smallest value of calculated

above 370•Ž. This is because the forma ‡™ H4•‹is 36.7 kcal. This value is given by

tion temperature of ore-bearing skarn is the equation, inferred to be below 400•Ž as indicated ‡™ H4•‹=1/5[1/2{5(‡™H1•‹+‡™H5•‹) from the stability and occurrence of

vesuvianite, the homogenization temper +‡™H7•‹)+‡™H9•‹],

atures of fluid incluision, and the decrepita and using each smallest value of ‡™H1•‹, ‡™H7•‹, and ‡™H9•‹. The reason why lines 4 and 5 tion temperatures of grandite garnet (Shoji, cross in Figs. 5 and 6 depends possibly on 1975; 1976b). That is, the equilibrium

boundary for Reaction (4) does not agree the fact that dH4 =45.3 kcal is greater than ‡™H5•‹=23.1 kcal, and not on the term, with the natural occurrence of ore-bearing ‡™ Vs•‹(pf-1). Using ‡™H4•‹=36.7 kcal, the in skarn. This disagreement seems to be tersecting point between lines 4 and 5 in caused by two reasons: (i) the error of Figs. 5 and 6 is shifted about 50•Ž to the thermodynamic calculation, and (ii) the low temperature side, and 0.6 in log Xco2

difference between the Fe2O3 containing to the low carbon dioxide side.

and Fe$Oa free systems. The above-mentioned facts suggest

Sources of the error of calculation are that the error caused by the thermodynamic

(i) inadequancy of the assumptions (‡™H•‹= calculation is, estimated to be •}15•Ž in the const. and ‡™S•‹=const. in equation (17), temperature of the isobaric invariant point

and Lewis, rule), (ii) uncertainty of used where calcite, wollastonite, grossular and

data, and (iii) uncertainty of fitting of quartz coexist, and 9 kcal in .the enthalpy

straight lines. Equations (10) and (11) change of Reaction (4). Accordingly, the

mean that the equilibrium boundaries for position and slope of each line drawn in

Reactions (1), (2), (5) and (6) pass always Figs. 5 and 6 involve an uncertainty of such

through the same invariant point on a T- degree at least.

Xco, diagram under a specified pressure In the system containing iron, lines in

The intersecting point between Reactions Figs. 5 and 6 seem to shift more. In the 38 Tetsuya- Shoji system containing Fe203, if the composition 2Ca2Al2Si3O10(OH)2+ Fe2O3 of grandite garnet does not change during = 2Ca2Al2FeSi3O12(OH)+H20, the decomposition, Reaction (3) is reduced to and that the left side assemblage is stable

calcite + epidote + quartz + hematite below about 200•Ž under a few kilobars. If =grandite+CO2+H2O (3') the assemblage prehnite-hematite is where the composition of epidote is Ca2(Fey, unstable in the system containing iron, the

Al1-y)3Si3O12(OH), and that of grandite is tie-line between them is cut by the as

Ca3(FeX,Al1-X)2Si3O12. According to the semblage grandite-epidote-quartz. Fig. 7 chemical analyses of coexisting grandite and shows the schematic phase relation among epidote (Kitamura, 1975), x is always grandite, epidote, prehnite and quartz. greater than y. Therefore, hematite (Fe2O3) Where the amount of Fe2O3 is richer than is only an added product formed by the the point XF, the assemblage grandite decomposition of grandite, and any other epidote-quartz is stable (Fig. 7a). In this product in the system CaO-A1203-SiO2(-H2O) case, the compositions of grandite and is not formed. If an oxygen fugacity is low, epidote vary with bulk composions -of the magnetite substitutes for hematite. How system. On the other hand, where the ever, the differences of thermodynamic amount of Fe2O3 is poorer than the point XF, properties between hematite and magnetite the assemblage grandite-epidote-prehnite are so small that it is possible to neglect the - quartz is stable (Fig. 7a). In this case, the effect, of oxygen fugacities on the mineral compositions of grandite and epidote are assemblage. determined independently of bulk composi

In the system containing Fe2O3, Reac tions to be grc and epc, respectively. The tion (9) is reduced to simplified phase relation is shown in Fig. 7b. prehnite+hematite Above 400•Ž, the assemblage grandite- grandite+epidote epidote-quartz is stable even in the Fe2O3-

+quartz+H2O. (9') poor region as shown in Fig. 7c, because

Reaction (3') and (9') are combined to give prehnite is unstable (Liou, 1971). The com calcite+hrenite+quartz+hematite positions of grandite and epidote (grc and =grandite+CO2+H2O . (4') epc, respectively), which coexist stably If the temperatures at which Reaction (9') with prehnite and quartz, vary with tem takes place decrease with the increasing peratures and pressures. Although the amounts of Fe2O3in the system, the isobaric relation between the compositions , grc and invariant point C, where calcite, grossular , epc, and temperatures and/or pressures epidote, prehnite, quartz and hematite has not been determined yet , the Fe2O3 coexist, shift toward the low temperature content in them seem to increase with the side. In this case, the grandite garnet field decreasing temperatures as shown in Figs . is extended to the lower temperature regions 7d and 7e. The occurrences of prehnite than that shown in Figs. 5 and 6. suggest that the temperature of the boun

However, the assemblage prehnite-hem dary between the assemblage grandite

atite (or magnetite) is not found in skarn . epidote-quartz and the assemblage grandite-

Seki (1972) considered that the stability limit epidote-prehnite-quartz , decreases steeply of epidote is determined by the reaction , with the increasing amount of Fe 2O3, The stability of grossular in H20-C02 mixtures 39

Fig. 7. Schematic phase relation among grandite, epidote, prehnite and quartz in the system CaO-Al2O3-Fe2O3-SiO2-SiO2-H2O. XF represents the intersecting point between the plane

qz-grc-epc and the line pr-Fe303, and grc and epc represent the compositions of grandite and epidote, respectively, which associate with prehnite and quartz. Abbreviations: ad=andradite, cz=clinozoisite, ep=epidote, gd=grandite, gd/ep=grandite and/or epidote,

gr=grossular, pr=prehnite, ps=pistasite, and qz=quartz. (a) A block diagram showing the phase relation among grandite, epidote, prehnite and quartz below 400•Ž. Where the amount of Fe2O3 is richer than the plane gz-grc- epc, the assemblage grandite-epidote-quartz is stable. On the other hand, where the amount of Fe203 is poorer than the plane, one of the assemblages grandite (grc)- epidote (epc)-prehnite-quartz, grandite-prehnite-quartz, epidote-prehnite-quartz, and

grandite-epidote-prehnite is stable in accordance with bulk compositions. The arrow shows the direction from which Figs. (b), (c) and (d) are projected.

(b) Schematic phase relation among grandite, epidote, prehnite and quartz below 400•Ž.

(d) A schematic diagram showing the change of the compatibility field of grandite (grc), epidote (epc), prehnite and quartz with temperatures. (e) A schematic diagram showing the stability field of prehnite in the system containing iron.

Prehnite occurs only in iron-poor skarn shift to the low temperature side. In this such as the Nambuhinokiyama deposit of case, the grandite garnet field extends to the Yaguki mine (Shoji, 1976a).' On the the lower tempreature side than that shown other hand, grandite and/or epidote in in Figs. 5 and 6. prehnite-free skarn contain a considerable As mentioned above, even if the as amount of iron (Shimazaki, 1969; Kita semblage prehnite-hematite is stable or not, mura, 1975). These facts suggest that the in the system containing Fe2O3 the assemblage grandite-epidote-phrenite is only equilibrium boundary for Reaction (9) stable under limited iron amounts. If this. passes through lower temperature regions assumption is true, in the system containing than the lines shown in Figs. 5 and 6 in Fe2O3 the equilibrium boundary for Reac corresponding to the amount of iron in the tion (9) and the isobaric invariant point C system. Accordingly, in the system con 40 Tetsuya Shoji

taining iron the equilibrium boundary for lastonite is not stable.

Reaction (3) must be extrapolated to the low temperature regions. This is the CONCLUSION reason why the extrapolated part of the In the system CaO-A12O3-SiO2H2O-CO2, equilibrium boundary for Reaction (3) is the equilibrium boundaries for Reactions shown with the dashed line in Figs. 5 and 6. (1), (2), (3) and (4) restricting the grossular The above-mentioned discussion in field have been calculated on the basis of dicates that in the system containing iron previous work. Reaction (3') placeg a strict limit on the con (1) Table 3 and Figs. 5 and 6 show the dition of formation of grandite garnet equilibrium boundary for each reaction in believed to have grown in equilibrium with H20-CO, mixtures under fluid pressures of a H2O-CO, fluid below 450°C. Typical ore 1000 bars and 2000 bars. bearing skarn contains calcite, quartz and (2) Three isobaric invariant points grandite garnet. The assemblage calcite are listed in Table 4. quartz is stable on the carbon dioxide-rich (3) In the system containing iron, the side of the boundary line 5 shown in Figs. 5 grandite garnet field is restricted by Reac and 6, while garnet is stable on the carbon tion (3') in the place of Reactions (3) and dioxide-poor side of the lines 1, 2 and 3. (4). The field where calcite-quartz and garnet (4) If the enthalpy change of Reac are stable is very narrow. It is not ac tion (3') is smaller than that of Reaction ceptable to consider that ore-bearing skarn (3), the grandite field is extended toward can be formed only under such narrow the carbon dioxide-richer side than the line conditions. If the enthalpy change of Reac 3 shown in Figs. 5 and 6. tion (3') is smaller than that of Reaction (5) Reactions (3) and (5) place ap (3), the grandite garnet field in H2O-CO2 proximately the upper and lower limits on fluids is extended toward the carbon dioxide the carbon dioxide contents in a 1120-CO, rich side. This suggests that grandite fluid, respectively, with which ore-bearing skarn consisting of calcite, grandite and garnet can be formed under the wider conditions than those shown in Figs. 5 and quartz was formed in equilibrium.

6. ACKNOWLEDGEMENT According to the hydrothermal phase relation in the system CaO-A1208 Si02 re I wish to thank Professor S. Takenouchi ported by Shoji (1974), the tie-line does not of the University of Tokyo for critical read exist between grossular and quartz, but ing of the manuscript and many valuable between anorthite and wollastonite above suggestions. 400•Ž. However, even if the experimental result in the carbon dioxide-free system is REFERENCE correct, the grossular field in H2O-CO2 mix Boettcher, A.L. (1970): The system CaO-Al tures is restricted by Reaction (2) and (3) 1O3 . SiO2-H2Oat high pressuresand temperatures, This is inferred by the fact that Petrol., 11, 337-379. , as shown in Burnham,C.W., Holloway, J.R. and Davis Figs. 5 and 6, grossular is stable in the ,, N.F. (1969) : Thermodynamic properties of water carbon dioxide-rich region where wol to 1,000•Ž and 10,000 bars, Geol. Soc . Amer., The stability of grossular in H20-C02 mixtures 41

Spec. Paper 132, pp. 96. Jap., 12, 1-15, in Japanese.- Gordon,T. M. and Greenwood,H. J. (l971): ; The (1975): Role of temperature and CO2 stability of grossularitein H2O-CO2mixtures, pressure in the formation of skarn and its Amer. Mineral, 56, 1674-1688. bearing on mineralization, Econ. Geol., 70, Greenwood,H. J. (1967):Wollastonite: stability in 739-749. H2O-CO2mixtures and occurrence in a - (1976a): (On tungsten-bearing skarn contact-metamorphic aureole near Salmo, from Yaguki mine, Fukushima Prefecture), British Columbia, Canada, Amer. Mineral., Mining Geol., 26, 44, in Japanese. 52, 1669-1680. (1976b): (Temperature, Xco, and fog on Harker, R.I. and Tuttle, O.F. (1956): Experimental the formation of ore-bearing skarn), Preprints data on the Pco2-T curve for the reaction: Ann. Meeting Geol. Soc. Japan., 322, in Japa calcite+quartz=wollastonite +carbon dioxide, nese.

Amer.Jour. Sci., 254, 239-256. - (1976c) : The stability of the assemblage Kitamura, K. (1975):Al-Fe partitioning between calcite-quartz in H2O-CO2 mixtures, Jour. Jap. garnet and epidote from the contact metas Mineral. Petrol. Econ. Geol., 71, 379-388. omatic copper deposits of the Chichibu mine, Skippen, G. (1974): An experimental model for low Japan, Econ. Geol.,70, 725-738. pressure metamorphism of siliceous dolomitic Lion, J.G. (1971): Synthesis and stability rela marble, Amer. Jour. Sci., 274, 487-509. tions of prehnite, Ca2Al2Si3O10(OH)2, Amer. Storre, B. and Nitsch, K.-H. (1972) : Die Reaktion Mineral. 56, 507-531. 2 Zoisit+1 CO2=3 Anorthit+1 Calcit+1 H2O, Newton, R.C. (1966): Some calc-silicateequilib Contr. Mineral. Petrol., 35, 1-10. rium relations, Amer.Jour. Sci., 264, 204-222. Strens, R. G. J. (1968) : Reconnaissance of the

Seki, Y. (1972): Lower-gradestability limit of prehnite stability field, Mineral. Mag., 36, epidote in light of natural occurrences.Jour. 864-867. Geol. Soc. Japan, 78, 405-413. Strunz, H. (1970): Mineralogische Tabellen (5 Auf.), Shimazaki, H. (1969): Pyrometasomatic copper Akademische Verlagsgesellsachaft Geest & and iron deposits of the Yaguki mine, Fuku Portig K.-G., Leipzig, pp. 621. shima. Prefecture, Japan, Jour. Fac. Sci., Menhmnt, TO.11. (1972) : Tepmo„t„yHaMNsecKNe CBONCTBa C„waTbLX Ta3OB HeKOTOPbIe OOOOeHHOCTH MeTaMOPƒ³„I„NecK„IX peak„D„I„J C y ?? acT„IeM BO„D„Z „I„D‚a„DBYOK„I‚b„I yc„Depo„Da, Teox„IM„I„`, 1972, 654-662. Univ. Tokyo,Sec. II, 17, Pt. 2, 317-350. Shoji, T. (1974): Phase relations in the system CaO-A1208SiO$ H2O, Jour. Mineral. Soc.

H2O-CO2混合流体中における灰バンザクロ石の安定領域

正路徹也

従来発表されているCaO-Al2O2-SiO2-H2O-CO2系の研究結果をもとに, H2O-CO2混合流体中における灰バンザクロ石の安定領域を求めた。灰パンザクロ石の安定領域を決める反応は,高温から低温へ,次の4つの反応

である。 (1)灰バンザクロ石+CO2=方解石+灰長石+珪灰石 (2)灰バンザクロ石+CO2=方解石+灰長石+石英 (3)灰バンザクロ石+CO2+H2O=方解石十ユウレン石+石英 (4)灰バンザクロ石+CO2+H2O=方解石+ブドウ石これらの反応の交点として, 3つの不動点が存在する。A点(方解石-珪灰石-灰バンザクロ石-灰長石-石英):1000パールで, T=564℃, Xco2=18.8mole%, 2000バールで, T=575℃, XCO2=13.9mo1e%; B点 (方解石一灰パンザクロ石一ユウレン石-灰長石-石英):1000バ ― ルで, T=461℃, Xco2=5.18mole%, 2000 パールで, T=503℃, Xco2=6.30mole%; C点(方解石一灰バンザクロ石一ユウレン石一ブドウ石一石英): 1000パールセで, T=406℃, Xco2=1.60mo1e%, 2000パ ― ルで, T=405℃, Xco2=0.919mole%。鉄を含む系で,ブドウ石と鉄酸化物の組合せが不安定な温度では,次の反応, (3') グランダイトザクロ石+CO2+H2O=方解石+緑簾石+石英+赤鉄鉄(又は磁鉄鉱) が,H2O-CO2.混合流体中で安定に存在しうるグランダイトザクロ石の生成条件の限界を決定する。方解石,ザクロ石および石英からなる鉱右鉱物を伴うスカルンの生成するCO2条件の上限と下限はそれぞれ,反応(3')および次の反応, 方解石+石英=珪灰石+CO2 により決定される。