J. Japan. Assoc. Min. Petr. Econ. Geol. 75, 221-229, 1980.

THE STABILITY OF CLINOPYROXENE OF THE DIOPSIDE-

HEDENBERGITE SERIES IN H2O-CO2 MIXTURES

TETSUYA SHOJI

Department of Mineral Development Engineering, the University of Tokyo

The stability field of hedenbergite in H2O-CO2 mixtures has been calculated on the basis of previous work on the systems FeOx-SiO2-O2, CaO-FeOx-SiO2-O2, CaO-FeOx-SiO2-CO2-O2 and CaO-FeOx-SiO2-H2O. At an oxygen fugacity higher than the FMQ buffer , where ferroactinolite is unstable, the hedenbergite field is restricted by the following reactions:

(1) hedenbergite+CO2+O2=calcite+magnetite+quartz, (2) hedenbergite+O2=andradite+magnetite+quartz. On the other hand, at an oxygen fugacity lower than the FMQ buffer, where ferroactinolite is stable, the hedenbergite field is restricted by the following reactions:

(3) hedenbergite+CO2=calcite+fayalite+quartz, (4) hedenbergite+CO2=calcite+ferroactinolite+quartz, (5) hedenbergite+O2=andradite+ferroactinolite+quartz. The equilibrium boundaries for Reactions (1), (3) and (4) are almost independent upon the fluid pressure and the oxygen fugacity in the system, and pass through the vicinity of the limit of the diopside field on the T-XCO2 diagram. It is estimated , therefore, that the clinopyroxene of the diopside-hedenbergite series is stable in the fluid containing less than 30 mole % CO2 at 500•Ž and 0.5 mole% at 300•Ž.

reaction was given experimentally by INTRODUCTION Gustafson (1974). On the other hand, at Clinopyroxene of the diopside-heden- elevated CO2 contents of fluid, hedenbergite bergite series is one of the most common breaks down to the assemblage calcite- gangue minerals of skarn-type ore deposits. magnetite-quartz by Reaction (1). The Especially, hedenbergitic pyroxene is the equilibrium boundary for this reaction was important constitutent of high-grade ore of discussed by Shoji (1978). On the contrary, copper and/or zinc (e.g. Shimazaki, 1969; diopside decomposes to the assemblage Shiobara, 1961). For this reason, the favor calcite-tremolite-quartz by Reaction (8). able condition of formation of ore-bearing The equilibrium boundary for this reaction skarn should be indicated by the stability of was determined by Skippen (1974). The clinopyroxene in the skarn-forming fluid purpose of this paper is to estimate quanti- consisting predominantly of water and tatively the limit of the hedenbergite field in carbon dioxide. H2O-CO2 mixtures, and to discuss the At elevated oxygen pressures, heden- stability of clinopyroxene of the diopside- bergite breaks down to the assemblage an hedenbergite series. dradite-magnetite-quartz by Reaction (2) The compositions of the phases con listed in Table 2. The fO2-Trelation of this sidered in this paper are shown in the CaO

(Manuscript received December 5, 1979) 222 Tetsuya Shoji

calculated from the data compiled by Strunz (1970) and those of Ernst (1966). In the following statements, salite and actinolite mean expediently the diopside- hedenbergite solid solution and the tremolite- ferroactinolite solid solution, respectively.

STABILITY RELATIONS OF HEDEN BERGITE Reactions Concerning Hedenbergite Field In a H2O-CO2 fluid with an oxygen fugacity higher than the FMQ buffer, where Fig. 1. Compositions of crystalline phases in the system CaO-FeOx-MgO-SiO2-H2O- ferroactinolite is unstable (Ernst, 1966), CO2, projected from the volatile com the hedenbergite field is restricted by ponent apexes. Reactions (1) and (2) (Shoji, 1978). On the contrary, at an oxygen fugacity lower Table 1. List of minerals. than the FMQ buffer, where ferroactinolite is stable (Ernst, 1966), the field is restricted by Reactions (3), (4) and (5), from high to low temperatures. Reactions (3), (4) and (5) are yielded by the linear combination of two or three of Reactions (1), (2), (6) and (7), as shown in Table 2. The enthalpy and entropy changes of Reaction (1) was calculated by Shoji * Calculated from the data compiled by Strunz (1970), except ferroactinolite (after Ernst, 1966). (1978). The equilibrium boundary for Reaction (2) was determined by Gustafson FeOx-MgO-SiO2 tetrahedron regardless of (1974), and its enthalpy and entropy CO2 and H2O (Fig. 1), and listed in Table 1 changes were calculated by Shoji (1978). with their abbreviations and molar volumes The enthalpy and entropy changes of Reac

Table 2. Volume changes, enthalpy changes and entropy changes of some stable reactions in the system CaO-FeOx-SiO2-H2O-CO2, and in the systems CaO-MgO-SiO2H2O-CO2 and CaO- FeOx-MgO-SiO2H2O.

* E=Ernst (1966), G=Gustafson (1974), H=Huebner (1971). Sh=Shoji (1978), Sk=Skippen (1974). The stability of clinopyroxene in H2O-CO2 mixtures 223

tions (6) and (7) can be calculated from the is calculated from the data described by

data of Huebner (1971) and Ernst (1966), Huebner (1971) (Table 2).

respectively, as shown in the following Reaction (7) was investigated by Ernst

section. (1966). Fig. 2 shows a {21n fHeO+‡™Vs (pf- The equilibrium boundaries for Reac 1)/RT} vs. 1000/T plot of the experimental

tions (3), (4) and (5), therefore, can be data of Ernst (1966). The line represents the

estimated by the thermodynamic procedure. equilibrium boundary for the reaction.

The procedure is the same as described by From this line, the enthalpy and entropy

Shoji (1976). On the calculation, Lewis' changes of Reaction (7) are calculated to

rule (ideal mixing of fluids) has been as be 20.5 kcal and 50.3 cal. deg-1, respectively sumed, and fugacity data of water, carbon (Table 2). With an foe defined by the dioxide and oxygen have been taken from FMQ buffer, ferroactinolite is stable up to

Burnham et al. (1969), Mel'nik (1972) and 500•Ž at 2000 bars Pfluid. The high-

Huebner (1971), respectively. temperature assemblage of equivalent com Enthalpy and Entropy Changes of Reactions position consists of the phases hedenbergitic (6) and (7) pyroxene+fayalite+magnetite+quartz+ fluid*. Reaction (6) is the FMQ buffer. The

enthalpy and entropy changes of the reaction Hedenbergite Field in H2O-CO2 Mixtures

The calculated enthalpy and entropy

changes of Reactions (3), (4) and (5) are

listed in Table 2. The equilibrium bound

aries for these reactions can be estimated

from the values shown in Table 2.

Fig. 3 shows the hedenbergite field under

the condition where the fluid pressure is 2000

bars and the oxygen fugacity is defined by

the FMQ buffer. The solid lines represent

the case that the oxygen fugacity is some

what lower than the FMQ buffer, while the

dashed lines represent the higher oxygen

fugacity (Shoji, 1978).

Under a fluid pressure of 2000 bars and

the oxygen fugacity defined by the FMQ

buffer, hedenbergite is stable below the CO2

content of 10 mole % at 450•Ž and 0.3 mole

Fig. 2. A {21n fH2O+‡™VS (Pf-1)/RT) vs. 1000/T % at 300•Ž (Fig. 3). Above these CO2

plot of the experimental data on Reaction contents, hedenbergite breaks down to

(7) reported by Ernst (1966). Open symbols the assemblage calcite-magnetite-quartz or indicate reaction to ferroactinolite; closed symbols to hedenbergite-fayalite-quartz. the assemblage calcite-ferroactinolite-quartz

* Forbes (1977) determined the stability limit of grunerite to be 690•Ž under the same condition.

This implies that ferroactinolite breaks down to hedenbergite+grunerite+quartz. 224 Tetsuya Shoji

Fig. 3. A calculated stability field of hedenbergite in H2O-CO2 mixtures under a 2000 bars pfluid. Numerals correspond to the number of reaction (see Table 2). Solid lines represent the stability limit of hedenbergite at the fO2defined by the FMQ buffer but somewhat lower, while dashed lines represent that at the same oxygen fugacity but somewhat higher (Shoji, 1978). A dotted line and a chain line represent the stability field of ferroactinolite (Ernst, 1966), and diopside (Skippen, 1974), respectively. Abbreviations are as shown in Table 1. in accordance with the oxygen fugacities. sure. Fig. 4a shows the schematic phase STABILITY OF DIOPSIDE-HEDENBERG diagram in the system CaO-MgO-FeOx- ITE SOLID SOLUTION SiO2-H2O-CO2at an oxygen fugacity lower Diopside breaks down to the assemblage than the FMQ buffer. It is inferred that calcite-tremolite-quartz with increasing CO2 the actinolite solid solution continues from pressures (Skippen, 1974). When the the tremolite end to the ferroactinolite one oxygen fugacity is lower than FMQ buffer, under this condition, because ferroactinolite hedenbergite breaks down to the assemblage is stable (Ernst, 1966), Consequently, the calcite-ferroactinolite-quartz, as stated pre tie-lines exist between salite and actinolite viously. These imply that salite of the in the whole range of composition (Fig. 4a). diopside-hedenbergite solid solution series Reaction (9) shows the Fe/Mg distribution breaks down to the assemblage of calcite, salite and actinolite. Although the enthalpy actinolite of the tremolite-ferroactinolite and entropy changes of this reaction listed series and quartz, with increasing CO2 pres in Table 2 have a large volume of error, they The stability of clinopyroxene in H2O-CO2 mixtures 225

Fig. 4. Schematic phase diagrams showing the decomposition sequence of clinopyrox ene of the diopside-hedenbergite series. The enthalpy and entropy changes of Reaction (9) implies that salite is associated with the relatively Fe-rich member of actinolite. The composition of the actinolite solid solution is changable from the tremolite end to the ferroactinolite end under a low fO2 condition (a), while that is not more than the point represented by acFe at a high fo2 (b). Accordingly, the decomposition sequence of clinopyroxene in illustrated by two diagrams corresponding to the former (c) and the latter (d). The letters, a, b, c, d, e and f, correspond to the compositions of salite solid solution, while acFe means the Fe-richest member of the actinolite solid solution. Abbreviations: acss=actinolite solid solution, sass=salite solid solution. and others as in Table 1. indicate that actinolite is richer in iron The produced actinolite is always richer in than salite at temperatures below 500•Ž. iron than the coexisting salite. Consequent

This is consistent with the fact that the ly, the composition of actinolite and salite lowest temperature of the hedenbergite field shift towards Fe-rich (Mg-poor) members is higher than that of the diopside field at a during the decomposition process. The specified CO2 pressure. initial composition of actinolite is controlled

With decreasing of temperature, calcite, by the stable tie-line from the original salite, actinolite and quartz are formed from salite. while the final one by inheriting all of the 226 Tetsuya Shoji

Fig. 5. Schematic diagrams showing the decomposition sequence of clinopyroxene. Differing from Fig. 4, this diagrams is drawn on the assumption that actinolite are associated with the relatively Fe-rich salite.

FeO amounts of the original salite. This diagram of the system CaO-FeOx-SiO2-H2O- decomposition sequence is illustrated sche CO2 at an oxygen fugacity higher than the matically in Fig. 4c. This sequence is scarsely FMQ buffer. Under this condition, the observed in field, because ferroactinolite has actinolite solid solution is inferred to have not been found (Ernst, 1966). a limit of FeO content, because ferroactino When the oxygen fugacity is higher than lite is unstable (Ernst, 1966). Fig. 4b is the FMQ buffer, hedenbergite breaks down drawn under consideration on this point to the assemblage calcite-magnetite-quartz and the assumption that iron concentrates (Shoji, 1978), because of the unstability of more in actinolite than salite. Let us con ferroactinolite. This implies that the sider the decomposition sequence of the mineral assemblage of the decomposition salite solid solution caused by the decrease products of the salite solid solution is dif of temperature at a specified pressure for ferent in accordance with its Fe/Mg ratio. the following three cases: the compositions Fig. 4b illustrates the schematic phase of salite are between (d) and (c), (c) and (b), The stability of clinopyroxene in H2O-CO2 mixtures 227

and (b) and (a), as shown in Fig. 4b. (i) oxygen fugacity. Accordingly, the same

Between (d) and (c) : the initial decomposi conclusion as stated by Shoji (1978) is tion products are calcite, magnetite and obtained in the present work. That is, the quartz. These minerals being formed, the CO2 condition favorable to the formation of FeO content of salite decreases continuously. clinopyroxene of the diopside-hedenbergite

When the composition of salite reaches the series is less than about 30 mole% at 500•Ž, point (b), the formation of actinolite whose and 0.5 mole% at 300•Ž. composition is acFe begins, and continues Shimazaki (1969) described the actino till the disappearence of salite. The final lite formed as an alteration product of products are calcite, actinolite (acFe),magnet clinopyroxene in the Yaguki iron-copper ite and quartz. (ii) Between (c) and (b): the deposit, Fukushima Prefecture. The com same decomposition sequence as that in the positions of coexisting clinopyroxene and case (i) continues until the appearence of actinolite in four specimens were determined the actinolite whose composition is acFe. by their indices. They are 90 and 65, 48 After this, magnetite is consumed by and 68, 84 and 74, and 87 and 93 in the formation of actinolite. After the hedenbergite and ferroactinolite molecules disappearence of magnetite, both of salite (mole%), respectively. In two pairs iron and actinolite decrease their FeO contents. concentrate more in salite than actinolite,

When the actinolite takes all of the magne and vice versa in another two pairs. The sium ions involved in the original salite, the fact that salite is richer in iron than actino clinopyroxene disappears complitely. This lite does not agree with the thermochemical final sequence is the same as that described data described in Table 2. This discrepancy in the case that ferroactinolite is stable. seems to be due to the error of estimated (iii) Between (b) and (a): the decomposi values and/or the non-ideality of the solid tion sequence is the same as the final one in solutions. If some member of actinolite are the case (ii). Fig. 4d shows schematically poorer in iron than the coexisting salites, these decomposition sequences. the phase diagrams shown in Fig. 5 should

be considered instead of Fig. 4. DISCUSSION As shown in Fig. 3, the equilibrium CONCLUSION boundaries for Reactions (1) and (4), which The stability field of hedenbergite in restrict the hedenbergite field, pass through H2O-CO2 mixtures has been determined on the vicinity of the equilibrium boundary for the basis of previous work, and the Reaction (8) restrincting the diopside field. decomposition of clinopyroxene of the diop With the increasing oxygen fugacities, shifts side-hedenbergite series has been considered. the equilibrium boundary for Reaction (1) (1) Fig. 3 shows the hedenbergite field towards the low CO2 side. The shift, how represented by temperature and CO2 content ever, is so small as to be negligible between of fluid. the NNO and FMQ buffers (Shoji, 1978). (2) Fig. 4 shows the schematic decom

These facts suggest that the upper limit of position relation of clinopyroxene at a low CO2 content of the clinopyroxene field do oxygen fugacity (Figs. 4a and 4c), and at a not depend on the composition, and the high oxygen fugacity (Figs. 4b and 4d). 228 Tetsuya Shoji

Huebner, J. S. (1971), Buffering techniques for (3) Figs. 3 and 4 indicate that the hydrostatic systems at elevated pressure. CO2 content of H2O-CO2 fluid in which "Research Technique for High Pressure and hedenbergite was formed is less than 30 mole High Temperature" (ed. Ulmer, G. C.), 123- 177, Springer-Verlag, New York. % at 500•Ž and 0.5 mole% at 300•Ž, Mel'nik, Yu. P. (1972), Thermodynamic parameters respectively. of compressed gases and metamorphic reactions involving water and carbon dioxide. Geochem.

ACKNOWLEDGEMENT Intern., 9, 419-426. Shimazaki, Hidehiko (1969), Pyrometasomatic I wish to thank Professor S. Takeno copper and iron deposits of the Yaguki mine,

uchi of the University of Tokyo for critical Fukushima Prefecture, Japan. Jour. Fac. Sci., Univ. Tokyo, Sec. II, 17, Pt. 2, 317-350. reading of the manuscript and many val Shiobara, Kanji (1961), Decrepitation temperatures uable suggestions. and chemical characteristics of the mineral species from the Kamioka mine. Mining Geol., 11, 344-349 (in Japanese with English REFERENCES abstract).

Burnham, C. W., Holloway, J. R. and Davis, N. F. Shoji, Tetsuya (1976), The stability of the as

(1969), Thermodynamic properties of water to semblage calcite-quartz in H2O-CO2 mxitures, 1,000•Ž and 10,000 bars, Geol. Soc. Amer., Jour. Jap. Assoc. Mineral. Petrol. Econ. Geol., Spec. Paper 132, pp. 96. 71, 379-388.

Ernst, W. G. (1966), Synthesis and stability rela •\ (1978), Phase relations in the system CaO- tions of ferrotremolite. Amer. Jour. Sci., 264, FeOx-SiO2 in H2O-CO2 mixtures. Jour. Jap.

37-65. Assoc. Mineral. Petrol. Econ. Geol., 73, 221- Forbes, W. C. (1977), Stability relations of grunerite, 240. Fe7Si8O22(OH)2. Amer. Jour. Sci., 277, 735- Skippen, G. B. (1974), An experimental model for 749. low pressure metamorphism of siliceous dolo Gustafson, Wm. I. (1974), The stability of an mitic marble. Amer. Jour. Sci., 274, 487-509. dradite, hedenbergite, and related minerals in Strunz, H. (1970), Mineralogische Tabellen (5 Auf.). the system Ca-Fe-Si-O-H. Jour. Petrol., 15, Akademische Verlagsgesellschaft Geest & 455-496. Portig K. -G., Leipzig, pp. 621. The stability of clinopyroxene in H2O-CO2 mixtures 229

H2O-CO2混 合 流 体 中 に お け る 透 輝 石-ヘ デ ン 輝 石 系 単 斜 輝 石 の 安 定 領 域

正 路 徹 也

H2O-CO2混 合 流 体 中 にお け るヘ デ ン輝石 の安 定 領 域 を従 来 の研 究 結 果 に もとつい て熱 力 学 的 に計 算 し た。 鉄 アクチ ノ閃石 が 不 安 定 な よ うなFMQバ ッフ ァー よ り高い 酸 素 フ ュガ シ テ ィー の下 で,ヘ デ ン輝 石 の 安 定領 域 を決め るの は次 の2つ の反 応 で あ る。 (1)ヘ デ ン輝石+CO2+O2=方 解石+磁 鉄 鉱+石 英 (2)ヘ ヂ ン輝 石+O2=灰 鉄 ザ ク ロ石+磁 鉄 鉱+石 英 一方,鉄 ア クチ ノ閃石 が安 定 なFMQバ ッ フ ァー よ り低 い 酸 素 フ ュガ シ テ ィ ー下 で,ヘ デ ン輝石 の 安 定領 域 を 限 るのは次 の3つ の 反 塔 で あ る。 (3)ヘ デ ン輝 石+CO2=方 解 石+鉄 カ ン ラン石+石 英 (4)ヘ デ ン輝 石+CO2=方 解 石+鉄 ア ク チ ノ閃 石+石 英 (5)ヘ デ ン輝石+O2=灰 鉄 ザ ク ロ石+鉄 ア ク チ ノ閃石+石 英 。 これ らの うち,反 応(1), (3), (4)の 平 衡 境 界 は,流 体圧 と酸素 フ ュガ シ テ ィーに は あ ま り依存 せ ず, T-XCO2 図中 でお 互 い に ほ ゞ同 じ位 置 を通 る。 そ の位 置 は ま た透輝 石領 域 の 限界 に も近 い 。 こ の こ とか ら, 500℃ で30 mole%以 上, 300℃ で0.5mole%以 上 のCO2を 含 む流 体 中 で は,透 輝石-ヘ デ ン輝石 系 の 単斜 輝 石は 生成 し え ない と結論 で き る。