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INTRODUCTION Clinopyroxene of the Diopside-Heden- Bergite Series Is

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INTRODUCTION Clinopyroxene of the Diopside-Heden- Bergite Series Is 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.
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