J. Japan. Assoc. Min. Petr. Econ. Geol. 73, 231-240, 1978 PHASE RELATIONS IN THE SYSTEM CaO-FeOX SiO2 IN H20-CO2 MIXTURES TETSUYA SHOJI Department of Mineral Development Engineering, The University of Tokyo The phase relations among calcite, wollastonite , ferrobustamite, andradite, heden- bergite, magnetite and quartz in H2O-CO2 mixtures have been estimated on the basis of previous work in the systems CaO-SiOa-CO, and CaO-Fe01 SiO2-O2. The upper limit of COs content of the condition favorable to the formation of skarn involving andradite and/or hedenbergite is restricted by the following reactions: (6) andradite+CO2 =calcite+magnetite+quartz+O2, (8) hedenbergite+CO2+O2=calcite+magnetite+quartz. From the andradite and hedenbergite fields, the skarn consisting of these minerals is inferred to be formed in H2O-CO2 fluids containing less 30 mole % CO2 at 500•Ž , and 0.5 mole % at 300•Ž, respectively. When ferrobustamite is stable, the phase relations are more complex. The above mentioned values, however, are independent on the stability of ferrobustamite. INTRODUCTION plot regardless CO, (Fig. 1), and listed in Table 1 with their abbreviations and molar Grandite garnet and/or chnopyroxene volumes calculated from the data complied of diopside-hedenbergiteseries are the most by Strunz (1970) and Yamanaka et al. (1977). important minerals which characterize the skarn-type ore deposits. They frequently Table 1. List of minerals. contain a considerable amount of iron. Besidesthem, the ore deposits of this type are usually composed of calcite, magnetite and quartz with sulfidesand oxides as ore minerals. For this reason, the phase rela tions among calcite, andradite, hedenbergite, * Calculated fron the data compiled by Strunz (1970). magnetite and quartz should indicate the except ferrobustamite (Yamanakaet al., 1977). favorable conditions of formation of ore bearing skarn. SOME STABLE REACTIONS IN THE SYSTEM Ca-Fe-Si-C-O AND RELA In this paper, the writer tries to estimate quantitatively the phase relations TIONSHIP AMONG THEM in the system CaO-FeOBSiO2 under the Fig. 2 shows the schematic, isobaric T fluidscomposed of water and carbon dioxide, f co, relations among calcite, andradite, on the basis of previous studies of the system hedenbergite and quartz under a controlled CaO-SiO2-CO2and CaO-FeORSiO2-O2. oxygen fugacity. In this diagram, there Compositions of phases considered in are apparently four invariant points and this paper are shown on a CaO-FeOx-SiO2 eleven univariant curves. These univariant (Manuscript received July 3, 1978) 232 Tetsuya Shoji curves are equilibirum boundaries for the reactions listed in Table 2. These reactions are combined one another as follows: Reaction(2)=1/2{9•EReaction(3) -Reaction (1)} (1) Reaction(3)=1/9{Reaction(1) +2• Reaction (2)} (2) Reaction(4)=1/3{Reaction(2) -Reaction(1)} (3) =1/2(3• Reaction(3) -Reaction (1)} Fig. 1. Compositions of crystalline phases in the Reaction(6)=9{Reaction(5) system Ca-Fe-Si-C-O projected on the -Reaction(1)} (4) CaO-FeOx SiO2 plane. Reaction(7)=Reaction(6)-Reaction(2) (5) +9•EReaction(5)} (6) Reaction(8)=1/3(Reaction(2)-Reaction (1) =1/3(Reaction(2)+Reaction(6)} Fig 2. Schematic analysis of phase relations in the system CaO-FeO-Fe2O3- SiO2-CO2 as a function of CO3 fugacity and temperature. Numerals correspond to the number of reaction (see Table 2). Capital letters in circles show isobaric, f02-defined invariant points (see Table 4). Abbreviations are shown in Table 1 and Fig. 1. Phase Relations in the System. CaO-FeOx-SiO2 in H2O-CO2 Mixtures 233 Table 2. Volume changes, enthalpy changes and entropy changes of some stable reactions in the system CaO-FeO-Fe202-Si02-C027% (at atmospheric pressure). * Gu=Gustafson (1974), Gw=Greenwood(1967), H-T=Harker and Tuttle (1956), K=Kurshakova (1971a,b), L=Liou (1974), S1=Shoji (1976b), S2=Shoji (1977), T-L=Taylor and Liou (1978) Reaction(9)=Reaction(3)-Reaction(5) (7) REACTIONS (1) TO (4) =1/9 {2•EReaction(2)-Reaction(6)} At first, let us consider the stability Reaction (10) relations among wollastonite, andradite, =2{Reaction(5)-Reaction(3)} (8) hedenbergite, magnetite and quartz. Fig. = Reaction(3)+2•EReaction (9) Reaction(11)=1/3{2•EReaction(1) +Reaction (7)} (9) Equations (1) to (9) mean that the number of independent reactions is three. Accord ingly, if three equilibrium boundaries among them, for example Reactions (1), (2) and (5), have been already determined, the other reactions can be calculated by the thermodynamic procedure which is derived from the reaction isobar of van't Hoff (Shoji, 1976b), Fig. 3. Schematic analysis of phase relations at •¬. On the estimation, Lewis' rule (ideal isobaric invariant point I, where the mixing of fluids) has been assumed, and assemblage wollastonite-andradite-heden- bergite-magnetite-quartz is stable. Solid fugacitydata of carbon dioxide and oxygen, lines represent the isobaric univariant except that of the Pb-PbO buffer (see phase relations, and dotted lines buffer Appendix), have been taken from Me,7ni curves. Numerals correspond to the num ber of reaction (see Table 2). Abbrevi- (1971)and Huebner (1971), respectively. ations are shown in Table 1. NNO and FMQ represent the Ni-NiO buffer and the fayalite-magnetite-quartz one, respec tively. 234 Tetsuya Shoji 3 shows the schematic, isobaric fo2-T rela This line is not necessarily inconsistent with tions in the system Ca-Fe-Si-O. In this the results of Iypmago$a (1971a, b). diagram, there are one invariant point and From this line, the equilibrium constant four univariant curves. The univariant of Reaction (2) is as follows: curves are equilibrium boundaries for Reactions (1) to (4). Equations (1) to (4) mean that the number of independent reactions is two. Phase relations in the system have been investigated by Rypma- •¬ The equilibrium boundary for Reaction EP Ba (1970,1971a, b), „K„…„‚‡V„p„{„€„Ž„p „y „@„Ž„u„„„y„ƒ„‘„~ (3) was determined by I ypmat oBa (1970, (1974), Gustafson (1974) and Liou (1974), 1971a), and by Liou (1974). Rypmaspa using solid phase oxygen buffers. The (1971a) stated that at 1000 bars Pffuid the equilibrium boundary for Reaction (1) was assemblage andradite-quartz is stable above determined by Gustafson (1974). The ex an oxygen fugacity of 10-13 bar at 800•Ž, periments were carried out between 500 and 10-20 bar at 500•Ž. On the contrary, bars and 3000 bars. The enthalpy and Liou (1974) concluded that at 2000 bars entropy changes, and equilibrium constant of Reaction (1) were calculated by Shoji (1977) (Table 1). The equilibrium boundary for Reaction (2) was determined by KypmanoBa (1971a, b), and by Gustafson (1974). IfiypinanoBa (1971b) stated that at 1000 bars Pflnid hedenbergite is stable below an oxygen fugacity of 10-14 bar at 750•Ž and 10-23 bar at 400•Ž. On the contrary, Gustafson (1974) concluded that at 2000 bars Pflnid hedenbergite is stable below an f o, of 10-13 bar at 800•Ž and 10-28 bar at 400•Ž. Both of the conclusions do not agree with each other at the low temperature. The data of their experiments, however, are ap proximately coincident with each other, as shown in Fig. 4, which represents a {2 ln fo2+‡™Vs(pf-1)/RT}vs. 1000/T plot of their results. The low temperature experiments performed by KypmanoBa (1971a, b) seem to have not been demonstrated as equilibrium, because the assemblage an Fig. 4. A (2 ln fo2+‡™Vs (iii-1)/RT) vs. 1000/T dradite-hedenbergite-magnetite-quartz was plot of the experimental data on Reac synthesized at 400°C within a wide fo, tion (2) reported by lcypmazoaa (1971a, b) and Gustafson (1974). Open symbols range. The straight line in Fig. 4 has been indicate reaction to andradite-magnetite- drawn for the data of Gustafson (1974). quartz; closed symbols to hedenbergite. Phase Relations in the System CaO-FeOx-SiO2 in H2O-C02 Mixtures 235 Pfluid this assemblage is stable above an fo, of 10-is bar at 800•Ž, and 10-26 bar at 500•Ž. Both of the conclusions do not agree with each other at the low tem perature. Liou (1974) suggested that the experiments performed by I£ypma iosa (1970, 1971a) had not been successfully demonstrated as equilibrium because of starting materials, run duration and experi mental methods. Fig. 5 shows a {1/2 In fo2 +‡™Vs(Pf-1)/RT}vs. 1000/T plot of their experimental results. The solid line in Fig. 5 have been drawn for the data of Liou (1974). From the line, the enthalpy and Fig. 5. A{1/2 ln fo2+‡™Vs(pf-1)/RT}vs. 1000/ entropy changes of Reaction (3) are calcu- T plot of the experimental data on Re lated to be 86.1 kcal and 52.5 cal.deg-1, action (3) reported by Kypmaxoea (1970, respectively. On the other hand, the 1971a) and Liou (1974). Open symbols indicate reaction to andraite-quartz; Gustafson's (1974) results suggest that closed symbols to wollastonite (or ferro- the enthalpy and entropy changes of bustamite ?)-hedenbergite. Reaction (3) are 71.5 kcal and 38.0 cal.deg-1, respectively (Table 2). These values have demonstrated as equilibrium. For this been calculated using equation (2). The reason, the writer believes that the values dashed line in Fig. 5 shows the calculated obtained from the results of Gustafson equilibrium boundary for Reaction (3). (1974)are more accurate than those on the The dashed line is not necessarily incon- results of I£ypniagosa (1970, 1971a) and sistent with the results of Kypluaaosa (1970, Liou (1974)*. 1971a), while it does not agree with the The enthalpy and entropy changes of results of Liou (1974).