Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

J. A. K. Tareen ¹, A. R. Fazeli ¹, B. Basavalingu ¹, G. T. Bhandage ²

Affiliations
1 Department of Studies in Geology, Mysore University, Manasagangotri, Mysore 570006, IN
2 Department of Chemistry, Regional College of Education (NCERT), Mysore 570006, IN

Abstract

The P-T stability field of rhombohedral double carbonates, CaMg (CO₃)₂ (dolomite) and CaMn (CO₃)₂ (kutnahorite) has been studied up to 2 kb of PCO₂. The dolomite decomposes to CaCO₃ (ss)+MgO+CO₂ and the kutnahorite to CaCO₃ (ss)+MnO+CO₂. The kutnahorite was considered to be completely disordered (R̅₃C) near the decomposition temperature while the dolomite was found to be in ordered state at the decomposition temperature, though both were considered to stabilise with ordered structures at lower temperatures. The standard enthalpy and free energy data retrieved for the ordered dolomite is ; ΔH⁰_f = -2331.64±2.07 kJ/mol, ΔG⁰_f = -2168.59±2.07 kJ/mol., and for the disordered kutnaborite, is; ΔH⁰_f = -2093.46±3.77kJ/mol, ΔG⁰_f = -1944.80±3.77kJ/mol. The new data on dolomite are atIeast 7 kJ more negative than the earlier reported values. In the low temperature region the double carbonate field is preceded by a metastable two carbonate field of M'CO₃ (ss) and M"CO₃ (M' = Ca, M" = Mg, Mn) phases. This is considered to be the likely stage in the natural process of dolomite or kutnahorite formation.

Keywords

Dolomite, Kutnahorite, Thermodynamics, Decarbonation Curves.

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J. A. K. Tareen ¹, A. V. Ganesha ¹

Affiliations
1 Department of Geology, Manasagangotri, University of Mysore, Mysore-570 006, IN

Abstract

An experimental study in the Mg-Al rich portion of K₂O-MgO-Al₂O,-Si₂O-H₂O (KMASH) system was carried out using bulk composition 2Phl+6Sil+9Qtz. Experiments were done using Internally Heated Pressure Vessel (Gas Bomb) in a P-T range of 3-10 kbar and 900-1250°C with water undersaturated conditions. Three distinct fields were noticed in the given P-T plane: (a) Field-I: Crd+L (up to 1000°C), (b) Field-II: Ind+Sp+L (between 1000 and 1250°Cl up to 6 kbar) and (c) Field-III: Spr+Ind+AI.En±Os±Qtz±L (between 1000 and 1250°C above 6 kbar). The appearance of hexagonal indialite (Ind) instead of orthorhombic cordierite above 1000°C is significant. The analysis of the mineral phases shows that sapphirine is of 2:2:1 type and enstatite is highly aluminous (up to 14 wt% Al₂O₃). The anafyses of indialite lie between 2:2:1 sapphirine and ideal cordierite on the (Al₂O,+K₂O)-MgO-SiO₂, triangular diagram. The analysis of melt is found to depend on the associated restites and the overall composition corresponds to 'granitic'.

Keywords

Mg-Al Granulites, KMASH System, Cordierite, Indialite, Sapphirine, Al-Enstatite, Spinel Stability, UHT Metamorphism.

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