Ca–Mn exchange between grossular and MnCl2 solutions at 2 kbar and 600°°C: reaction mechanism and evidence for non-ideal mixing in spessartine-grossular garnets |
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Authors: | I Gavrieli Alan Matthews J B Holland |
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Institution: | (1) Geological Survey of Israel, 30 Malkei Israel street, Jerusalem, 95501, Israel, IL;(2) Institute of Earth Sciences, The Hebrew University, Jerusalem, 91904, Israel, IL;(3) Department of Earth Sciences, University of Cambridge, Cambridge, CB2 3EQ, UK, GB |
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Abstract: | The hydrothermal reaction between grossular and 1 molar manganese chloride solution was studied at 2 kbar and 600 °C at various
bulk Ca/(Ca+Mn) compositions:
Ca3Al2Si3O12+3Mn2+(aq) ⇔ Mn3Al2Si3O12+3Ca2+(aq)
The reaction products are garnets of the spessartine-grossular solid-solution series which discontinuously armour the dissolving
grossular grains. The first garnet to crystallize is spessartine rich (X
gt
Mn≥0.95), reflecting the high Mn content of the solution, but as the reaction proceeds more calcium-rich garnets progressively
overgrow the initial products. The armouring product layer is detached from the dissolving grossular, which allows the progressive
overgrowth to occur on both its external and internal surfaces and results in the development of a two directional Ca/(Ca+Mn)
zoning pattern in the product grains. The compositional changes in the run products are consistent with attainment of heterogeneous
equilibrium between the external rims of the spessartine-grossular garnets and the bulk solutions in runs of duration ≥24
hours. Plots of ln KD versus X
gt
Ca maxima show linear variations that are not consistent with the ideal mixing that has been proposed for spessartine-grossular
garnets at temperatures of 900 to 1200 °C. The data rather fit a regular solution model with the parameters ΔG° (600 °C, 2 kbar)=−8.0±0.8 kJ/mol and w
gt
CaMn=2.6±2.0 kJ/mol. Existing solubility measurements and thermodynamic data from other Ca and Mn silicates support the calculated
data. Grossular activities calculated using the w
gt
CaMn parameter indicate that even in manganese-rich metapelites pressure estimates calculated using the garnet-plagioclase-Al2SiO5-quartz barometer will not be increased by more than 0.2 kbar.
Received: 18 January 1995/Accepted: 4 June 1996 |
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