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

Ittai Gavrieli*, Alan Matthews, Tim J.B. Holland

*Corresponding author for this work

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5 Scopus citations

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 spessartinegrossular solid-solution series which discontinuously armour the dissolving grossular grains. The first garnet to crystallize is spessartine rich (XMngt ≥ 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 In KD versus XCagt 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 wCaMngt = 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 wCaMngt 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.

Original languageEnglish
Pages (from-to)251-262
Number of pages12
JournalContributions to Mineralogy and Petrology
Volume125
Issue number2-3
DOIs
StatePublished - 1996

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