Analysis of Mn-bearing lawsonite occurring in meta-siliceous rocks in Hakoishi serpentinite mélange of Kurosegawa Belt, Central Kyushu, Japan

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Mn-bearing lawsonite was discovered in meta-siliceous rocks metamorphosed under the lawsonite-blueschist facies formed at less than 350 °C and 0.8-1.0 GPa in the Hakoishi serpentinite melange of the Kurosegawa Belt, Central Kyushu, Japan. The lawsonite deposits were accompanied by those of hematite, braunite (Mn2+Mn63+SiO12), and quartz, indicating that the rock had metamorphosed under high Oxygen fugacity (fO2) (∼ -20 < log fO2 < -5) at the abovementioned P-T conditions. The Oxygen fugacity led to the conversion of all the iron into ferric compounds and some of the manganese into its trivalent form. As a result, the lawsonite was found to contain a significant amount of (Mn3+ + Fe3+), substituting ∼ 5-11 mol% of Al in the ideal lawsonite formula. Lawsonite in meta-siliceous rocks metamorphosed under low fO2 state (∼ log fO2 < -20), and metabasites collected from the same melange contained lesser amount of (Mn3+ + Fe3+), substituting ∼ 3-5 mol%. The amount of Sr and Ba in the analyzed lawsonite were below the detection limit of energy-dispersive microprobe analysis, ∼ <0.1 wt%. This indicated the existence of an unknown endmember of the lawsonite group of minerals, CaFe23+Si2O7(OH)2·H2O or CaMn23+Si2O7(OH)2·H2O on the basis of the observed substitution. The extremely high fO2 state that results in the conversion of all the iron into ferric compounds along with the Mn-rich local chemical compositions would necessarily enhance (Mn3+ + Fe3+) ⇔ Al substitution in lawsonite.