Analysis of Mn-bearing lawsonite occurring in meta-siliceous rocks in Hakoishi serpentinite mélange of Kurosegawa Belt, Central Kyushu, Japan
Journal of Mineralogical and Petrological Sciences
Mnbearing lawsonite was discovered in metasiliceous rocks metamorphosed under the lawsoniteblueschist facies formed at less than 350 °C and 0.8 -1.0 GPa in the Hakoishi serpentinite mélange of the Kurosegawa Belt, Central Kyushu, Japan. The lawsonite deposits were accompanied by those of hematite, braunite (Mn 2+ Mn 6 3+ SiO 12 ), and quartz, indicating that the rock had metamorphosed under high Oxygen fugacity (f O 2 ) (~ -20 < log f O 2 < -5) at the abovementioned P -T conditions. The Oxygen
... itions. 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 (Mn 3+ + Fe 3+ ), substituting ~ 5 -11 mol% of Al in the ideal lawsonite formula. Lawsonite in metasiliceous rocks metamorphosed under low f O 2 state (~ log f O 2 < -20), and metabasites collected from the same mélange contained lesser amount of (Mn 3+ + Fe 3+ ), substituting ~ 3 -5 mol%. The amount of Sr and Ba in the analyzed lawsonite were below the detection limit of energydispersive microprobe analysis, ~ <0.1 wt%. This indicated the existence of an unknown endmember of the lawsonite group of minerals, CaFe 2 3+ Si 2 O 7 (OH) 2 •H 2 O or CaMn 2 3+ Si 2 O 7 (OH) 2 •H 2 O on the basis of the observed substitution. The extremely high f O 2 state that results in the conversion of all the iron into ferric compounds along with the Mnrich local chemical compositions would necessarily enhance (Mn 3+ + Fe 3+ ) ↔ Al substitution in lawsonite.