Manganoan grossular-piemontite association in a low grade metamorphic manganese ore from the Dainichi mine, Hadano City, Kanagawa Prefecture, Japan.

Abstract

A ferrian manganoan grossular of the chemical composition (Ca,Mn2+)3(Al,Fe3+)2[SiO4]3 with Ca : Mn2+≈2 : 1 and Al : Fe3+ ≈2 : 1 was found in a lower grade metamorphic bedded manganese ore deposit of the Dainichi mine, Hadano City, Kanagawa Prefecture, in association with piemontite of the similar Ca : Mn and Al : Fe3+ ratios, where the metamorphic grade belongs to prehnite-pumpellyite facies. Grossular and piemontite occur as fine disseminations in a metabasite forming the wallrock of ore beds. The co-existence of Mn2+ with Mn3+ favoured the formation of two discrete but chemically related phases after their preferential lodging. Thus, in the grossular-spessartine continuous solid solution series the manganoan grossular with Ca : Mn2+ ≈2 : 1 is to be partnered with a piemontite having Mn3+: (Al+Fe3+) ≈1 : 2 as its oxidized derivative unless minor fluctuations of ratios and OH in the latter are considered. Apart from the occurrence in metabasite, manganoan grossular is found as a massive form involving hausmannite, caryopilite, chlorite, johannsenite, manganoan calcite, and an unknown mineral with the probable chemical formula CaMn3[SiO4]2·H2O. The metabasite and some ores are penetrated by the veinlets of piemontite with lower Mn2O3 contents. The massive manganoan grossular is superficially converted into rancieite through weathering.