A ferrian manganoan grossular of the chemical composition (Ca,Mn
2+)
3(Al,Fe
3+)
2[SiO
4]
3 with Ca : Mn
2+≈2 : 1 and Al : Fe
3+ ≈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 : Fe
3+ 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 Mn
2+ with Mn
3+ 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 : Mn
2+ ≈2 : 1 is to be partnered with a piemontite having Mn
3+: (Al+Fe
3+) ≈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 CaMn
3[SiO
4]
2·H
2O. The metabasite and some ores are penetrated by the veinlets of piemontite with lower Mn
2O
3 contents. The massive manganoan grossular is superficially converted into rancieite through weathering.
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