Manganoan grossular-piemontite association in a low grade metamorphic manganese ore from the Dainichi mine, Hadano City, Kanagawa Prefecture, Japan
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A ferrian manganoan grossular of the chemical composition (Ca,Mn<SUP>2+</SUP>)<SUB>3</SUB>(Al,Fe<SUP>3+</SUP>)<SUB>2</SUB>[SiO<SUB>4</SUB>]<SUB>3</SUB> with Ca : Mn<SUP>2+</SUP>≈2 : 1 and Al : Fe<SUP>3+</SUP> ≈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<SUP>3+</SUP> 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<SUP>2+</SUP> with Mn<SUP>3+</SUP> 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<SUP>2+</SUP> ≈2 : 1 is to be partnered with a piemontite having Mn<SUP>3+</SUP>: (Al+Fe<SUP>3+</SUP>) ≈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<SUB>3</SUB>[SiO<SUB>4</SUB>]<SUB>2</SUB>·H<SUB>2</SUB>O. The metabasite and some ores are penetrated by the veinlets of piemontite with lower Mn<SUB>2</SUB>O<SUB>3</SUB> contents. The massive manganoan grossular is superficially converted into rancieite through weathering.
- Mineralogical Journal
Mineralogical Journal 17(5), 211-218, 1995-03-01
Japan Association of Mineralogical Sciences