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Cummingtonite
Here's today's science lesson.....
Cummingtonite or magnesium iron silicate hydroxide is a metamorphic index mineral with the chemical composition (Mg,Fe)7Si8O22(OH)2. It is a member of the cummingtonite-grunerite solid solution series which ranges from Fe2Mg5Si8O22(OH)2 for cummingtonite to the iron rich grunerite endmember Fe7Si8O22(OH)2. Manganese also substitutes for iron producing a manganese rich variety. The cummingtonite series is polymorphic with the anthophyllite-gedrite series and often requires optical or X-ray diffraction analysis to distinguish the polymorphs.
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