Doolan, Barry L. Department of Geology, Memorial University of Newfoundland, St. John's, Canada.
Last reviewed:December 2019
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The name given to magnesium-rich monoclinic calcium amphibole Ca2Mg5Si8O22(OH)2. The mineral is white to gray, but colorless in section and optically negative. It usually exhibits long prismatic crystals with prominent (110) amphibole cleavage. Unlike other end-member compositions of the calcium amphibole group, very pure tremolite is found in nature. Substitution of Fe for Mg is common, but pure ferrotremolite, Ca2Fe5Si8O22(OH)2, is rare. Intermediate compositions between tremolite and ferrotremolite are referred to as actinolites and are green in color and encompass a large number of naturally occurring calcium amphiboles. The substitution of Na, Al and Fe3+ ions into the amphibole structure is common in actinolites. The nature of these substitutions is complex and leads, under some conditions, to miscibility gaps between actinolites and aluminous calcium amphiboles called hornblendes. For the most part, however, the physical and chemical variations between actinolite and hornblende are continuous so that an arbitrary actinolite-hornblende division is necessary. Most of the amphibole classifications divide actinolite from the hornblende series on the basis of Al substitution for Si with actinolite containing less than 0.5 atom of Si replaced by Al per formula unit. See also: Hornblende
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