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(Fe,Al,Mg,Mn,Zn)7Si8O22(OH)2
Monoclinic
Cummingtonite, an iron magnesium silicate hydroxide mineral of the amphibole group, was first reported from Sterling Hill (as grunerite) by Reilly (1983). It occurred as small dark-brown needle-like crystals, as an alteration product of diopside, and was associated with franklinite, titanite, and calcite in drill-hole #124, 111 feet from its origin on the 340 level. Cummingtonite was also found as an alteration product of hedenbergite and iron oxides, associated with andradite, ferroactinolite, franklinite, and calcite in drill-hole #124, 119 feet from its origin. No symmetry determination was given by Reilly (1983); the monoclinicity was apparently inferred optically. The mineral described as zinc-manganese-cummingtonite by Bauer and Berman (1930) is now known as tirodite.
Microprobe analyses by Reilly (1983) of the occurrences noted above yielded SiO2 48.63, 46.42; CaO 0.14, 0.36; Al2O3 6.87, 8.00; FeO 29.92, 29.96; MnO 1.77, 2.30; MgO 2.33, 2.49; K2O 1.61, 1.32; ZnO 5.09, 5.65; TiO2 0.49, 0.73; total = 96.85, 97.23 wt. %. Iron occupies 3.96 to 4.00 of the 7 cations and is insufficient for the designation grunerite according to the nomenclature conventions of Leake (1978).
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| Copyright © 1995 by Pete J. Dunn |
Website
by Herb Yeates
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Link
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