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(Mn,Mg)MgZn(SiO4)(OH,F)2
Orthorhombic, Bbcm, a = 8.185, b = 18.65, c =
6.256 Ĺ, Z = 8.
Gerstmannite is a rare mineral from Sterling Hill and has not been found at Franklin. It was described by Moore and Araki (1977a).
Moore and Araki (1977a) provided the crystal structure. Simonov et al. (1978) pointed out a structural relationship to clinohedrite. The gerstmannite structure is based on oxygen cubic close-packing and is composed of [MnMgO3(OH)2] octahedral sheets linked to [ZnSiO4] tetrahedral sheets which are parallel to (010).
Gerstmannite is light pink to white and occurs as bunched, flattened sprays of radiating prismatic crystals, up to several cm in composite length. It is translucent to opaque, with vitreous luster, good cleavage on {010}, and a density of 3.68 g/cm3.
Optically, gerstmannite is biaxial, negative, 2V = 50-60o, with a = 1.665, b = 1.675, and g = 1.678. The orientation is X // b, Y // c, and Z // a. There is no discernible fluorescence in ultraviolet.
Gerstmannite is a manganese magnesium zinc silicate hydroxide mineral. Chemical analyses of gerstmannite are given in Table 3 and show little variation in composition from specimen to specimen. Re-analysis of type gerstmannite by the writer found 5.0 wt. % fluorine which was missed in the original study; it is presumed to be in substitution for (OH), comprising 0.7 of the 2.0 (OH,F). Analysis by DTA-EGA confirmed the presence of H2O.
Gerstmannite is known from secondary vein occurrences. Moore and Araki (1977a) described the type gerstmannite as occurring on calcite-rich willemite/franklinite ore from Sterling Hill. They described this ore as coated with magnesian calcite, followed by a mixture of brown manganpyrosmalite and sphalerite. Calcite and gerstmannite coat this assemblage. They described a second sample occurring with apple-green willemite and pink magnesian alleghanyite. The writer has identified impure fluoborite as white veins crosscutting gerstmannite.
Gerstmannite was named in honor of Ewald Gerstmann, a collector and enthusiast of Franklin minerals; he is responsible for stimulating many persons to collect the local minerals.
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| Copyright © 1995 by Pete J. Dunn |
Website
by Herb Yeates
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