MCGOVERNITE

(Mn,Mg,Zn)₂₇₃As³⁺_~12As⁵⁺_~30Si_~42O₃₂₄(OH)₂₅₂
Hexagonal, R3c, a = 8.22, c = 203.15 Å, Z = 1

	Figure 18-41. Splayed rosettes of mcgovernite (white-gray) on ore composed of franklinite, willemite, and calcite from Sterling Hill. Specimen is 7 cm in maximum dimension. Smithsonian Institution, #153611. Photo by the author.

Mcgovernite was described by Palache and Bauer (1927b) from Sterling Hill. They recognized its complex chemical nature and did not propose a precise chemical formula. The possible relation of mcgovernite to dixenite was pointed out by Bauer and Berman (1928). The crystallography of mcgovernite was studied by Wuensch (1960), who also discussed relations with dixenite and who subsequently (1968) compared the crystallography of these phases. Moore and Araki (1978) discussed the relations in the hematolite group and provided the formula M²⁺₁₉Zn₃(OH)₂₁(AsO₃) (AsO₄)₃(SiO₄)₃. The chemical composition of mcgovernite was considered as part of a study of kraisslite by Dunn and Nelen (1980). They verified the original ratio of Bauer (for As³⁺:As⁵⁺) and provided new chemical data. Mcgovernite is not known from Franklin.

Description

Mcgovernite occurs as platy hexagonal crystals identical to those of kraisslite. Visually, the two minerals can be nearly indistinguishable. On some specimens, mcgovernite occurs as large (6 cm) radial flattened sprays and splayed rosettes, a habit not seen to date in kraisslite. Mcgovernite is reddish-brown with perfect {0001} cleavage and is visually similar to kraisslite in luster and most aspects. The density is 3.719 g/cm³ (Palache and Bauer, 1927). Optically, mcgovernite is uniaxial, positive, with w = 1.754. Mcgovernite is distinguished easily from kraisslite by its index of refraction and X-ray powder pattern. There is no discernible response to ultraviolet.

Composition

Mcgovernite is a complex ferric-iron magnesium manganese zinc arsenite arsenate silicate hydroxide mineral. Microprobe analyses of mcgovernite are given in Table 15. As shown by Dunn and Nelen (1980), the composition of mcgovernite, like that of kraisslite, is remarkably constant from specimen to specimen. The similarity in chemical constituents supports the close relation between these species, and the presence of ferric iron in both mcgovernite and kraisslite may further support the relation to hematolite and similar phases. Although both minerals have As in two oxidation states, the ratio of As³⁺:As⁵⁺ is different. There is no definitive chemical formula for mcgovernite; the structure is unknown. However, a mcgovernite-like mineral was described from the Kombat Mine in Namibia by Dunn et al. (1988); the material they described may be the Zn- and Al-analogue of mcgovernite proper. They proposed a tentative formula for Sterling Hill material, given above in the header.

Occurrence and paragenesis

Mcgovernite was found in 1927 on the 900 level at Sterling Hill. It remained a relatively uncommon mineral, but was found again in the early 1980’s. The earlier material occurred as a massive vein filling, sometimes having a breccia texture, and as splayed rosettes, some browner than others, on fracture and vein surfaces. It was somewhat notable for having few associated arsenate species, unlike kraisslite which occurs with many such minerals. The later find of mcgovernite provided material of lesser richness, but some of it was vuggy, and secondary hodgkinsonite was found in mcgovernite-lined vugs.

Name

Mcgovernite was named for J. J. McGovern, a prominent collector of Franklin minerals.

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
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