MANGANPYROSMALITE

Mn₈Si₆O₁₅(OH,Cl)₁₀
Hexagonal, P3m1, a = 13.36, c = 7.16 Å, Z = 2

	Figure 18-11. Columnar parallel-growth of manganpyrosmalite crystals from Sterling Hill. Field of view is 0.5 mm in maximum dimension.

Manganpyrosmalite was first described as a new species from Sterling Hill by Frondel and Bauer (1953), and the formula was further discussed by Hey (1956). It has been found sporadically since, but only at Sterling Hill. One occurrence was described by Parker and Guy (1990), but most of the occurrences remain largely unstudied. Some specimens may be mislabeled as friedelite in older collections. 

Description

Manganpyrosmalite occurs as both massive material (Figure 18-14) and as fine euhedral crystals (Figures 18-11, 18-12, and 18-13) which are composed of prisms, pinacoids, and pyramids. Such crystals may be randomly intergrown or may exhibit strong columnar parallel growth. Massive manganpyrosmalite is distinctly brown to yellow-brown in all specimens seen by the writer, except for slightly altered material; hand-sized masses are known. Massive, fine-grained aggregates have a decidedly dull to very slightly bronzy to resinous luster, and a distinct, radial fibrosity may be readily apparent on polished surfaces (Figure 18-14).

	Figure 18-12. Barrel-shaped, hexagonal crystals of manganpyrosmalite from Sterling Hill. Field of view is 1.5 mm in maximum dimension.

Euhedral crystals are slightly brownish pink and faintly colored. The luster is vitreous on cleavage surfaces and crystal faces. The density is 3.13 g/cm³, and cleavage is perfect on {0001}. Optically, manganpyrosmalite is uniaxial, negative, with w = 1.669 and e = 1.631 (Frondel and Bauer, 1953) and w = 1.680 and e = 1.647 (Parker and Guy, 1990). There is no discernible fluorescence in ultraviolet. It is best distinguished from friedelite, nelenite, and schallerite using X-ray methods.

Composition

Manganpyrosmalite is a manganese silicate hydroxide chloride mineral of the friedelite group and is polymorphous with friedelite. Several analyses of manganpyrosmalite are given in Table 15. That of #C6216 is of the massive type material; several additional unpublished analyses of specimens from this assemblage indicate that the composition of this material varies only slightly. Analysis # 147471 is a partial analysis by the writer of the euhedral crystals described by Parker and Guy (1990); additional unpublished analyses of these crystals indicate that they, too, are relatively uniform in composition. The data for local material show extensive substitution of Fe for Mn in manganpyrosmalite, as compared with friedelite, in which such solid solution is quite limited. An analogous relation is seen in schallerite and nelenite, wherein nelenite is receptive to much Fe substitution for Mn, and schallerite is not.

Occurrence and paragenesis

	Figure 18-13. Stout hexagonal crystals of manganpyrosmalite from Sterling Hill. Field of view is 1 mm in maximum dimension.

The type manganpyrosmalite consists of large 10-15 cm specimens of brown, platy to tight-fibrous masses which have inclusions of franklinite and calcite sparsely distributed within them. figure 18-14 shows the breccia-like texture of this material.

Little is known of the occurrence of this assemblage, although one of Lawson Bauer’s specimens lists “black willemite” as an associated mineral. This would be consistent with the anomalously high iron content, inasmuch as there was high Fe-silicate activity associated with the black-willemite, fayalite, and loellingite assemblage.

	Figure 18-14. Massive manganpyrosmalite is the fine-grained fibrous mineral surrounding the mineral grains in this breccia from Franklin. The visible surface is polished. Maximum dimension of the specimen is 7 cm in maximum dimension. Smithsonian Institution, #C6215-1. Photo by Vic Krantz.

The superb crystals shown in figures 18-11, 18-12, and 18-13 were found in 1980 on the 1400 level of Sterling Hill. They were found in a 25-cm pod associated with calcite, diopside, amphibole, and sphalerite (Parker and Guy, 1990) and are the best manganpyrosmalite crystals found here.

Additional occurrences have also been noted at Sterling Hill, intimately mixed with friedelite. Among them are: (1) manganpyrosmalite with willemite, friedelite, calcite, and chlorite from the 1010 stope; (2) manganpyrosmalite with friedelite and a Mn-carbonate from the 1500 level; and (3) very light brown, extremely fine-grained, manganpyrosmalite with friedelite, calcite, and franklinite, from the 1100 level.

Name

Manganpyrosmalite was named for the composition and relation to pyrosmalite.

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
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This page created: January 14, 2001