RAMMELSBERGITE

NiAs₂ 
Orthorhombic

	Figure 21-36. Bright-white rammelsbergite dendrites in cross-section. Gray gersdorffite rims rammelsbergite. Associated minerals are ferrostilpnomelane (black) and calcite (gray angular with cleavage). Specimen is 7 cm in maximum dimension. Smithsonian Institution, #R822. Photo by Vic Krantz.

Rammelsbergite, together with gersdorffite and loellingite, comprise the bulk of the opaque minerals associated with nickeline at Franklin and were first noted, as chloanthite, by Koenig (1889, 1890). The assemblage is described in detail under nickeline. Rammelsbergite subsequently was shown to occur here by Holmes (1935, 1936, 1945, and 1947), who used the term “white arsenides” to describe the predominant rammelsbergite-gersdorffite mixture. It is not known from Sterling Hill.

Description

Rammelsbergite is silver-white to silver-gray,  opaque, and has metallic luster. Crystals, if discernible, are modified pseudo-octahedrons. Surfaces are commonly dull, perhaps due to alteration of associated gersdorffite. Aggregates are bulbous in part, representing the outermost section of a cauliflower-like termination of an arborescent dendrite (Figures 21-35 and 21-36). It is easily confused with loellingite, with which it is isostructural and associated, and with arsenopyrite; it requires chemical or X-ray verification.

Composition

Rammelsbergite is a nickel arsenide mineral of the loellingite group. The analysis of chloanthite by Koenig (1890) may have been of impure rammelsbergite. Accurate analyses were given by Oen et al. (1984); several are given in Table 18.

Occurrence and paragenesis

Rammelsbergite occurs intimately associated with nickeline, gersdorffite, loellingite, and other species as dendrites, which were described in substantial detail by Oen et al. (1984). In general, rammelsbergite, sometimes intimately associated with gersdorffite in concentric aggregates, coats the trunks of arborescent nickeline dendrites and forms the uppermost part of such dendrites, as shown in figures 21-33, 21-34, 21-35, and 21-36. Cross-sections of the top of such dendrites (Figures 21-35 and 21-36) show rammelsbergite surrounded by gersdorffite. Remnant nickeline is sometimes seen at the cores of such aggregates.

The gangue minerals are sphalerite, fluorite, calcite, barite, and rarely ferrostilpnomelane (Figures 21-35 and 21-36). Masses of rammelsbergite with gersdorffite have been found in which the dendritic habit is not evident. See the discussion under nickeline.

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