NICKELINE

NiAs
Hexagonal

	Figure 21-31. Dendrites of nickeline in fluorite from Franklin. White calcite (upper right) is in contact with magnetite (gray). Ends of the dendrites (darker color) have gersdorffite and rammelsbergite present. Specimen is 9 cm in maximum dimension. Privately owned. Photo by Vic Krantz.

Nickeline, also called niccolite, occurred in an isolated, anomalous assemblage in the Trotter Shaft at Franklin. It was a rare occurrence, apparently very limited in extent, and has not been found elsewhere in the mine, nor at Sterling Hill. Relations to the ore are unknown. Franklin nickeline is associated with rammelsbergite, pararammelsbergite, safflorite, loellingite, gersdorffite, arsenopyrite, and other species described herein, but the overall assemblage description and history is given here but once.

The nickeline assemblage was first reported by Koenig (1889, 1890), who had observed some 30 pounds of material. He reported only small grains of nickeline and gave a longer description of the white nickel arsenides, which he referred to as chloanthite  and which he said formed the bulk of the material observed. These white arsenides were later shown to be other arsenide species listed here. Although Palache (1935) reported hearsay of several hundred pounds of material, the bulk of which was found after Koenig’s description, relatively little specimen material is known today. This might have been incorrect or large amounts may have been lost or consumed in chemical experiments. Crude crystals were reported by Palache (1935) to occur where nickeline is in contact with calcite or fluorite.

The assemblage was subsequently studied by Holmes (1935, 1936, 1945, 1947), who established the absence of chloanthite and the presence of rammelsbergite, pararammelsbergite, and skutterudite.

The definitive chemical and textural study was performed by Oen et al. (1984), who described the assemblage in great detail, added loellingite and safflorite as associated minerals, and provided a detailed model for the crystallization sequence and phase equilibria. This assemblage hosts the Fe-, Ni-, and Co-bearing members of the loellingite group, which are loellingite, rammelsbergite, and safflorite, respectively. Breithauptite was added by Verbeek and Sutphin (1990).

Description

Nickeline is light orange-pink, opaque, and has metallic luster and no cleavage. It commonly occurs as bleb-like grains and massive aggregates and as dendrites, some of which serve as the core nucleus for dendrites of the white arsenides. It is not confused with other local minerals, except freshly-broken bornite, which is always associated with sulfides and tarnishes readily.

Composition

	Table 18. Chemical analyses of gersdorffite and nickel-, cobalt-, and iron-arsenides.

Nickeline is a nickel arsenide mineral. The analyses by Oen et al. (1984) show it to be fairly constant in composition and slightly antimonian (Table 18).

Occurrence and paragenesis

Franklin nickeline occurs as dendrites in a matrix of sphalerite, fluorite, calcite, and barite. Koenig (1890) noted that it occurred at a depth of 340 feet within an abundance of sphalerite and fluorite, under a layer of yellow andradite. Nothing more is known of the geologic nature of the occurrence.

	Figure 21-32. Nickeline dendrites with calcite and fluorite gangue from Franklin. The visible surface is polished. Specimen is 15 cm in maximum dimension. Smithsonian Institution, #C503. Photo by the author.

The nickeline assemblage is one of dendritic growth, as shown in figures 21-30, 21-31, and 21-32. The dendrites are about 7-8 cm in length, several mm thick at their bases, and have trunks rimmed with white arsenides.

Toward their ends, the trunks become more arborescent and rimmed with enlarged, cm-wide, rounded, cauliflower-like, concentric aggregates of gersdorffite and rammelsbergite (Figures 21-35 and  21-36). Some nickeline dendrites exhibit very little, if any, replacement by the white arsenides (rammelsbergite and pararammelsbergite) or gersdorffite; these simpler dendrites are narrower. Calcite interstitial to the dendrites is recrystallized.

In the general case, nickeline is the first formed of these minerals and is followed in the sequence of crystallization by rammelsbergite and pararammelsbergite, and then by gersdorffite and loellingite, and lastly, but only locally, by the rarer minerals skutterudite and safflorite. Magnetite occurs sporadically in calcite near the ends of some dendrites (Figures 21-33 and 21-34). The outward growth of the dendrites is accompanied by depletion of nickel and enrichment of cobalt.

	Figure 21-33. The rims of these Franklin dendrites exhibit rammelsbergite (mottled, in bottom of photographs), rimmed by gersdorffite (gray) which occurs as discontinuous or skeletal growths in its outer portions. Dark gray beyond dendrites is calcite. These are images of polished sections. Field of view is 0.8 mm in maximum dimension. Photographs from Oen et al. (1984).		Figure 21-34. Drawings showing the growth-sequence relations of the tips of nickel arsenide dendrites. Growth direction is toward calcite (white). Drawings from Oen et al. (1984).

The gangue is commonly calcite, sphalerite, fluorite, barite, and rarely ferrostilpnomelane (Figures 21-35 and 21-36). Some fluorite is colored violet in contact with arsenides. Subsequent alterations have produced minor amounts of pimelite and annabergite. Few specimens have vugs; where present they enclose 5-7 mm crystals of fluorite and, in one case, superb nickeline crystals, which are in the Hancock Collection at Harvard University.

Some extant dendritic specimens (Figures 21-35 and 21-36) show only minor residual nickeline, with relatively larger amounts of the white arsenides and gersdorffite, in keeping with Koenig’s original description of relative abundance, but others are almost wholly nickeline (Figure 21-32). Koenig did not mention dendrites, but they are commonly not apparent until the assemblage is sliced into slabs. Considering the statements of Koenig and Palache, it is very surprising that only a small number of specimens are known or could be located by the writer. The writer’s dissolution of some calcite matrix in search of other associated species found insoluble residues of rutile, chlorite, and pararammelsbergite.

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
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This page created: January 11, 2001