LOELLINGITE

FeAs₂
Orthorhombic, Pmnn, a = 2.88, b = 5.29, c = 5.98 Å, Z = 2

	Figure 21-28. Crystal drawings of loellingite from Franklin. The bottom one is of a crystal from the Buckwheat Mine; the others are of crystals from the Trotter Mine. The top two drawings are projections of one individual crystal. Drawings are from Palache (1935) who provided crystallographic data.

Loellingite is the most common arsenide at both deposits; the other common arsenic-bearing mineral is arsenopyrite. In general, loellingite is found within the orebodies, whereas arsenopyrite is found predominantly in the marble.

Loellingite was first reported from Franklin by Brush (1871) and subsequently by Bauer and Berman (1927); the results of the latter study were summarized by Palache (1935).

The crystal structure was described by Buerger (1932) and the unit-cell was refined by Peacock (1941, 1944), who also provided X-ray powder data; both obtained data from the material of Bauer and Berman. Berry and Thompson (1962) provided additional powder data and gave a unit-cell refined by R. J. Traill (a = 2.8802, b = 5.3023, c = 5.9818 Å). Davis (1985) provided descriptions of the occurrence in the black-willemite zone at Sterling Hill.

Description

Loellingite occurs in euhedral crystals; those seen by the writer resemble the habits illustrated by Bauer and Berman (1927) and Palache (1935) (Figure 21-28). Most crystals are slightly tabular and prismatic to dipyramidal in habit; many have rounded coigns. It also occurs as masses, interstitial fillings, and granules in high-temperature assemblages. The color is brilliant silver-white; the luster is metallic; and it is opaque and brittle. The density is 7.53 g/cm³ (Buerger, 1932), 7.43 g/cm³ (Peacock, 1941). It is easily confused with arsenopyrite, although arsenopyrite is commonly prismatic in habit; mineral associations are helpful in distinguishing them, inasmuch as arsenopyrite is only rarely found in the orebodies. Although only a general rule-of-thumb, Franklin loellingite crystals are commonly euhedral and have bright luster, whereas those at Sterling Hill are more rounded, distorted, and have duller luster. 

Composition

Loellingite is an iron arsenide mineral of the loellingite group. Few analyses have been made of local material. That of Franklin loellingite by Bauer and Berman (1927) is given in Table 18, together with analyses of nickeloan loellingite (Oen et al., 1984). Radcliffe and Berry (1968; unpublished data on file at the Smithsonian Institution) found Franklin loellingite to contain 99.8 mole % FeAs₂. The writer’s unpublished analyses of several loellingites found them to have near end-member composition.

Occurrence and paragenesis

In general, loellingite is found within the orebodies and is commonly associated with calcite or sphalerite in addition to other species. It is moderately common in small amounts, but is not generally abundant.

	Figure 21-29. Ore from the black-willemite zone at Sterling Hill showing willemite and fayalite (mixed, black, and both colored by grains of other minerals), loellingite (white), and an area of impure calcite (extreme upper right). The visible surface is polished. Specimen is 9 cm in maximum dimension. Smithsonian Institution, #147586. Photo by the author.		Figure 21-30. Dendrites of nickeline in fluorite matrix from Franklin. White calcite (upper right corner) is in contact with magnetite (gray). Specimen is 9 cm in maximum dimension. Mineralogical Museum, Harvard University, #117576. Photo by Vic Krantz.

At Franklin, the first report was of an association with cubic gahnite by Brush (1871). The crystals described by Bauer and Berman (1927) from the 750 level were associated with pyroxene, calcite, and franklinite. Bauer and Berman also reported an occurrence with gahnite at the Trotter Shaft. Studies by the writer have found loellingite associated with almandine, magnetite, and fayalite. A significant Franklin occurrence is of nickeloan material associated with gersdorffite, rammelsbergite, and nickeline from the Trotter Mine. In this assemblage, described herein under nickeline and in great detail by Oen et al. (1984), loellingite is among the last arsenides to form (Figure 21-34).

At Sterling Hill, loellingite is found in a number of diverse assemblages, most commonly associated with sphalerite. It is also found with bornite and magnetite slightly above the 1300 level; with calcite on the 340, 1000, 1100, 1200, 1500, and 1600 levels; and variously associated with sphalerite, fluorite, pyroxene, magnetite, franklinite, and calcite in numerous places in the mine. The most significant Sterling Hill occurrence is one in which it is associated with black willemite, fayalite, and calcite (Figure 21-29); this material has provided fine crystals. Loellingite may be a source of As for secondary arsenate minerals.

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