ARSENOPYRITE

FeAsS
Monoclinic

	Figure 21-1. Crystal drawings of twinned arsenopyrite from Franklin. These are two projections of the same twinned crystal. Drawings are from Palache (1935) who provided crystallographic data.

Arsenopyrite is one of the two dominant, opaque arsenic minerals occurring locally; the other is loellingite.

	Figure 21-2. Crystal drawings of arsenopyrite from the Franklin Iron Company Quarry in Franklin. These are sets (A and B) of projections of two individual crystals. Drawings are from Palache (1935) who provided crystallographic data.

In general, arsenopyrite is found in the Franklin Marble, and loellingite is found in the orebodies. Arsenopyrite was first noted from Franklin by Nuttall (1822) and was mentioned in subsequent lists. Fine crystals of arsenopyrite from the Franklin Marble were found in a number of the Franklin quarries in 1905 and were described by Palache (1910, 1935) and Palache et al. (1930). He also described superb crystals from the realgar occurrence at Sterling Hill (1941b). The crystal structure of arsenopyrite was described using Franklin crystals by Buerger (1936a, 1936b).

Description

Arsenopyrite occurs well-crystallized in euhedral crystals up to 22 mm in length, although most are much smaller. They are generally prismatic, highly lustrous, and of simple to complex habit.

	Figure 21-3. Crystal drawings of arsenopyrite from Sterling Hill . Drawings are from Palache (1941b) who provided crystallographic data.

The morphology has been described in great detail by Palache (1910, 1935, 1941b) (Figures 21-1, 21-2, 21-3, 21-4, and 21-5). Many crystals were illustrated by Dunn (1979c). Crystals are commonly isolated and have chevron-like striations (Figure 21-7), but intergrowths of crystals are known and are rarely arcuate (Figure 21-6). Arsenopyrite twins on {012} to form cruciform or star-shaped trillings (Palache et al., 1944); massive material is known as well. Arsenopyrite is silver-white when freshly broken, is opaque, has a metallic luster which is commonly dulled by exposure, and has two commonly indistinct cleavages. The density is 6.223 g/cm³.

	Figure 21-4. Crystal drawings of arsenopyrite from the Franklin Iron Company Quarry in Franklin. These drawings are projections of the same crystal. Drawings are from Palache (1935) who provided crystallographic data.

Composition

Arsenopyrite is an iron arsenic sulfide mineral and the major host for arsenic in the Franklin Marble. Few analyses exist; one of cobaltoan material was given by Palache (1935). Several semiquantitative analyses by the writer indicate that local material is relatively close to end-member composition. The arsenopyrite associated with the nickel arsenides from Franklin has Fe 32.5, As 47.6, S 19.3, total = 99.4 wt. % (Oen et al., 1984).

Occurrence and paragenesis

The vast preponderance of local arsenopyrite occurs as crystals associated with calcite in the Franklin Marble. It has been found in the local quarries, notably the Fowler Quarry and the Franklin Iron Company Quarry, and within the ore deposits, the latter only sparsely. In the marble, it is associated with pyrrhotite, graphite, amphiboles, uvite, titanite, norbergite, and many other species.

Arsenopyrite was found in the Franklin orebody associated with the nickel arsenide assemblage from the Trotter Shaft (Oen et al., 1984) and was reported from the 900 level at Sterling Hill by Palache (1941b), associated with calcite, realgar, graphite, arsenic, pyrite, diopside, and zinkenite.

	Figure 21-5. Crystal drawings of arsenopyrite from Franklin; the upper drawing is of a crystal from the Fowler Quarry. Drawings are from Palache (1935) who provided crystallographic data.

Arsenopyrite also occurs in the margarite assemblage in the marble adjacent to the Sterling Hill orebody (Figures 21-6,  21-7, and 21-8); the assemblage is described under margarite (Dunn and Frondel, 1990). An occurrence from the 1100 level at Sterling Hill was reported by Dunn (1979c) who found arsenopyrite associated with calcite, realgar, microcline, and clinopyroxene.

Additionally, massive arsenopyrite was found in the late 1970’s associated with bornite and calcite on the 1680 level. It also was found on the 1300 level, and in the north orebody, and near both the hanging wall and footwall on the 1000 level, all at Sterling Hill.

Cook (1973) and Peters et al. (1983) reported arsenopyrite from the Buckwheat Dolomite, wherein it is a rare mineral.

	Figure 21-6. Curved array of arsenopyrite crystals from the Franklin Marble at Sterling Hill.		Figure 21-7. Prismatic, typical arsenopyrite crystals from the margarite occurrence near the Sterling Hill deposit. Crystal on the left is 1 mm, and that on the right is 4 mm, in maximum dimension.

	Figure 21-8. Doubly-terminated crystals of arsenoprite on acid-etched anorthite from the margarite occurrence near the Sterling Hill deposit. Crystals are 0.7 mm in maximum dimension.

 

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