Habit
The crystals, of brilliant metallic luster, nearly an inch long and doubly terminated, are embedded in limestone but because of their brittleness can rarely be extracted without fracture. They are prismatic parallel to the vertical axis and are generally slender, but some are stout and thick. (See Figure 19.)

Figure 19 A stout prismatic crystal of arsenopyrite showing the forms b(010), m(110), e(101), u(014), t(013), n(012), q(011), k(021), t(031), o(112), g(111), and the new forms A(532), p(143), and f(132). Franklin Iron Company quarry. A, Plan; B, clinographic projection.

Brachydomes, of which n and q are the commonest, are the chief terminal faces, as is ordinarily the case with this mineral. The most marked characteristic of these crystals, however, is the unusual development of pyramid faces, chiefly in two zones—between m(110) and e(101) and between m(110) and q(011). The first zone is developed on every crystal and is generally marked by deep striations on the prism faces, parallel to the intersection of m and e. In this zone are found the pyramids i(312), A(532) (new to the species and seen on all the crystals), and B(514) (seen but once). The existence of A and B has since been confirmed on crystals of arsenopyrite from Hiddenite, N.C. (Palache, Davidson, and Goranson, 276).

The forms of the second zone, marked like the other by striations on the prism faces, were seen only on a large brilliant crystal (Figure 19), one of whose terminations was destroyed in freeing it from the matrix.

They are p(143), f(132), and the doubtful forms (154), (275), and (594), each seen but once. The pyramid w is characterized by distinct plane facets; the forms o(112) and g of the unit series were seen only on the crystal of Figure 19 as small facets, and on the same crystal was seen the doubtful form (188), probably to be regarded as vicinal to q(011).

Figures 17 and 18 show characteristic crystals of the common habit, accurately illustrating the perfect symmetry that many of them possess.

Figure 17 Prismatic crystal of arsenopyrite, showing the forms q(011), n(012), e(010), m(110), w(212), i(312), and the new pyramid A(532). Franklin Iron Company quarry. A, Plan; B, clinographic projection.
	Figure 18 One end of a prismatic crystal of arsenopyrite, showing the forms u(014), w(027), n(012), q(011), k(021), e(101), w(212), and the new pyramid A(532). Franklin Iron Company quarry. A, Plan; B, clinographic projection.

Figure 20 shows a crystal from the Fowler quarry with new pyramid d(754), lying between m and w(212).

Figure 20 Crystal of arsenopyrite showing the forms m(110), e(101), n(012), w(212), i(312), and the new form d(754). Fowler quarry.

The crystal elements, computed from measurements of 30 faces of the forms m, q, e, and g, on 6 crystals, gave the following values for the axial ratio of the crystals, which agree closely with those used in Dana’s "System."

The data upon which the new forms are established are presented in the following table of angles:

[Angles of faces on crystals of arsenopyrite]

Composition
Crystals from the Franklin Iron Company’s quarry were sent to the laboratory of the United States Geological Survey for analysis. The result, given below, is especially interesting as showing the presence of a small amount of cobalt, an element likewise found in small amount in the pyrite of the same locality. (See page 30.) The specific gravity is 6.199 (Buerger).

A note by Sullivan states that the analysis was made on 0.25 gram, that the percentage of arsenic is probably too high, and that silicon and organic matter were also present.

Occurrence
Arsenopyrite was recognized at Franklin by Nuttall (7) and has occasionally been reported by other writers. It is uncommon there and is sparsely represented in collections of Franklin minerals. The crystals just described, which are of unusual perfection and complexity, were collected by the author in 1905 in limestone of the quarry of the Franklin Iron Company. As they add materially to our knowledge of the crystallography of the mineral, they have been described in detail. Intimately associated with them in the limestone are pyrrhotite, pyrite, graphite, edenite, brown tourmaline, titanite, and norbergite. Similar crystals in the same association were found in the limestone of the east wall of the Buckwheat open cut and in the Fowler quarry, but they were neither so abundant nor so well developed.

In the collection of Mr. Fiss was seen a mount of microscopic crystals of arsenopyrite from a Franklin locality not exactly designated. As shown in Figures 21 and 22, the habit is rather different from that of the crystals collected by the author, and most of the crystals are twinned in a manner somewhat rare for the mineral—in contact with the prism m(110).

Figure 21 Simple prismatic crystal of arsenopyrite showing the forms m(110) and u(014). Fiss collection.
	Figure 22 Two crystals of arsenopyrite of the form of Figure 21, twinned on the prism plane m(110). A, Plan; B, clinographic projection. Fiss collection.

Massive arsenopyrite was found in the Trotter mine near the deposit of nickel arsenides described on page 29, and minute crystals were found there intermixed with desaulesite. Arsenopyrite is also recorded from the Parker shaft, associated in one place with roeblingite, in another with quartz, pyroxene, and molybdenite.

Minute crystals of arsenopyrite were seen in a specimen of massive epidote and axinite from the Gooseberry iron mine, but there seems to be no record of its occurrence at Sterling Hill.

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