Habit
Crystals of sphalerite have been found in considerable abundance at Franklin. They are remarkable for the sharpness of their form and for the variety of forms and combinations displayed. Several individual or twin crystals, some an inch across, were found in the Stanton collection. All the forms listed were determined by measurement, the distinction between positive and negative forms being based on the presence of twinning, always assumed to be on the positive tetrahedron. There seems to be no rule governing the luster of the two tetrahedrons, either being the more lustrous or both being either bright or dull in different crystals. Both are generally developed, but the negative tetrahedron seems more likely to be dominant.

Figure 1 Crystal of sphalerite showing d(110), o(111), o1(111), and q(331), with o dominant. Franklin.

Figure 2 Crystal of sphalerite twinned on the octahedron, showing o(111), o1(111), and m1(311). Franklin.

Figure 3 Crystal of sphalerite of uncommon habit, showing the forms a(100), o(111), o1(111), and the rare form t1(722). Franklin.

The mineral is found also in coarse granular form and in massive forms with medium to finest grain.

Physical properties
The mineral has a wide-ranging color, from white or only faintly green or yellow, through deep oil-green, resinous yellow, gray, and brown, to black. At Sterling Hill it is generally a peculiar reddish brown, much like some willemite and friedelite. The specific gravity of cleiophane, the variety free from iron, was determined by Henry (53) to be 4.063.

Composition
The white sphalerite or cleiophane consists of almost pure zinc sulphide, with a trace of cadmium, but the colored material contains considerable iron and minute amounts of other elements. Some of the material contains an appreciable amount of manganese.

[Analyses of sphalerite]

Analysis 1 confirms the absence of iron indicated by the white color of the cleiophane. The black sphalerite (no. 2) is rich in iron and is interesting because, like no. 1, it contains a trace of cadmium. Analysis 3 was derived by computation from the analysis of a mixture of sphalerite and bornite in a ratio of 2 to 3, the complete analysis of the mixture being given under bornite (page 29).

Occurrence
Sphalerite is fairly common at Franklin, though not common enough to be accounted an ore of zinc. It is found but rarely in the normal zinc ore body but is characteristic of secondary veins, of pneumatolytic zones, and of the pegmatite.

In the veins its commonest form is a dull-white or gray compact phase, associated with granular or chalcedonic quartz and with carbonates of calcium, magnesium, manganese, and zinc, rather commonly in alternate layers with a fine ribbon structure parallel to the vein walls. Where this white sphalerite is more coarsely crystalline it has the normal adamantine luster, and to this variety, noticed by the earliest writers on Franklin minerals, Nuttall gave the name cleiophane. It was especially abundant at the Trotter mine and could be collected freely on the old dump there. In the same mine was found a crystallized form of white sphalerite in flattened twin octahedrons, embedded with clear yellow prisms of willemite and cleavage fragments of calcite in thin veins, whose black matrix has the appearance of hardened mud.

A very different phase of sphalerite from the secondary veins, more common in collections, is the brilliant yellow, green, and brown crystalline form found in the porous gray dolomite veins of the Buckwheat mine. The crystals are generally combinations of positive and negative tetrahedrons, some of them in perfect balance, forming apparent octahedrons. In most specimens there is repeated twinning on the octahedron, producing complex groups. Only a few showed the uncommon hextetrahedron (753) or the trigonal tristetrahedron (522), as shown in Figures 4 and 5, illustrating combinations 13 and 14.

Figure 4 Crystal of sphalerite showing the positive and negative tetrahedrons and the trigonal tristetrahedron b(522). Buckwheat mine, Franklin.

The crystals are attached to the walls of the open cavities in the dolomite, other cavities being entirely filled with granular resinous sphalerite.

The sphalerite specimens in the Stanton collection were found in the mine at the 300-foot and 450-foot levels in the 1100 stope north and came from what must have been a series of dolomite veins. The crystals coat the walls of cavities, together with brilliant plates of specular hematite, dolomite, and quartz and a later coating of delicate plates of chlorite. The crystals are of the various combinations No. 3 to 12 in the list of combinations, as well as of an almost infinite variety of intermediate types. The color is chiefly light greenish yellow, but in very coarse cleavage masses with grains an inch or more across the color ranges from beautiful oil-green to reddish yellow, the luster being wonderfully brilliant.

Sterling Hill sphalerite is more widely distributed than at Franklin. It forms veins traversing the ore and in their vicinity may be distributed in fine grains throughout the ore. In places it is so abundant as to constitute nearly 1 percent of the willemite concentrate. The sphalerite of these veins, reddish to pale brown, and fine granular to cryptocrystalline, is quite unlike any other sphalerite is easily mistaken for friedelite.

The principal pneumatolytic zone containing sphalerite was found in the Trotter mine, at a depth of 350 feet, where the mineral was abundantly developed in yellow granular form, with fluorite, chloanthite, niccolite, and arsenopyrite.

In the pegmatite sphalerite was noted megascopically as a subordinate constituent, generally visible only near the limestone contacts, at both Franklin and Sterling Hill. It is disseminated as cleiophane, in minute grains with galena, amidst the normal pegmatite minerals. Concentration of the sphalerite was commonly evident in the vicinity of jeffersonite grains. It was also seen enveloped by rhodonite, and some granular masses several inches in diameter were seen in the pegmatite or admixed with yellow garnet at the limestone contact.

The conclusion has been reached that sphalerite, or at least the sulphur required for its formation, was introduced by the pegmatite magma, that from the pegmatite it found its way into pneumatolytic and other types of contacts zones, and that from both those sources, by subsequent movement of solutions, it past into secondary veins filling fissures that had in all likelihood been formed by the same pegmatitic intrusion.

Sphalerite was listed by Spurr and Lewis (234) among the sulphides found in late veins cutting the Kittatinny limestone, referred to on page 20.