Smithsonite

Physical properties
Crystals of smithsonite are rare at Franklin, but some were described by the author (257) in 1928. They were in a fragment of coarse franklinite-willemite ore from the picking table at Franklin that contained a few solution cavities, some lined with needles of calamine and one with pure-white crystals of smithsonite. The crystals are of typical calcite habit, as shown in figure 62.

Figure 62
End of prismatic crystal of smithsonite showing the forms m(1010), f(0221), and v(2131). Franklin.

The scalenohedron is dominant, with small faces of the prism and the rhombohedron. The faces are brilliant and, unlike most crystals of smithsonite, gave excellent angle readings, agreeing perfectly with those computed from the accepted elements based on the cleavage angle. The refractive indices are w = 1.850, e = l.625, and the specific gravity is 4.43. Although not analyzed, the crystals are almost certainly practically pure zinc carbonate.

Occurrence
Smithsonite was observed very early at both Franklin and Sterling Hill as an earthy white film on the fracture surfaces of zincite, the result of surface carbonatization. At Franklin, however, it is more abundant in secondary transverse veins associated with other carbonates and willemite, quartz, and sphalerite. Probably no such material is pure zinc carbonate, as all specimens tested show the presence of some manganese, calcium, and magnesium. No analyses of carbonates rich in zinc have been made at Franklin.

The limestone on the west side of the zinc-ore body, near the Trotter shaft, was cut by a vein about a foot thick, consisting almost wholly of massive smithsonite, together with a little hydrozincite and calamine and much limonite. Traces of sphalerite and pyrite were also visible, affording proof that there, at least, the smithsonite was formed by the oxidation of zinc sulphide.

At Sterling Hill more or less smithsonite was found in the calamine deposit, partly earthy, partly in stalactitic masses. Rarely these are translucent and pale blue.


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