Franklinite, a mineral composed of peroxide of iron, oxide of zinc, and oxide of manganese, in appearance much like the magnetic oxide of iron. It is found in considerable quantity only in Sussex co., N. J., although it is also mentioned as accompanying ores of zinc in amorphous masses at the mines of Altenberg (Yieille-Montagne), near Aix-la-Chapelle. The composition of the franklinite of New Jersey is:

Its hardness is 5.5-6.5; specific gravity, 5-5.09. It occurs in large veins or beds at the mines of the New Jersey zinc company at Stirling hill and Mine hill in Sussex co., accompanied by the red oxide of zinc, lying between the crystalline limestone and the gneiss rocks. At Stirling hill it constitutes the main substance of two beds of considerable magnitude, lying in immediate contact with each other, divided only by a parting seam, running S. W. and N. E., and dipping S. E. about 40° from the hill against which the beds seem to repose, toward and under the bed of the Walkill river. The upper of these beds, lying immediately under the crystalline limestone, is composed chiefly of the red oxide of zinc with the franklinite interspersed in granular masses, often assuming the appearance of imperfect crystals. It presents a thickness varying from 3 to 8 ft., and is traced with great uniformity of structure. At times almost perfect crystals of franklinite are found, particularly where the bed comes in contact with the superincumbent limestone; these crystals are of the regular octahedral form with the edges replaced. The franklinite constitutes about 45 per cent. of the mineral contents, the rest being mainly red oxide of zinc.

This bed is extensively worked for the manufacture of white oxide of zinc, which is used for paint; the residuum, after the oxide of zinc is driven off, being franklinite, is smelted into iron. The underlying bed appears on the surface or outcrop to be almost a pure massive franklinite, amorphous in structure, although occasionally also exhibiting very large and nearly perfect crystals of the franklinite; it contains no red oxide of zinc, which fact is the distinctive feature between this and the overlying bed, which is generally known as the bed of red zinc. The other locality where the franklinite is found in large masses is on Mine hill, about 1 1/2 m. N. E. of Stirling hill, following the course of the Walkill to the village of Franklin. Here there are also found two distinct beds in immediate juxtaposition; but their relative position, as compared with that at Stirling hill, is reversed, the franklinite being the easternmost and uppermost, and the zinc being the underlying and westernmost.