Physical characters
An octahedral parting is marked in some material but is generally absent. The hardness is 5.5 to 6.5, and the specific gravity is 5.07 to 5.22. The color is black, rarely with an iridescent coating, and the luster is brilliant metallic to dull. The streak and thin fragments are reddish brown, (See, however, the description of the unusual phase on page 45.) Index of refraction, n_Li = 2.36 ±0.02. The magnetism in some material, to which the local name of "magnofranklinite" was given by Canfield, is as strong as that of magnetite, but other material is not sensibly affected by a pocket magnet. This difference probably depends on the relative amount of ferrous iron present, and all varieties are strongly attracted by the electromagnets in the separating machines.

Composition
Although conforming to the spinel formula, the composition of franklinite shows a wide range, both in the relative proportions of iron, manganese, and zinc and in the state of oxidation of the iron and manganese. Although but one analysis in the following table actually records the presence of ferrous iron, the ordinary magnetism of the substance leads to the conviction that some ferrous iron is usually present; its accurate determination is impossible in the presence of manganous and manganic oxides.³ Manganese is probably present in both states of oxidation, and the general formula given at the head of this description probably applies to most specimens of the mineral. Special tests for titanium made on samples of franklinite from different parts of the deposit showed its presence only in traces.
    ³ Schaller, W. T., personal communication.

[Analyses of franklinite]

Occurrence
Franklinite is the dominant mineral of the ore body at Franklin, either forming thick beds free from any other material or mixed with various amounts of calcite, willemite, zincite, or rhodonite. It is commonly granular, as shown in plate 7, B, or in isolated grains. Crystals are rare and are confined chiefly to parts of the ore body which appear to have undergone recrystallization, the ore constituents being embedded in calcite. In such places the crystals are likely to be very sharply formed and of brilliant luster. Such crystals, scarcely more than half an inch in diameter and with most brilliant luster, were not rare in material from the Parker shaft. At the Trotter mine slightly larger crystals, as much as 1-½ inches in diameter, were found. They show a tendency toward the dodecahedral habit, and some dodecahedrons are striated like magnetite, parallel to the intersection edge with the octahedron, the faces being dull. Many similar groups, taken from the Buckwheat and Taylor mines, were also seen.

Figure 39 Crystal of franklinite showing the octahedron with a narrow truncation by the dodecahedron—a common type.
	Figure 40 Crystal of franklinite showing the octahedron in balance with a trapezohedron, m(311), and small faces of the dodecahedron.

At Sterling Hill also franklinite is the dominant ore mineral and occurs prevailingly in granular masses. In the first explorations of the mines, however, at the very surface, crystals of extraordinary dimensions were found, the best of which are now preserved in the Canfield collection. Several of the finest are illustrated in plate 6, A, B. The crystals shown in the photographs are octahedrons, alone or with slight modification by the dodecahedron, measuring 7 inches or less on an edge. They are attached to massive franklinite and with them is troostite in dull, lusterless crystals. They are said to have been embedded in dark-brown or black claylike material or wad, presumably the residue left from solution of the manganiferous limestone in which they wore originally developed. They are of dull luster but of wonderfully perfect form.

From the Noble mine also came many dodecahedral crystals of nearly perfect form, dull and somewhat rounded as if etched, with small or no octahedral faces and very rarely the faces of the trapezohedron m(311). One of them is shown in plate 7, A. Much of this material was almost wholly lacking in zinc and was correspondingly rich in ferrous iron. This was the so-called "magnofranklinite" and was used as an ore of iron and manganese without previous roasting for zinc oxide. (See appendix, c, page 130.)

Figure 41 Crystal of franklinite showing the dodecahedron and the octahedron and small faces of the trapezohedron m(311).
	Figure 42 Crystal of franklinite showing the dodecahedron, the octahedron, and the trapezohedron n(211).

Lustrous octahedral crystals of franklinite with rounded angles, embedded in granular zincite, were also characteristic of much of the ore at Sterling Hill, especially of the outcrop of the front vein.

Figure 48 Dodecahedral crystal of franklinite modified and striated by the octahedron. Sterling Hill.

Historical notes
Bruce, who, discovered the zinciferous nature of zincite, considered the black mineral associated with it to be magnetite. This it was generally supposed to be, and several unsuccessful efforts were made to work this refractory material as an ore of iron.

The true character of the mineral was first determined by the French chemist Berthier (3), who named it franklinite "in order to remind us that it was first found in a place to which the Americans have given the name of a great man." He gave an approximate analysis and the correct mineralogic characters.

Although thus early recognized as a distinct species, with a considerable content of zinc, it was for many years ignored as an ore of zinc, the whole attention of the zinc miners being given to the richer zincite. This oversight and the persistent consideration of franklinite as an ore of iron led to the peculiar division of mineral rights in the Franklin deposits into ores of zinc and ores of iron, which resulted in the long-fought lawsuits that for years retarded the development of the mines.

The value of franklinite as an ore of zinc was not recognized until later, when the invention by Wetherill of a furnace in which the zinc could be roasted off to form zinc white revolutionized the local practice, and the further improvement in treatment brought about by the perfection of the magnetic separator established it as the major zinc ore of the district.