Franklinite

Formula:	(Zn, Mn2+,Fe2+)(Fe3+,Mn3+)2O4
Crystal system:	Cubic	Space group:	Fd3m

Occurence

	Franklinite. Large sample of octahedral crystals, in calcite matrix. Franklin, NJ. Width 12 cm.

Franklinite is a major component of the Zn-Mn-Fe orebodies at Franklin and Sterling hill, and was mined in great quantity as an ore of zinc.

Franklinite is black in color, and transparent reddish-brown in small fragments. Some samples, particularly when found with tephroite, have an iridescence not unlike "peacock ore" (bornite).

Unlike the other two zinc ore minerals found in the district, willemite and zincite, the composition of local franklinite varies considerably.

	Iridescent franklinite. Associated minerals include willemite (green), zincite (red) and tephroite (not visible). Franklin, NJ. Width 6 cm.

Franklinite has a normal spinel-type structure and contains zinc, manganese and iron.

	Crystal structure of franklinite. Coordination polyhedra shown. Red: tetrahedral sites occupied by Zn+2, Mn+2. Blue: octahedral sites occupied by Mn+3, Fe+3.

Franklinite is found in massive, granular and, less often, in crystallized forms. Grains range from sub-millimeter to several centimeters in size, and occur as rounded through subhedral to finely formed crystals.

Crystals many centimeters in size, generally with the octahedron dominant, are not uncommon. Many of these are crudely formed, or fracture easily, however. In vuggy material, minute crystals with cubic habit are occasional found.

	Franklinite crystal group in calcite. Franklin, NJ. Width 12 cm.

	Franklinite crystal. Franklin, NJ. Width 4 cm.

Specimens of euhedral franklinite are still field-collectable today at Franklin and Sterling Hill.

For further information on franklinite, consult Dunn (1995).

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