Franklinite Composition, Crystallization & Structure

Composition. (Fe,Zn, Mn) O. (Fe, Mn)₂O₃. Shows Wide variation in the proportions of the different elements, but conforms to the general formula, RO.R₂O₃.

Crystallization. Isometric. Habit strongly octahedral. Dodecahedron sometimes as truncations. Other forms rare. Crystals often rounded.

Structure. Massive, coarse or fine granular, in rounded grains or crystallized.

Physical Properties. H. = 6. G. = 5.15. Metallic Luster. Color iron-black. Streak  dark brown. Not magnetic.

Test. infusible. Becomes strongly magnetic on heating in R.F. Gives a bluish green color to sodium carbonate bead in O.F. (manganese). When very fine powder is mixed with sodium carbonate and heated intensely on charcoal gives a coating of zinc oxide. Distinguished by above tests and its black color and brown streak.

Occurrence. Found practically only in the zinc deposits at Franklin Furnace, New Jersey, which are in the form of large beds, enclosed in granular limestone. Associated chiefly with zincite and willemite, with which it is often intimately intergrown.

Use. As an ore of zinc and manganese. The zinc is converted into zinc white and the residue is smelted to form an alloy of iron and manganese, spiegeleisen, which is used in the manufacture of steel.