Abstract

The mineral deposits of the Franklin area are remarkable alike for their great size, their wealth of mineral species, and the unusual character of the minerals composing them. More than 140 minerals, described in this paper, have been found in the district, 32 of which were first found there, and 30 are not known elsewhere in the world. This proportion is the more noteworthy because among the 30 are included 2 of the 3 principal ore minerals of the district.

The chemical composition of the ore bodies is not less unusual than their mineral complexity. Oxides of zinc, iron, and manganese, in a ratio of 3 to 2-½ to 1, make up two thirds of their mass, and the remainder consists chiefly of calcite and silicates in a ratio of about 3 to 1. No other ore has an even remotely similar composition. It is therefore not remarkable that the interest of mineralogists has long been attracted to the deposits.

Geologists, too, find interesting problems in the area. The occurrence of similar deposits of roughly identical form at Franklin (formerly called Franklin Furnace) and at Sterling Hill, the age of the limestone containing them and its relation to other rock formations, the genesis of the ores and the processes through which they have acquired their present peculiar characters, and several minor problems—all have been and in part still are subjects of interested study and speculation.

Investigation of the Franklin ore deposits and minerals has extended over more than a century, but none of the many papers published concerning them includes an adequate description of both the deposits as a whole and of all the minerals found in them. Especially is there no comprehensive statement of the grouping of the minerals, as the lists of species issued from time to time give the reader little clue to the association of the minerals in the rocks. The preparation of this paper was first suggested by the realization of this lack, and the knowledge that the local collections are exceptionally complete and yet are inadequately described inspired the hope that it might be possible, even after a century of mining, to prepare a fairly complete and detailed description of the mineralogy of these remarkable deposits, in many respects the most interesting that this country possesses.

This paper contains a full description of the minerals of the district, preceded by a brief statement of their occurrence, geologic relations, and associations in the rocks and a resume of the several hypotheses of the origin of the ore deposits.


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