Minerals in the Franklin limestone

The Franklin limestone, commonly called the "white limestone" from its color, is a coarsely crystalline rock, ranging in composition from nearly pure calcium carbonate to a manganesian carbonate with almost the ratio of dolomite. The differences in composition do not affect its appearance or degree of crystallinity. Near the ore deposits the rock is manganesian and weathers black.

The white limestone, long quarried as a flux at Franklin, Sterling Hill, and Rudeville, is well exposed and has yielded much material to mineral collectors. It contains a group of minerals that carry neither zinc nor manganese and are either very rare or unknown in the zinc ore deposits. In some places these minerals are found near dikes of pegmatite or of basic rocks, and formation was probably due to interaction with the igneous material. More commonly, however, they are quite isolated in the limestone and either are products of its recrystallization during metamorphism or were formed by the introduction from intrusive rocks of new elements that had migrated far from their source.

The minerals in the pegmatite contact zone in the limestone include amphiboles, mainly tremolite and edenite; pyroxenes, especially diopside and leucaugite; scapolite; green and brown magnesian tourmaline; and chondrodite and norbergite, intergrown or separate. The author has never seen garnet in the limestone away from the zinc ores, though Spurr and Lewis (234) describe it from the wall of the ore body. All the minerals named above except pyroxene contain some "mineralizer" such as hydroxl, chlorine, fluorine, or boron—a fact that hardly supports the statement of Spurr and Lewis that the minerals are due to simple heat reactions without the presence of gas.

The minerals found in isolated crystals or grains in the Franklin limestone include all those named in the preceding paragraph and also graphite, molybdenite, chalcopyrite, pyrite, pyrrhotite, arsenopyrite, fluorite, quartz, corundum, hematite, ilmenite, magnetite, spinel, rutile, anorthite, phlogopite, titanite, and apatite. This group is typical of such highly metamorphosed limestones, and it is duplicated, with minor changes, at the well-known mineral localities in Orange County, New York, a score of miles to the north in the same belt of Franklin limestone, and in many other limestones. In its formation it clearly owed nothing to the agencies that produced the ore bodies.


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