DOLOMITE

CaMg(CO₃)₂
Hexagonal

	Figure 23-14. Dolomite crystals from the Buckwheat Dolomite in Franklin. Field of view is 0.5 mm in maximum dimension.

Dolomite is a common mineral at Franklin and Sterling Hill. It is also found in the Franklin Marble. Much dolomite is probably unrecognized as such. It was first identified by Palache (1929, 1935) but has been little studied. The manganesian dolomite of Roepper (1870) is likely kutnahorite.

Description

Much local dolomite is massive, white to gray, gray-brown, and nondescript in appearance. Small (< 1 cm) crystals are common in vein assemblages and are commonly curved and/or saddle-shaped. A slightly pearly vitreous luster is common, and the rhombohedral cleavage is well developed. No physical or optical data exist.

Composition

Dolomite is a calcium magnesium carbonate mineral. The only published analysis is by Yau et al. (1984) who gave the composition of a specimen associated with margarite in the Franklin Marble as (Ca_0.49Mg_0.48Mn_0.1Fe_0.2)₂(CO₃)₂. The paucity of analytical data precludes general statements about solid solution towards ankerite, minrecordite, and kutnahorite. Compositional data given herein under kutnahorite suggest very little Mg-Mn solid solution in the specimens studied to date. However, as noted by Frondel (1972), Mn is to be expected in the as-yet unstudied orebody dolomites.

Occurrence and paragenesis

Dolomite was present as the major consituent of a small geologic feature, perhaps formed in part by replacement, and known as the “Buckwheat Dolomite” (Figure 23-14). This was found in a large vein-like mass in the west wall of the Buckwheat Open Cut. It was mostly removed in the overburden-stripping process, and much of it was deposited on the now-named Buckwheat Dump. This dolomite is well known to collectors of micromounts, because it is very vuggy, and at least eighteen well-crystallized mineral species have been reported from its vugs, as noted and illustrated by Peters et al. (1983). This adds yet another mineralogical dimension to Franklin. It is discussed at the end of Chapter 12. 

Dolomite plays a considerable role in late veins in the Franklin orebody. These are referred to as “carbonate veins” in the willemite discussion and consist of willemite (commonly of radial habit) with serpentine and calcite and, locally, with rhodochrosite and talc. Dolomite forms part of the outside contact edges of such veins, which commonly crosscut the bedding of the ores and are thought to be Paleozoic in age. Additionally, in other, perhaps genetically related assemblages, dolomite is associated with a number of other minerals, such as calcite, hematite, sphalerite, lennilenapeite, and magnesioriebeckite (Figure 21-22).

The occurrence of dolomite at Sterling Hill is less well represented in the specimen collections available for study. Given the abundance of Mg at Sterling Hill, dolomite is likely much more common than has been recognized. It has been found in carbonate veins similar to those from Franklin, but  thinner and less abundant. Large amounts of dolomite are known from the north orebody and areas affected by the introduction of Mg-bearing solutions along the Zero fault. 

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
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This page created: January 11, 2001