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C
Hexagonal
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| Figure 20-2. Crystal drawings of graphite from Sterling Hill. Drawings are in plan-view except for that in the lower left which shows twin-lamellae. Drawings are from Palache (1941b) who provided crystallographic data. | ||
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Graphite, elemental carbon, is common in the Franklin Marble but, with one significant exception, it is not commonly found in the Franklin or Sterling Hill orebodies nor from the Franklin Marble in immediate contact with them. It is found associated with calcite and magnetite in the Furnace Magnetite Bed which underlies part of the Franklin orebody. It was noted by Nuttall (1822) and Pierce (1822) and appeared as plumbago in lists of the 1820’s. It was also reported from the Buckwheat Dolomite by Peters et al. (1983). The best study of local graphite is that of Palache (1941b), who illustrated a number of crystal habits and described the morphology in detail; a fine review is given by Jaszczak (1994).
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| Figure 20-3. Round graphite nodule in calcite from Franklin or Sterling Hill. Specimen is 9 cm in maximum dimension. Smithsonian Institution, #C66. Photo by the author. | ||
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Graphite occurs in fine 0.5-2.0 mm, rock-locked, euhedral crystals, as illustrated by Palache (1941b). They are commonly tabular, hexagonal in outline, but not always ideally formed; some are distorted along one of the a axes (Palache, 1941b). Crystals such as these are highly lustrous, opaque, have sharp edges, and may have growth- or etch-trigons on the pinacoidal faces. These crystals are commonly twinned, apparently on {111} (Figure 20-2).
The bulk of local graphite, however, is blackish gray, massive, and opaque with a greasy submetallic luster; it is very soft. It occurs in very small to large (up to 15 cm) masses and plates. A radial texture is known from Franklin, as described below. Isotopic data were given by Davis (1993). No chemical analyses have been performed.
Graphite is common in the Franklin Marble where it occurs widely disseminated in individual crystals and clusters. It is commonly deformed, but generally the layering of platy crystals is conformable with dolomitic or other bands in the marble. There is a graphite-free zone, about 2 meters in thickness, at the contacts with both of the zinc orebodies. Graphite is commonly associated with phlogopite, other silicates, and sulfides. It is generally found in the local quarries where it may form as masses.
A remarkable occurrence at the Franklin Mine was mentioned briefly by Palache (1935). Here graphite occurs as spherules, up to 2 cm in diameter, with a coarsely radial structure of platy crystals (Figure 20-3). The core of such spherules, as well as the crust, is fine-grained; the surface is slightly mammillary. Unlike the common graphite from the marble, this spheroidal graphite occurs within a coarse pink calcite, associated with fine-grained willemite/franklinite ore and sphalerite of varying fluorescence in ultraviolet. The catalogue of Lawson Bauer indicates this material came from the Buckwheat Mine at Franklin (# C6053).
One well-described Sterling Hill occurrence provided the crystals noted by Palache (1941b) from the 900 level, probably from an area of the Franklin Marble accessed from within the orebody. Here, graphite occurred with pyrite, realgar, arsenopyrite, arsenic, diopside and zinkenite. It also occurs as fine crystals in the margarite/corundum assemblage (Dunn and Frondel, 1990). Spherulitic graphite, similar to that found at Franklin, was found at Sterling Hill in 1965-66, about 30-40 feet above the 430 level; information on this find and on graphite in the Sterling Hill ore was given by Lemanski (1991). Of potential geologic significance is the occurrence of graphite in the black-willemite zone at Sterling Hill (Davis, 1993).
See Kümmel (1908) for a discussion of the occurrences of graphite in the regional rocks and Miller (1912), Crawford and Valley (1990), and others for a discussion of graphite in the Franklin Marble in Pennsylvania.
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| Copyright © 1995 by Pete J. Dunn |
Website
by Herb Yeates
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This
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