UNUSUAL GRAPHITE CRYSTALS FROM
THE LIME CREST QUARRY, SPARTA, NEW JERSEY

The Franklin Marble at the Lime Crest quarry near Sparta, New Jersey is also host to graphite crystals of exceptional quality. Besides well-formed crystals, some very unusual graphite morphologies can also be found.

Bulk samples of graphite-bearing marble were collected from a graphite-rich zone on the lower level of the quarry by John Rakovan on April 29, 1991, and generously supplied to the author for examination. Bulk material was subsequently trimmed and examined with an optical stereo microscope. A dilute solution of hydrochloric acid (HCl) was used to partially etch away the calcite in many specimens in order to expose the enclosed, protected graphite crystals. Although graphite crystals occur in the marble to over 1 cm across, the best crystals are usually less than 2 mm across. Associated minerals include crystals of phlogopite, pyrite, pyrrhotite, molybdenite, arsenopyrite, sphalerite and a few unidentified species. The identities of the pyrrhotite, pyrite, molybdenite and arsenopyrite were confirmed by the use of energy dispersive spectroscopy (EDS) capabilities of a scanning electron microscope (SEM) at Michigan Technological University's Institute of Materials Processing.

Material similar to that described here is no doubt readily available, and simply requires some one�s careful examination to find good crystals. One hand-sized sample can yield many micromounts. Similar material from Lime Crest was subsequently given to the author by Scott Stepanski. These samples contained graphite crystals in marble with morphologies very similar to those that are the focus in this paper, but associated with a somewhat different assemblage of minerals, which appears (but has not been confirmed) to include serpentine, zircon, muscovite, apatite and diopside, in addition to those already mentioned. The geology and mineralogy of Lime Crest are briefly described by Widmer (1962), Metsger (1977) and Tracy (1991).

Graphite Morphology

Fig. 1. Idealized crystal structure of hexagonal graphite showing sheets of carbon atoms bonded in a honeycomb pattern. The c-axis of the graphite is perpendicular to the sheets.

As a result of the crystal structure, the symmetry and the nature of the bonding, the {0001} basal pinacoid faces (c-faces) of graphite crystals tend to be the lowest energy surfaces, and growth of the crystals is typically fastest at the sheet edges. These factors lead to tabular hexagonal plates as being the typical morphology of graphite crystals (Jaszczak 1995). By far the most common crystal form for graphite at Lime Crest and elsewhere is the basal pinacoid (figs. 2 to 7).

Fig. 2. Idealized graphite crystal showing the basal pinacoid c{0001}, the first-order prism m{10-10}, and the first-order dipyramid p{10-11}.

The c-faces of the graphite crystals from most localities are typically highly striated (fig. 5) due to the crystals being mechanically bent and deformed by the various stresses and forces on and in the host-rock. The bending of the crystals induces so-called "mechanical twinning" that is evidenced by the striations on c-faces (see Palache 1941). Many small graphite crystals from Lime Crest, however, can be found with no striations on the c-faces, indicating their rare escape from the many factors that could have deformed them. A few crystals have been observed that show growth twins that are evident as small triangular crystals protruding from the c-faces, as shown in fig. 3 (Jaszczak 1991).

Fig. 3. (a) Graphite crystal (0.7mm) showing a smaller growth twin (brightly reflecting), twinned according to graphite's most common twin law: by reflection and contact on a {11-21} plane. (b) Idealized graphite crystal illustrating the same twin law.

Fig. 4. A well-formed, lustrous graphite crystal (0.6 mm) showing part of one of the typical basal pinacoid faces as well as several modifying faces. The crystal is partially embedded in calcite and associated with an orange-brown phlogopite crystal. JAJ specimen #1556h and photo.

Fig. 5. An unusually thick graphite crystal with uncommon faces, on a basal pinacoid face of a more typical, striated, tabular graphite crystal (1.3 mm). JAJ specimen #1556i and photo.

A few graphite crystals from Lime Crest have been found that display spiral growth steps on the c-faces. The spirals can usually only be observed by orienting the crystals to reflect the light "just right" to make the steps visible, and are difficult to photograph (fig. 6). Such growth spirals are thought to emerge from defects in the crystal structure called screw dislocations, which play an important role in the growth of the crystals (Weiner and Hager 1987). A crystal exhibiting a different of spiral is shown in fig. 7. Rather than just having spiral steps on teh c-faces, this entire crystal seems to be coiled as a macroscopic spiral, similar to a few turns of a spiral staircase. The overall appearance suggests that the growth of such "macrospirals" also might be related to growth twinning (compare fig. 3).

Fig. 6. Graphite crystal (0.7mm) showing spiral steps on the c-face. Author's specimen #1626 and photo.

Fig. 7. Graphite crystal (0.7mm) showing a "macrospiral". Author's specimen #1626 and photo.

Occasionally graphite crystals will show well-formed prism faces. These are often striated as well, not because of mechanical twinning, but because of the graphite's layered crystal structure. Only rarely do other forms show themselves in well-formed, lustrous crystal faces (see figs. 4 and 5).

Fig. 8. A 1.1-mm crystal of molybdenite in partially etched calcite with minor pyrite. JAJ specimen #1556e and photo.

Molybdenite, which can be easily confused with graphite, also occurs in the marble at Lime Crest as well-formed, lustrous, platy crystals (fig. 8), but is much less common. However, graphite and molybdenite crystals submerged in a shallow bath of water and viewed with a microscope can readily be distinguished from one another by the difference in their color. Compared to graphite, the molybdenite shows a distinctly bluish hue. A few graphite crystals have been found having molybdenite overgrown on graphite�s c-faces. Although both the graphite and the molybdenite have their c-axes parallel in these samples, an epitaxic relationship has not yet been confirmed in these samples.

An interesting set of crystals that occur at Lime Crest shows secondary graphite overgrowths on earlier-formed, larger graphite crystals (figs. 9 to 11). Similar graphite overgrowths from Precambrian Grenville Series marble at Eagle Lake, Essex County, New York have been described by Weis (1980). Such overgrowths have also been noted by the author from the Sterling Mine, Ogdensburg, New Jersey, the Treadway Quarry, Port Henry, New York, the Limberg quarry, Pargas, Finland, a roadcut near Dresden, New York, and a roadcut south of Gooderham, Ontario, Canada. It is interesting to note that graphite occurs at all of these localities also as spherical aggregates in sizes ranging from microns in diameter at Pargas, up to a centimeter in diameter at the Sterling Mine. This correlation suggests a possible relationship between the growth mechanisms, growth conditions, and growth sequence of the graphite spheres and the graphite overgrowths.

Fig. 9. A well-formed hexagonal graphite crystal (1.2 mm) with a thick secondary overgrowth of graphite. The crystal is on partially etched calcite and is associated with ora nge phlogopite that is included with tiny graphite spheres, and an unidentified pale green mineral at the mid-lower right. JAJ specimen #1556b and photo.

A whole spectrum of varying degrees of graphite overgrowth on earlier-formed graphite can be identified in material from Lime Crest. At one end of the spectrum there are the lustrous crystals with no overgrowth whatsoever. On some crystals, overgrowth is primarily localized on the crystal edges leaving areas of lustrous, earlier-formed graphite still visible (fig. 9). This suggests that either the secondary graphite growth was nucleated by the edges of the earlier- generation graphite, or perhaps that the secondary growth took place rapidly compared to the diffusion of carbon to the crystal surface. On other graphite crystals the whole surface can be overgrown by the second-generation graphite. At the opposite end of the spectrum are particularly thick overgrowths that seem to take on a spherical morphology (fig. 11), similar to crystals also observed by Weis. Whereas in the partially coated crystals the secondary graphite shows some degree of order in their alignment with each other and with the substrate crystal, the thickly coated crystals appear to have more randomly oriented overgrowths. The preservation of the delicate secondary crystallites and the undistorted morphology of the first-generation graphite crystals implies that the metamorphic processes that commonly deform rock and distort or destroy the enclosed graphite crystals must have been greatly diminished by the time the graphite crystals formed.

Fig. 10. SEM photo of hexagonal graphite crystals (to approximately 0.2 mm) showing asecondary overgrowth of graphite crystals as minute plates. Associated are a spray of arsenopyrite crystals at the lower right and a plate of pyrrhotite at the lower left. SEM photo by Ruth I. Kramer. JAJ specimen #1556.

Fig. 11. A 1-mm graphite crystal (inclined) thickly overgrown by a second generation of graphite to such an extent that spherical clusters appear. JAJ specimen #1556g and photo.

Some of the most unusual graphites observed from Lime Crest are pseudocubic aggregates of crystals that reach up to 0.5 mm across and have a rather velvety luster (figs. 12 to 14). In shape they are reminiscent of small balls of putty that have been dropped several times on a flat surface, giving both rounded and flattened areas. It appears, however, that the flattened areas form 90-degree angles with each other, as would cube faces. The aggregates are composed of flat crystal flakes (figs. 13 and 14), which show that the dominant growth mechanism is still fastest at the edges of the graphite sheets. The cause of the overall pseudocubic morphology could be pseudomorphism after some other mineral, although this has not been verified.

Fig. 12. An aggregate of graphite crystals (0.4 mm)
showing a pseudo-cubic habit, with phlogopite on calcite.
JAJ specimen #1556d and photo.

Fig. 13. SEM photo of a graphite pseudo-cube (nearly 0.1 mm) on phlogopite, partially etched from calcite. Individual graphite crystals in the aggregate seem to be partially aligned. SEM photo by Ruth I. Kramer. JAJ specimen #1556.

Fig. 14. SEM photo of a somewhat larger graphite pseudo-cube than shown in Fig. 13, viewed such that two of the cube sides are at the top left and at the right. In the area of the cube corner at the lower left, the individual graphite flakes appear to be randomly oriented. SEM photo by Ruth I. Kramer. JAJ specimen #1556.

Finally, distinctly spherical aggregates of graphite occur up to 0.5 mm in diameter and are commonly clustered around phlogopite crystals (fig. 15). It will be an interesting future study to investigate if these spheres are related to the pseudocubic graphite aggregates or possibly to spherical graphite that is known to occur at a number of other localities, such as Sterling Hill and Franklin, New Jersey, Chilson Hill, New York, Gooderham, Ontario, and others (Lemanski, 1991; Dunn 1995; Jaszczak 1995 and unpublished).

Fig. 15. Cluster of graphite spheres (up to 0.4 mm)
on orange-brown phlogopite, partially etched from calcite.
JAJ specimen #1556c and photo.

Acknowledgments

References