SPHALERITE

ZnS
Cubic, F43m, a = 5.41 Å, Z = 4

	Figure 21-18. Crystal drawings of sphalerite from Franklin. Drawings are from Palache (1935) who provided crystallographic data.

Sphalerite, in keeping with the Zn-rich character of the orebodies, is the dominant sulfide here and the major host for sulfur in the orebodies; it is found commonly in small quantities in the Franklin Marble. It occurs sporadically at Franklin and much more abundantly at Sterling Hill. Sphalerite was first reported by Silliman (1822) and Pierce (1822) and also by Fowler (1825). Local material of high quality has been called cleiophane and cramerite, the latter of obscure origin (Henry, 1851; Kemp, 1893). Mining terms for sphalerite were “blende” and “shines,” the latter used by local miners less commonly.

	Figure 21-19. Crystal drawings of sphalerite from the Buckwheat Mine at Franklin. Drawings are from Palache (1935) who provided crystallographic data.

Subsequent observations by Ries and Bowen (1922) and especially by Palache (1935) and Baum (1986a) added much to our knowledge of this mineral’s occurrence here. Sphalerite from the black-willemite zone at Sterling Hill was studied by Davis (1993) using microscopy, cathodoluminescence, microprobe analyses, fluid inclusion studies, and stable-isotope studies. Additional references are cited below under composition. Local sphalerite has no economic use.

Description

In general, sphalerite from Franklin occurs as freely-formed crystals more commonly than that from Sterling Hill. Some Franklin crystals were described in substantial detail by Palache (1935) (Figures 21-18 and 21-19); others were figured by Peters et al. (1983) (Figure 21-20). Among the best of these are dark brown euhedral crystals from the Buckwheat Dolomite. Most are tetrahedral or pseudo-octahedral in habit, equant, and up to 8 mm in diameter; many are twinned on the spinel law. Superb crystals also have been found in veins in the Franklin Mine; these are up to 5 cm in size; twinning on the spinel law is common. A cut gem of 59.5 carats is in the Smithsonian Institution. Much Sterling Hill sphalerite is massive, granular, and locally abundant in the common ore. Additionally, some is fine-grained and cherty in appearance.

	Figure 21-20. Sphalerite twinned on the spinel law from the Buckwheat Dolomite at Franklin. Field of view is approximately 4 mm in maximum dimension. Photograph courtesy of Tom Peters and the Paterson Museum.

Sphalerite occurs in diverse colors, from colorless, grading into light yellow, light green, light brown, and darker hues of all these, plus light blue, gray, and apparent black when rich in iron.

Cleavage is perfect on {111}. The luster is very variable: that of fine cleavage fragments is adamantine to slightly greasy; that of light green material is adamantine to oily; and that of fine-grained material is distinctly resinous to dull. The luster of crystal faces also varies substantially: most are vitreous to resinous; some are satiny. Natural etching is common and form-selective. The density is 4.063 g/cm³ for the highest-quality, nearly-colorless material formerly called cleiophane. Reflectance data were given by Criddle and Stanley (1993).

Much local sphalerite is fluorescent and phosphorescent in ultraviolet; the effects are strongest in longwave, and the response color is generally orange to pale-orange to yellow of medium intensity. Fluorescence also occurs in an enormous range of hues. Some material has blue fluorescence, and some of this occurs in isolated, small grains generally associated with the more common orange-fluorescent material, and commonly as thin borders where orange-fluorescent sphalerite contacts franklinite or calcite. Some sphalerite is strongly triboluminescent (Wick, 1937).

Fine-grained sphalerite is easily confused with friedelite and manganpyrosmalite, from which it is easily differentiated by its inferior hardness and/or an evolved odor of sulfur when sphalerite is scratched vigorously or powdered. Some massive and/or fine-grained material is confused with some willemite or serpentine.

The confusion with willemite engendered a interesting local “test” which is now part of the vast Franklin folklore. The late Lawson Bauer, chemist of the New Jersey Zinc Company and a friend to many, kept a small collection of eighty 1-2 inch specimens in a wooden case (Figure 7-5) and would challenge visiting students and geologists to sort out the sphalerite and the willemite. Few persons, if any, were fully successful on the initial attempt! Nonetheless, word of this “test” spread, and many were eager to try. Having attempted it, regardless of outcome, became a common bond for some Franklin mineralogists and collectors.

Composition

Sphalerite is a zinc sulfide mineral. The most colorless material from Franklin conforms to nearly pure zinc sulfide (Henry, 1851; Skinner and Barton, 1960). Sphalerite from the black-willemite zone at Sterling Hill is nearly pure ZnS, containing only small amounts of As, Cd, and Mn (Davis, 1993).

Substitution of Fe for Zn can vary considerably, as shown by Palache (1935). The absence of significant gallium was proven by Cornwall (1880) and confirmed by Papish and Stilson (1930). Skinner and Barton (1960) showed that zinc oxide substitution in local sphalerite was minimal. A number (>8) of unpublished analyses by Bauer showed minimal Cd contents of 0.05 to 0.97 wt. % Cd in material from both Franklin and Sterling Hill, with the higher values in Franklin material. This likely provided a Cd-source for some of the small amount of greenockite or hawleyite found locally.

Occurrence and paragenesis

Franklin sphalerite, in general, occurs in veins and is better crystallized than that from Sterling Hill, and colorless, light yellow, and light green material is more common here than at Sterling Hill. Associated minerals vary substantially. At Franklin, sphalerite is commonly associated with post-ore-formation dolomite veins (Baum, 1986a) (Figure 21-23).

	Figure 21-21. Twinned tetrahedral crystals of sphalerite on quartz crystals on the Buckwheat Dolomite from Franklin. Specimen is 9 cm in maximum dimension. Mineralogical Museum, Harvard University, #80813. Photo by Chip Clark.		Figure 21-22. Superb sphalerite crystals on calcite (white) rimmed by lennilenapeite (dark at edges) from Franklin. Specimen is 9 cm in maximum dimension. Smithsonian Institution, #R13385. Photo by the author.

	Figure 21-23. Twinned sphalerite crystals with calcite from Franklin. Specimen is 7 cm in maximum dimension. Mineralogical Museum, Harvard University, #97906. Photo by Chip Clark.		Figure 21-24. Sharp, euhedral sphalerite crystals on fine-grained lennilenapeite from Franklin. Specimen is 6 cm in maximum dimension. Smithsonian Institution, #C6068-1. Photo by the author.

Within the ore units, it is commonly associated with andradite, franklinite, calcite, serpentine, amphiboles, and pyroxene, and less commonly with a number of other species including fluorite, rhodonite, and bannisterite in part. An association with zincite, ZnO, is rare. Ries and Bowen (1922) noted the molding of sphalerite around granular willemite and franklinite and interstitial to them.

	Figure 21-25. Sphalerite fragments (dark gray) cemented by magnesioriebeckite (light gray) to form a breccia from Franklin. Specimen is 7 cm in maximum dimension. Smithsonian Institution, #163940. Photo by the author.

It is found in breccias and fracture-zones and is younger than willemite in both. A more detailed exposition of Franklin occurrences is given by Baum (1986). Assemblages are very diverse; a few worthy of special mention are:

a)   As euhedral dark brown crystals in the Buckwheat Dolomite, associated with calcite, dolomite, albite, and other species (Palache, 1935; Peters et al., 1983) (Figure 21-21).

b)   As light yellow-green fragments (angular or rounded) in magnesioriebeckite associated with lennilenapeite (Figures 17-40 and 21-25). Also from this assemblage may have come some of the best Franklin sphalerite of all, occurring in 2-3 cm light green, euhedral crystals on lennilenapeite with dolomite (Figures 21-22 and 21-24). These came from dolomite veins and are the crystals reported by Palache (1935) from the 300 and 450 levels at Franklin.

c)   As massive brown material with the nickeline assemblage at a depth of 340 feet in the Trotter Mine.

d)   As colorless grains in andradite from the Trotter Mine.

e)   As veins of fine-grained material up to 2-3 cm thick, occasionally associated with willemite and commonly  cross-cutting the older franklinite/willemite ore.

f)   As superb 1-2 cm yellow-brown crystals from the 750 level in the north end of the Franklin orebody.

g)   With rhodonite and calcite in calcium-silicate units.

h)   As irregular segregations in ore.

	Figure 21-26. Sphalerite in spherical aggregates in white calcite from Sterling Hill. Specimen is 11 cm in maximum dimension. Smithsonian Institution, #147218. Photo by the author.

Sterling Hill sphalerite, in contrast, is not commonly found in large crystals, although superb, small ones are found rarely in secondary seams. At Sterling Hill, the association with willemite is much more common than at Franklin, and sphalerite, in general, is granular, pale yellow to pale red or colorless in the ore. It occurs in large segregated masses to 30 cm. Some cherty material is commonly reddish-brown and darker colored, reflecting a likely higher content of iron, consistent with the generally higher iron content of many Sterling Hill minerals, relative to those from Franklin.

	Figure 21-27. Rounded fragments of willemite- franklinite-calcite ore form a breccia cemented in part by sphalerite (gray to black) from Sterling Hill. Note that the enclosing sphalerite is much darker at its contact with the ore fragment. The visible surface is polished. Specimen is 13 cm in maximum visible dimension. Privately owned. Photo by the author.

At Sterling Hill sphalerite is locally abundant in the primary ore, occurring in grain-sizes comparable to those of willemite and franklinite and, like them, occurring in gneissic textures. The origins of sphalerite in the black-willemite zone were discussed by Davis (1993). Sphalerite and zincite are almost never found together; exceptions are known. In general, sphalerite may be more common in the black-willemite ore than in the red-willemite ore, but the keel of the Sterling Hill orebody has sphalerite associated with red willemite. Sphalerite is also common in the sheared and recrystallized assemblages and is a common constituent of vein assemblages, especially late-stage ones; it also persists in some weathered assemblages. Locally abundant pinkish-yellow material with a distinct yellow fluorescence is found on the 2250-2350 levels, near the Zero fault at Sterling Hill.

In general, the commonly associated minerals are calcite, willemite, franklinite, and pyroxene. In addition to the associations noted above, it occurs as light yellow masses with loellingite, fayalite, and black willemite and also with a peculiar yellow-orange fluorescing willemite, locally mislabeled as beta-willemite. Sphalerite was found in an oxidized sulfide occurrence below the 700 level at Sterling Hill by Jenkins and Misiur (1994). Some notable Sterling Hill assemblages are:

a)   As segregations and granules in the black-willemite zone (Davis, 1993).

b)   In late-stage sulfide veins.

c)   As large, coarsely-crystallized masses with franklinite, calcite, willemite, and some rhodonite in the north orebody.

d)   With spessartine, pyroxene, feldspar, and calcite.

e)   As fine-frained, reddish-brown, swirly “mahogany”- colored masses (Figure 21-27).

f)   As nearly pure colorless material in the Nason fault (Johnson, 1990).

Sphalerite occurs in the Franklin Marble, commonly associated with galena and other sulfides, and also is found in the Kittatinny Limestone.

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