GYPSUM
CaSO4 · 2H2O,
calcium sulfate hydrate
Gypsum is one of the minor and more underappreciated minerals in the Franklin-Sterling Hill area, probably in part because it is a common species worldwide, and in part because it is dominantly a late-stage mineral unrelated to the ores. Indeed, Frank Edwards, who for years served as editor of The Picking Table, wrote in a 1966 issue that “Gypsum is not often found at Sterling Hill, but nobody cares.” What little attention has been paid to gypsum to date often centered more on its fluorescence than its genesis. Nevertheless, one of the Sterling Hill finds produced large, transparent cleavage plates of the selenite variety (Kolic and Sanford, 1993), and small but well-formed crystals have also been recovered on occasion.
The first report of gypsum from the area is that of Palache (1928), but it is difficult to believe it had not been found earlier and had either passed unrecognized or was ignored. In Palache’s later (1935) treatise he mentioned a single specimen from Franklin and another from Sterling Hill, neither of which, to judge from his descriptions, furnished gypsum of much caliber. The major finds of that mineral would all come later. By the early 1970s, however, some of the large cleavage plates were in circulation (Frondel, 1972, p. 59), and in 1974, John MacDonald, in a letter to the editor of The Picking Table (see Edwards, 1974b) reported a specimen of gypsum as “perfect monoclinic crystals” as much as 1.3 cm long. Interest was beginning to pick up.
The major find of gypsum in the Sterling mine, in January 1990, deserves special mention, for it produced numerous specimens of the selenite variety, some quite large, and a few more than 20 cm across. These came from 780 pillar on 1100 level, at mine coordinates 780N, 670W, which places the locality not far north of the keel of the orebody. The specimens were found in “a series of solution cavities within lean franklinite ore” toward the footwall of the East limb of ore and were “filled to about one-third of their volume with massive transparent selenite” (Kolic and Sanford, 1993, p. 14). Numerous specimens were recovered and quickly dispersed by sale by the Sterling Hill Mining Company (later the Sterling Hill Mining Museum). Most of the specimens include scattered grains of franklinite and of red-fluorescent calcite, all readily visible within their transparent selenite host, and some also contain tiny masses of a secondary copper mineral reported by Kolic and Sanford (1993) to be brochantite. Older specimens, as reported by the same authors, may have come from similar localities anywhere between the 1400 and 700 levels.
In addition to the above-mentioned occurrences, gypsum occurs locally in a variety of ways:
▪ as a hydration product of primary anhydrite in the Franklin Marble;
▪ as a low-temperature hydrothermal vein mineral, often in association with calcite;
▪ as an alteration product of charlesite in specimens from the Parker Shaft suite; and
▪ as small, post-mining crystals on drift walls.
The general paucity of gypsum specimens precluded definitive statements on its fluorescence until the late 1970s. An early report of pale blue fluorescence SW (Edwards, 1974a) included the caveat that such fluorescence was seen only in bruised areas. That same report claimed the existence of red-fluorescent microcrystals of selenite from Sterling Hill, but this was based on a miscommunication and was promptly corrected in the next issue (Edwards, 1974b). As more specimens were examined, from different assemblages, the true nature of gypsum fluorescence gradually became clear and was carefully documented in print by Bostwick (1977, 1992, 2016). Although most gypsum from Franklin and Sterling Hill is nonfluorescent or only feebly so, numerous specimens are now known to show a weak to moderately bright fluorescence in various pastel colors: white, pale yellow, and pale blue. Other specimens show a ghostly yellowish-green fluorescence, often ranging from bright to dim to absent in the same specimen. Here, however, the fluorescence is not due to the gypsum itself, but to micrograin inclusions of willemite.
The cause of pale-colored fluorescence in local gypsum has not yet been studied, but as a mostly late-stage, near-surface mineral it is likely that molecular inclusions of organic molecules are a contributing or main cause in some specimens. Crystal-defect activators are a possibility as well.
Gypsum in most of its forms is readily identified by its softness (it is easily scratched by a fingernail), and its perfect cleavage. Its lack of effervescence in dilute acids helps to distinguish it from calcite. King et al. (2021, p. 12) offered a simple means of distinguishing gypsum from anhydrite, which in some specimens is associated with gypsum and is sometimes confused with it.
References
Bostwick, R.C. (1977), The fluorescent minerals of Franklin and Sterling Hill, New Jersey: a progress report for 1977, Part II. The Picking Table, vol. 18, no. 2, p. 11-24. [Brief description of gypsum fluorescence on p. 13]
Bostwick, R.C. (1992), The Check List of Franklin-Sterling Hill Fluorescent Minerals. The Picking Table, vol. 33, no. 2, p. 4-11. [Four-line mention of gypsum fluorescence on p. 10]
Bostwick, R.C. (2016), Fluorescent Minerals of Franklin and Sterling Hill, N.J., Part 1. The Picking Table, vol. 57, no. 2, p. 10-22. [Update on fluorescence of gypsum on p. 19]
Dunn, P.J. (1995), Franklin and Sterling Hill, New Jersey: the world’s most magnificent mineral deposits. Privately published in five volumes, 755 pp. [Description of gypsum on p. 633]
Edwards, F.Z. (1966), The minerals of Sterling Hill, 1962-65. The Picking Table, vol. 7, no. 2, p. 3-15. [Mention of gypsum from the mud zone at Sterling Hill on p. 9]
Edwards, F.Z. (1974a), Freak fluorescents. The Picking Table, vol. 15, no. 1, p. 7-8. [Erroneous mention of red-fluorescing gypsum from Sterling Hill; corrected in next issue]
Edwards, F.Z. (1974b), Fluorescent corner: Gypsum/selenite/quartz: The Picking Table, vol. 15, no. 2, p. 7-11. [Correction to previous statement of red-fluorescing gypsum from Sterling Hill]
Frondel, Clifford (1972), The minerals of Franklin and Sterling Hill, a check list. Wiley-Interscience, 94 pp. [Brief statement on gypsum on p. 59; mentions “cleavage surfaces several inches across”]
King, V.T., King, N.E., Betancourt, P.P., Bostwick, R.C., Chin, Peter, Hecht, T.J., Kuitems, S.M., Moritz, Harold, Nemetz, J.D., Nikischer, A.J., Sanford, Stephen, Van Fleet, J.A., and Verbeek, E.R. (2021), The Mineralogy of Franklin and Ogdensburg, New Jersey—A Photographic Celebration. Privately published, 1400 pp. [Fluorescence of gypsum described on p. 20. Specimen photos on numerous pages, including, on p. 913, two photomicrographs of well-formed gypsum crystals]
Kolic, John and Sanford, Steve (1993), Recent mineral finds from the Sterling mine, Ogdensburg, New Jersey. The Picking Table, vol. 34, no. 2, p. 12-21. [Description, with mine coordinates, of the major find of gypsum at Sterling Hill]
Palache, Charles (1928), Mineralogical notes on Franklin and Sterling Hill, New Jersey. The American Mineralogist, vol. 13, no. 7, p. 297-329. [Lists gypsum as a mineral definitely identified from both Franklin and Sterling Hill. List only; no description]
Palache, Charles (1935), The minerals of Franklin and Sterling Hill, Sussex County, New Jersey. U.S. Geological Survey Professional Paper 180, 135 pp. [Short description of gypsum on p. 129]
Verbeek, E.R. and Penny, R.N. (2022), A Raman study of minerals in Palmerton, Pennsylvania smelter slag. The Picking Table, vol. 63, no. 2, p. 19-23. [Shows the SW blue-fluorescent crusts in Palmerton slag are gypsum rather than hydrozincite]
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