SCHALLERITE
(Mn,Fe)16Si12As3O36(OH)17,
manganese arsenic silicate hydroxide
Franklin is the type locality for this mineral species, which was first found there in 1924 and described the following year (Gage et al., 1925). This original schallerite occurred in reddish-brown, granular masses greatly resembling massive rhodonite; specimens SCH3 and SCH10 are good examples. The exact locality that furnished the first specimens is unclear. Foshag et al. (1927) maintained they were found between the 500 and 600-foot levels of the Franklin mine, and Gage et al. (1925) stated they came “from about seven hundred feet below the surface.” Both sets of authors stated that schallerite was first found in the same workings that furnished specimens of chlorophoenicite, recognized as a new mineral during the same time period. Dunn (1995, p. 651) repeated the information of Foshag et al. (1927) but added that the locality was in the east limb of ore at Franklin.
Several years after the original find of schallerite, a new find of that mineral, not massive but in small, rounded masses (“augen”) showing concentric structure when broken, was mentioned by Bauer and Berman (1928). Those authors, however, did not describe the new material but simply referred to it as “type II” schallerite in a data table, the original material being designated “type I.” Palache (1935, p. 90) provided a brief description of type II schallerite, but it was not until 1982 (Dunn, 1982) that this material was described in detail. Beyond these two types, schallerite has also been found as veins in calcite-rich ore (SCH5, SCH7, SCH8, SCH9), implying one or more additional finds, and it also occurs in small amounts in other assemblages, most notably with manganberzeliite.
The spherules or “augen” of schallerite referred to above are illustrated here by several specimens (SCH1, SCH2, SCH4, SCH6), but the best published example of their internal concentric structure is Figure 18-18 on p. 483 of Dunn (1995), a black-and-white photograph of a cut-and-polished section through numerous spherules in a specimen in the Smithsonian Institution. The description of these spherules in an earlier paper by Dunn et al. (1981) is worthy of careful attention. Schallerite forms tiny, hemimorphic crystals shaped like hexagonal pyramids with steep sides and a flat base; the SEM image of one such crystal in Figure 1 of that paper is illustrative. Numerous such crystals compose the augen; they are arranged radially, their apices pointed inward and their flat bases (pedion faces) outward (see also Figure 18-16 of Dunn, 1995, p. 482). The intervening spaces between the schallerite crystals are filled with microcrystalline rhodonite. The concentric layers of schallerite crystals that compose the augen apparently formed as successive coatings over tiny, angular fragments of franklinite, calcite, willemite, or barite (Dunn et al., 1981).
The description of the augen (German for “eyes”) as spherules should not be taken literally, for most are ovoid rather than truly spherical, and some are notably elongate. Nearly all, however, have smoothly curved exteriors, the result of successive layers of schallerite encrusting a tiny, broken fragment of ore or a mineral grain. Where the parent fragments were highly irregular, however, and especially where the encrusting schallerite is thin, the augen can depart markedly from a spheroidal shape. Most augen range in apparent diameter from 1 to 4 mm, but some approach nearly 1 cm (SCH4) and others are of submillimeter size (parts of SCH2).
Abundant evidence exists of faulting during the formation of type II schallerite: multiple shear surfaces in the adjacent wallrock (SCH1, SCH2, SCH4), elongate fragments of ore torn from the wallrock and suspended in rhodonite, common breakage of these fragments into smaller ones, etc. Both the large fragments and the small, angular fragments of ore that form the augen cores likely are clasts of a fault breccia that formed as the two sides of the fault ground together (“attrition breccia” in geologic parlance).
Schallerite is commonly difficult to identify by sight, save for type II examples with their characteristic spherulitic form, and has often been confused with specimens of massive rhodonite, friedelite, and bustamite (Gage et al., 1925; McHugh, 1976; Dunn, 1982). Vein specimens of dark red, fine-grained “cherty” sphalerite, and even some rare specimens of fine-grained willemite, can have a similar appearance. In practice this means that the number of specimens optimistically (or misleadingly) labeled schallerite, especially in old collections, vastly outnumber those correctly identified. The high indices of refraction of schallerite relative to friedelite are an easy and useful discriminator for those possessing a good microscope and the relevant immersion oils. In recent decades, too, the rising popularity and affordability of portable X-ray fluorescence instruments offers a rapid means of distinguishing schallerite – an arsenic mineral – from the other minerals mentioned above, which are lacking, or nearly so, in that element. Schallerite is also readily identified by Raman spectroscopy.
Though schallerite remains a rare mineral locally, with good specimens always in high demand, Gage et al. (1925) maintained that it probably occurred in the mine in “considerable quantity.” Owing to its confusion with other species at the time, however, “no effort whatever was made to collect it,” and in consequence it was “crushed with the other zinc minerals in the mill.”
Schallerite for 64 years was on the list of species unique to the Franklin-Sterling Hill area but was removed from that list by Dunn and Baum (1988), who noted its discovery elsewhere.
References
Bauer, L.H. and Berman, H. (1928), Friedelite, schallerite, and related minerals. American Mineralogist, vol. 13, no. 7, p. 341-348. [First recognition of “type II” schallerite; established relationship of schallerite to friedelite]
Berman, H. (1937), Constitution and classification of the natural silicates: American Mineralogist, vol. 22, no. 5, p. 342-408. [Mentions schallerite as a member of the friedelite group of minerals and speculates that a series might exist between friedelite, schallerite, and pyrosmalite; see p. 377]
Dunn, P.J. (1982), Additional information on schallerite, friedelite, and carypopilite. The Picking Table, vol. 23, no. 1, p. 11. [Reiterates McHugh’s earlier finding that most specimens labeled schallerite are not that mineral; a “large number” examined by Dunn proved to be rhodonite or friedelite instead. Provides new chemical analyses of type I and type II schallerites]
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 schallerite, with five SEM images, on p. 481-484]
Dunn, P.J. and Baum, J.L. (1988), Notes from the laboratory and changes to the list of species from Franklin and Sterling Hill. The Picking Table, vol. 29, no. 2, p. 3. [Deletes schallerite from the list of minerals unique to the local area]
Dunn, P.J., Peacor, D.R., Nelen, J.A., and Norberg, J.A. (1981), Crystal-chemical data for schallerite, caryopilite, and friedelite from Franklin and Sterling Hill, New Jersey. American Mineralogist, vol. 66, nos. 9 and 10, p. 1054-1062. [Proposes a new chemical formula for schallerite based in part on five new chemical analyses and describes, in detail, the nature of “type II” schallerite]
Edwards, F.Z. (1968), The exclusive minerals of Franklin/Ogdensburg, N.J. The Picking Table, vol. 9, no. 1, p. 11-17. [Mentions that schallerite remains on the list of minerals exclusive to the Franklin-Sterling Hill area (but 20 years later it was removed; see Dunn and Baum, 1988)]
Foshag, W.F., Berman, H.M., and Gage, R.B. (1927), The occurrence and properties of chlorophoenicite, a new arsenate from Franklin, New Jersey. Proceedings of the United States National Museum, vol. 70, article 20, p. 1-6. [Mentions on p. 2 that schallerite was found in the same workings, between the 500 and 600-foot levels at Franklin, that furnished specimens of chlorophoenicite, then recently recognized as a new mineral]
Frondel, C. (1972), The minerals of Franklin and Sterling Hill - a checklist. Wiley Interscience, New York, 94 pp. [Short description of schallerite on p. 75]
Frondel, C. and Bauer, L.H. (1953), Manganpyrosmalite and its polymorphic relation to friedelite and schallerite. American Mineralogist, vol. 38, nos. 9 and 10, p. 755-760. [Suggests that manganpyrosmalite, friedelite, and schallerite are polytypes]
Gage, R.B., Larsen, E.S., and Vassar, H.E. (1925), Schallerite, a new arseno-silicate mineral from Franklin Furnace, New Jersey. American Mineralogist, vol. 10, no. 1, p. 9-11. [First description of schallerite as a newly discovered mineral species]
Kashaev, A.A. and Drits, V.A. (1970), The polytypism of pyrosmalite minerals. Kristallografiya, vol. 15, p. 52-56.
Kashaev, A.A. and Drits, V.A. (1970), The polytype properties of pyrosmalite minerals. Soviet Physics: Crystallography, vol. 15, no. 1, p. 40.
Kato, T. and Watanabe, I. (1992), The crystal structures of schallerite and friedelite. Yamaguchi University, College of Arts Bulletin, vol. 26, p. 51-63. [Determination of the crystal structure of schallerite]
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. [Photos of schallerite specimens on p. 1068-1071]
McConnell, D. (1954), Crystal chemistry of schallerite. American Mineralogist, vol. 39, nos. 11 and 12, p. 929-936. [Discusses a possible crystal-structural formula for schallerite, addresses the role of arsenic in the structure, and challenges the assumption that schallerite is a phyllosilicate mineral]
McHugh, Daniel (1976), Mineral Notes: An examination of the species schallerite. The Picking Table, vol. 17, no. 2, p. 4-5. [XRD and SEM results from 12 specimens labeled schallerite in four different collections showed that none were schallerite but were rhodonite instead; concludes that schallerite is even rarer than originally thought. Also mentions purported schallerite from Sterling Hill (but none ever found)]
Palache, Charles (1935), The minerals of Franklin and Sterling Hill, Sussex County, New Jersey: U.S. Geological Survey Professional Paper 180, 135 pp. [Description and chemical analyses of schallerite on p. 90]
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