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Rare and unusual manganese-arsenate minerals occur in secondary veinlets which commonly transect the bedding of the ores at Sterling Hill. They may have been as common at Franklin, but possibly were unrecognized due to paucity of readily accessible microscopes; this is conjecture. Some of these rare minerals are known only from here and from Långban and/or Pajsberg, in Sweden. They are listed and discussed in greater detail under the introduction to arsenates.
There are nine lead silicate minerals found at Franklin. No species is a simple lead silicate; most contain Ca, Mn, or Zn. Unlike esperite, which was widespread in the northern end of the Franklin orebody, and larsenite, associated with it locally, the occurrences of the other lead-silicate species (margarosanite, barysilite, kentrolite, nasonite, hancockite, roeblingite, and ganomalite) were, in general, limited to one part of the deposit near the Parker Shaft, as described above in the discussion of the “Parker-Shaft minerals.”
These lead silicate minerals are associated with much andradite, manganaxinite, and microcline in the restricted assemblage described by Dunn (1985b) and Yeates (1991). They occurred within calcium silicate units and not, generally, within the ore units. The study of Dunn (1985b) is inadequate; much remains to be done.
Found only once, in 1890, described by Koenig (1890), and subsequently studied by Holmes (1945, 1946) and Oen et al. (1984), the nickel arsenide assemblage from the Trotter Shaft at Franklin remains wholly enigmatic. Only this one occurrence was found, underlying a stratum of yellow garnet; no relations to the ore were given. Nickel plays no known geochemical role at Franklin or Sterling Hill; indeed, no other nickel minerals are known from here. The cause of this isolated, rich, anomalous occurrence is unknown; some have assigned its genesis to the massive pegmatite at the Trotter Mine. Although wholly without proof, this argument is tentatively accepted by the writer; no other explanation has been offered. The assemblage, consisting of nickeline, rammelsbergite, pararammelsbergite, gersdorffite, skutterudite, breithauptite, arsenopyrite, and safflorite, with secondary minerals, is described in more detail under nickeline. These arsenides form dendrites in a calcite-barite-sphalerite-stilpnomelane gangue.
There are six manganese arsenosilicates known from Franklin and Sterling Hill; excepting schallerite, they have not been found elsewhere. Kolicite and holdenite both contain pentavalent arsenic and are known from both deposits. Schallerite and nelenite both contain trivalent arsenic, are related to the friedelite group, and occur at Franklin, but not Sterling Hill. Kraisslite and mcgovernite contain both pentavalent and trivalent arsenic, are related to hematolite, and locally are known only from Sterling Hill. Additionally, although not manganese arsenosilicates per se, kittatinnyite and wallkilldellite should be mentioned here; they are the first silicate/arsenate analogues with layer structures, and are found at Franklin and Sterling Hill, respectively, and exclusively.
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| Copyright © 1995 by Pete J. Dunn |
Website
by Herb Yeates
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This
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