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Ca4Si2O7(F,OH)2
Monoclinic
Cuspidine was originally reported from Franklin by Palache (1910, 1935), as a result of his interpretation of an 1899 analysis by C. H. Warren of colorless crystals associated with nasonite. Samples of this material, unknown for most of the 20th century, were rediscovered by the writer, and a brief description was included in a paper on glaucochroite by Leavens et al. (1987). Cuspidine has not been found at Sterling Hill.
Cuspidine occurs as colorless, clear, 2-5 mm distorted crystals with no discernible forms. The density is between 2.965 and 2.989 (Palache, 1910). Franklin cuspidine is readily recognized by its moderately-strong, yellow fluorescence in longwave ultraviolet. Very small crystals may not be discernibly fluorescent. The fluorescence in short-wavelength ultraviolet is similar, but weaker.
Cuspidine is a calcium silicate fluoride mineral. Microprobe analyses of Franklin cuspidine yielded SiO2 32.3, 32.8; CaO 63.5, 61.1; F 9.8, 9.5; less O = F 4.1, 4.0; total 101.5, 99.4 weight percent and conform closely to the ideal composition. Substitution of (OH) for F is apparently minimal. Additionally, these analyses are in reasonably good agreement with that of Warren.
Franklin cuspidine occurs in an apparently uncommon assemblage of hardystonite, clinohedrite, nasonite, andradite, and euhedral crystals of glaucochroite. It is in apparent chemical equilibrium with glaucochroite and with hardystonite, which is altered in some specimens to clinohedrite. Cuspidine also occurs within assemblages of glaucochroite crystals as a late-stage phase, forming druses with clinohedrite in solution vugs in willemite. It is rare at Franklin.
In 1986, the writer examined a similar assemblage of glaucochroite and cuspidine with vesuvianite from the Kombat Mine in Namibia, which suggested that glaucochroite-cuspidine might be a stable assemblage where bulk rock compositions are appropriate.
The best samples are in the collections at Yale and Harvard Universities, the American Museum of Natural History, and the Smithsonian Institution.
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| Copyright © 1995 by Pete J. Dunn |
Website
by Herb Yeates
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