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Mg3Si4O10(OH)2
Monoclinic
Talc is known from Franklin, Sterling Hill, and the Franklin Marble, but is largely unstudied. The materials earlier described as pyrallolite (Robinson, 1825; Alger, 1845) and steatite (Fowler, 1825) are largely talc.
Local specimens are quite ordinary in appearance, varying from white to gray to light green and other faint colors. The luster is commonly greasy and pearly, but sometimes dull in fine-grained aggregates. The extremely low hardness is a useful though not diagnostic indicator. Talc occurs as massive aggregates, rosettes, and coatings and as pseudomorphs. Few data have been obtained using local material. Franklin talc is fluorescent in ultraviolet, with a light yellowish-white color, more intense in longwave than shortwave. This fluorescent response is best observed on fresh surfaces; it is diminished on surfaces which have been extensively handled.
Talc is a magnesium silicate hydroxide mineral. Local material has not been the subject of many analyses; one specimen (HU- #113992) contains FeO 1.8, MnO 1.5, and ZnO 6.0 wt. %.
At Franklin, talc occurs as massive material, commonly associated with calcite and replaces crystals of many species such as spinel, willemite, hemimorphite, and pyroxene. It was reported in massive form in the Buckwheat Dolomite (Palache, 1935). The analytical data given above are for talc which occurs interstitially with nelenite (Dunn and Peacor, 1984). Mamillary masses are known, up to several cm, and may represent replacements of willemite. In the Franklin Marble, talc is host to fine 1-2 cm crystals of quartz. Additionally, it occurs sporadically with calcium silicates in the Franklin Marble. Germine (1987) noted the replacement of Franklin talc by sepiolite.
Talc is found associated with sphalerite, franklinite, and willemite in the north orebody at Sterling Hill. The occurrence has not been studied in detail, but the mineral was common, as was serpentine. Talc replaces red willemite locally at Sterling Hill and might be the unknown mineral of Squiller (1976). It also forms, with serpentine, a thin but visually indiscernible rim on some tephroite crystals, not to be confused with dark brown, very obvious, sonolite rims.
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| Copyright © 1995 by Pete J. Dunn |
Website
by Herb Yeates
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