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NaCa2Al5Si5O20.6H2O
Orthorhombic
Thomsonite is known from Franklin and Sterling Hill, but is not found in the assemblages in which other local zeolites occur. It was first reported as calciothomsonite Gordon (1923b); this was shown to be thomsonite by Palache (1935). It was also verified from Franklin by Cook (1973). Much material labeled “thomsonite” from Franklin is in fact xonotlite, which occurs in the same assemblage, and is much more abundant. Thomsonite is a very rare mineral locally.
Thomsonite is white to colorless, occurring in clusters of prismatic crystals. The cleavage is perfect, and the luster is vitreous. Optically, it is biaxial, positive, 2V = 48o, with a = 1.530, b = 1.532, and g = 1.542. These indices of refraction are supported by additional studies of Franklin material by Gennaro (1929) and Hey (1932). Thomsonite has a weak white to light yellow fluorescence in both longwave and shortwave ultraviolet (Bostwick, 1982). X-ray diffraction and optical methods are best for distinguishing thomsonite from xonotlite.
Thomsonite is a sodium calcium aluminum silicate hydrate mineral of the zeolite group. There are no modern analyses; that given by Gordon (cited in Palache, 1935) is the only one known.
Thomsonite has been verified by the writer on one Franklin specimen, on which it occurs as a 2 cm bundle of subparallel, colorless to white, acicular crystals. This is the same specimen verified by Cook (1973) and comes from the part of the mine described by Hurlbut and Baum (1960). Thomsonite occurs in a vug surrounded by prehnite, which is enclosed in a mixture of ganophyllite, unanalyzed mica, and garnet. One very different specimen in the Harvard collection is labeled as being from Sterling Hill and consists of white, rock-locked, 1-cm sprays of crystals; its identity, but not its provenance, was verified by the writer.
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| Copyright © 1995 by Pete J. Dunn |
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by Herb Yeates
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