MIMETITE

Pb₅(AsO₄)₃Cl
Hexagonal

Mimetite was reported from Franklin by Rouse et al. (1984). It is also known from Sterling Hill, but is rare locally.

Description

Mimetite is yellow to whitish gray, with adamantine to greasy luster; cleavage is not evident, but is presumably present on {0001}. It occurs as subhedral crystals to several mm and as small masses to several cm. No other physical or optical data exist. Franklin mimetite resembles hedyphane, but the latter is much more abundant. Identification requires chemical analysis and X-ray methods.

Composition

Mimetite is a lead arsenate chloride mineral of the apatite group. Franklin material is calcian; Sterling Hill mimetite contains up to 93 mole % of the end-member. Representative analyses are given in Table 24.

Occurrence and paragenesis

Mimetite from Franklin is rare; the specimen cited in Table 24 occurs as yellow-orange, lamellar, 2-cm-thick aggregates in a sheared vein with willemite, amphibole, and calcite in franklinite/calcite  ore. It is not visually distinguishable from hedyphane.

Sterling Hill mimetite occurs as thin colorless crusts and yellow hemispherules, associated with spessartine, cerussite, and an amphibole similar to hastingsite, in a severely altered augite from the north wall of the Noble Mine at Sterling Hill. Few valid specimens are known. The “pyromorphite” reported from Sterling Hill by Frondel (1972) is this material. In general, yellow material with cerussite on galena is mimetite and not hedyphane.

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
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