BRUCITE

Mg(OH)₂ 
Hexagonal

Brucite was first reported from Franklin by Bauer and Berman (1930); they described it using a now defunct varietal name, manganbrucite, which was also utilized by Palache (1935). It has not been studied since. Brucite has also been found at Sterling Hill. The name brucite also was used for chondrodite prior to 1824; a history of this matter is given by Frondel (1972).

Description

Franklin brucite occurs as acicular to stout prismatic 1-2 mm crystals with a tapering trigonal pyramidal habit and triangular cross-section. Some crystals terminate in points, but most have a pearly-lustered, slightly curved termination formed of slightly offset pinacoidal faces. Such crystals form parallel and subparallel aggregates. Bauer and Berman (1930) reported the color as varying from white to deep brown. Brucite seen by the writer is of very high quality, semitransparent, and varies from colorless to very light pink and light violet, not unlike the color of very weakly colored barysilite. Highly manganoan material weathers to a black color. Some Sterling Hill brucite is intimately mixed with zincite, giving it a false orange color.

Cleavage is perfect on {0001} and easily produced; crystals are easily broken. The luster of cleavage surfaces is distinctly pearly. Optically, brucite is uniaxial, positive, with w = 1.59 and e = 1.60; the indices of refraction are quite variable, suggesting much solid solution, perhaps toward pyrochroite. There is no discernible fluorescence in ultraviolet. It is best identified using X-ray methods.

Composition

Brucite is a magnesium hydroxide mineral and the Mg-analogue of pyrochroite. Local material is the most manganoan known, with up to 17 wt. % MnO and 3 wt. % ZnO in solid solution for MgO in one analyzed Franklin specimen. The limits of solid solution between local brucite and pyrochroite remain uninvestigated. There are no modern analytical data.

Occurrence and paragenesis

Brucite was first reported from Franklin in a narrow veinlet cutting franklinite-willemite  ore. At Sterling Hill, brucite has been found as 1.0 cm platy crystals, intimately mixed with fine-grained orange zincite and associated with calcite, dypingite, and sussexite in the north orebody. This material weakly resembles kraisslite or a weathered mica. Brucite was reported with oxidized sulfides at Sterling Hill by Jenkins and Misiur (1994).

There is a sparseness of reported occurrences, but brucite is likely more common than is recognized, due to its colorless nature and small grain size.

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
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This page created: January 11, 2001