BOSTWICKITE

CaMn³⁺₆Si₃O₁₆.7H₂O
Orthorhombic?            

	Figure 19-8. Thin bladed crystals of bostwickite from Franklin. Field of view is approximately 0.3 mm in maximum dimension.

Bostwickite was known to mineralogists for over a century, but was undescribed, in part due to the soft and friable nature of the aggregates and the paucity of clean material. It was considered to be arseniosiderite by Palache (1935) and was shown to be a manganese silicate by Frondel (1972) and Dunn (1979c). It was described and named bostwickite by Dunn and Leavens (1983). It is known only from Franklin. The unit-cell and crystal structure are unknown. 

Description

	Figure 19-9. Acicular bladed crystals of bostwickite with minor rhombic calcite; these crystals are especially small compared with the common crystals. Field of view is 0.5 mm in maximum dimension.

Bostwickite occurs as hemispherules and radial aggregates up to 3 mm in diameter. It is brownish-red; weathered surfaces are darker brownish-red. It has vitreous luster and is very soft. The crystals are bladed, composite units, made up of many crystallites. Representative photographs are shown in figures 19-8 and 19-9. The density is 2.93 g/cm³ or greater.

Optically, bostwickite is biaxial, negative, with 2V = 25^o, a = 1.775, b = 1.798, and g = 1.800; dispersion is strong, r < v; the absorption is X = Y > Z; pleochroism is strong with X = Y = reddish brown and Z = yellowish brown. There is no discernible fluorescence in ultraviolet. Bostwickite is best distinguished using its unique X-ray diffraction data.

Composition

Bostwickite is a calcium manganese silicate hydrate mineral. The oxidation state of Mn is inferred to be trivalent, in part because of the color and in part based on Gladstone-Dale calculations. The chemical analysis yielded MgO 0.9, CaO 5.1, Mn₂O₃ 56.3, Fe₂O₃ 0.5, Al₂O₃ 1.0, As₂O₅ 1.0, SiO₂ 20.1, H₂O (by difference) 15.1, total = 100.0 wt. percent.

Occurrence and paragenesis

Bostwickite is known from but one occurrence from which a number of specimens were saved; most of these are in the collection at Harvard University. It was found in the Taylor Mine (later part of the Franklin Mine) in l874. Bostwickite occurs as flattened hemispherules on medium-grade ore consisting of franklinite, calcite, and fluorite, with minor willemite. Bostwickite occurs directly on the ore surface and on secondary carbonates which coat the ore in part. The “back” side of the sample contains the type material for kittatinnyite, an even rarer mineral.

Name

Bostwickite is named in honor of Richard C. Bostwick, a sophisticated collector of Franklin and Sterling Hill minerals. He has contributed much to our knowledge of their fluorescence and has done much to spread the excitement of minerals from Franklin and Sterling Hill far and wide. 

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