BANNISTERITE

Ca_0.5(K,Na)_0.5(Mn,Fe,Mg)₁₀(Si,Al)₁₆O₃₈(OH)₈.nH₂O
Monoclinic, A2/a, a = 22.265, b = 16.368, c = 24.668 Å,
b = 94.285^o, Z = 8

	Figure 18-24. Platy, curved bannisterite (black) with impure calcite (white at right), sphalerite (light gray at left), and dull-lustered fine-grained, impure mixtures of minerals (dark gray) from Franklin. Specimen is 9 cm in maximum dimension. Smithsonian Institution, #102731. Photo by the author.

Bannisterite was originally described as an uncommon form of ganophyllite (Foshag, 1936) and was redefined as a unique species by Smith and Frondel (1968). A second occurrence, of highly ferroan material from Australia (Plimer, 1977), led to a re-examination of the species by Dunn et al. (1981b). Bannisterite is known from Franklin but not from Sterling Hill. Bannisterite is a layer silicate closely related to ganophyllite.

Crystal structure

The crystal structure of bannisterite was determined by Threadgold (1979) using Australian ferroan bannisterite. The structure of Franklin material was further described and refined by Heaney et al. (1990, 1992), who stated that “bannisterite has a modified 2:1 trioctahedral layer structure in which some of the tetrahedra are inverted toward the interlayer region and linked to inverted tetrahedra in the opposite layer. The octahedral sheet is strongly corrugated along b.” The tetrahedral sheet consists of 5-, 6-, and 7-fold rings; Al is concentrated into two of the four inverted tetrahedra; and the interlayer cations are highly disordered. A review is given by Guggenheim and Eggleton (1988).

Description

Bannisterite occurs as large massive hand- specimens, up to 250 cm³, composed almost entirely of dark brown platy crystals with perfect {001} and imperfect {010} cleavages and slightly offset or curved surfaces (Figure 18-24). Bannisterite is brittle and the density is 2.83 g/cm³. Optically, it is biaxial, negative, with small 2V; a = 1.544, b = 1.586, and g = 1.588; dispersion is weak, r < v; pleochroism is moderate with X nearly colorless, Y = Z = brown. There is no discernible fluorescence in ultraviolet. It is best distinguished from similar species on the basis of optical properties and/or X-ray powder diffraction data.

Composition

Bannisterite is a calcium potassium manganese aluminosilicate hydroxide hydrate mineral. Franklin samples are remarkably constant in composition, suggesting they might have come from one localized occurrence. Numerous analyses have been published (Dunn et al., 1981b), and a representative analysis is given in Table 14. Bannisterite from Franklin has Fe, Al, and Mg approximately 12, Mn approximately 48, Zn approximately 8, and both Ca and K nearly 4 atoms per unit-cell. Altered Franklin material is enriched in Mg and deficient in Ca and Mn.

Occurrence and paragenesis

Bannisterite was found on the picking table at the Franklin Mine, but nothing is known of its geologic occurrence. The major associated minerals are richterite, barite, quartz, and sphalerite. Several ill-defined manganese layer-silicates are present.

Name

Bannisterite was named in honor of Dr. Frederick A. Bannister of the British Museum.

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