JERRYGIBBSITE Chapter15. Nesosilicates

JERRYGIBBSITE

Mn₉(SiO₄)₄(OH)₂
Orthorhombic, Pbmn or Pbn2₁, a = 4.85, b = 10.70, c = 28.17 Å,
Z = 4.

Jerrygibbsite was first described from Franklin by Dunn et al. (1984d). Subsequent local finds were mentioned by Dunn (1985a). Jerrygibbsite has not been found at Sterling Hill, but was found at the Kombat Mine in Namibia (Dunn et al., 1988). Yau and Peacor (1986) studied the relations between jerrygibbsite and leucophoenicite.

Crystal structure

The crystal structure of jerrygibbsite was described by Kato et al. (1989). They found it to be the equivalent of a unit-cell-twinned sonolite in which the cells are related by a b/4 glide plane; some details of their structure determination were questioned by Freed et al. (1993).

Description

Jerrygibbsite has been found only as massive material. It is slightly violet-pink with vitreous luster; cleavage is imperfect on {001}; and the density is 4.00 g/cm³. Optically, it is biaxial, negative, 2V = 72^o, with a = 1.772, b = 1.783, and g = 1.789. Pleochroism is absent; dispersion is moderate to strong, r > v; the orientation is Z= a, X= b, and Y= c. There is no discernible fluorescence in ultraviolet.

Composition

A microprobe analysis of jerrygibbsite is presented in Table 1. Manganese does not exceed 32 of the possible 36 octahedral cations (full cell contents with Z = 4), but this may be an artifact of the small number of analyses. The apparently low Mg content of both leucophoenicite and jerrygibbsite is especially noteworthy because Mg is very easily accommodated in the Mn-humites.

Occurrence and paragenesis

Jerrygibbsite occurs in simple assemblages consisting of only zincite, willemite, tephroite, franklinite, and/or leucophoenicite, and are notable for the lack of any Ca-bearing minerals. Of the five known jerrygibbsite samples, four are texturally distinct from each other, suggesting they occurred somewhat spatially distributed in the Franklin orebody. Zincite is present in all samples.

The equilibrium relations which favor the formation of leucophoenicite or jerrygibbsite, rather than the manganese humites, are worthy of additional study. Similarly, the effects of zinc on the phase relations for the Mn-humite group and leucophoenicite group remain unstudied, and very little is known of the partitioning of cations among co-existing Mn-humites.

Name

Jerrygibbsite was named for Professor Gerald (Jerry) V. Gibbs, of Virginia Polytechnic Institute and State University, in recognition of his contributions to the mineralogy of the humite group.

FOOTER LBI




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