FRANKLINFURNACEITE

Ca₂Fe³⁺Mn²⁺₃Mn³⁺Zn₂Si₂O₁₀(OH)₈
Monoclinic, C2, a = 5.483, b = 9.39, c = 14.51 Å, b = 97.04^o, Z = 2

	Figure 18-32. Cluster of superb franklinfurnaceite crystals from Franklin. Field of view is 0.3 mm in maximum dimension.

Franklinfurnaceite was first described from Franklin by Dunn et al. (1987b); additional X-ray data were given by Bailey (1988a), and a review was given by Bailey (1988b).

Crystal structure

The crystal structure of franklinfurnaceite, described by Peacor et al. (1987b, 1988), is intermediate between those of chlorite and the brittle micas, having both interlayer cations and an octahedral interlayer. The structure is the first known example of: (a) a type II structure, (b) a tri-dioctahedral chlorite, and (c) a structure with calcium occupying octahedral sites between tetrahedral sheets and octahedral layers. Alternating zinc and silicon are ordered in the tetrahedral layers.

Description

Franklinfurnaceite occurs as platy 0.3-0.5 mm crystals, tabular on {001}, and composed of the pinacoid {001} modified by a serrated, unindexed, edge form (Figures 18-32 and 18-33). Additional morphological description was provided by Yeates (1989). Polycrystalline aggregates are up to 1.0 cm in size. The color is invariantly dark brown, nearly black; cleavage is perfect on {001}; the luster is vitreous; the density is 3.66 g/cm³; and the mineral is extremely brittle.

Optically, it is biaxial, negative, 2V = 79^o, with a = 1.792, b = 1.798, and g = 1.802. Pleochroism is intense with X = very dark brown, Y = brown, and Z = deep brown; absorption is X >> Z > Y; orientation is Z = b with both X and Y in the (010) plane. There is no discernible fluorescence in ultraviolet. Franklinfurnaceite can be locally confused only with bannisterite, ganophyllite, and yeatmanite, none of which occur in the host assemblage and all of which have diagnostic properties which aid in their identification.

Composition

Franklinfurnaceite is a calcium ferric-iron manganese zinc silicate hydroxide mineral. A microprobe analysis is given in Table 15; others were given by Dunn et al. (1987b). Solid solution of non-essential elements is limited to Mg and Al; both are minor substituents. In the original description zinc was assumed to be in tetrahedral coordination, and this was validated by the solution of the crystal structure (Peacor et al., 1987b, 1988).

Occurrence and paragenesis

Franklinfurnaceite is found in unaltered, vuggy intergrowths of hodgkinsonite, clinohedrite, willemite, franklinite, and hetaerolite from the Franklin Mine (Figures 18-32 and 18-33).

	Figure 18-33. Franklinfurnaceite crystals from Franklin in subparallel growth. Field of view is 0.5 mm in maximum dimension.

Barite and rhodonite are minor associated minerals. Although it was not noted at the time of the original description, Lawson Bauer’s catalogue indicates that one of the known franklinfurnaceite specimens was found in the 436 pillar, in the 3^rd slice below the 700 level, and was acquired by him from another person in 1949. A number of specimens have been found in such vuggy assemblages, and more specimens may be found by collectors and curators.

Name

Franklinfurnaceite is named for the old name (Franklin Furnace) of the community in which the Franklin Mine is located.   

 

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
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This page created: January 14, 2001