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CaMnSiO4
Orthorhombic, Pbnm, a = 4.913, b = 11.151, c =
6.488 Å, Z = 4.
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| Figure 15-10. Crystal drawings of glaucochroite from Franklin. Drawings are from Palache (1935) who provided crystallographic data. | ||
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Glaucochroite was initially described from Franklin by Penfield and Warren (1899, 1900), and goniometric data were provided by Palache (1910, 1935). Crystals described by Gordon (1922a) were later shown to be tephroite (Gordon, 1923a). The crystal structure was published by O’Daniel and Tscheischwili (1944) using Franklin material, and unit-cell parameters were also given by O’Mara (1951). The crystal chemistry of glaucochroite was studied by Brown (1970), and a high-temperature structural study was published by Lager and Meagher (1978). Leavens et al. (1987) provided analytical and paragenetical data. Although locally abundant at Franklin, glaucochroite has not been found at Sterling Hill. Impure material has been called calcotephroite locally.
Glaucochroite was first noted in 1-3 mm euhedral prismatic crystals (Figure 15-10), which are similar to those of tephroite.
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| Figure 15-11. Prismatic crystals of glaucochroite in parallel growth, from Franklin. Field of view is 0.6 mm in maximum dimension. | ||
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Glaucochroite rarely forms subparallel, columnar, 1 cm clusters of such crystals (Figure 15-11); less than 20 specimens of such crystals were observed by the writer. Massive material was found in 1927 and in subsequent years; it constitutes the bulk of the known specimens and occurs in hand-sized specimens. Glaucochroite is bluish-gray to pink in euhedral crystals. The color of massive glaucochroite is very variable; the gemmiest material is distinctly blue to blue-gray, but brown material is very common. Much brown material has been mislabeled as tephroite, or overlooked due to its nondescript appearance. Some material has an “alexandrite effect.” Severely altered glaucochroite, known locally as calcotephroite, is very fine-grained, with dull luster; it is light-brown on freshly broken surfaces, and it darkens on exposure. Most glaucochroite has a vitreous luster and exhibits only traces of the poor cleavage. The density is 3.41 g/cm3. Optically, glaucochroite is biaxial, negative, 2V = 61o, with a = 1.686, b = 1.722, and g = 1.735 (calc.). The dispersion is marked, r > v (Penfield and Warren, 1899). There is no discernible fluorescence in ultraviolet.
Although brown glaucochroite resembles tephroite, it is easily distinguished by optical measurements. Additionally, the willemite exsolution common in much tephroite is absent in glaucochroite. Identification of altered material requires X-ray powder diffraction techniques.
Glaucochroite is a calcium manganese silicate mineral of the olivine group. The analytical data provided by Leavens et al. (1987) demonstrated that there is very little solid solution between glaucochroite and tephroite. Additionally, there is little substitution of Mg, Fe, or Zn for Mn. The low but nearly constant zinc content, however, should be considered in light of similar low but constant Zn concentrations in the closely related Mn-humites (Dunn, 1985a). Representative analyses of glaucochroite are presented in Table 1. The low Pb content reported by Penfield and Warren (1899) was likely due to nasonite impurities. Most glaucochroite conforms very closely to the theoretical composition of the end-member.
Glaucochroite occurs as both primary and recrystallized material. Glaucochroite was mapped in a number of places in the north end of the Franklin orebody, where it occurs as crystals and as coarse-grained and fine-grained massive material, as described below.
Glaucochroite was first found in a recrystallized assemblage at Franklin (Penfield and Warren, 1899) as euhedral crystals in nasonite with garnet and purported axinite, which was shown by Leavens et al. (1987) to be fine-grained andradite. The description by Palache (1935) of glaucochroite crystals in willemite led to the writer’s reinvestigation of the various assemblages of both crystals and massive material, as published by Leavens et al. (1987), from which this description is taken.
Glaucochroite crystals are found embedded in nasonite with associated franklinite and andradite. This assemblage is rare (Figure 15-11). Minor amounts of barite, clinohedrite, willemite, and a late-stage Mn-chlorite are present. Crystals of glaucochroite also occur embedded in willemite which encrusts common, granular, willemite/franklinite ore; cuspidine and clinohedrite occur as a younger druse of crystallites. This occurrence has the appearance of a vein assemblage, but proof is lacking.
Glaucochroite crystals also occur within massive clinohedrite and hardystonite, which both postdate andradite of several generations. Breccia-like textures are evident; andradite fragments are commonly cemented by the glaucochroite-bearing assemblage. Cuspidine is present in 2-3 mm crystals, enclosing glaucochroite euhedra, in apparent equilibrium with hardystonite and glaucochroite. In the apparent glaucochroite-in-clinohedrite association, clinohedrite undoubtedly is derived from hydrothermal alteration of hardystonite, which formed in equilibrium with glaucochroite. Hardystonite is known as a host for glaucochroite crystals, associated with nasonite and andradite.
Crystals of glaucochroite were also observed in vugs in massive glaucochroite, pinkish in color, and associated with 1.0 cm, white diopside crystals and stilpnomelane.
Massive glaucochroite occurs in many of the calcium-silicate assemblages, in both high-temperature primary assemblages and, less commonly, in veins; they are conveniently described in terms of coarse-grained and fine-grained assemblages.
a) Blue glaucochroite occurs with green willemite and franklinite; hardystonite, andradite, and calcite are common but not ubiquitous; leucophoenicite and zincite are sparse. The specimens are commonly sheared. This assemblage was very likely historically overcollected relative to others, because of its attractiveness.
b) Blue glaucochroite occurs with leucophoenicite, willemite, and franklinite; hardystonite and calcite are present but only in traces.
c) Blue glaucochroite occurs with andradite, willemite, and calcite in a coarsely crystallized intergrowth.
d) Blue glaucochroite occurs with 2 cm aggregates of andradite, bustamite, and willemite.
a) Brown glaucochroite occurs with esperite, hodgkinsonite, calcite, willemite, zincite, and franklinite in specimens wherein fine-scale shearing is quite common. In a few specimens, shards of blue glaucochroite occur within brown fine-grained glaucochroite. This may have been the assemblage which led to Palache’s (1935) report of coexisting glaucochroite and tephroite, a rare association not observed by the authors at the time of the publication of Leavens et al. (1987), but subsequently observed by this writer in an assemblage of bustamite+tephroite+ glaucochroite+hardystonite, with minor calcite.
b) Dense, fine-grained, dark-brown material, known locally as calcotephroite, occurs associated inconsistently with calcite, willemite, and, most commonly, franklinite. This material is chemically inhomogeneous on a small scale, as determined using microprobe techniques. It is found oxidized to a black surface coating which is unstudied.
Glaucochroite was derived from the Greek words for blue-green and color.
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| Copyright © 1995 by Pete J. Dunn |
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