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CaMnSi2O6
Monoclinic
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| Figure 17-5. Fibrous johannsenite in epitactic overgrowth on rhodonite from Franklin. Field of view is 2.6 mm in maximum dimension. | ||
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Johannsenite was first reported from Franklin and from numerous other localities by Schaller (1933b, 1938). Additional data were provided by Frondel (1965), but local material has been little studied since. It has not been reported from Sterling Hill.
Franklin johannsenite varies in color and habit. The type material, occurring as exsolved lath-like crystals, is dark red; that occurring in radial sprays of tightly bound crystals associated with nasonite is blue to green to light brown; and that occurring as fibrous, epitactic overgrowths on rhodonite (Figure 17-5) is medium brown. The luster is vitreous; the cleavage is good; and the density is 3.52 g/cm3. Optically, the material associated with nasonite is biaxial, positive, with a = 1.699, b = 1.712, and g = 1.734. There is no discernible fluorescence in ultraviolet.
Johannsenite is a calcium manganese silicate mineral of the pyroxene group and the Mn-analogue of diopside. Representative analyses are given in Table 10. Solid solution between johannsenite and diopside, hedenbergite, and petedunnite is limited; although Deer et al. (1978) have reported Fe/Mn as the usual solid solution at other localities, this might be precluded at Franklin by the low Fe content of silicates in general. An exception to this observation of limited solid solution is represented by specimen #144636, from the same assemblage as the type material. In this anomalous assemblage, solid solution is extensive and the sodium content is significant.
Johannsenite occurs in at least three distinct assemblages at Franklin. Given its quite varied physical appearance, others may exist unrecognized inasmuch as johannsenite may mimic other species.
The type assemblage is of dark red johannsenite exsolved in pinkish orange, massive bustamite, together with exsolved willemite (Schaller, 1938). The assemblage is worthy of additional study. The material was locally abundant, and much has been preserved (#144636).
The assemblage described by Frondel (1965) from the 910 pillar on the 900 level is likely the same as that which provided the lead silicates (Dunn, 1985b). This assemblage consists of radial sprays of johannsenite with abundant manganaxinite, nasonite, willemite, unanalyzed mica, and lesser quantities of clinohedrite. The material may have been locally abundant.
One of the less abundant assemblages for johannsenite is the occurrence of stiff, fibrous, light-brown, epitactic overgrowths on rhodonite (Figure 17-5) from Franklin. This is a vein assemblage, formed on franklinite/willemite ore, with manganaxinite and epidote occurring in superb 1-2 mm crystals, partly overgrown by rhodonite, the principal species in the vein, and barite. Euhedral recrystallized willemite occurs within some of the rhodonite. Johannsenite occurs as a surface alteration of rhodonite.
Johannsenite was named in honor of Dr. Albert Johannsen, a noted petrologist.
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| Copyright © 1995 by Pete J. Dunn |
Website
by Herb Yeates
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