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Ca(Fe,Mg)Si2O6
Monoclinic
Hedenbergite is found at both Franklin and Sterling Hill, but is an uncommon mineral at both localities, being restricted to a few anomalous assemblages, in keeping with the dominant Mn/Mg local solid solution and the paucity of iron silicates.
Hedenbergite is massive; well-formed crystals are uncommon or unknown. In the dominant Franklin assemblage, noted below, hedenbergite is dark green, visually black, with imperfect cleavage and vitreous luster. Its optical properties have not been determined.
Hedenbergite is a calcium iron silicate mineral of the pyroxene group and the Fe-analogue of diopside. Hedenbergite from both Franklin and Sterling Hill is highly magnesian; most samples have intermediate compositions in solid solution to diopside. Representative analyses are given in Table 10, and others were given by Reilly (1983) and Johnson (1990). Solid solution toward johannsenite or petedunnite is quite limited locally. Hedenbergite associated with magnetite at Balls Hill in Franklin approximates to Ca(Fe0.58Mg0.36Mn0.06)Si2O6 in composition. That described in 4 analyses from Sterling Hill by Reilly (1983) approximates 85 mole % of the end-member.
The predominant and most abundant hedenbergite occurrence is at Franklin, where it occurs as dark green intergrowths with epidote; it is described under fluorapophyllite (Betancourt, 1989). This assemblage has been well preserved in systematic collections, in large part due to the presence of large euhedral fluorapophyllite crystals, together with euhedral crystals of epidote, zircon, ferroaxinite, pyrite, andradite, and natrolite. Another Franklin occurrence, albeit a rare one, consists of dark green prismatic crystals within Franklin ferristilpnomelane (Dunn et al., 1984b); these crystals approach 80 mole % of the end-member.
At Sterling Hill, hedenbergite occurs in several assemblages. It was found on the 1600 level, associated with feldspar, epidote, and quartz. It also was found on the hanging wall of the black-willemite zone, on the 430 level, associated with molybdenite, powellite, calcite, and feldspar. Johnson (1990) reported it from the 340 level. Hedenbergite was found in an assemblage of calcite, franklinite, ferroactinolite, and andradite from drill-hole #124, 119 feet from its origin on the 340 level, surrounded by andradite and altering to cummingtonite and ferroactinolite (Reilly, 1983). See also the comments of Jenkins (1994).
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| Copyright © 1995 by Pete J. Dunn |
Website
by Herb Yeates
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This
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