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(Ca,Na)2(Mg,Fe)5Si8O22(OH)2
Monoclinic, C2/m, a = 9.851, b = 18.143, c
= 5.295 ล,
b = 104.47o, Z = 2
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| Figure 17-37. Superb crystals of actinolite from the Sterling Hill tennantite assemblage. Field of view is 0.2 mm in maximum dimension. Harvard University, #113070. | ||
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Actinolite, first reported by Nuttall (1822), is the most abundant amphibole in the Franklin orebody, occurring in the calcium-silicate units (Figure 12-24) and occasionally with highly calcic ores. The amianthus of Fowler (1825) and the asbestos of Koenig (1887a) were likely actinolite. The extant study on the Franklin actinolites is that of Klein and Ito (1968), from which many of these data are taken. A zincian actinolite of asbestiform habit was studied by Dorling and Zussman (1984); they found triple-chain fibrils to be present. Actinolite has been reported from Sterling Hill, but not in great quantities.
Franklin actinolite is light-brown, green, and dark green, with vitreous luster, and normal amphibole prismatic cleavages; the density is 3.2 g/cm3. Massive actinolite occurs in hand-sized masses; crystals, if freely-grown, are prismatic in habit. Optically, it is biaxial, negative, with a = 1.629, b = 1.642, and g = 1.652; pleochroism is moderate. There is no discernible response in ultraviolet.
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| Table 12. Chemical analyses of amphiboles. | ||
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Actinolite is a calcium magnesium iron silicate hydroxide mineral of the amphibole group. Actinolites from the Franklin orebody are invariably zincian and manganoan. However, the analyses of Klein and Ito (1968), together with many by the writer, strongly suggest that Fe, Mn, and Zn are limited in many Franklin actinolite samples and may be ordered. The compositions of many samples from diverse assemblages are largely invariant, with FeO, MnO, and ZnO, approximately 4, 5, and 9 weight percent, respectively. Representative analyses are given in Table 12. Germine and Puffer (1989) reported 3.8 wt. % Cr2O3 in one fibrous specimen from Franklin.
Because actinolite is found in many Franklin silicate-ore assemblages, only general comments are given here. Many specific actinolite assemblages were listed by Klein and Ito (1968). Actinolite apparently captures available Fe in such assemblages, and the Fe-bearing assemblages have very little or no willemite present.
The specimens with the Fe, Mn, and Zn values noted above are mostly coarse-grained and commonly associated with calcite, andradite, and rhodonite; numerous other minerals are locally associated. These specimens include some of the more esthetic, colorful, massive, and coarse-grained specimens of the calcium-silicate assemblages found in many systematic collections of Franklin minerals. Carbonates are common in these assemblages, and there may be shearing and fracturing of the actinolite.
Two analyses (Table 12; # C6219, #HU-92791a) are of dark green, high-Fe actinolites associated with nelenite. The assemblage is a rare breccia composed of Fe-rich nelenite, actinolite, and willemite, coated in part with a stilpnomelane-group mineral, and cemented with calcite; details and discussion were given by Dunn and Peacor (1984).
Actinolite is markedly less abundant at Sterling Hill, but it has been found locally, associated with manganaxinite.
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| Copyright © 1995 by Pete J. Dunn |
Website
by Herb Yeates
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This
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