MICROCLINE

KAlSi₃O₈
Triclinic

	Figure 19-4. Crystal drawings of typical microcline from the Trotter Mine at Franklin. Drawings are from Palache (1935) who provided crystallographic data.

Microcline, a potassium aluminum silicate mineral of the feldspar group, was first verified from Franklin by Kemp (1893a) and is surely the “green feldspar” reported by Nuttall (1822). Some material has been called amazonstone or amazonite. Microcline was reported by Palache (1935) as large crystals at Sterling Hill and also as a common constituent of the local pegmatites, in crystals to 30 cm. Peters et al. (1983) reported and illustrated microcline from the Buckwheat Dolomite. Frondel et al. (1966), in an extensive study of the Franklin feldspars, found 2/3 of the 59 studied specimens to be microcline or sodian anorthoclase, having BaO values from nil to approximately 3.0 wt. %. A survey by Langer (1994) found the trace-element composition ranges of three specimens of green microcline to be: Rb 202-327, Pb 938-2547, Ba 1699-6808, and Zn 507-830 ppm.

The colorless to white feldspar associated with the lead silicates is largely microcline (Dunn, 1985b); it is Pb-free at the microprobe level. A barian microcline from Sterling Hill was reported by Frondel et al. (1966). There are no full chemical analyses.     

Much of the common green feldspar found at Franklin, especially on the Trotter dump, is perthitic microcline. This green microcline, when in contact with calcite, occurs in fine euhedral crystals up to 30 cm, as figured by Palache (1935) (Figure 19-4); the writer has seen crystals up to 7 cm, but the preponderance of local microcline is massive.

	Figure 19-5. Massive microcline (light gray) with andradite (black) from Franklin. Specimen is 10 cm in maximum dimension. Smithsonian Institution, #147234. Photo by the author.

Franklin microcline also occurs in yellow-brown, olive-green, and pale white, as well as a very bright green color. Local microclines may fluoresce with a weak red or moderate blue color under shortwave ultraviolet.

Microcline is one of very few minerals associated with quartz, an uncommon mineral locally. It is also associated with rhodonite, allanite (Figure 16-2), andradite, calcite, franklinite, diopside, and rarely willemite or bustamite. In addition to occurrences in association with pegmatite, microcline is found in the calcium silicate units of the Franklin orebody (Figures 12-20, 12-21, 12-27, and 19-5). The pseudomorphs after possible microcline, described by Palache (1935), are discussed herein under pumpellyite. Microcline was reported from Sterling Hill by Jenkins (1994).

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
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This page created: January 11, 2001