WGNHS
Wisconsin Geological and Natural History Survey

Polylithionite

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Crude crystal of polylithonite. Size is about 4.5 cm across. From Ramshorn Creek, near Sheridan, Madison County, Montana. (Photo by W. Cordua.)

Formula: KLiFe2+Al(AlSi3O10)(F,OH)2 Monoclinic

Description:Polylithionite refers to a dark lithium-rich mica, often an unusual brown, found in some granitic pegmatites. “Zinnwaldite”  is a technically discredited name that appears in older literature and refers to this mineral.FLORENCE COUNTY: Koehler (1989) reports “zinnwaldite” as occurring in small euhedral books less than 4.5 mm across in a complex zoned pegmatite in Fern Township (NE NE NW sec. 22 T.39N. R.17E). This is likely more correctly called polylithionite. Associated minerals are amblygonite, spodumene, beryl and quartz.

MARATHON COUNTY: Weidman (1907) described a lithium-rich mica from a quartz-bearing pegmatite in the NW sec. 22 T.29N. R.6E., within the Stettin pluton. He called it a new species, naming it irvingite. Irvingite was later discredited as a species and the mineral described was probably lepidolite. New work suggests the mineral is actually “zinnwaldite,” now renamed polylithionite. (Falster et al., 2000). The mineral was found as grayish-white to bright yellow to pale pink micaceous grains up to 2.0 cm across. The mineral also displayed a parting causing it to break into laths or needles. It is associated with quartz, K feldspar, riebeckite, aegirine, rutile, fluorite, pyrochlore, and zircon.