Fuchsite

Fuchsite is a chromium-bearing variety of the mica muscovite, K(Al,Cr)₃Si₃O₁₀(OH)₂ . Chromium gives the green color to the muscovite.

This specimen was collected by Theresa Schwartz and given to me by Susan Vuke. Theresa reports it was a clast in a pediment near Little Antelope Valley, northwest of Harrison, Montana. That leaves the question open as to where it formed and came from, but a source in the Tobacco Root Mountains is most likely. The pediment gravel itself was probably deposited fairly recently, the last 50 million years or so, a result of the uplift of the Tobacco Roots. There are at least two chromite occurrences regionally nearby, 3 miles southwest of Silver Star (in the Highlands Mountains), and 5 miles southeast of Sheridan (southwestern Tobacco Roots) (Chadwick, 1941 Montana Tech thesis).

A reasonable idea for the protolith is a well-sorted, clean, quartz sandstone, with a very few resistant detrital chromite grains that during metamorphism had the chromium mobilized to find potassium and aluminum impurities from wherever you want to form the mica. The chromite could have come (with obvious challenges and difficulty) from the Stillwater Complex, the chromite- and platinum-rich ultramafic deposit on the north flank of the Beartooth Mountains, supposing some fluvial system to transport the sand that became this quartzite. That could have been a Precambrian river system, eroding either from what is now the Beartooths to the east or from the minor chromite occurrences to the west.

Fuchsite was named in 1842 by Karl F. Emil von Schafhäutl in honor of Johann Nepomuk von Fuchs [May 15, 1774 Mattenzell, near Bremberg, Lower Bavaria, Germany - March 5, 1856 Munich, Germany] professor of chemistry and mineralogy at University of Landshut and curator of the mineralogy collection. (from MinDat)