Show Notes
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)
More from History of the Earth
The mineral here is just calcite (even though it’s mostly almost black), but it shows interesting features. Cone-in-cone structures are nested cones, seen here in cross section. The inset shows them a little better – in the main photo, they are represented by very narrow vertical triangles.
Mineral Monday + Tectonic Tuesday. Blue quartz is uncommon and is usually colored by inclusions of unusual minerals like crocidolite, tourmaline, or dumortierite. The purplish-blue quartz here, from north of Llano, Texas, is colored by inclusions of ilmenite (iron-titanium oxide). This rock is called llanite for its occurrence in the Llano Uplift of central Texas, and although similar rocks are found in other parts of the world, the variety name llanite really only applies to this location. On a sunny day, the blue quartz in the rocks has an opalescent sheen that sometimes seems to “wink” at you from the outcrop.
Epitaxy, from Greek words meaning “upon” or “above” and “ordered arrangement,” in minerals means crystals of one (or the same) mineral growing in a particular crystallographic position on another (or the same) mineral. It happens because the molecular spacings and orientations happen to be similar, allowing, even encouraging the crystal structure of the second, later mineral to mesh with that of the first. Some mineralogists might say that epitaxy requires the two minerals to be different minerals, but I do not – just two distinct generations of crystallization.
In the far northwest corner of the flat, flat Texas panhandle, extending into New Mexico, there’s a narrow, elongate magnetic low. The intensity of the anomaly – 250 nanoTesla or more – says it’s fundamentally the expression of a lithologic change rather than a structure; i.e. it represents something pretty strongly magnetic. Its long narrow geometry is that of a dike. And its negative value suggests that it’s reversely polarized, solidifying from magma during a time when the earth’s magnetic field was in the orientation opposite to that today.