Fuchsite
Sermons in Stones
And this our life, exempt from public haunt,
Finds tongues in trees, books in the running brooks,
Sermons in stones, and good in everything.
—William Shakespeare, As You Like It, Act II, Scene I
Today’s sermon comes from the stone at the top of this post and the tiny green crystals in it.
That 8-cm cobble of quartz was collected by geologist Theresa Schwartz and given to me by geologist Susan Vuke. It looks like simple quartz, with a distinct greenish tinge that could be many things. Under the microscope you can see many tiny transparent green crystals scattered through the quartz. They are fuchsite, not a defined mineral but a chromium-bearing variety of the mica muscovite, K(Al,Cr)3Si3O10(OH)2. Chromium gives the green color to the muscovite.
Theresa reports the rock was a clast in a pediment near Little Antelope Valley, northwest of Harrison, Montana USA.
A reasonable idea for the protolith (the original rock before metamorphism) 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 from the Stillwater Complex, the chromite- and platinum-rich ultramafic deposit on the north flank of the Beartooth Mountains, the most significant chromium-rich body in North America (and one of the most significant in the world). It’s about 2,700 million years old, but it was not uplifted and subject to erosion until about 55-70 million years ago.
Some fluvial system might have transported chromite-bearing sand that became this quartzite from the Stillwater Complex, but it is 200 km away, on the northeast side of the Beartooth Range, not draining toward the site of the deposit near Harrison, so it would have been a complex river system that carried it.
A source in the Tobacco Root Mountains is probably more likely. The pediment gravel itself was probably deposited fairly recently, the last 40 million years or so, a result of the uplift of the Tobacco Roots. There are at least two small chromite occurrences relatively nearby, 3 miles southwest of Silver Star in the Highland Mountains, and 5 miles southeast of Sheridan in the southwestern Tobacco Roots (Chadwick, R.H.W., 1941, Some Chromite Deposits in Madison County, Montana: Montana Tech thesis). They are in Archean (early Precambrian) rocks, comparable in age to the Stillwater Complex, but like the Beartooths the core of the Tobacco Roots was not uplifted and subject to erosion until around 55 million years ago. The quartzite cobble was probably eroded into one of the Cenozoic rock packages on the flank of the Tobacco Roots between about 40 and 15 million years ago, then eroded again more recently, perhaps by glacial outwash or more modern streams and debris flows, into the loose rock that comprises the pediment where Theresa found it.
Both of those small chromite deposits are on the western side of the Tobacco Roots opposite from the gravel near Harrison, so we need either a somewhat complex river system or unknown deposits on the east side of the Tobacco Roots (or perhaps most likely, from rocks now gone, eroded off the top of the Tobacco Roots) to get chromite to Harrison. There isn’t all that much fuchsite in the rock (and such rocks are unusual in southwestern Montana), so you wouldn’t need much chromium to produce it.
Fuchsite was named in 1842 by Karl F. Emil von Schafhäutl in honor of Johann Nepomuk von Fuchs (1774-1856), professor of chemistry and mineralogy at University of Landshut and curator of the mineralogy collection there (from MinDat).
So what does the presence of fuchsite in this quartzite tell us? It might help guide us to the original deposits where the chromite came from, possibly leading to an economic chromium ore body, if it hasn’t been eroded into nothing but sediment. Apart from recycling the US is 100% dependent on imports for chromium, mostly from South Africa and Kazakhstan, and chromium is a critical element in the manufacture of stainless steel, which is far more important than just your kitchen sink and utensils – medical, dental, and pharmaceutical equipment, airplane and automobile parts, the construction industry and more require stainless steel. Even with the bits of chromium in the Stillwater Complex, the US has far less than 1% of the identified chromium reserves in the world. You can’t mine what you don’t have.
If we can figure out the geometry of the river systems that eroded the fuchsite-bearing quartzite and transported the cobble to its final location, we might recognize some unknown fault systems, now inactive, that focused the old river channels. Identifying such ancient faults that might be rejuvenated someday can help us understand earthquake risk. Much of the relatively recent (the past 10 million years or so) geometry of the river systems of southwestern Montana was probably guided by faulting that was the consequence of uplift at Yellowstone (Thomas and Sears, 2018, Middle Miocene through Pliocene sedimentation and tectonics in Montana: A record of the outbreak and passage of the Yellowstone Hot Spot: MBMG Special Pub. 122). Before that, old mountain uplifts constrained the river systems and the depositional basins adjacent to them (Vuke, 2018, The Eocene through early Miocene sedimentary record in western Montana: MBMG Special Pub. 122).
The positions and geometry of those same old river systems can help us understand the locations and the nature (coarse gravel, sand, clay) of subsurface aquifers. As Montana attracts more and more people, the demand for water is increasing significantly, so studying the geology and hydrogeology of aquifers is vitally important. “Whiskey’s for drinking, water’s for fighting,” they say in the arid West.
Likewise understanding the old river systems can lead us to valuable sand and gravel deposits. Sand and gravel are mundane but together with crushed stone and cement comprise the top three most valuable non-fuel mineral commodities produced in the United States, so finding new, high-quality sources for sand and gravel is a viable economic goal.
Understanding the nature of the river systems that transported the quartzite cobble at the top of this post helps us understand past climate conditions: rainfall, stream gradients and topography, vegetation, erosion, and much more that may seem to be mostly academic, but that knowledge helps us understand the active climate changes going on today and might help us mitigate some climate disasters.
You can find much more about the evolution of the river systems in southwestern Montana that may have carried my cobble in this presentation by my friend Rob Thomas (1 hour 20 min):