ISINGLASS AND MUSCOVITE: BLACK HILLS

What is it?  Width ~7 cm.

As a kid growing up in Kansas I was always interested in the sciences and especially in the “natural world.”   I always wanted to become a Forest Ranger, mostly due to my love of the “outdoors” and the correspondence school advertisements in publications such as Field and Stream.  I wanted to go out and measure giant trees and trap wild beaver and wear those knee-high, lace-up boots.  In the right jobs you could also wear the flat-banded Smoky the Bear Hats.  I also unmercifully quizzed my father about hills and streams and trees and mountain lions.  He answered to the best of his ability but certainly had no formal training in the sciences.  At any rate, he was fond of calling one local spot “Isinglass Hill” due to shiny clear crystals in a black shale reflecting sunlight.  Furthermore, I learned that isinglass (Muscovy Glass in Russia) was used as windows in the older model natural gas heating stoves.  I was duly impressed and asked to stop and collect, which we did.  The crystals were small and did not seem large enough for stove windows.  But, oh well I was certain that bigger crystals were found in other hills. 

OK, on a field trip to Isinglass Hill several years later in an introductory geology class, I learned that my father was only “half-right.” Isinglass is used as a stove window; however, the crystals in the hill (Cretaceous: Graneros Formation) were selenite gypsum!  But, hold on.  As they say in the movies, "maybe the old man was right", or at least he was not dreaming up something!  Modern maps do not show an Isinglass Hill in Ellsworth County; however, a 1918 Atlas has a location for Isinglass Hill Farm in Carneiro Township along the Union Pacific Railroad.  That is good enough for me.  Trust your father! Isinglass is a mica-mineral, usually muscovite [KAl2(AlSi3O10)(OH)], and at one time was mined at many places in the United States including North Carolina and the pegmatites of the Black Hills of South Dakota.  I find the latter pegmatites fascinating and often wish that I was more of a mineralogist/petrologist.  Norton and Redden (1990) believed there were somewhere near 20,000 pegmatite bodies located within the Precambrian Harney Peak Granite (the Granite is home to Mt. Rushmore).  And, several of these bodies were mined early in the Hills’ mining history---for stove windows.  As I understand, the McMackin Mine started producing sheet mica in 1879.  To put that date in perspective, it was only five years before (1874) that Lt. Colonel George Custer “discovered” gold in the Blacks Hills and three years (1876) since he lost his life at the Battle of the Little Bighorn. Mica is not really a mineral but the name of a group of sheet-like phyllosilicate minerals (www.Mindat.com lists 45 or so), the most common being muscovite (potassium-rich), biotite (iron-rich), phlogopite (magnesium-rich), and lepidolite (lithium-rich).  Muscovite in the Black Hills has been mined as scrap mica, small pieces that were later shred and ground, and used in paints, roofing, concrete and mortar, caulking, fireproofing, lubricants, and host of other applications. At one time the Hills produced something like one-third of the nation’s mica.  As far as I know, Pacer Minerals in the small Hills town of Custer is the only producer left in the area. Sheet mica consists of large sheets of muscovite and was used in insulating radios, fuse boxes, lamp sockets, and other types of electronics (and stove windows).  Westinghouse Electric arrived early in the history of mica mining and operated four or five mines near Custer.  It is my understanding (from visiting with local collectors) that thousands of pounds of mica were produced as part of the “war effort” (World War II) but that need almost evaporated after the war.  Roberts and Rapp (1965) reported that a book of mica at the White Spar Mica Mine measured three feet wide by four feet long.  I have not collected anything that large but certainly have noted isolated sheets five inches by five inches (no visible crystal faces).  Since muscovite has perfect basal cleavage, a knife may be used to cleave quite thin sheets. One of the interesting byproduct of the early electronics industry around Custer is the presence of muscovite sheets that have been “punched out.”  That is, some sort of a machine, presumably hand-operated, punched out specific shapes of muscovite that were then used as insulators (again I presume).  Local collectors were uncertain about the exact age of the specimens but “thought” World War II.  These pieces are sometimes available on local hillsides or near the factory (that was pointed out to me by a collector).  I have four of these punched out sheet mica and they add a distinct flavor to my collection.    Width ~7 cm. Width ~7 cm. By-the-way, later in life during the summers of 1971, 1972, 1973, I served as a seasonal Park Ranger Naturalist at Dinosaur National Monument.  Thus, I achieved one of my boyhood dreams of wearing a Smokey the Bear hat.  I still have that hat as it brings back many pleasant memories. REFERENCES CITED Norton, J.J. and J.A. Redden, 1990, Relations of Zoned Pegmatites, Granite, and Metamorphic Rocks in the Southern Black Hills: American Mineralogist, v. 75. Roberts, W.L. and G. Rapp Jr., 1965, Mineralogy of the Black Hills:  South Dakota School of Mines Bulletin 18.