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| 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.
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.
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