Well, it appears that I have been off the Blogosphere for several weeks and need to get my writing in gear. Not really nifty excuses but: 1) the house/home needed some repairs in order to prepare for cold weather and winter; 2) do you know how many colorful maple leaves are on three mature trees (and they all need to pitter-patter down to the lawn); 3) receiving a letter from the state department of revenue with their interpretation (not mine) of my 2021 income tax; 4) construction of three power point programs (each an hour long) for meeting/conference presentations (done) is time consuming. Fun, but a real suck of time.
So,
I just finished reading a Blog Posting by Hollis over at In the Company of
Plants and Rocks and it was a barn burner about the geology of the House
Range in the West Desert of Utah. I
could tell that the author put in tens of hours of observing, reading, and
writing to construct a fine piece of work. But I am going to start with
something very simple. Two interesting photos of muscovite, one from Brazil and
the second from South Dakota!
Muscovite [KAl2(AlSi3O10)(OH)2] is one of those minerals that was easy to identify in my beginning geology course. It was the soft (~2.5 Mohs), “light colored” (pale yellow to pale light brown to pale green to silver white) mineral that could be split into very thin, transparent, elastic sheets; it had perfect basal cleavage. A similar dark colored (iron rich) mineral was always labeled biotite. These two minerals, commonly referred to as mica, were essentially freebies on the lab tests. However, at times the class examined very fine-grained river sand, or perhaps a piece of sandstone or gneiss and it became tougher to identify the shiny, shimmering flakes as muscovite.
A specimen almost completely composed of muscovite books, quarter for scale (lower). Collected a few miles west of Custer at an abandoned quarry.
In
areas of igneous and metamorphic outcrops these weathered and eroded flakes in
river sands shimmer with a slow current and clear water and are a major cause
of “gold fever.” Tourists and other
visitors to Colorado see these shimmering flakes when the kids bound out of the
car and head to the stream and believe they have hit the jackpot. Alas, no
shiny gold but just tiny flakes of muscovite.
Muscovite
is easily identified “in the field” by its formation of massive crystalline
books found in many types of igneous and metamorphic rocks but especially
prevalent in granite and granitic pegmatites.
The larger the mineral grains, as in pegmatites, the larger the
muscovite books. Roberts and Rapp (1965) described many large muscovite
crystals from the Black Hills of South Dakota, “one of the most productive [mica]
districts in the United States”: 1) books of muscovite as much as three feet
across from the Crown Mica Mine; 2) a book of muscovite three feet wide, four
feet long and eight inches in thickness from the White Spar Mica Mine; and 3) a
crystal of mica weighing 36 pounds from the Lake Mica Prospect. MinDat listed 37 different varieties of muscovite,
most were due to different cations appearing in the crystal structure. Some of
these cations impart “colors” to muscovite such as in fuchsite, a greenish
chromium-bearing variety.
Muscovite
is also a member of the Mica Group, a “term for the sheet silicates that can be
parted into flexible or brittle sheets”---as defined by MinDat. The data base
also listed 70 members of the Group although not all are “officially” valid
minerals. For example, zinnwaldite, a
somewhat common “mineral” found in rocks associated with the Pikes Peak Batholith
is now discredited and placed as a dark mica containing lithium found in the
siderophyllite-polylithionite series (see Blog posting November 24, 2013).
Lepidolite, a pink to light purple Mica Group member has been discredited and is now assigned to the
polylithionite-trilithionite series (see MinDat). Rock/mineral shops, and
purveyors of rock/mineral trinkets, found across the U.S. have lepidolite “for
sale.” In fact, “lepidolite” collected
from the Black Hills was the first mineral specimen I purchased as a kid.
Rose or pink or lithium muscovite collected from the Harding Mine, Taos County, New mexico. As Rob Lavinsky noted on Mindat, "on close inspection, you can see these muscovite crystals are translucent to transparent. They are actually masses of sheety crystals stacked densely together." The color is probably not due to lithium but to manganese and iron. Width of crystal mass ~1.9 cm.
OK,
back to my simple photos of muscovite.
Most “books” of muscovite that are commonly found by rockhounds have a
hexagonal shape in a cross-sectional view. That, along with its tabular form,
elasticity, and transparent cleavage sheets, is usually enough to identify the
mineral. However, one of the more
collectable micas in the Black Hills comes from the Diamond Mica Mine in the
Keystone Mining District. Here the single crystals are, you guessed it, diamond
shaped.
Muscovite crystals collected from the Diamond Mica Mine, Width of each crystal ~3.2 cm.
Muscovite crystal collected from the Diamond Mica Mine (Courtesy of Dakota Matrix). Note sharp crystal face boundaries.
Galleries.com noted that muscovite tabular crystals have a prominent pinacoid termination, that is the crystal has a pair of opposite parallel faces. The diamond shaped books have four dominant crystal faces with two pairs of opposite parallel faces. If another pair of parallel crystal faces form, then the books have a hexagonal shape. However, the diamond shaped muscovite from the Diamond Mine varies in shape and pinacoid terminations. Notice in the lower figure above, lifted from DakotaMatrix.com, the sharp corners of the diamond. Compare that with my specimens from the mine where an additional two parallel faces occur on the diamond. Although the specimens are coarsely diamond shaped, the crystal has six faces. As I understand the situation (a stretch), muscovite is Monoclinic; however, the symmetry is not easily observed. The diamonds simulate Orthorhombic symmetry with prism faces meeting at ~ 60 degrees, The six-sided forms have prism angles of ~120 degrees and appear to have Hexagonal symmetry. I have never pretended to be a crystallographer and my low pay grade does not allow me to speculate on the mechanics of those designs! Crystal Systems and Crystal Classes are one of the reasons I became a paleontologist!
So,
just when I think that muscovite is “easy” to identify I notice that the
Handbook of Mineralogy throws the proverbially wrench into the equation: muscovite is also described as “stellate
aggregates, plumose, globular, scaly, granular, compact and massive.” One needs
to consult MinDat to locate photos of these different habits.
Muscovite with twined crystals forming yellow five-pointed (or less) stars. Most star specimens come from pegmatites in the Jenipapo District, Minas Gerais, Brazil. FOV width ~4.0 cm.
Of
these forms, the stellate aggregates, or star muscovite, are perhaps the most
collectable, appealing, and attractive of the many habits of muscovite. Star
muscovite involves twinning of the crystals in a manner that remains confusing
to me. I spent several hours reading the
literature and my noggin still does not comprehend: Twins in micas are difficultly identified
due to mica’s hexagonal pseudosymmetry. In this paper, we present an electron
backscattered diffraction analysis to identify twins in the muscovite… A
trilling twin with twin law <310>/{110} is common in the muscovite. A
six-couplet twin consisting of two trilling twins related by twin laws
<110>/{130} and <001>/{001}(or <100>/{100}) has been
discovered (Zhao and others, 2019). Remember I am a softrocker!
Regardless
of my snarky comments I learned much from this little exercise, not the least
of which is that a tremendous amount of literature is available for such a
common, rather nondescript, easy-to-identify mineral. I certainly did not realize that muscovite is
such a complex mineral. As such I fell into the habit of pursuing the
literature over a ten-day period and therefore did not reach my goal of just
getting something into the Blogosphere. Oh well, life is interesting.
One
item I did not miss out on during this final exam study (at least it felt like
such) was observing the full moon, known as the Beaver Moon, on the evening of
November 7. Officially the full moon, the peak illumination, was very early on
the morning of November 8. Unlike many
moon watchers I was sound asleep at 4:00 am and as such missed the lunar
eclipse and the Blood Moon phase. This reddish hue happens when the moon is
completely in the earth’s shadow and the rays from the sun are blocked. Over the course of several hours the full
moon goes from a bright white ball to the copper hued object. I probably should have got my derriere out of
the sack and observed the event as the next full lunar eclipse is something
like three years distant. However, I caught the eclipse several times on the
magic of the Internet and did watch the previous lunar eclipse in May 2022 (the
Super Blood Moon as pushed by the media) although it bounced in and out of the
clouds in later stages. At any rate, lunar eclipses are great astronomical
events.
As
I daydream through my life and think of these astronomical events, I remain
fascinated by members of the ancient Mesoamerican civilization of Central America.
They built their pyramids, ball courts, palaces, temples, and other structures
in accordance with astronomical events and positions of the sun, moon, planets,
and stars over the course of a year. Of course, as a person of Scandinavian
descent I am partial to the views of the Norsemen who believed that a large
wolf, Fenrir, was constantly chasing the sun and at times got lucky and caught
the orb and the lights go out--a lunar eclipse.
REFERENCES CITED
Roberts,
W.L. and G. Rapp Jr., 1965, Mineralogy of the Black Hills: South Dakota School
of Mines and Technology, Bull. No. 18.
Zhao,
Shan-Rong, Chang XU and Chuan Li, 2019, Identification of twins in muscovite”
an electron backscattered diffraction study: Zeitschrift fur
Kristallographie—Crystalline Materials 234(5). DOI:
10.1515/ZKRI-2018-2139
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