PETROGENESIS
Crystalline magma
Solid solution forming
Behold, microlite!
Surprise, surprise. It is sometimes quite interesting to find a nifty mineral stuck on a shelf in the local rock/mineral store that has been displayed for a “long time.” This fall I was visiting such a store and asked about a specimen marked $2. The genial proprietor told me that the specimen had been part of a purchased estate and to please take it off his hands since it had not sold for months. First, in this tourist locality no one really knew what it was, and secondly most could not even spot the advertised mineral on the specimen. So, I shelled out two one-dollar bills for my very own specimen of microlite (well really Microlite Group) but more on that later. I really didn’t know zip about microlite except that it contained tantalum, and at times, a Rare Earth Element or two.
In getting the specimen home and trying to locate additional information I did find out that microlite is not a valid mineral name but is a generic moniker for the 13 members of the Microlite Group (as listed by MinDat). In turn, the Group is part of a larger classification of minerals named the Pyrochlore Supergroup. Members of both the Group and the Supergroup are difficult to distinguish between and correctly identify without the help of magic laboratory gizmos.
Microlite, a tantalum (along with some other cations) oxide, may contain trace amounts of niobium (a rare earth mineral, and perhaps even scandium and yttrium) since it is in solid solution with the niobium oxide pyrochlore. MinDat notes a very complex chemical formula with many substitutions for microlite and pyrochlore; however, the $2 Amelia Courthouse specimen in my collection seems to be fluorcalciomicrolite [(Ca,Na)2(TaNb)2O6F] although the literature seems replete with authors simply calling the mineral microlite (especially in older literature).
A single crystal of "microlite" embedded in albite var. cleavelandite. Width of crystal ~3.5 mm.
Pyrochlore also is no longer recognized as valid mineral name since specimens may now be identified (usually not visually) as belonging to one of the 21 members of the Pyrochlore Group, part of the Pyrochlore Supergroup. In generic terminology pyrochlore usually means the tantalum has been replaced by niobium but scientifically such minerals are fluorcalciopyrochlore [(Ca,Na)2(Nb)2O6F] and as stated, there is a solid solution relationship between the these two minerals. The most common occurrence of generic microlite and/or generic pyrochlore is as scattered grains in lithium-rich granites.
Microlite
received its original name from the size of most crystals---small, tiny. When one does spot a crystal of microlite, it
usually has a resinous luster, a dark yellow to yellow brown to reddish brown
to greenish brown color, and a yellow streak. It is subtranslucent and crystals
have an octahedral outline with a hardness of ~5.5. Crystals may be slightly radioactive. Mostly "microlite" forms rather nondescript,
small, yellow brown, resinous, slightly translucent crystals. As Simon &
Schuster’s Guide To Rocks And Minerals point out, microlite is “of interest to
mineralogists and collectors.”
As previously stated, I know very little about microlite (and now I am really confused); however, I know less about the Amelia Courthouse mines except they are situated in the Piedmont Physiographic Province of the Appalachian Mountains, are/were a “famous” collecting locality for pegmatite minerals, especially industrial mica, and amazonite. Yes, the same greenish-blue variety of microcline that is a collector’s favorite in the Pikes Peak region. The mines were also noted for producing a large number of rather uncommon minerals. Grier (1994) reported that the “Rutherford Mines Nos. 1, 2, and 3 were a series of world- famous excavations into pegmatite bodies in the vicinity of Amelia Courthouse, Amelia Co., VA. The earliest recorded mining was for muscovite in the No. I mine in 1873, but operation in prehistoric times has been indicated…From 1912 to 1932, 15 tons of gem amazonite were taken from the No. 2 mine by the American Gem and Pearl Company of New York. Commercial operations ceased in late 1959…
Outstanding
specimens of microlite occur at the Rutherford Mines, finds having been recorded
as early as 1881. Masses weighing at least 8 lbs. were observed in No. 1 mine,
and well-formed octahedra up to 7 cm across were found in No. 2 mine… Microlite
is ordinarily found in extremely small crystals, some of microscopic size,
hence, the name… The composition of Rutherford microlite approaches that of the
Nb end-member of the series, pyrochlore. All of the microlite specimens are
radioactive due to trace amounts of uranium. The crystals vary from opaque to
nearly transparent. They are brown, greenish-brown, brownish-yellow or
reddish-brown in color. The crystals occur both in the interstices of
cleavelandite and in the blocky albite.”
MinDat listed 54 valid minerals collected from the mines including Rare Earth Minerals, several containing tantalum and/or columbite, and others that are rare or uncommon (including some gemstones). John Sinkankas (1968) stated the mines were “classic mineral occurrences.” So, although my specimen is not much to look at, it adds something to my collection coming from a classic locality!
Don't be afraid to be confused. Try to remain permanently confused. George Saunders
References Cited
Grier,
D., Jr., !994, The Rutherford Mines: https://tgms.weebly.com/uploads/3/3/6/9/3369036/the_rutherford_mines.pdf
Sinkankas,
J., 1968, Geology and mineralogy of the Rutherford pegmatites, Amelia, Virginia:
American Mineralogist, vol. 53.