Well, it has finally arrived, or
at least I have arrived in Tucson to bum around at the Tucson Gem, Mineral and
Fossil Shows. At my best count there are
~60+ different venues/shows this year anchored by the Tucson Gem and Mineral
Show® (TGMS) scheduled for the Tucson Convention Center February 8-11. Most of the other venues/shows run during the
two weeks preceding the end of the TGMS—in other words about the first two
weeks of February, give or take a few days on either end. My goal this year is to visit 15 or so of
these ancillary shows. Some of these
other venues are huge; for example, the G&LW Tucson Show/Holidome has over
500 booths (as best as I count count) while other small shows have a half-dozen
booths. Some shows are in hotels and
utilize individual rooms while others, such as the large 22nd Street
Show, erect giant, and I mean really large, tents. Other more “mom and pop” shows have dealers
selling from trailers or individual sun/rain protectors or pickup beds open to
the elements. There are several venues
that specialize in specific areas---perhaps beads, fossils, high-end minerals;
however, most are a mixture of “everything” from clothes to rugs to dinosaur
mounts to all sort of minerals and rocks.
In addition, specific shows such as the large JOGS Tucson Gem and
Mineral Show are restricted to wholesale buyers and “buyer tags” are required
for entrance. All in all, the shows/venues represent an amazing experience.
One of the first shows I headed
for was the 22nd Street Mineral Fossil and Gem Show, a cornucopia of
pleasures. My major purchase was a
grouping of Brazilian tourmaline crystals.
No, the crystals were not gemmy but I liked the piece for display and it
was in my monetary range (cheap) of $5.
Tourmaline is now classified as
the Tourmaline Group with several end-member species created by common solid
solution series. Virtually every member
of the Group seems in solid solution with other members. Every few years a new classification pops
up. For example, on 1 February 2018, I
noticed an abstract by Bosi stated that his new study “has direct implications
for the tourmaline nomenclature, as well as on petrogenetic and provenance
information. Some assumptions behind the classification scheme of tourmaline
have been reformulated, revealing major agreement and significant improvements
compared to earlier proposed scheme.”
So, if you want the newest scoop locate his paper in the American Mineralogist. Otherwise I will offer some generalities.
Tourmalines (refers to the Group) are
boron silicate minerals with a large variety or combination of calcium,
potassium, sodium, aluminum, iron, lithium, vanadium, magnesium, manganese,
chromium cations (the positive charged elements) thrown in. For example, the well known variety elbaite is: Na(Al1.5Li1.5)Al6(Si6O18)(BO3)3(OH)3OH. The chemical
formula is really hard to understand but I will toss out the general formula
from www.mindat.org:
A(D3)G6(T6O18)(BO3)3X3Z
A = Ca, Na,
K, or is vacant (large cations);
D = Al, Fe2+, Fe3+, Li, Mg2+, Mn2+ (intermediate
to small cations - in valence balancing combinations when the A site is
vacant);
G = Al, Cr3+, Fe3+, V3+ (small cations);
T = Si (and sometimes minor Al, B3+);
X = O and/or OH;
Z = F, O and/or OH.
As you can guess there is much
cation substitution, so the group has many different varieties; MinDat lists 39
different “kinds” of tourmaline. A
couple of members that many of us recognize would include the common black schorl (iron rich form and the most common variety in rocks)
and the pink elbaite. Bosi (2018) noted that “tourmalines form the most
important boron rock-forming minerals on Earth.”
 |
| Pink elbaite crystals in a off white matrix that is some sort of a feldspar mineral (maybe microcline, maybe not). Width of photo ~8 cm. |
Elbaite, a sodium lithium aluminum
boron silicate (in the formula above sodium would occupy Position A of large
cations while lithium and aluminum occupy Position D of smaller cations). It is
probably the most colorful of any of the Group members and in some case even
bi-or tri-colored (for example the popular watermelon tourmaline with zoned crystals displaying a green outer layer over a pink center). The best known colored elbaite is probably
the pink variety (sometimes termed rubellite), and that is what I purchased. Elbaite belongs to the
Hexagonal Mineral System and often is found as elongated prismatic, striated
crystals without cleavage. They also have a very distinguishing
rounded triangular cross section. Gemmy varieties
are vitreous and transparent while less gemmy crystals are more
translucent. Tourmaline is about the
hardness of quartz at 7.0-7.5 (Mohs) and gemmy crystals of many colors are in
high demand for faceting, carving and cabochons.
The seller stated that the specimen
was from one of the mines in the State of Minas Gerais, Brazil-----an area of
hundreds or thousands of mines. MinDat
states that “probably the best known of these are the pegmatite minerals like
tourmaline, beryl, quartz etc. that come from the pegmatites in the north west
granitic region of the state and whose center, business wise if not
geologically is the town of Governador Valadares.” So, I will settle for that general area!
The pink color of elbaite is
probably due to the high lithium content; however, this variety also occurs in about every color of the rainbow and I am uncertain about other color chromophores.
In fact, I have heard of elbaite referred to as lithium tourmaline. I also understand that the lithium (and the
boron) is very important in lowering the magma solidification temperature.
So, the Tourmaline Group is a very
complex assemblage of minerals and understanding much of the mineral chemistry
is above my pay grade. However, the
various tourmalines often make spectacular gemstones and display
specimens. I was impressed in observing
a very large “pile” of these masses of crystals at the venue.
REFERENCES CITED
Bosi,
F.,2018, Tourmaline crystal chemistry: American Mineralogist, v. 103.
