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| BEAUTIFUL GREEN AND GEMMY SPHENOID CRYSTALS, SOME TWINNED, OF TITANITE WITH GEMMY PORCELAIN-WHITE ORTHOCLASE VAR. ANDULARIA. WIDTH ~3.8 CM. COLLECTED TORMIQ VALLEY, GILGIT-BALISTAN, PAKISTAN. |
I grew up in geology classes with the name
"sphene" firmly entrenched in my repertoire of mineral names (CaTiSiO5). I really didn't know too much about the
mineral except that study collection housed a really nice greenish
crystal. During those halcyon days of my
youth I believed that all crystals in the field certainly would be almost identical
to those specimens in the study collection!
Wow, that thought was one of those "non-truisms they teach you in school" moments.
Later on in sedimentary geology class I discovered
that sphene is classified as a "heavy mineral" (specific gravity 3.5+ for sphene) and may be separated (for
example in sandstones), along with other "heavies" such as zircon, ilmenite,
epidote, apatite, magnetite, garnets, rutile and others. An analysis of these heavy mineral concentrations
often help geologists understand the original source area for the particles in
the sandstone--many heavy minerals are distinctive of a specific source area. For example, one class project determined
that the provenance for a particular sandstone was a series of metamorphic
rocks a few hundred miles away---the abundance of rutile in the heavy mineral
separation lead to that speculation.
Rutile is a common mineral in high temperature-high pressure metamorphic
rocks.
Heavy minerals (density greater than ~2.9) may be
separated from "lights" (such as quartz and feldspars) by the use of
a heavy liquid such as bromoform and a centrifuge. Further separation may be made with the use
of magnetic attraction.
Rockhounds are quite familiar with mining of placer
gold. However, several other minerals
form important placer deposits such as the titanium-rich ilmenite beach sands
in Alaska. Cassiterite placers are important sources of tin in Malaysia and
Indonesia.
Sphene is a common accessory mineral in many igneous
and metamorphic rocks; however, the grains are usually small and tough to
distinguish with a hand lens. But, with the
use of cut thin sections and a petrographic microscope, a whole new world opens
up for the geologist. Sphene is observable
and identifiable.
My next experience with sphene occurred in the shop
of a faceter who had stunning specimens of this green to yellow-green, high
dispersion, gemstone. However, these
faceted specimens are mostly for exhibit since their hardness is quite low (~5-5.5)
and the mineral is brittle and easily breaks.
I now look for sphene stones to examine; however, they are often tough to locate. The faceted stones have a brilliant luster, some unique colors of green to yellow-green, and maybe even tending to a brownish-green or reddish-green. In fact, the stones, if cut correctly, exhibit a nice pleochroism-changing colors when looked at via different angles. A jeweler told me that some of his faceted specimens have a very strong fire especially when displayed with a brilliant cut (refractive index of over 2). Although I have not seen such, it is my understanding that heat-treated stones will lose their "greenness" and turn red.
I now look for sphene stones to examine; however, they are often tough to locate. The faceted stones have a brilliant luster, some unique colors of green to yellow-green, and maybe even tending to a brownish-green or reddish-green. In fact, the stones, if cut correctly, exhibit a nice pleochroism-changing colors when looked at via different angles. A jeweler told me that some of his faceted specimens have a very strong fire especially when displayed with a brilliant cut (refractive index of over 2). Although I have not seen such, it is my understanding that heat-treated stones will lose their "greenness" and turn red.
So, I have seen many "grains" of sphene via
heavy mineral separation and thin sections but was still on a quest for one of
those nice green crystals! Where would I find one? Well, the specimen finally came to me via an
unexpected route--in a flat of mixed minerals at an estate auction! Whatever, I now have these beautiful green
crystals--of titanite? Yep. Seems like in 1982 the International
Mineralogical Association Commission on New Minerals and Mineral
Names adopted the name titanite and discredited the name sphene (for its wedge shaped crystals) since the
mineral has a high percentage of titanium (CaTiSIO5). How could they do such a thing? RIP sphene.
mike
Post Script: I recently was able to acquire another crystal of titanite collected from Kennesaw Mountain, Georgia. It is perched, by itself, on a mass of quartz.
mike
Post Script: I recently was able to acquire another crystal of titanite collected from Kennesaw Mountain, Georgia. It is perched, by itself, on a mass of quartz.
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| WIDTH OF CRYSTAL ~.88 MM. |