I am often confused about identification of minerals
in the Columbite-Tantalite series; of course numerous other ideas offer
confusion to my mind! As I understand
the situation, columbite (Fe, Mn)(Nb,Ta)2O6 [niobium-rich]
is in a solid solution gradation with tantalite Fe,Mn)(Ta,Nb)2O6
[tantalum-rich) and individual specimens are very difficult to accurately
name (without some sophisticated instrumentation). In fact, pure end members may be rather rare
in nature. The amount of manganese also
varies in the specimens. The best bet for field identification seems to be the
high specific gravity, (~7.9) for iron-rich tantalite, compared to ~5.3 for
columbite. Both have a subconcoidal
fracture, good cleavage in one direction, black to brownish-black color, and a submetallic
luster. Add that to the note of www.MinDat.org:
iron-tantalite is fairly rare and many specimens are actually misidentified as
iron-rich tapiolite (tetragonal dimorph of Fe orthorhombic tantalite). With that in mind, I am uncertain what an ole
stratigrapher like me is suppose to do?
Columbite-Tantalite is often found in lithium-and
phosphate-rich pegmatites and associated with such minerals as spodumene, beryl
and lepidolite. Mining of these minerals is ongoing as
tantalum is used in the manufacture of electronic capacitors. Niobium has uses in strengthening iron alloys,
and in superconducting medical magnets.
Brazil, Australia and Canada seem to be the major producers of tantalum
and niobium at the current time.
However, in past years the pegmatites of the Black Hill in South Dakota have
produced many tons of the minerals.
Roberts and Rapp (1965) state: "The
Black Hills have received world-wide recognition for the many excellent
specimens of columbite-tantalite collected from pegmatites in the area since
first reported in 1884...In addition to specimens, over 65 tons of
columbite-tantalite have been produced since 1918 as a by-product of mining
other minerals". I am unaware of
current mining for columbite-tantalite in the Black Hills.
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| TWINNED PARTIAL CRYSTAL OF TANTALITE. WIDTH ~2 CM. |
I have a small specimen of "columbite-tantalite"
collected many years (decades) ago from somewhere near Custer, South Dakota. At times in my life, especially when younger,
my note taking and locality information was not the best; I thought my memory
would last forever! I know this specimen
came from near Custer and my best guess is the Tin Mountain Mine west of the
city. The specimen is a section of a twinned
crystal and actually is pretty nice. My
guess is that the mineral specimen is an iron-rich tantalite (rather than
columbite) since the specific gravity (heft) seems high. At the Tin Mountain Mine Precambrian
metamorphic rocks, mostly a schist, host the zoned pegmatite.
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| COLUMBITE CRYSTALS SET IN CRYSTALLINE QUARTZ. LENGTH ~7 CM. |
My second specimen from the columbite-tantalite
series was collected closer to home, somewhere west of Colorado Springs. I purchased this specimen from an
out-of-state rock/mineral shop so only have the following information: "Tantalite,
St. Peter's Dome, CO". The specimen
exhibits massive blocky "slabs" of the mineral surrounded by
crystalline quartz. I suspect the
mineral is columbite since Eckel (1997)
stated, "Columbite is
probably more common that tantalite in Colorado because of the limited degree
of differtiation of the host granites and pegmatites...The earlist reported
occurrence of columbite in the state was from Pikes Peak, possibly the Crystal Park or Stove Mountain areas." Since Stove Mountain is near St. Peter's Dome
perhaps the specimen in my collection was mislabeled since the "Dome"
is much better known.