Thursday, October 11, 2012


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 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.

  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.

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.