TELLURITE, A TELLURIUM OXIDE

The element tellurium is a silver-white metalloid (possesses properties of both metals and non-metals) with the symbol Te and the atomic number of 52 (number of protons in the nucleus of the atom). Tellurium exhibits oxidation states of 6+, 5+, 4+, 3+, 2+. 1+, 1-, 2-; however, only 6+, 4+ and 2- are stable. Tellurium can act as a cation with a 4+ oxidation state (IV) as in the uncommon mineral tellurite, TeO₂, or with a 6+ oxidation state (VI) as in jensenite, Cu₃TeO₆-2H₂O.   The telluride anion with a charge of 2- can combine with gold and silver cations in the minerals calaverite (AuTe₂) and sylvanite (AuAgTe₄); both minerals form major gold ores at Cripple Creek, Colorado.

Tellurium is an extremely rare element as most rocks contain about 3 parts per billion and is 8 times less abundant than gold. Tellurium is rarely found in a native form in rocks of the earth’s crust. MinDat listed 135 valid species containing essential tellurium: 1) 1 element; 2) 71 sulfides and sulfosalts; 3) 2 halides; 4) 44 oxides; 5) 13 sulfates; 6) 1 phosphate,1 vanadate, and 1 arsenate; and 7) 1 silicate.  So, most tellurium minerals are oxides and sulfates.

I previously posted about emmonsite (February, 13, 2017), a fairly rare iron (ferric) tellurite [Fe₂(TeO₃)₃-2H₂O], about the only tellurium mineral in my collection.  However, I recently picked up a specimen of the oxide tellurite, although certainly I feel out of my comfort zone in describing it. It was collected in 2000 by David Shannon (Arizona).

This is the best I can do with the photomicrographs!  The largest crystal (up or left), appears hexagonal and is only about .33 mm in length.  perhaps it is zemannite?  The dark green minerals attached to this large crystal may be denningite.  The really tiny acicular, straw yellow crystals are tellurite.

The specimen comes from the Moctezuma Mine (Bambolla) Mine, Moctezuma Municipality, Sonora Mexico.  It is a hydrothermal, gold-tellurium mine that produced a bonanza of tellurium minerals in the oxidized zone, something like 15 new species plus several previously named. The problem with my specimen is that all crystals are extremely small, less than one mm in length, and beyond the capability of my digital microscope.  However, they are visible (barely) in my binocular scope. The tellurite crystals are acicular or needle-like, light or straw yellow to light tan in color, less than adamantine in luster.  The tiny crystals are in “tuffs” or scattered, are translucent, and have a measured hardness of ~2.0 (Mohs). The quartz matrix also contains two other types of crystal that are larger and more robust than tellurite, and of a different color (green to brown), are striated, and some are terminated (pyramid-like).  My best guess is denningite [(Mn,Ca,Zn)Te₂O₅].  The other unknown appears to be hexagonal and might be zemannite [Mg₀.₅ZnFe(TeO₃)₃-4.5H₂O].  My mineralogical capability with these tiny crystals is lacking!  But, I have learned much from this little exercise.

Anyone who stops learning is old, whether at twenty or eighty. Anyone who keeps learning stays young. The greatest thing in life is to keep your mind young.  Henry Ford