New Mexico Mineral Symposium — Abstracts

Bismuth minerals are relatively uncommon and are usually associated with vein-type or hydrothermal deposits, particularly gold and silver ore deposits. Bismuth sulfide is the primary mineral and is often associated with other sulfides of arsenic and antimony. Bismuth also occurs in telluride-type deposits where it may be present in complex suites of Ag, Au, Pb, Cu, Sb, and Hg sulfosalt and telluride minerals. However, primary and secondary bismuth minerals can also be found in pegmatites, although it is rare in these deposits.

Bismuth has been noted in the pegmatites of San Diego County (California) and also in a few pegmatites in New England, Black Hills (South Dakota), Colorado, and New Mexico. The most famous, and probably most abundant, pegmatite occurrence in New Mexico is at the Harding mine although these minerals have also been reported in other New Mexico pegmatites such as the Petaca, Ojo Caliente, El Porvenir, and Rociada districts. Of the 20 reported bismuth occurrences in New Mexico (Lueth, 1996; Northrop, 1996), seven are found at the Harding and two are found only there.

The Harding deposit, located in the Picuris Range 30 km south of Taos and 10 km east of Dixon, is a rare-element pegmatite. The pegmatite was mined more than 50 years: before World War II for the lithium minerals lepidolite, lithium muscovite, and spodumene; during the war for tantalum in the form of microlite and columbite/tantalite; and in the 1950s for beryl as an ore of beryllium. The bismuth minerals were never commercially produced and as a result can be picked up on the mine dumps with some careful searching (and maybe some luck). A scan of the Harding mine collection at UNM yielded 18 samples of bismuth minerals and 4 more containing unidentified minerals that are possibly Bi-bearing minerals.

Bismuth minerals have been identified from three lithologic units in the pegmatite: the beryl zone (quartz+albite+muscovite±perthite), the quartz+albite+muscovite unit and the quartz+lath spodumene unit. Nearly all the samples contain a suite of colorful bismuth minerals, interstitial and surface coatings on coarse-grained quartz, albite, spodumene, and perthite. Bismutite [Bi₂(CO₃)O₂] appears to be the most common mineral and occurs as blue-green crystals or as green to yellow-green, waxy masses and coatings. Beyerite [(Ca,Pb)Bi₂(CO₃)₂O₂] occurs intergrown with bismutite in various shades of yellow and yellow green. The oxide bismite occurs as grayish-green to yellowish powdery coatings and masses. The rare Bi-vanadate pucherite occurs as yellow-orange to orange-brown coatings and microcrystals on spodumene, quartz, and muscovite. In addition to the colorful yellow-green and yellow of the secondary bismuth minerals, some specimens host brilliant-green mottramite [PbCu(VO₄)OH] and malachite [Cu₂(CO₃)(OH)₂] (Hlava, 1986). The primary bismuth minerals from which these secondary minerals have altered are probably bismuthinite (Bi2S₃), native bismuth, and bismutotantalite [BiTaO₄] that have all been reported as residual grains within secondary bismuth minerals (Hlava, 1986; Jahns and Ewing, 1976). In addition, bismuth occurs in minor amounts in primary microlite [(Ca,Na)₂Ta2O₆ (O,OH,F)] from the quartz+lath spodumene unit of the pegmatite.

A note on collecting at the Harding mine: the property is owned by the University of New Mexico and is one of the few mine properties where collectors are actively welcomed. Some restrictions apply, mainly that collecting is limited to personal use and that a signed release form is given to the mine caretaker. Release forms and information about the mine may be obtained from the Department of Earth and Planetary Science or at http://eps.unm.edu/harding/harding.htm.

Primary and secondary bismuth minerals from the Harding Mine

References:

1. Hlava, P. F., 1986, Unusual minerals from the Harding pegmatite; in Colorado pegmatites: Colorado Pegmatite Symposium, May 30-June 2, 1986, pp. 55-57.
2. Jahns, R. H., and Ewing, R. C., 1976, The Harding mine, Taos County, New Mexico; in Ewing, R. C., and Kues, B. S. (eds.), Vermejo Park: New Mexico Geological Society, Guidebook 27, pp. 263-276.
3. Lueth, V. W., 1996, Secondary bismuth minerals from the Apache mine, Hidalgo County, New Mexico: 17th Annual New Mexico Mineral Symposium, abstracts, p. 4; New Mexico Geology, 1997, v. 19, p. 26.
4. Northrop, S. A., and LaBnizza, F. A., 1996, Minerals of New Mexico: University of New Mexico Press, 3rd edition.