New Mexico Mineral Symposium — Abstracts

Over 120 years ago, argentite was first reported in Colorado (Endlich, 1878). Since that time, all of the argentite studied in Colorado has been found to be acanthite, the monoclinic form of silver sulfide, which is the stable form of Ag2S below 177°C (?). In Colorado, acanthite is usually found in secondary enriched zones of epithermal silver-base metal deposits such as the limestone replacements at Leadville and Aspen or the veins at Creede; however, it has been reported from at least 27 additional districts and undoubtedly occurs sporadically throughout the state (Eckel et al., 1997).

Collectable and much sought after silver-bearing minerals such as native silver, Ag-sulfides (primarily acanthite, stromeyerite, and jalpaite), Ag-sulfosalts (primarily pyrargyrite, pearceite, matildite, polybasite, schirmerite, proustite, and stephanite), Ag-tellurides (primarily hessite, petzite, and sylvanite), and Ag-halides (primarily bromargyrite-chlorargyrite) are known from many mining districts within Colorado (Eckel et al., 1997). The most famous of these districts include: Caribou, Gold Hill, and Ward in Boulder County; Georgetown-Silver Plume in Clear Creek County; Rico in Dolores County; Gold Brick in Gunnison County; Creede in Mineral County; Montezuma in Park and Summit Counties; Aspen in Pitkin County; Silverton in San Juan County; and Telluride in San Miguel County.

Colorado's silver mining tradition dates back to 1864, when its first silver lode was discovered "on Glacier Mountain, about a mile south of Montezuma" (Lovering and Goddard, 1950). Interestingly, samples from this district were the catalysts for this investigation. The samples are examples of high grade silver mineralization, but they were unusual for two other reasons. First, they contain microscopic arborescent growths. Second, the fine growths were not noticed until a decade after they were collected. In fact, it is believed that they have formed since they were collected, similar to some polished sections studied from Creede (Raines, pers. comm. 2000; Plumlee, pers. comm. 2000) and elsewhere (Mozgova et al., 1994).

To confirm this last point, earlier this year a trip to the mine was made where the specimens originated in the early 1980s, the Burke—Martin mine, also known as the Bell, California, Meteor, Silver Wing, Sunburst, or Wing lode (Lovering, 1935). Many samples were taken from the exact 1980s stockpiles of mineralized material from the Albany or Old Timer vein by three different mineral collectors. Each collector found native silver and various hypogene and supergene associates, yet none found any of the arborescent growths.

Laboratory investigations using standard X-ray diffraction failed to establish the identity of the growths. Neither did comparisons with silver minerals observed in a talk given at last year's symposium by Regis and Regis (1999). However, scanning electron microscopy verified that they are acanthite and indeed delicately exquisite. In fact, the name acanthite is from the Greek for thorn, referring to the shape of the crystals" (Mason, 1997).

One explanation why the X-ray diffraction study proved fruitless is that acanthite is very sectile and does not pulverize well using a mortar and pestle. Hence, it could not diffract the X-ray beam to produce characteristic peaks, as most other minerals do.

Disappointingly, a literature search combining the terms primary or arborescent, or acicular or dendritic or needle or hair or tree with acanthite, or argentite failed to obtain any matches. The combination of monoclinic and acanthite only found four marginally useful references. The combinations of habit or form or morphology and acanthite proved more productive.

What many authors really mean when they use the term primary acanthite is acanthite that crystallized as itself, rather than forming as a paramorph after argentite, the higher-temperature, isometric form of AO. However, the use of the word primary as a descriptor for acanthite is actually a misnomer because much acicular acanthite is formed by supergene processes after some other Ag phase. Use of morphological terms such as acicular, arborescent, or needle-like are far clearer in describing not only the form of the acanthite in question (monoclinic), but also its genesis (independent crystallization and not paramorphic replacement after primary, isometric argentite).

Illustrations will be provided showing acicular acanthite from Colorado. It has been reported from at least seven other districts: Creede (Plumlee, pers. comm. 2000), Gilman (this study), Leadville (Raines, pers. comm. 2000), Rico (Chester, 1894), Ruby/Irwin (this study), Silver Plume (crystals coating argentite and later coated by pyrite), and Ward (crystals coating native silver, pearceite, and galena); (Eckel et al., 1997).

Further questions to be discussed include: Is there anything unique to Creede, Leadville, Montezuma, Rico, Silver Plume, and Ward that causes their deposits to generate acicular acanthite? Why wasn't the acicular acanthite on the Montezuma specimens observed in the field? How does acicular acanthite form? Why doesn't acicular acanthite form on household silverware or jewelry?
 

References:

1. Chester, A. H., 1894, Acanthite from Colorado: Columbia University School of Mines Quarterly, v. 15, pp. 103-104.
2. Eckel, E. B., and others, 1997, Minerals of Colorado: Fulcrum Publications, Colorado, 665 pp.
3. Endlich, F. M., 1878, Mineralogical report: catalogue of minerals found in Colorado; in Hayden, F. V., U.S. Geological and Geographical Survey 10th Annual Report for 1876.
4. Lovering, T. S., 1935, Geology and ore deposits of the Montezuma quadrangle, Colorado: U.S. Geological Survey, Professional Paper 178,119 pp.
5. Lovering, T. S., and Goddard, E. N., 1950, Geology and ore deposits of the Front Range, Colorado: U.S. Geological Survey, Professional Paper 223,319 pp.
6. Mason, B., 1997, Acanthite; in Gaines, R. V., and others, Dana's new mineralogy: John Wiley and Sons, New York, p. 42.
7. Mozgova, N. N., and others, 1994, New growth of sulphides on the surface of polished sections: International Mineralogical Association, abs. 16, p. 288.
8. Regis, A. J., and Regis, K. A., 1999, The ugly ores of silver???part II, a photo gallery: Twentieth Annual New Mexico Mineral Symposium, abs., pp. 20-21.