Aguilarite

Aguilarite is bright lead-gray on fresh surfaces but becomes dull iron black when exposed to air.[2] The mineral occurs with massive habit, as elongated pseudododecahedral crystals up to 3 cm (1.2 in), or as intergrowths with acanthite or naumannite.[4]

In the late 19th century, Ponciano Aguilar, superintendent of the San Carlos mine in Guanajuato, Mexico, found several specimens of a mineral thought to be naumannite.[4][5] The samples were given to F. A. Genth for identification, who, along with S. L. Penfield, discovered that it was a new mineral. The mineral was described in the American Journal of Science in 1891 and named aguilarite in honor of Ponciano Aguilar.[5] When the International Mineralogical Association was founded, aguilarite was grandfathered as a valid mineral species.[6]

Aguilarite is uncommon, and forms at relatively low temperatures in hydrothermal deposits rich in silver and selenium but deficient in sulfur.[4] The mineral is known from a number of countries in North and South America, Europe, Asia, and Australasia.[2][4] Aguilarite occurs in association with acanthite, calcite, naumannite, pearceite, proustite, silver, stephanite, and quartz.[4]

In 2013, aguilarite's chemistry and crystal structure were reexamined by Bindi and Pingitore.[1] The significant reevaluation of aguilarite did not discredit its status as a valid mineral, but it was established as the selenium analogue of acanthite instead of sulfur-rich naumannite.[7] The sample primarily studied came from the Gem and Mineral Collection of the Department of Geosciences at Princeton University.[8]

The work of Petruk et al. in 1974 formed the basis of knowledge regarding the silver–sulfur–selenium system for about forty years. They indexed their x-ray diffraction patterns of aguilarite on an orthorhombic cell similar to naumannite.[9] Bindi and Pingitore determined that aguilarite is, in fact, monoclinic and is isostructural to acanthite and not naumannite.[1] Bindi and Pingitore believe that Petruk et al. were unable to resolve closely spaced peaks due to low resolution equipment, making aguilarite appear similar to naumannite. Additionally, a number of inconsistencies in unit cell dimensions in the 1974 work show that aguilarite does not have the same structure as naumannite.[10]

The crystal structure of aguilarite consists of planes nearly parallel to (010) composed of tetrahedrally coordinated nonmetal atoms and AgX₃ triangles (where X is a nonmetal). The planes are joined by twofold-coordinated silver atoms.[11]

Aguilarite is part of the acanthite-like solid solution series Ag₂S–Ag₂S_0.4Se_0.6. The mineral comprises the range from 50 atomic percent selenium up to the transition from monoclinic to orthorhombic.[7]

Bibliography

• Bindi, L.; Pingitore, N. E. (February 2013). "On the symmetry and crystal structure of aguilarite, Ag₄SeS". Mineralogical Magazine. 77 (1): 21–31. doi:10.1180/minmag.2013.077.1.03.CS1 maint: ref=harv (link) (subscription required)
• Genth, F. A. (1891). Dana, James D.; Dana, Edward S. (eds.). "Aguilarite, a new species". American Journal of Science. 141 (241–246).CS1 maint: ref=harv (link)

• Petruk, W.; Owens, D. R.; Stewart, J. M.; Murray, E. J. (August 1974). "Observations on acanthite, aguilarite and naumannite" (PDF). The Canadian Mineralogist. 12 (6): 365–369.

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