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From Wikipedia, the free encyclopedia
Aguilarite is an uncommon sulfosalt mineral with formula Ag4SeS. It was described in 1891 and named for discoverer Ponciano Aguilar.
| Aguilarite | |
|---|---|
 Aguilarite – San Carlos Mine, La Luz, Guanajuato municipality, Mexico. Specimen height is 3.9 cm. | |
| General | |
| Category | Sulfosalt minerals |
| Formula (repeating unit) | Ag4SeS |
| IMA symbol | Agu[1] |
| Strunz classification | 2.BA.55 |
| Dana classification | 2.4.1.3 |
| Crystal system | Monoclinic |
| Crystal class | Prismatic (2/m) (same H-M symbol)[2] |
| Space group | P21/n[2] |
| Unit cell | a = 4.2478(2), b =6.9432(3) c = 8.0042(5) [Å] β = 100.103(2)° Z = 4[2] |
| Identification | |
| Color | Bright lead-gray on fresh surfaces; dull iron-black on exposure to air. |
| Cleavage | None observed |
| Fracture | Hackly |
| Tenacity | Sectile |
| Mohs scale hardness | 2.5[3] 1–1.5[2] |
| Luster | Metallic |
| Streak | Gray-black |
| Diaphaneity | Opaque |
| Ultraviolet fluorescence | Non-fluorescent[4] |
| References | [3] |
Aguilarite is bright lead-gray on fresh surfaces but becomes dull iron black when exposed to air.[3] The mineral occurs with massive habit, as elongated pseudododecahedral crystals up to 3 cm (1.2 in), or as intergrowths with acanthite or naumannite.[5]
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.[5][6] 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.[6] When the International Mineralogical Association was founded, aguilarite was grandfathered as a valid mineral species.[7]
Aguilarite is uncommon, and forms at relatively low temperatures in hydrothermal deposits rich in silver and selenium but deficient in sulfur.[5] The mineral is known from a number of countries in North and South America, Europe, Asia, and Australasia.[3][5] Aguilarite occurs in association with acanthite, calcite, naumannite, pearceite, proustite, silver, stephanite, and quartz.[5]
In 2013, aguilarite's chemistry and crystal structure were reexamined by Bindi and Pingitore.[2] 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.[8] The sample primarily studied came from the Gem and Mineral Collection of the Department of Geosciences at Princeton University.[9]
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.[10] Bindi and Pingitore determined that aguilarite is, in fact, monoclinic and is isostructural to acanthite and not naumannite.[2] 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.[11]
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.[12]
Aguilarite is part of the acanthite-like solid solution series Ag2S–Ag2S0.4Se0.6. The mineral comprises the range from 50 atomic percent selenium up to the transition from monoclinic to orthorhombic.[8]
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