Aguilarite

Aguilarite
Aguilarite - San Carlos Mine, La Luz, Guanajuato municipality, Mexico. Specimen height is 3.9 cm.
General
Category	Sulfosalt minerals
Formula (repeating unit)	Ag4SeS
Strunz classification	2.BA.55
Dana classification	2.4.1.3
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/n
Unit cell	a = 4.2478(2), b =6.9432(3) c = 8.0042(5) [Å] β = 100.103(2)° Z = 4
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 1–1.5
Luster	Metallic
Streak	Gray-black
Diaphaneity	Opaque
Ultraviolet fluorescence	Non-fluorescent
References

Aguilarite is an uncommon sulfosalt mineral with formula Ag₄SeS. It was described in 1891 and named for discoverer Ponciano Aguilar.

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

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. 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. When the International Mineralogical Association was founded, aguilarite was grandfathered as a valid mineral species.

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

In 2013, aguilarite's chemistry and crystal structure were reexamined by Bindi and Pingitore. 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. The sample primarily studied came from the Gem and Mineral Collection of the Department of Geosciences at Princeton University.