Chemistry:Aguilarite

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Aguilarite
Aguilarite - San Carlos Mine, La Luz, Guanajuato, Mexico.jpg
Aguilarite - San Carlos Mine, La Luz, Guanajuato municipality, Mexico. Specimen height is 3.9 cm.
General
CategorySulfosalt minerals
Formula
(repeating unit)
Ag4SeS
Strunz classification2.BA.55
Dana classification2.4.1.3
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)[1]
Space groupP21/n[1]
Unit cella = 4.2478(2), b =6.9432(3)
c = 8.0042(5) [Å]
β = 100.103(2)°
Z = 4[1]
Identification
ColorBright lead-gray on fresh surfaces; dull iron-black on exposure to air.
CleavageNone observed
FractureHackly
TenacitySectile
Mohs scale hardness2.5[2] 1–1.5[1]
|re|er}}Metallic
StreakGray-black
DiaphaneityOpaque
Ultraviolet fluorescenceNon-fluorescent[3]
References[2]

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

Description

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]

Discovery

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]

Occurrence

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]

Chemistry and structure

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–Ag2S0.4Se0.6. The mineral comprises the range from 50 atomic percent selenium up to the transition from monoclinic to orthorhombic.[7]

References

  1. 1.0 1.1 1.2 1.3 1.4 1.5 Bindi & Pingitore 2013, p. 21.
  2. 2.0 2.1 2.2 2.3 "Aguilarite". Mindat. http://www.mindat.org/min-59.html. 
  3. "Aguilarite". Webmineral. http://webmineral.com/data/Aguilarite.shtml. 
  4. 4.0 4.1 4.2 4.3 4.4 Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W. et al., eds. "Aguilarite". Handbook of Mineralogy. Chantilly, VA: Mineralogical Society of America. http://www.handbookofmineralogy.org/pdfs/aguilarite.pdf. 
  5. 5.0 5.1 Genth 1891, p. 401.
  6. "The New IMA List of Minerals – A Work in Progress – Update: November 2012". Commission on New Minerals, Nomenclature and Classification. International Mineralogical Association. p. 3. http://pubsites.uws.edu.au/ima-cnmnc/IMA_Master_List_(2012-11).pdf. 
  7. 7.0 7.1 Bindi & Pingitore 2013, p. 30.
  8. Bindi & Pingitore 2013, p. 23.
  9. Bindi & Pingitore 2013, p. 22.
  10. Bindi & Pingitore 2013, p. 29.
  11. Bindi & Pingitore 2013, p. 26.
Bibliography

Further reading

External links