Tetrahedrite

Tetrahedrite
Tetrahedrite
General
Category	Sulfosalt mineral
Formula (repeating unit)	(Cu,Fe) 12Sb 4S 13
Strunz classification	2.GB.05
Crystal system	Cubic
Crystal class	Hextetrahedral (43m) H-M symbol: (4 3m)
Space group	I43m
Unit cell	a = 10.39(16) Å; Z = 2
Identification
Color	Steel gray to iron-gray
Crystal habit	Groups of tetrahedral crystals; massive, coarse to fine compact granular
Twinning	Contact and penetration twins on {111}
Cleavage	None
Fracture	Uneven to subconchoidal
Tenacity	Somewhat brittle
Mohs scale hardness	3+1⁄2 –4
Luster	Metallic, commonly splendent
Streak	Black, brown to dark red
Diaphaneity	Opaque, except in very thin fragments
Specific gravity	4.97
Optical properties	Isotropic
Refractive index	n greater than 2.72
References	[1] [2]

Tetrahedrite is a copper antimony sulfosalt mineral with formula: (Cu,Fe)
_¹²Sb
_⁴S
_¹³. It is the antimony endmember of the continuous solid solution series with arsenic-bearing tennantite. Pure endmembers of the series are seldom if ever seen in nature. Of the two, the antimony rich phase is more common. Other elements also substitute in the structure, most notably iron and zinc, along with less common silver, mercury and lead. Bismuth also substitutes for the antimony site and bismuthian tetrahedrite or annivite is a recognized variety. The related, silver dominant, mineral species freibergite, although rare, is notable in that it can contain up to 18% silver.

Mineralogy

Tetrahedrite gets its name from the distinctive tetrahedron shaped cubic crystals. The mineral usually occurs in massive form, it is a steel gray to black metallic mineral with Mohs hardness of 3.5 to 4 and specific gravity of 4.6 to 5.2.

Tetrahedrite occurs in low to moderate temperature hydrothermal veins and in some contact metamorphic deposits. It is a minor ore of copper and associated metals. It was first described in 1845 for occurrences in Freiberg, Saxony, Germany. Historically, it was an important ore of copper, the formula Cu₃SbS₃ corresponding with 57.5% of the metal; it was also worked as an ore of silver, of which element it sometimes contains as much as 30%. ^[3]

Applications

The now-defunct company Alphabet Energy announced plans to offer a thermoelectric device based on tetrahedrite to turn heat into electricity. The company claimed that other thermoelectrics typically produce about 2.5 percent efficiency, while tetrahedrite could achieve 5 to 10 percent. ^[4]

Other thermoelectrics are either scarce, expensive ($24–146/kg vs $4 for tetrahedrite) and/or toxic. Working with a natural material also reduces manufacturing costs. ^[4]

Images

• 
  Tetrahedrite crystals with chalcopyrite and sphalerite from the Casapalca Mine, Peru (size: 8.2 x 6.4 x 4.7 cm)
• 
  Tetrahedrite from Casapalca Mine, Casapalca, Huarochiri Province, Lima Department, Peru
• 
  Rhodochrosite with fluorite, tetrahedrite and quartz; the tetrahedrite occurs as sharp, metallic crystals
• 
  Tetrahedrite crystals to several inches on size in combination with quartz crystals

See also

• Bornite
• Cuprite
• List of minerals

References

1. Handbook of Mineralogy
2. "Tetrahedrite: Tetrahedrite mineral information and data". Mindat.org. 2014-07-12. Retrieved 2014-07-17.
3.  One or more of the preceding sentences incorporates text from a publication now in the public domain :  Spencer, Leonard James (1911). "Tetrahedrite". In Chisholm, Hugh (ed.). Encyclopædia Britannica . 26 (11th ed.). Cambridge University Press. pp. 670–671.
4. Jacobs, Suzanne (2014-07-12). "Cheaper Thermoelectric Materials | MIT Technology Review". Technologyreview.com. Retrieved 2014-07-17.

External links

Media related to Tetrahedrite at Wikimedia Commons

• Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, ISBN   0-471-80580-7
• Mineral galleries
• Webmineral data