Smoky quartz

Smoky quartz is a brownish grey, translucent variety of quartz that ranges in clarity from almost complete transparency to an almost-opaque brownish-gray or black crystal.[6] Like other quartz gems, it is a silicon dioxide crystal. The smoky colour results from free silicon formed from the silicon dioxide by natural irradiation.

Smoky quartz
General
CategorySilicate minerals
Formula
(repeating unit)
SiO2
Strunz classification04.DA.05
Dana classification75.01.03.01
Crystal systemα-quartz: trigonal trapezohedral class 3 2; β-quartz: hexagonal 622[1]
Space groupTrigonal 32
Unit cella = 4.9133 Å, c = 5.4053 Å; Z=3
Identification
ColourBrown to grey, opaque
Crystal habit6-sided prism ending in 6-sided pyramid (typical), drusy, fine-grained to microcrystalline, massive
TwinningCommon Dauphine law, Brazil law and Japan law
Cleavage{0110} Indistinct
FractureConchoidal
TenacityBrittle
Mohs scale hardness7 – lower in impure varieties (defining mineral)
LustreVitreous – waxy to dull when massive
StreakWhite
DiaphaneityTransparent to nearly opaque
Specific gravity2.65; variable 2.59–2.63 in impure varieties
Optical propertiesUniaxial (+)
Refractive indexnω = 1.543–1.545
nε = 1.552–1.554
Birefringence+0.009 (B-G interval)
Pleochroismweak, from red-brown to green-brown
Melting point1670 °C (β tridymite) 1713 °C (β cristobalite)[1]
SolubilityInsoluble at STP; 1 ppmmass at 400 °C and 500 lb/in2 to 2600 ppmmass at 500 °C and 1500 lb/in2[1]
Other characteristicslattice: hexagonal, Piezoelectric, may be triboluminescent, chiral (hence optically active if not racemic)
References[2][3][4][5]

Varieties

Morion is a very dark brown to black opaque variety. Morion is the German, Danish, Spanish and Polish synonym for smoky quartz.[7] The name is from a misreading of mormorion in Pliny the Elder.[8] It has a density of 5.4.

Cairngorm is a variety of smoky quartz found in the Cairngorm Mountains of Scotland.[9] It usually has a smoky yellow-brown colour, though some specimens are greyish-brown. It is used in Scottish jewellery and as a decoration on kilt pins and the handles of sgianan-dubha (anglicised: sgian-dubhs or skean dhu). The largest known cairngorm crystal is a 23.6 kg (52 lb) specimen kept at Braemar Castle.

Uses

Smoky quartz is common and was not historically important, but in recent times it has become a popular gemstone, especially for jewellery.[10]

Sunglasses, in the form of flat panes of smoky quartz, were used in China in the 12th century.[11]

See also

References

  1. Deer, W. A., R. A. Howie and J. Zussman, An Introduction to the Rock Forming Minerals, Logman, 1966, pp. 340–355 ISBN 0-582-44210-9
  2. Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C. (eds.). "Quartz". Handbook of Mineralogy (PDF). III (Halides, Hydroxides, Oxides). Chantilly, VA, US: Mineralogical Society of America. ISBN 0962209724.
  3. Quartz. Mindat.org. Retrieved on 2013-03-07.
  4. Quartz. Webmineral.com. Retrieved on 2013-03-07.
  5. Hurlbut, Cornelius S.; Klein, Cornelis (1985). Manual of Mineralogy (20 ed.). ISBN 0-471-80580-7.
  6. "Smoky Quartz: Smoky Quartz mineral information and data".
  7. http://www.mindat.org/min-6270.html Morion on Mindat
  8. New Oxford American Dictionary (2nd ed., 2005), p. 1102.
  9. Shaw, Philip; Thompson, Des B. A. The Nature of the Cairngorms: Diversity in a Changing Environment. The Stationery Office. p. 22. ISBN 978-0-11-497326-1.
  10. "The Gemstone Smoky Quartz". minerals.net. Retrieved 14 December 2015.
  11. Joseph Needham, Science & Civilisation in China (Cambridge, England: Cambridge University Press, 1962), volume IV, part 1, page 121. Needham states that dark glasses were worn by Chinese judges to hide their facial expressions during court proceedings.


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