Spinel

Spinel ( /spɪˈnɛl/) is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula MgAl2O4 in the cubic crystal system. Its name comes from the Latin word "spinella", which means spine in reference to its pointed crystals.[4]

Spinel
Large terminated spinel crystal from the Lena River Valley, Siberia, measuring 8 x 8 x 7 cm in size.
General
Category
  • Oxide minerals
  • Spinel group
  • Spinel structural group
Formula
(repeating unit)		MgAl2O4
Strunz classification4.BB.05
Crystal systemCubic
Crystal classHextetrahedral (43m)
H-M symbol: (43m)[1][2][3]
Space groupF 4 3 m (No. 216)
Unit cella = 8.0898(9) Å; Z = 8
Identification
ColorVarious; red, pink, blue, lavender/violet, dark green, brown, black, colourless
Crystal habitOctehedra or flat triangular plates caused by twinning
Twinningcommon
CleavageNone
FractureConchoidal
Mohs scale hardness7.5–8.0
LusterVitreous
StreakWhite
DiaphaneityTransparent to opaque
Specific gravity(depending on the composition) the rare, Zn-rich spinel can be as high as 4.40, otherwise it averages from 3.58 to 3.61
Optical propertiesIsotropic
Refractive index1.719
PleochroismAbsent
Solubilitynone
Other characteristicsWeak to medium magnetic, sometimes fluorescent (red synthetic yes, natural red sometimes)
References[4][5]

Properties
Cut spinel

Spinel crystallizes in the isometric system; common crystal forms are octahedra, usually twinned. It has an imperfect octahedral cleavage and a conchoidal fracture. Its hardness is 8, its specific gravity is 3.5–4.1, and it is transparent to opaque with a vitreous to dull luster. It may be colorless, but is usually various shades of pink, rose, red, blue, green, yellow, brown, black, or (uncommon) violet. There is a unique natural white spinel, now lost, that surfaced briefly in what is now Sri Lanka. Some spinels are among the most famous gemstones; among them are the Black Prince's Ruby and the "Timur ruby" in the British Crown Jewels, and the "Côte de Bretagne", formerly from the French Crown jewels. The Samarian Spinel is the largest known spinel in the world, weighing 500 carats (100 g).

The transparent red spinels were called spinel-rubies or balas rubies. In the past, before the arrival of modern science, spinels and rubies were equally known as rubies. After the 18th century the word ruby was only used for the red gem variety of the mineral corundum and the word spinel came to be used. "Balas" is derived from Balascia, the ancient name for Badakhshan, a region in central Asia situated in the upper valley of the Panj River, one of the principal tributaries of the Oxus River. Mines in the Gorno Badakhshan region of Tajikistan constituted for centuries the main source for red and pink spinels.

Occurrence

Geologic occurrence

Spinel is found as a metamorphic mineral, and also as a primary mineral in rare mafic igneous rocks; in these igneous rocks, the magmas are relatively deficient in alkalis relative to aluminium, and aluminium oxide may form as the mineral corundum or may combine with magnesia to form spinel. This is why spinel and ruby are often found together. The spinel petrogenesis in mafic magmatic rocks is strongly debated, but certainly results from mafic magma interaction with more evolved magma [6] or rock (e.g. gabbro, troctolite).[7][8]

Spinel, (Mg,Fe)(Al,Cr)2O4, is common in peridotite in the uppermost Earth's mantle, between approximately 20 km to approximately 120 km, possibly to lower depths depending on the chromium content.[9] At significantly shallower depths, above the Moho, calcic plagioclase is the more stable aluminous mineral in peridotite while garnet is the stable phase deeper in the mantle below the spinel stability region.

Spinel, (Mg,Fe)Al2O4, is a common mineral in the Ca-Al-rich inclusions (CAIs) in some chondritic meteorites.

Geographical occurrence

Spinel has long been found in the gemstone-bearing gravel of Sri Lanka and in limestones of the Badakshan Province in modern-day Afghanistan and Tajikistan; and of Mogok in Myanmar. Over the last decades gem quality spinels are found in the marbles of Lục Yên District (Vietnam), Mahenge and Matombo (Tanzania), Tsavo (Kenya) and in the gravels of Tunduru (Tanzania) and Ilakaka (Madagascar).

Since 2000 in several locations around the world have been discovered spinels with unusual vivid pink or blue color. Such "glowing" spinels are known from Mogok (Myanmar),[10] Mahenge plateau (Tanzania), Lục Yên District (Vietnam) [11] and some more localities. In 2018 bright blue spinels have been reported also in the southern part of Baffin Island (Canada).[12] The pure blue coloration of spinel is caused by small additions of cobalt.[13]

Synthetic spinel

Synthetic spinel, accidentally produced in the middle of the 18th century, has been described more recently in scientific publications in 2000 and 2004.[14] By 2015, transparent spinel was being made in sheets and other shapes through sintering.[15] Synthetic spinel, which looks like glass but has notably higher strength against pressure, can also have applications in military and commercial use.[16]

See also
  • Minerals portal
  • Black Prince's Ruby
  • Ceylonite
  • The Samarian Spinel: the largest known spinel in the world, part of the Iranian Crown Jewels

References
  1. Robert John Lancashire. "Normal Spinels". CHEM2101 (C 21J) Inorganic Chemistry - Chemistry of Transition Metal Complexes. University of the West Indies.
  2. N. W. Grimes; et al. (Apr 8, 1983). "New Symmetry and Structure for Spinel". Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences. 386 (1791): 333–345. Bibcode:1983RSPSA.386..333G. doi:10.1098/rspa.1983.0039. JSTOR 2397417.
  3. L. Hwang; et al. (Jul 1973). "On the space group of MgAl
    2    O
    4     spinel"    . Philosophical Magazine. doi:10.1080/14786437308217448.
  4. Spinel. Mindat.org
  5. Barthelmy, Dave. "Spinel Mineral Data". Webmineral.
  6. Irvine TN (1977). "Origin of chromite layers in the Muskox intrusion and other stratiform intrusions: a new perspective". Geology. 5 (5): 273. doi:10.1130/0091-7613(1977)5<273:ooclit>2.0.co;2.
  7. Leuthold J, Blundy JD, Brooker RA (2015). "Experimental petrology constraints on the recycling of mafic cumulate: A focus on Cr-spinel from the Rum Eastern Layered Intrusion, Scotland". Contributions to Mineralogy and Petrology. 170 (2): 12. Bibcode:2015CoMP..170...12L. doi:10.1007/s00410-015-1165-0.
  8. O Driscoll B, Emeleus CH, Donaldson CH, Daly JS (2009). "The roles of melt infiltration and cumulate assimilation in the formation of anorthosite and a Cr-spinel seam in the Rum Eastern Layered Intrusion, NW Scotland". Lithos. 111 (1–2): 6–20. Bibcode:2009Litho.111....6O. doi:10.1016/j.lithos.2008.11.011.
  9. Klemme, Stephan (2004). "The influence of Cr on the garnet–spinel transition in the Earth's mantle: Experiments in the system MgO–Cr2O3–SiO2 and thermodynamic modelling" (PDF). Lithos. 77 (1–4): 639–646. Bibcode:2004Litho..77..639K. doi:10.1016/j.lithos.2004.03.017.
  10. Vincent Pardieu (2014). "Hunting for "Jedi" Spinels in Mogok". Gems & Gemology. 50 (1).
  11. Wondermondo (16 June 2019). "Finds of cobalt blue spinel in Lục Yên, Vietnam".
  12. Mining.Com (5 April 2019). "Scientists figure out origin of cobalt-blue spinel in Canada's Arctic".
  13. Boris Chauviré, Benjamin Rondeau, Emmanuel Fritsch, Phillipe Ressigeac, and Jean-Luc Devidal (Spring 2015). "Blue Spinel From the Luc Yen District of Vientam". Gems & Gemology.CS1 maint: multiple names: authors list (link)
  14. "SSEF – Leader in coloured gemstone, diamond and pearl testing and certification". Retrieved 17 March 2015.
  15. "Researchers finding applications for tough spinel ceramic". Phys.org. 24 April 2015.
  16. "Transparent Armor from NRL; Spinel Could Also Ruggedize Your Smart Phone". Naval Research Laboratory. 23 April 2015.

Bibliography
  • Deer, Howie and Zussman (1966). An Introduction to the Rock-Forming Minerals, Longman, pp. 424–433, ISBN 0-582-44210-9.
  • Shumann, Walter (2006). Gemstones of the World 3rd edition, Sterling, pp. 116–117.
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