Spinel crystallizes in the isometric system; common crystal forms areoctahedra, usuallytwinned. It has no truecleavage, but shows an octahedralparting and aconchoidal fracture.[8] Itshardness is 8, itsspecific gravity is 3.5–4.1, and it is transparent to opaque with a vitreous to dullluster. It may be colorless, but usually has various shades of red,lavender, blue, green, brown, black, or yellow.[9] Chromium(III) causes the red color in spinel from Burma.[10]
Some spinels are among the most famous gemstones; these include theBlack Prince's Ruby and theTimur Ruby in the BritishCrown Jewels,[11] and the "Côte de Bretagne", formerly from the French Crown jewels.[12] TheSamarian Spinel, the largest known spinel in the world, weighs 500 carats (100 g).[13]
The transparent red spinels were called spinel-rubies[14] or balas rubies.[15] Before the development of modern science and the establishment of a formal definition of "ruby", the name was also applied to spinels.[16] "Balas" derives from Balascia, the ancient name forBadakhshan, a region in centralAsia situated in the upper valley of thePanj River, one of the principal tributaries of theOxus River. However, "Balascia" itself may be derived fromSanskritbālasūryaka, which translates as "crimson-coloured morning sun".[17] Mines in theGorno Badakhshan region of present-dayTajikistan constituted for centuries the main source for red and pink spinels.[16]
Spinel is found as ametamorphic mineral in metamorphosedlimestones and silica-poormudstones.[9] It also occurs as a primary mineral in rare maficigneous rocks; in these igneous rocks, themagmas are relatively deficient inalkalis relative toaluminium, and aluminium oxide may form as the mineral corundum or may combine with magnesia to form spinel. This is why spinel andruby 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[18] or rock (e.g. gabbro, troctolite).[19][20]
Spinel,(Mg,Fe)(Al,Cr)2O4, is common inperidotite in the uppermostEarth's mantle, between approximately 20 km to approximately 120 km, possibly to lower depths depending on the chromium content.[21] At significantly shallower depths, above theMoho, calcicplagioclase is the more stable aluminous mineral in peridotite whilegarnet is the stable phase deeper in the mantle below the spinel stability region.[22]
Since 2000, in several locations around the world, spinels have been discovered with unusual vivid pink or blue colors. Such "glowing" spinels are known from Mogok (Myanmar),[25] Mahenge plateau (Tanzania), Lục Yên District (Vietnam)[26] and some more localities. In 2018 bright blue spinels have been reported also in the southern part ofBaffin Island (Canada).[27] The pure blue coloration of spinel is caused by small additions ofcobalt.[28]
Synthetic spinel can be produced by similar means to synthetic corundum, including theVerneuil method and theflux method pioneered byEdmond Frémy. It is widely used as an inexpensive cut gem inbirthstonejewelry for the month ofAugust. Light blue synthetic spinel is a good imitation of aquamarine beryl, and green synthetic spinel is used as an emerald or tourmaline simulant.[29] By 2015, transparent spinel was being made in sheets and other shapes throughsintering.[30] Synthetic spinel, which looks like glass but has notably higher strength against pressure, can also have applications in military and commercial use.[31]
^Robert John Lancashire."Normal Spinels".CHEM2101 (C 21J) Inorganic Chemistry – Chemistry of Transition Metal Complexes. University of the West Indies. Archived fromthe original on 2018-08-08.
^Biswas, A.K. (2001). "Minerals and their Exploitation in Ancient and Pre-modern India". In Ramachandra Rao, P.; Goswami, N.G. (eds.).Metallurgy in India : a retrospective. New Delhi: India International Publisher. pp. 1–22.ASINB002A9M6QU.
^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.
^Philpotts, Anthony R.; Ague, Jay J. (2009).Principles of igneous and metamorphic petrology (2nd ed.). Cambridge, UK: Cambridge University Press. p. 17.ISBN9780521880060.
