Spinel

Spinel
Spinel
	Small spinel crystal on top of a white calcite from Mogok, Myanmar, measuring 1.5 × 1.1 × 1 cm in size
General
Category	Oxide minerals; Spinel group; Spinel structural group;
Formula; (repeating unit)	MgAl; 2O; 4
IMA symbol	Spl
Strunz classification	4.BB.05
Crystal system	Cubic
Crystal class	Hextetrahedral (43m); H–M symbol: (43m)
Space group	F 4 3 m (No. 216)
Unit cell	a = 8.0898(9) Å; Z = 8
Identification
Color	Various; red, pink, blue, lavender/violet, dark green, brown, black, colourless
Crystal habit	Octahedral or flat triangular plates caused by twinning
Twinning	common
Cleavage	None
Fracture	Conchoidal
Mohs scale hardness	7.5–8.0
Luster	Vitreous
Streak	White
Diaphaneity	Transparent 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 properties	Isotropic
Refractive index	1.719
Pleochroism	Absent
Solubility	None
Other characteristics	Weak to medium magnetic, sometimes fluorescent (red synthetic yes, natural red sometimes)
References

Spinel (/spɪˈnɛl, ˈspɪnəl/^[7]) is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula MgAl
₂O
₄ in the cubic crystal system. Its name comes from the Latin word spinella, a diminutive form of spine, in reference to its pointed crystals.^[5]

Properties

Spinel crystallizes in the isometric system; common crystal forms are octahedra, usually twinned. It has no true cleavage, but shows an octahedral parting and a conchoidal fracture.^[8] 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 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; among them are the Black Prince's Ruby and the "Timur ruby" in the British Crown Jewels,^[11] and the "Côte de Bretagne", formerly from the French Crown jewels.^[12] The Samarian Spinel is the largest known spinel in the world, weighing 500 carats (100 g).^[13]

The transparent red spinels were called spinel-rubies^[14] or balas rubies.^[15] 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.^[16] "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. However, "Balascia" itself may be derived from Sanskrit bālasūryaka, which translates as "crimson-coloured morning sun".^[17] Mines in the Gorno Badakhshan region of Tajikistan constituted for centuries the main source for red and pink spinels.^[16]

Occurrence

Geologic occurrence

Spinel is found as a metamorphic mineral in metamorphosed limestones and silica-poor mudstones.^[9] It also occurs 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 ^[18] or rock (e.g. gabbro, troctolite).^[19]^[20]

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.^[21] 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.^[22]

Spinel, (Mg,Fe)Al₂O₄, is a common mineral in the Ca-Al-rich inclusions (CAIs) in some chondritic meteorites.^[23]

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).^[16]^[24]

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 of Baffin Island (Canada).^[27] The pure blue coloration of spinel is caused by small additions of cobalt.^[28]

Synthetic spinel

Synthetic spinel can be produced by similar means to synthetic corundum, including the Verneuil method and the flux method pioneered by Edmond Frémy. It is widely used as an inexpensive cut gem in birthstone jewelry for the month of August. 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 through sintering.^[30] Synthetic spinel, which looks like glass but has notably higher strength against pressure, can also have applications in military and commercial use.^[31]

References

^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
^ Robert John Lancashire. "Normal Spinels". CHEM2101 (C 21J) Inorganic Chemistry – Chemistry of Transition Metal Complexes. University of the West Indies. Archived from the original on 2018-08-08.
^ 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. S2CID 96560029.
^ L. Hwang; et al. (Jul 1973). "On the space group of MgAl
₂O
₄ spinel". Philosophical Magazine. doi:10.1080/14786437308217448.
^ ^a ^b Spinel, Mindat.org
^ Spinel Mineral Data, WebMineral.com
^ "spinel". The Chambers Dictionary (9th ed.). Chambers. 2003. ISBN 0-550-10105-5.
^ Nesse, William D. (2000). Introduction to mineralogy. New York: Oxford University Press. pp. 362–363. ISBN 9780195106916.
^ ^a ^b Klein, Cornelis; Hurlbut, Cornelius S. Jr. (1993). Manual of mineralogy : (after James D. Dana) (21st ed.). New York: Wiley. ISBN 047157452X.
^ "Minerals Colored by Metal Ions". minerals.gps.caltech.edu. Retrieved 2023-03-01.
^ Sir Thomas Butler (1989). The Crown Jewels and Coronation Ceremony. Pitkin. p. 6. ISBN 978-0-85372-467-4.
^ Pardieu, V.; Farkhodova, T. (Summer 2019). "Spinel from Tajikistan". InColor: 30–33. Retrieved 28 April 2021.
^ "Samarian spinel". Dictionary of Gems and Gemology: 657–737. 2005. doi:10.1007/3-540-27269-0_19.
^ Lytvynov, L.A. (2011). "On the words used as names for ruby and sapphire" (PDF). Functional Materials. 18 (2): 275. Retrieved 29 April 2021.
^ Hughes, R.W. (1994). "The rubies and spinels of Afghanistan: A brief history" (PDF). Journal of Gemmology. 24 (4): 256–267. doi:10.15506/JoG.1994.24.4.256. Retrieved 29 April 2021.
^ ^a ^b ^c Pardieu & Farkhodova 2019.
^ 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. ASIN B002A9M6QU.
^ 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.
^ 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. hdl:1983/43578f76-07c8-4676-84d1-d763d5228efb. S2CID 129562202.
^ 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.
^ 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.
^ Philpotts, Anthony R.; Ague, Jay J. (2009). Principles of igneous and metamorphic petrology (2nd ed.). Cambridge, UK: Cambridge University Press. p. 17. ISBN 9780521880060.
^ MacPherson, G.J. (2007). "Calcium–Aluminum-Rich Inclusions in Chondritic Meteorites". Treatise on Geochemistry: 1–47. doi:10.1016/B0-08-043751-6/01065-3. ISBN 9780080437514.
^ Pardieu, Vincent; Hughes, R. W.; Boehm, E. (2008). "Spinel: Resurrection of a classic". InColor Magazine: 10–18. Retrieved 29 April 2021.
^ Pardieu, Vincent (2014). "Hunting for "Jedi" Spinels in Mogok". Gems & Gemology. 50 (1): 46–57. doi:10.5741/GEMS.50.1.46.
^ Wondermondo (16 June 2019). "Finds of cobalt blue spinel in Lục Yên, Vietnam".
^ Mining.Com (5 April 2019). "Scientists figure out origin of cobalt-blue spinel in Canada's Arctic".
^ Boris Chauviré, Benjamin Rondeau, Emmanuel Fritsch, Phillipe Ressigeac, and Jean-Luc Devidal (Spring 2015). "Blue Spinel From the Luc Yen District of Vietnam". Gems & Gemology.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^ Simon & Schuster's Guide to Gems and Precious Stones, K. Lyman, 1996
^ "Researchers finding applications for tough spinel ceramic". Phys.org. 24 April 2015.
^ "Transparent Armor from NRL; Spinel Could Also Ruggedize Your Smart Phone". Naval Research Laboratory. 23 April 2015. Archived from the original on 4 August 2023. Retrieved 4 August 2023.

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.