Spinel

Spinel
Spinel: 1.83 and 4.13 ct
General
Category	Oxide minerals Spinel group Spinel structural group
Formula (repeating unit)	MgAl2O4
Strunz classification	4.BB.05
Crystal system	Isometric
Unit cell	a = 8.0898(9) Å; Z = 8
Identification
Color	Various; red and pink to blue to mauve, dark green, brown, black
Crystal habit	Cubic, octahedral
Crystal symmetry	Isometric – hexoctahedral H-M symbol (4/m 3 2/m) Space group: Fd3m
Cleavage	Indistinct
Fracture	Conchoidal, uneven
Mohs scale hardness	7.5–8.0
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	3.6–4.1
Optical properties	Isotropic
Refractive index	1.719
Pleochroism	Absent
Solubility	none
Other characteristics	Nonmagnetic, non-radioactive, sometimes fluorescent (red)
References	[1][2]

Spinel /ˈspɪnɛl/ 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 Latin "spina" (arrow).^[1] Balas ruby is an old name for a rose-tinted variety.

Properties of true spinel

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 red, blue, green, yellow, brown, or black. 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 Kokcha River, one of the principal tributaries of the Oxus River. The Badakshan Province was for centuries the main source for red and pink spinels.

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 of Mogok in Burma. Recently gem quality spinels were also found in the marbles of Luc Yen (Vietnam), Mahenge and Matombo (Tanzania), Tsavo (Kenya) and in the gravels of Tunduru (Tanzania) and Ilakaka (Madagascar). 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.

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.^[3] 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.

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.^[4] By 2015, transparent spinel was being made in sheets and other shapes through sintering.^[5] Synthetic spinel which looks like glass but has notably higher strength against pressure, can also have applications in military and commercial use.^[6]

See also

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

References

1. Spinel. Mindat.org
2. Barthelmy, Dave. "Spinel Mineral Data". Webmineral.
3. Klemme, Stephan (2004). "The influence of Cr on the garnet–spinel transition in the Earth's mantle: Experiments in the system MgO–Cr₂O_3–SiO2 and thermodynamic modelling" (PDF). Lithos 77: 639. doi:10.1016/j.lithos.2004.03.017.
4. "SSEF – Leader in coloured gemstone, diamond and pearl testing and certification". Retrieved 17 March 2015.
5. "Researchers finding applications for tough spinel ceramic". Phys.org. 24 April 2015.
6. "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.

External links

• Spinel structure at the University of Wisconsin - Green Bay
• Spinel structure at the Institut for materials science of the University of Kiel
• Value of Spinel