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Spinel
""Spinel-Pargasite-Marble-66637.jpg
Spinel on top of a "pargasite, and both perched and centered atop a marble matrix
General
Category "Oxide minerals
"Spinel group
Spinel structural group
"Formula
(repeating unit)
MgAl2O4
"Strunz classification 4.BB.05
"Crystal system "Cubic
"Crystal class Hexoctahedral (m3m)
"H-M symbol: (4/m 3 2/m)
"Space group Fd3m
"Unit cell a = 8.0898(9) Å; Z = 8
Identification
Color Various; red, pink, blue, lavender/violet, dark green, brown, black, colourless
"Crystal habit Octehedra 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 3.58-3.61
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 [1][2]

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 Latin "spina" (arrow).[1] Balas ruby is an old name for a rose-tinted variety of spinel.

Contents

Properties[edit]

""
""
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[edit]

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 "Burma. Recently gem quality spinels 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. The spinel petrogenesis in mafic magmatic rocks is strongly debated, but certainly results from mafic magma interaction with more evolved magma [3] or rock (e.g. gabbro, troctolite)[4][5].

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.[6] 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[edit]

Synthetic spinel, accidentally produced in the middle of the 18th century, has been described more recently in scientific publications in 2000 and 2004.[7] By 2015, transparent spinel was being made in sheets and other shapes through "sintering.[8] Synthetic spinel which looks like glass but has notably higher strength against pressure, can also have applications in military and commercial use.[9]

See also[edit]

References[edit]

  1. ^ a b Spinel. Mindat.org
  2. ^ Barthelmy, Dave. "Spinel Mineral Data". Webmineral. 
  3. ^ Irvine TN (1977). "Origin of chromite layers in the Muskox intrusion and other stratiform intrusions: a new perspective". Geology. 5: 273. "doi:10.1130/0091-7613(1977)5<273:ooclit>2.0.co;2. 
  4. ^ 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: 12. "doi:10.1007/s00410-015-1165-0. 
  5. ^ 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: 6–20. 
  6. ^ 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: 639. "doi:10.1016/j.lithos.2004.03.017. 
  7. ^ "SSEF – Leader in coloured gemstone, diamond and pearl testing and certification". Retrieved 17 March 2015. 
  8. ^ "Researchers finding applications for tough spinel ceramic". Phys.org. 24 April 2015. 
  9. ^ "Transparent Armor from NRL; Spinel Could Also Ruggedize Your Smart Phone". Naval Research Laboratory. 23 April 2015. 

Bibliography[edit]

External links[edit]

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