Spinel

Spinel
General
Category	Mineral
Chemical formula	MgAl2O4
Identification
	{{{molweight}}}
Color	Various, red to blue to mauve. Dark green, brown. Black
Crystal habit	Cubic, octahedral
Crystal system	Isometric
Cleavage	Indistinct
Fracture	Conchoidal, uneven
Mohs Scale hardness	8.0
Luster	Vitreous
Refractive index	1.712-1.762
Pleochroism	Absent
Streak	White
Specific gravity	3.54-3.63
	{{{density}}}

The spinels are an important group of oxide minerals with similar crystalline structures. Their general chemical formula may be written as (X)(Y)₂O₄, in which X and Y represent cations of various metals or silicon. They are named after the representative mineral spinel, which corresponds to magnesium aluminum oxide (MgAl₂O₄).

This group consists of more than 20 minerals, including valuable ores such as chromite, a source of chromium; magnetite, a source of iron; and franklinite, once a source of iron, manganese, and zinc. The representative member, spinel, produces gem-quality stones.

Characteristics

Spinels crystallize in the isometric (cubic) system, usually with an "octahedral habit"—that is, outwardly shaped as octahedrons. The general chemical formula is (X)(Y)₂O₄, in which X and Y represent various divalent, trivalent, and quadrivalent cations. They include the cations of magnesium, zinc, iron, manganese, aluminum, chromium, titanium, and silicon.

In the normal spinel structure, the oxide anions are arranged in a cubic close-packed structure, the X cations occupy tetrahedral sites, and the Y cations, octahedral sites. For "inverse spinels," half the Y cations occupy the tetrahedral sites, and both X and Y cations occupy octahedral sites.

Spinel group members

The spinel group includes the following minerals:

• Spinel (MgAl₂O₄): This representative member of the group can be found in a range of colors, including red, pink, blue, green, violet, brown, and black.
• Chromite ((Fe,Mg)Cr₂O₄): This black mineral is an important ore of chromium.
• Franklinite ((Fe,Mn,Zn)(Fe,Mn)₂O₄): This black mineral was named in honor of Benjamin Franklin. It was once a source of iron, manganese, and zinc.
• Gahnite (ZnAl₂O₄): A rare mineral, it can be found in various colors, including dark blue-green, yellow, brown, and black.
• Hercynite (FeAl₂O₄): Dark blue-green, yellow, or brown in color, it was discovered in Silva Hercynia, the Latin name for the Bohemian Forest.
• Jacobsite (MnFe₂O₄): Named after Jakobsberg, Sweden, it is usually black or gray in color.
• Magnetite (Fe₃O₄): This black or gray mineral is one of several iron oxides and an important ore of iron.
• Trevorite (NiFe₂O₄): This rare variety of spinel is black with a brown or green tint.
• Ulvöspinel (TiFe₂O₄): This black or brown spinel was named after Ulvö Island, Sweden, a mining area for iron, titanium, and vanadium.

Occurrence

True spinel has long been found in the gemstone-bearing gravel of Sri Lanka and in limestones of Myanmar and Thailand.

Spinel usually occurs in isometric crystals, octahedrons, 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 lustre. 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. Another famous spinel is the Black Prince's Ruby in the British Crown Jewels.

The transparent red spinels are called spinel-rubies or balas-rubies and were often confused with actual rubies in ancient times. "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. Yellow spinel is called rubicelle and violet-colored manganese-bearing spinel is called almandine.

Spinel is found as a metamorphic mineral and also as a primary mineral in basic rocks, because in such magmas the absence of alkalis prevents the formation of feldspars and any aluminium oxide present will form corundum or 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 the Mohorovicic discontinuity (the Moho) and a depth of 70 kilometers or so; below that depth, the spinel (if present) becomes increasingly rich in chromium, as with increasing depth, pyrope-rich garnet becomes the more stable aluminous mineral in peridotite. At depths significantly shallower than the Moho, calcic plagioclase is the more stable aluminous mineral in peridotite.

See also

• Gemstone
• Mineral

References

ISBN links support NWE through referral fees

• Farndon, John. 2006. The Practical Encyclopedia of Rocks & Minerals: How to Find, Identify, Collect and Maintain the World's best Specimens, with over 1000 Photographs and Artworks. London: Lorenz Books. ISBN 0754815412 and ISBN 978-0754815419.

• Klein, Cornelis, and Barbara Dutrow. 2007. Manual of Mineral Science. 23rd ed. New York: John Wiley. ISBN 0471721573 and ISBN 978-0471721574.

• Pellant, Chris. 2002. Rocks and Minerals. Smithsonian Handbooks. New York: Dorling Kindersley. ISBN 0789491060 and ISBN 978-0789491060.

• Shaffer, Paul R., Herbert S. Zim, and Raymond Perlman. 2001. Rocks, Gems and Minerals. Rev. ed. New York: St. Martin's Press. ISBN 1582381321 and ISBN 978-1582381329.

• Deer, Howie and Zussman. 1966. An Introduction to the Rock Forming Minerals. Longman. ISBN 0-582-44210-9.

• Shumann, Walter. 2006. Gemstones of the World 3rd ed. Sterling.

External links

• Spinel Structure, by Steven Dutch. Retrieved April 6, 2007.

• Ionic Crystals, including the structure of spinel. Retrieved April 6, 2007.

• Spinel Compounds: Background and Historical Perspective. Retrieved April 6, 2007.

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