Spinel
General
Category	Mineral
Chemical formula	MgAl2O4
Identification
Color	Various, red to blue to mauve. Dark green, brown. Black
Crystal habit	Cubic, octahedral
Crystal system	Isometric
Cleavage	Absent
Fracture	Conchoidal, uneven
Mohs Scale hardness	8.0
Luster	Vitreous
Refractive index	Transparent to opaque
Streak	White
Specific gravity	3.6 - 3.7

The spinels are any of a class of minerals which crystallize in the isometric system with an octahedral habit. The general formula is as (X)(Y)₂O₄, with X representing cations occupying tetrahedral sites and Y cations occupying octahedral sites. Divalent, trivalent, and quadrivalent cations can occupy the X and Y sites, and they include magnesium, zinc, iron, manganese, aluminium, chromium, titanium, and silicon. Image File history File linksMetadata Download high-resolution version (1673x1254, 538 KB) Spinel. ... Minerals are natural compounds formed through geological processes. ... A chemical formula (also called molecular formula) is a concise way of expressing information about the atoms that constitute a particular chemical compound. ... In mineralogy, shape and size give rise to descriptive terms applied to the typical appearance, or habit of crystals. ... In mineralogy and crystallography, a crystal structure is a unique arrangement of atoms in a crystal. ... Cleavage, in mineralogy, is the tendency of crystalline materials to split along definite planes, creating smooth surfaces, of which there are several named types: Basal cleavage: cleavage parallel to the base of a crystal, or to the plane of the lateral axes. ... A fracture is the separation of a body into two, or more, pieces under the action of stress. ... Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer. ... Lustre (American English: luster) is a description of the way light interacts with the surface of a crystal, rock or mineral. ... The refractive index (or index of refraction) of a material is the factor by which the phase velocity of electromagnetic radiation is slowed in that material, relative to its velocity in a vacuum. ... Minerals are natural compounds formed through geological processes. ... Relative density (also known as specific gravity) is a measure of the density of a material. ... Minerals are natural compounds formed through geological processes. ... For other senses of this word, see crystal (disambiguation). ... In crystallography, the cubic crystal system (or isometric crystal system) is the most symmetric of the 7 crystal systems. ... General Name, Symbol, Number oxygen, O, 8 Chemical series Nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Atomic mass 15. ... An ion is an atom or group of atoms with a net electric charge. ... General Name, Symbol, Number magnesium, Mg, 12 Chemical series alkaline earth metals Group, Period, Block 2, 3, s Appearance silvery white Atomic mass 24. ... General Name, Symbol, Number zinc, Zn, 30 Chemical series transition metals Group, Period, Block 12, 4, d Appearance bluish pale gray Atomic mass 65. ... General Name, Symbol, Number iron, Fe, 26 Chemical series transition metals Group, Period, Block 8, 4, d Appearance lustrous metallic with a grayish tinge Atomic mass 55. ... General Name, Symbol, Number manganese, Mn, 25 Chemical series transition metals Group, Period, Block 7, 4, d Appearance silvery metallic Atomic mass 54. ... General Name, Symbol, Number aluminium, Al, 13 Chemical series poor metals Group, Period, Block 13, 3, p Appearance silvery Atomic mass 26. ... General Name, Symbol, Number chromium, Cr, 24 Chemical series transition metals Group, Period, Block 6, 4, d Appearance silvery metallic Atomic mass 51. ... General Name, Symbol, Number titanium, Ti, 22 Chemical series transition metals Group, Period, Block 4, 4, d Appearance silvery metallic Atomic mass 47. ... General Name, Symbol, Number silicon, Si, 14 Chemical series metalloids Group, Period, Block 14, 3, p Appearance as coarse powder, dark gray with bluish tinge Atomic mass 28. ...

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

Important members of the spinel group include:

• Spinel – MgAl₂O₄, after which this class of minerals is named
• Gahnite - ZnAl₂O₄
• Franklinite - (Fe,Mn,Zn)(Fe,Mn)₂O₄
• Chromite - (Fe·Mg)Cr₂O₄
• Magnetite - Fe₃O₄
• Hercynite - FeAl₂O₄
• Ulvospinel - TiFe₂O₄
• Jacobsite - MnFe₂O₄
• Trevorite - NiFe₂O₄
• Ringwoodite - SiMg₂O₄, an abundant olivine polymorph within the Earth's mantle from about 520 to 660 km depth, and a rare mineral in meteorites

True spinel has long been found in the gemstone-bearing gravel of Sri Lanka and in limestones of Myanmar and Thailand. Gahnite, ZnAl2O4, is a rare mineral belonging to the spinel group. ... Spinel is one of a group of minerals which crystallize in the isometric system with an octahedral habit, and whose chemical compositions are analogous. ... Chromite, iron magnesium chromium oxide: (Fe,Mg)Cr2O4, is an oxide mineral belonging to the spinel group. ... // Headline text Magnetite is a ferrimagnetic mineral form of iron(II,III) oxide, with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. ... Emery (also known as iron spinel and hercynite) is a common impure variety of the mineral corundum. ... ... Jacobsite is a manganese iron oxide mineral. ... Trevorite is a very rare nickeliferous variety of spinel. ... Olivine basalt The mineral olivine is a magnesium iron silicate with the formula (Mg,Fe)2SiO4. ... Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. ... Earth cutaway from core to exosphere. ... A selection of gemstone pebbles made by tumbling rough rock with abrasive grit, in a rotating drum. ... Limey shale overlaid by limestone. ...

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. Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. ... Relative density (also known as specific gravity) is a measure of the density of a material. ... Red may be any of a number of similar colours at the lowest frequencies of light discernible by the human eye. ... For other uses, see Blue (disambiguation). ... For other uses, see Green (disambiguation). ... Yellow is any color of light that stimulates both the red and green cone cells of the retina, but not the blue cone cells. ... The color brown is a dark red or orange of very low intensity. ... Unlucky black cat. ... The Black Princes Ruby set in the Maltese cross in the front of the Imperial State Crown of England. ... Coronation Chair and Regalia of England The collective term Crown Jewels denotes the regalia and vestments worn by the sovereign of the United Kingdom during the coronation ceremony and at various other state functions. ...

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. Ruby is a red gemstone,Rubies can vary from a light pink to a blood red, a variety of the mineral corundum (aluminium oxide). ... Badakhshan is a region comprising parts of northeastern Afghanistan and of Tajikistan. ... World map showing the location of Asia. ... The Amu Darya (in Persian آمودریا; Darya means river in Persian) rises in the Pamirs and flows mainly north-west through the Hindu Kush, Turkmenistan, and Uzbekistan to join the Aral Sea in a large river delta. ...

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. Quartzite, a form of metamorphic rock, from the Museum of Geology at University of Tartu collection. ... Magma is molten rock located beneath the surface of the Earth (or any other rocky planet), and which often collects in a magma chamber. ... For other uses of the word alkali see Alkali (disambiguation). ... Lunar Ferroan Anorthosite #60025 (Plagioclase Feldspar). ... Corundum is the crystalline form of aluminium oxide and one of the rock-forming minerals. ...

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. Peridotite Peridotite is a dense, coarse-grained rock, consisting mostly of the minerals olivine and pyroxene. ... Earth cutaway from core to exosphere. ... Mohorovičić discontinuity. ... General Name, Symbol, Number chromium, Cr, 24 Chemical series transition metals Group, Period, Block 6, 4, d Appearance silvery metallic Atomic mass 51. ... The pyrope is a garnet. ... Garnet is a group of minerals that have been used since the Bronze Age as gemstones and abrasives. ... Lunar Ferroan Anorthosite #60025 (Plagioclase Feldspar). ...

References

• Deer, Howie and Zussman (1966) An Introduction to the Rock Forming Minerals, Longman, pp.424-433, ISBN 0-582-44210-9
• Spinel structure by Steven Dutch
• Spinel structure

Weblinks

• Spinel: History and Mystery of a Magnificient Gem