Difference between revisions of "Spinel" - New World Encyclopedia
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| − | The '''spinels''' are | + | The '''spinels''' are an important group of [[oxide]] [[mineral]]s with similar [[crystal]]line structures. Their general chemical formula may be written as (X)(Y)<sub>2</sub>[[oxygen|O]]<sub>4</sub>, in which X and Y represent [[cation]]s of various [[metal]]s or [[silicon]]. They are named after the representative mineral ''spinel'', which corresponds to magnesium aluminum oxide (MgAl<sub>2</sub>O<sub>4</sub>). |
| − | The oxygen anions are arranged in a cubic close-packed structure | + | 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 [[Crystal|isometric]] (cubic) system, usually with an "octahedral [[Crystal|habit]]"—that is, outwardly shaped as octahedrons. The general chemical formula is (X)(Y)<sub>2</sub>[[oxygen|O]]<sub>4</sub>, 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 == | == Spinel group members == | ||
| − | + | The spinel group includes the following minerals: | |
| − | * [[Chromite]] (( | + | * Spinel (MgAl<sub>2</sub>O<sub>4</sub>): The representative member of the group. |
| − | * [[Franklinite]] ((Fe,Mn,Zn)(Fe,Mn)<sub>2</sub>O<sub>4</sub>) | + | * [[Chromite]] ((Fe,Mg)Cr<sub>2</sub>O<sub>4</sub>): An important ore of chromium. |
| − | * [[Gahnite]] (ZnAl<sub>2</sub>O<sub>4</sub>) | + | * [[Franklinite]] ((Fe,Mn,Zn)(Fe,Mn)<sub>2</sub>O<sub>4</sub>): Once a source of iron, manganese, and zinc. |
| + | * [[Gahnite]] (ZnAl<sub>2</sub>O<sub>4</sub>): A rare mineral. | ||
* [[Hercynite]] (FeAl<sub>2</sub>O<sub>4</sub>) | * [[Hercynite]] (FeAl<sub>2</sub>O<sub>4</sub>) | ||
| − | * [[Jacobsite]] (MnFe<sub>2</sub>O<sub>4</sub>) | + | * [[Jacobsite]] (MnFe<sub>2</sub>O<sub>4</sub>): Named for the location of Jakobsberg, Sweden. |
| − | * [[Magnetite]] (Fe<sub>3</sub>O<sub>4</sub>): one of the main ores of iron. | + | * [[Magnetite]] (Fe<sub>3</sub>O<sub>4</sub>): One of several iron oxides, it is one of the main ores of iron. |
| − | * [[Ringwoodite]] (SiMg<sub>2</sub>O<sub>4</sub>): | + | * [[Ringwoodite]] (SiMg<sub>2</sub>O<sub>4</sub>): An abundant [[olivine]] [[polymorphism (materials science)|polymorph]] within the [[Earth's mantle]] (about 520 to 660 kilometer depth), and a rare mineral in meteorites. |
| − | * [[Trevorite]] (NiFe<sub>2</sub>O<sub>4</sub>) | + | * [[Trevorite]] (NiFe<sub>2</sub>O<sub>4</sub>): A rare, black variety of spinel. |
| − | * [[Ulvöspinel]] (TiFe<sub>2</sub>O<sub>4</sub>) | + | * [[Ulvöspinel]] (TiFe<sub>2</sub>O<sub>4</sub>): Named after the Ulvö Islands (Ångermanland, [[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 [[limestone]]s of [[Myanmar]] and [[Thailand]]. | True spinel has long been found in the [[gemstone]]-bearing gravel of [[Sri Lanka]] and in [[limestone]]s of [[Myanmar]] and [[Thailand]]. | ||
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The transparent red spinels are called spinel-rubies or balas-rubies and were often confused with actual [[ruby|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. | The transparent red spinels are called spinel-rubies or balas-rubies and were often confused with actual [[ruby|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 rock|metamorphic mineral]] | + | Spinel is found as a [[metamorphic rock|metamorphic mineral]] and also as a primary mineral in basic rocks, because in such [[magma]]s the absence of [[alkali]]s prevents the formation of [[feldspar]]s 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)<sub>2</sub>O<sub>4</sub>, 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. | Spinel, (Mg,Fe)(Al,Cr)<sub>2</sub>O<sub>4</sub>, 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. | ||
Revision as of 17:26, 6 April 2007
| 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): The representative member of the group.
- Chromite ((Fe,Mg)Cr2O4): An important ore of chromium.
- Franklinite ((Fe,Mn,Zn)(Fe,Mn)2O4): Once a source of iron, manganese, and zinc.
- Gahnite (ZnAl2O4): A rare mineral.
- Hercynite (FeAl2O4)
- Jacobsite (MnFe2O4): Named for the location of Jakobsberg, Sweden.
- Magnetite (Fe3O4): One of several iron oxides, it is one of the main ores of iron.
- Ringwoodite (SiMg2O4): An abundant olivine polymorph within the Earth's mantle (about 520 to 660 kilometer depth), and a rare mineral in meteorites.
- Trevorite (NiFe2O4): A rare, black variety of spinel.
- Ulvöspinel (TiFe2O4): Named after the Ulvö Islands (Ångermanland, 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
Notes
ReferencesISBN 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.
- Spinel structure. Retrieved April 6, 2007.
- Spinel: History and Mystery of a Magnificient Gem Retrieved April 6, 2007.
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