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Chemical Formula
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MgAl2O4
The Mg may be partially or fully replaced by Fe, Zn, and/or Mn. This leads to a mineral series with the formula (Mg,Fe,Zn,Mn)Al2O4. The end-members of the Spinel group are individually categorized:
Spinel (or Magnesium Spinel) - MgAl2O4
Gahnite (Zinc Spinel) - ZnAl2O4
Hercynite (Iron Spinel) - FeAl2O4
Galaxite (Manganese Spinel) - MnAl2O4
The above formula for Galaxite is only for pure Galaxite. However, Galaxite will always contain traces of magnesium, iron, and aluminum, so its formula is better recognized as:
(Mn,Fe2+,Mg)(Al,Fe3+)2O4
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Composition
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Series of magnesium, iron, zinc, and/or manganese aluminum oxide.
Spinel is Magnesium aluminum oxide
Gahnite is Zinc aluminum oxide
Hercynite is Iron aluminum oxide
Galaxite is Manganese aluminum oxide, with some magnesium, iron, and aluminum
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Variable Formula
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(Mg,Mn,Fe2+)(Al,Fe3+)2O4
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Color
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Red, pink, purple, blue, green, aqua, orange, yellow, brown, gray, and black.
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Streak
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White
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Hardness
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7.5 - 8
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Crystal System
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Isometric
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Crystal Forms
and Aggregates
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Spinel occurs in octahedral crystals, often in perfectly shaped octahedrons. Crystals often have parallel layer growths or heavy etchings. Crystals can also be distorted as well as have complex faces. Crystal clusters and twinned octahedrons are very common, especially spinel twinning, which this mineral is famous for. An interesting habit occasionally observed in Spinel is the macle, which is a triangular form of two flattened crystal twins. Spinel also occurs grainy and as rounded, waterworn pebbles.
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Transparency
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Transparent to opaque
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Specific Gravity
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3.5 - 4.1
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Luster
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Vitreous, submetallic, or dull
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Cleavage
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None, but may exhibit slight octahedral parting
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Fracture
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Conchoidal
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Tenacity
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Brittle
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Other ID Marks
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Red, pink, and violet Spinel often fluoresces red or neon-yellow in shortwave ultraviolet light.
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Complex Tests
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Insoluble in acids
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In Group
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Oxides; Multiple Oxides
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Striking Features
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Crystal habit and high hardness
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Environment
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In metamorphic marbles and hornfels, as well as in plutonic rocks such as gabbro. Also in alluvial placerdeposits.
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Rock Type
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Igneous, Metamorphic
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Popularity (1-4)
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1
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Prevalence (1-3)
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3
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Demand (1-3)
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1
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