Chromite - Fe2+Cr2O4

The spinel group minerals is represented by a dozen minerals with a generalized formula A2+B23+O2 With a an essentially cubic close-packed oxygen arrangement with two different cation types in sixfold (octahedral) and fourfold (tetrahedral) coordination between the (111) close-packed layers.

Three series are defined by the dominant trivalent ion:
Spinel series: with dominant Al3+ ion
Magnetite series: with Fe3+ ion
Chromite series: with Cr3+ ion

Each series is further defined by the dominant bivalent ion:

Spinel series:

Common spinel: MgAl2O4
Hercynite: FeAl2O4
Gahnite: ZnAl2O4
Galaxite: MnAl2O4

chromite series:

Chromite: Fe2+Cr2O4
Magnesiochromite: MgCr2O4
magnetite series:

Magnetite: Fe2+Fe23+O4
Magnesioferrite: MgFe23+O4
Franklinite: ZnFe23+O4
Jacobsite: MnFe23+O4
Trevorite: NiFe23+O4

Two structural modification are recognized in the spinel group:

Normal spinel and Inverse spinel

The normal spinel the bivalent ion (A2+) appears in tetrahedral coordination with oxygen and all trivalent ion (B3+) in octahedral coordination, even though the bivalent ion are larger. The member of spinel and chromite series are "normal spinel".
The inverse spinel structure is characterized by all bivalent ion (A2+) in octahedral coordination and the trivalent ion (B3+) distributed evenly between tetrahedral and remaining octahedral sites. Magnetite spinel series are "inverse spinel".

Optical Properties

• Color: spinel are opaques , and grey in reflected light
• Form: normally forms octahedral crystals that yield triangular, square and trapezoid section

Magnetite is one of the most ubiquitous of all minerals, appears in a vast variety of igneous rocks, metamorphic and sedimentary. All plutonic rocks contain magnetite as highly disseminated accessory crystals, it may be also concentrated by crystal fractionation to form the major or only constituent of large rock masses, like in the Bushveld Igneous Complex in south Africa.

Bibliography



• Deer, W.A., Howie, R.A., Zussman, J. (1998) Rock-forming Minerals.
• Optical Mineralogy : The Nonopaque Minerals by Phillips / Griffen
• E. WM. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc


Photo
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Chromite crystals in a Venanzite from S.Venanzo (Italy). PPL image, 10x (Field of view = 2mm)
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Chromite crystals in a Venanzite from S.Venanzo (Italy). PPL image, 2x (Field of view = 7mm)
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Chromite crystals with magnetite unmixing in a Venanzite from S.Venanzo (Italy). PPL image, 10x (Field of view = 2mm)
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Chromite crystals with magnetite unmixing in a Venanzite from S.Venanzo (Italy). PPL image, 10x (Field of view = 2mm)
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Chromite crystals in a Venanzite from S.Venanzo (Italy). PPL image, 20x (Field of view = 1mm)
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Chromite crystals with magnetite unmixing in a Venanzite from S.Venanzo (Italy). PPL image, 10x (Field of view = 2mm)
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Chromite crystals and rounded olivine crystals set in a groundmass composed by calcite. Kimberlite from Bloemfontein (S.Africa). PPL image, 2x (Field of view = 7mm)
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Chromite crystals and rounded olivine crystals set in a groundmass composed by calcite. Kimberlite from Bloemfontein (S.Africa). PPL image, 2x (Field of view = 7mm)
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Chromite crystals and rounded olivine crystals set in a groundmass composed by calcite. Kimberlite from Bloemfontein (S.Africa). PPL image, 2x (Field of view = 7mm)
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Chromite crystals and rounded olivine crystals set in a groundmass composed by calcite. Kimberlite from Bloemfontein (S.Africa). PPL image, 2x (Field of view = 7mm)
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Chromite crystals and rounded olivine crystals set in a groundmass composed by calcite. Kimberlite from Bloemfontein (S.Africa). PPL image, 2x (Field of view = 7mm)
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Chromite crystals and rounded olivine crystals set in a groundmass composed by calcite. Kimberlite from Bloemfontein (S.Africa). PPL image, 2x (Field of view = 7mm)
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Chromite crystals and rounded olivine crystals set in a groundmass composed by calcite. Kimberlite from Bloemfontein (S.Africa). PPL image, 2x (Field of view = 7mm)
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Chromite crystals and rounded olivine crystals set in a groundmass composed by calcite. Kimberlite from Bloemfontein (S.Africa). PPL image, 2x (Field of view = 7mm)
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Chromite crystals and rounded olivine crystals set in a groundmass composed by calcite. Kimberlite from Bloemfontein (S.Africa). PPL image, 2x (Field of view = 7mm)