Magnetite - Fe²⁺Fe₂³⁺O₄

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: Al₂O₄

chromite series:

chromite: Fe²⁺Cr₂O₄; magnesiochromite: MgCr₂O_4.

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

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

Photo
Skeletal pyroxenes and skeletal magnetite. Komatiite from Alexo, Canada. PPL image, 10x (Field of view = 2mm)	Skeletal pyroxenes and skeletal magnetite. Komatiite from Alexo, Canada. PPL image, 10x (Field of view = 2mm)	Skeletal pyroxenes and skeletal magnetite. Komatiite from Alexo, Canada. PPL image, 10x (Field of view = 2mm)
Skeletal pyroxenes and skeletal magnetite. Komatiite from Alexo, Canada. PPL image, 10x (Field of view = 2mm)	Skeletal pyroxenes and skeletal magnetite. Komatiite from Alexo, Canada. PPL image, 10x (Field of view = 2mm)	Skeletal pyroxenes and skeletal magnetite. Komatiite from Alexo, Canada. PPL image, 10x (Field of view = 2mm)
Magnetite crystals in a vosegite (lamprophyre). PPL image, 10x (Field of view = 2mm)	Magnetite crystals in a vosegite (lamprophyre). PPL image, 10x (Field of view = 2mm)	Magnetite crystals in a vosegite (lamprophyre). PPL image, 10x (Field of view = 2mm)
Magnetite crystals in a basalt. PPL image, 10x (Field of view = 2mm)	Magnetite crystals in a basalt. PPL image, 10x (Field of view = 2mm)	Magnetite crystals in a basalt. PPL image, 10x (Field of view = 2mm)
Magnetite crystals in a basalt. PPL image, 10x (Field of view = 2mm)