Minette

 Minette is a variety of igneous rock. More specifically, minette is a type of lamprophyre with phenocrysts of biotite, and with or without phenocrysts of hornblende, augite with a high diopside content, and olivine.

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Lamprophyres (including minette) traditionally have been defined as: (Le Maitre, 2002)

• normally occurring as porphyritic dikes
• containing matrix restricted feldspars and/or feldspathoids if present
• biotite or phlogopite is an essential mineral phase
• commonly extensively hydrothermally altered
• may contain primary calcite, zeolites and other more typically hydrothermal minerals
• relatively higher than normal contents of K₂O and/or Na₂O, H₂O, CO₂, S, P₂O₅, and Ba

Although lamprophyre nomenclature typically has been applied to igneous rocks found in dikes, the name "minette" also has been used to refer to extrusive rocks with appropriate mineralogy and texture. Examples include minettes in the Navajo Volcanic Field of the Colorado Plateau (Roden and Smith, 1979) and in the Mexican Volcanic Belt (Wallace and Carmichael, 1989). The minette lavas commonly provide better opportunities to study the magmas, because hydrothermal alteration of minette dikes is common (Wallace and Carmichael, 1989). On a purely chemical basis, an extrusive minette might be classified as potassic trachybasalt, shoshonite, or latite using the total alkali-silica diagram (see TAS classification), or as absarokite, shoshonite, or banakite using a classification sometimes applied to potassium-rich lavas. Such chemical classifications ignore the distinctive textures and mineralogies of minette.

A historical view of minette was provided by Johannsen (1937). He wrote that the name was " … used by the miners in the Vosges apparently for oolitic or granular iron ore, and possibly derived from the valley of Minkette, where it occurs…." (Vosges is in France near the German border.) Johannsen (1937) further wrote that ".. minettes are lamprophyric rocks of the syenite series. They are porphyritic and are composed essentially of orthoclase and biotite, the latter mineral forming the phenocrysts. Pure types are rare."

In a few occurrences, rocks identified as lamprophyre contain diamonds, but these rocks are relatively unimportant as sources of diamonds.

References

• Johannsen, A., 1937, A Descriptive Petrography of the Igneous Rocks: Volume III, The Intermediate Rocks. University of Chicago Press, Chicago, Illinois
• Le Maitre, R. W., editor, 2002, Igneous Rocks: A Classification and Glossary of Terms. Recommendations of the International Union of Geological Sciences, Subcommission of the Systematics of Igneous Rocks. Cambridge University Press.
• Roden, M. F. and Smith, D., 1979, Field geology, chemistry, and petrology of Buell Park minette diatreme, Apache County, Arizona: In Kimberlites, Diatremes, and Diamonds: Their Geology, Petrology, and Geochemistry, Boyd, F. R., and Meyer, H. O. A., eds., American Geophysical Union: Proceedings of the Second International Kimberlite Conference, v 1, p 364-381.
• Wallace, P., and Carmichael, I. S. E., 1989, Minette lavas and associated leucitites from the Western Front of the Mexican Volcanic Belt: petrology, chemistry, and origin. Contributions to Mineralogy and Petrology, v 103, p 470-492.