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Clinopyroxene Group: Overview of Chemical Composition and Geological Significance

This presentation by Joyleen Desai and Paul Sandlin delves into the chemical composition, structural cell parameters, and geological occurrences of the clinopyroxene group. It includes information on metamorphic facies, geothermobarometry, thermodynamic modeling, and thermobarometers. The significance of clinopyroxenes in the upper mantle and their stability at extreme pressures is also discussed. Sources cited include webmineral.com and research by renowned authors in the field. Learn more about the importance of clinopyroxenes in geological studies.

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Clinopyroxene Group: Overview of Chemical Composition and Geological Significance

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  1. Clinopyroxene GroupABSi2O6 Presented by Joyleen Desai Paul Sandlin

  2. Diopside Hedenbergite Acmite Jadeite Spodumene

  3. Chemical CompositionABSi2O6

  4. Structural Cell Parameters Monoclinic Space Group: C2/c Point Group: 2/m Source: http://webmineral.com

  5. Structure

  6. Thermal Expansion Source: Cameron et al. (1973)

  7. Compression Source: http://www.ciw.edu/schutt/depletion/depletion_intro.html

  8. Occurrences • Clinopyroxenes are found in mafic and ultramafic igneous rocks • They are also found in medium- to high-grade metamorphosed mafic rocks

  9. Metamorphic Facies Source: Spear (1995)

  10. Geothermobarometry • Inferring T and P • Thermodynamic modeling and calculations, electron microprobe • Assume equilibrium for sub-assemblages • Possible to infer PT conditions at various stages in rock history • Can’t find ages, but can construct PT time path

  11. Garnet-Cpx Thermobarometer Source: Spear (1995)

  12. Diopside-Enstatite Geothermometer • Below the solvus, a pyroxene of composition X will unmix to produce two pyroxenes • The degree of unmixing is a function of temperature Source: Perkins (1998)

  13. Crystal Structure

  14. Hydrous Sodic Cpx in the Upper Mantle Source: Smyth et al. (1991)

  15. Hydrous Importance • Cpx in eclogites as mantle samples • X-ray crystal structure refinement and analytical transmission electron microscopy (McCormick, 1996) • OH at dominant 3470 cm-1 band • O2 site is underbonded according to Pauling bond strength • M2 site is the most likely candidate for OH, 250-1840 ppm by weight • Cpx is stable at P=12 GPa, 30-400 km • Cpx is 5-10 times more abundant in mantle rocks

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