Chemistry:Clinopyroxene thermobarometry

Compositional diagram of pyroxenes. Diopside, hedenbergite and augite are main sub-minerals of clinopyroxene. Jadeite contains aluminium and is therefore not shown in the diagram.

In petrology, the mineral clinopyroxene is used for temperature and pressure calculations of the magma that produced igneous rock containing this mineral. Clinopyroxene thermobarometry is one of several geothermobarometers. Two things make this method especially useful: first, clinopyroxene is a common phenocryst in igneous rocks and easy to identify; second, the crystallization of the jadeite component of clinopyroxene implies a growth in molar volume being thus a good indicator of pressure.

The minerals and liquids involved in clinopyroxene thermobarometry are:

• Augite – (Ca,Mg,Fe)SiO₃
• Diopside – MgCaSi₂O₆
• Hedenbergite – CaFeSi₂O₆
• Jadeite – Na(Al,Fe³⁺)Si₂O₆

When these minerals are still molten, the metals will exchange with each other. The extent to which they react depends on the environmental conditions and the chemical equilibrium. Therefore, the proportion of metals in each mineral and the chemical equilibrium can be used to determine the temperature and pressure each mineral formed at.

Magnified image of komatilites that have clinopyroxene crystals within. The crystals could be used for thermobarometry of the komatilite mineral. Cpx represents clinopyroxene, ol represents olivine, and gl represents glass.