Electron Petrography of Exsolution Textures in an Iron-Rich Clinopyroxene

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Abstract
The stability of chloritoid, FeAl2SiO6H2O, was investigated at fluid pressures less than 10 kb. At oxygen fugacities defined by the Ni-NiO buffer, chloritoid reacts to Fe-cordierite and hercynite spinel at 550 and 575 °C at 1 and 2 kb fluid pressure. At pressures between 2.5 and 3.5 kb the assemblage aluminous ferro-anthophyllite, staurolite and hercynite spinel appears. The breakdown of chloritoid to this assemblage takes place at 625, 650, and 675 °C at 5.5, 7.0, and 8.7 kb, respectively. The aluminous ferro-anthophyllite assemblage is stable only over 50 °C, reacting with increasing temperature to almandine, staurolite, and hercynite spinel. Under the QFM buffer, the same equilibria are displaced to higher temperatures and the aluminous ferro-anthophyllite bearing field is further restricted with respect to temperature. The 7 Å chamosite assemblage, previously considered to be the metastable equivalent of chloritoid at low pressures, is shown to be unstable and chloritoid can be synthesized at pressures as low as 1 kb.

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