Abstract

Experiments on the stability of chlorite in the presence of muscovite and quartz were performed in the system K2O–MgO–FeO–Al2O3–SiO2–H2O. The hydrothermal experiments were done at the quartz–fayalite–magnetite oxygen buffer using both natural and synthetic crystalline starting materials. Results of the experiments show that chlorite of intermediate Fe/Mg is stable to higher temperatures (at a given pressure) than the end-member compositions. Chlorite is more magnesian than co-existing biotite but plots to the Fe-side of the tie-line connecting coexisting biotite and cordierite on the AFM diagram.

Magnesian chlorite breaks down in metapelitic rocks by the continuous reaction  
chlorite + muscovite = biotite + cordierite + H2O.

The assemblage chlorite + biotite + cordierite + muscovite + quartz exists stably over a narrow temperature range at a given pressure and for a very small range of rock composition.

The terminal reaction involving an Al2SiO5 phase, by which chlorite disappears is  
chlorite + muscovite = cordierite + biotite + Al2SiO5 + H2O.

The reversed experiments have constrained the univariant assemblage chlorite–cordierite–biotite–Al2SiO5 as lying between the points 640±10°C at 4 kbar and 612±12°C at 2 kbar.

This implies that chlorite (of intermediate Fe/Mg) is stable to higher temperatures in metapelites than has been hypothesized, and chlorite can coexist stably with sillimanite according to the Al2SiO5 triple point of Holdaway (1971).

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