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3.3 b. Crystal chemistry and thermodynamics of phengite (M.W. Schmidt)

In nature the solid solution series muscovite-celadonite (KAl2[AlSi3]O10(OH)2-KMgAl[Si4]O10(OH)2) ranges from pure muscovite (cel0.00) to about cel0.50. In experiments on natural (basalts) and synthetic (system K2O-MgO-Al2O3-SiO2-H2O) compositions the entire celadonite-rich part of the solid solution was synthesized. In natural compositions the endmember cel1.00 was reached and in the iron- and sodium-free synthetic system, cel0.94 was synthesized. On the basis of this first synthesis of the celadonite-rich portion of the celadonite-muscovite solid solution, the thermodynamic model for this solid solution is revised. In general, celadonite contents increase with pressure but decrease slightly with temperature. The buffer reaction garnet + celadonite = clinopyroxene + muscovite, which controls the variation of celadonite content in white mica, constitutes a powerful geobarometer for all potassium-bearing eclogites and has been calibrated experimentally.

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