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In nature the solid solution series muscovite-celadonite (KAl₂[AlSi₃]O₁₀(OH)₂-KMgAl[Si₄]O₁₀(OH)₂) ranges from pure muscovite (cel_0.00) to about cel_0.50. 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 cel_1.00 was reached and in the iron- and sodium-free synthetic system, cel_0.94 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.