The equation of state of (KAl3Si3O10(OH)2) muscovite has been measured to 20 GPa using X ray diffraction through the diamond anvil cell. The isothermal bulk modulus (KOT) of miscovite is 61.4(¿4.0) GPa with a pressure derivative of 6.9(¿1.4), in good agreement with Brillouin spectroscopic and shock wave measurements. Above 18 GPa at room temperature, muscovite loses its long-range crystalline order and amorphizes near 27 GPa. The phase diagram of muscovite between 2 and 22 GPa at approximately 800 ¿C has been investigated using the laser-heated diamond anvil cell. Between 1.8 and 3.7 GPa, muscovite transforms into an assemblage containing KAlSi3O8-sanidine, Al2O3-corundum, H2O, and cymrite-structured KAlSi3O8⋅H2O, consistent with previous piston-cylinder studies. The K-cymrite phase appears to persist metastably to greater than 10 GPa. Between 3.7 and 4.0 GPa, muscovite breaks down to wadeite-structured K2Si4O9, Al2SiO5-kyanite, Al2O3-corundum, and H2O, and between 10.9 and 12.0 GPa, muscovite transforms to an assemblage containing hollandite-structured KAlSi3O8, Al2O3-corundum, and H2O: this assemblage remains stable to at least 21 GPa. In accord with previous work on granitic and K-feldspar-phlogopite systems our results indicate that muscovite may transform into the hydrous phase K-cymrite under deep crustal PT conditions. ¿ American Geophysical Union 1994