The equation of state, amorphization, and high‐pressure phase diagram of muscovite

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 (K0T) of muscovite 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.