HIGH-PRESSURE STRUCTURAL STUDY OF MUSCOVITE

The compressibility and structural variations of two 2M(1) muscovites having compositions (Na0.07K0.90 are(0.02))(Al1.84Ti0.04Fe0.07Mg0.04)( Si3.02Al0.98) O-10 (OH)(2) (7 mole % paragonite) and (Na(0.37)K(0.60)s quare(0.03))(Al1.84Ti0.02 Fe0.10Mg0.06)(Si3.03Al0.97) O-10(OH)(2) (37 mole % paragonite) were determined at pressures between 1 bar and 35 k bar, by single-crystal X-ray diffraction using a Merrill-Bassett diamo nd anvil cell. Isothermal bulk moduli, setting K'=4, were 490 and 540 (+/-30) kbar for the Na-poor and Na-rich samples respectively. Both sa mples show highly anisotropic compressibility patterns, with beta(a):b eta(b):beta(c)=1:1.15:3.95 for the Na-poor sample and beta(a):beta(b): beta(c)=1:1.19:3.46 for the Narich one. HP structural refinements show ed that the different compressibility was largely due to the partial s ubstitution of Na for K in the interlayer region. Moreover, the differ ent compressibility of the tetrahedral and octahedral layers, observed in both micas, increased the alpha rotation of the tetrahedral layer by about 2 degrees in 28 kbar, as also indicated by the evolution of i nterlayer cation bond lengths. This increases the repulsion of oxygens of the basal layers and between the high-charged cations of the tetra hedral layer. As a consequence, phengitic substitution, reducing alpha rotation, would increase the baric stability of mica. Comparison betw een the HP structures of muscovite and phlogopite indicated the lower compressibility of the latter, mainly due to the greater compressibili ty of the dioctahedral layer with respect to that of the trioctahedral layer. The HT and HP behaviour of di- and trioctahedral micas showed an anisotropy in the compressional pattern which was markedly greater than that observed in the dilatation pattern. This unexpected result w as explained by the different evolution with P and T of alkali-O bond lengths. By combining HP and HT data, a tentative equation of state of muscovite is proposed.