SYNTHESIS AND CHEMICAL VARIABILITY OF MG-STAUROLITE IN THE SYSTEM MGO-AL2O3-SIO2-H2O AS A FUNCTION OF WATER-PRESSURE

Mg-staurolites were synthesized from a gel with the oxide ratio 4 MgO : 9 Al2O3 : 8 SiO2 at water pressures from 20 to 50 kbar and the const ant temperature of 800 degrees C. They form euhedral grains up to 10 m u m in length. The chemical composition is strongly correlated with wa ter pressure and,compared to the ideal formula of Mg3-4Al18Si8O46(OH)( 2-4) the magnesium content increases only slightly between pressures o f 20 and 30 kbar, whereas a strong decrease in the aluminum content is accompanied by an increase in the amount of hydrogen. This suggests t he following substitutions: 3 H+ <-> Al3+ and Mg2+ + H+ <-> Al3+. At h igher pressures, a further increase in hydrogen is accompanied by a de crease in magnesium content related to the substitution 2 H+ <-> Mg2+. The change in substitution mechanisms may reflect an increase in the amount of sixfoId coordinated magnesium at high pressure. Surprisingly , the lattice parameters of Mg-staurolites are not measurably influenc ed by their chemical variability.