COMPRESSIBILITY OF TUGTUPITE AT HIGH-PRESSURE

Single crystal X-ray determinations of the lattice constants of tugtupite (Na-8[Al(2)Be(2)Si(8)O24] (Cl)(2) (idealized), tetragonal, I ($) over bar 4, specimen from Ilimaussaq, Greenland) were performed using a modified Merrill-Bassett type high pressure diamond anvil cell. The maximum pressure reached was 6.2 GPa. The data obtained suggest that the compressibility of the crystal structure changes between 2 and 3 GPa; the bulk modulus K computes to 0.62 . 10(11) Nm(-2) below and to 0.94 . 10(11) Nm(-2) above this region. Plots of the relative volume reduction of tugtupite, sodalite and helvite illustrate that the reaction of the crystal structure of tugtupite and sodalite to pressure corresponds, whereas the structure of helvite is less compressible.