COMPRESSIBILITY OF TUGTUPITE AT HIGH-PRESSURE

Single crystal X-ray determinations of the lattice constants of tugtup ite (Na-8[Al(2)Be(2)Si(8)O24] (Cl)(2) (idealized), tetragonal, I ($) o ver bar 4, specimen from Ilimaussaq, Greenland) were performed using a modified Merrill-Bassett type high pressure diamond anvil cell. The m aximum pressure reached was 6.2 GPa. The data obtained suggest that th e 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 0.9 4 . 10(11) Nm(-2) above this region. Plots of the relative volume redu ction of tugtupite, sodalite and helvite illustrate that the reaction of the crystal structure of tugtupite and sodalite to pressure corresp onds, whereas the structure of helvite is less compressible.