Single crystal structure investigations under high-pressure of the mineral cordierite with an improved high-pressure cell

Abstract

An improved single-crystal high-pressure anvil cell with beryllium-gaskets was used for the investigations of structure and lattice parameters of cordierite which had been heated in an Ar stream at about 1,000° C to remove natural water from its structural channels. The influences of pressure transmitting media were studied by using water as a pressure medium at pressures of 0.3, 0.9, 1.2, and 2.3 GPa and fluorcarbon, a liquid consisting of large molecules, at 2.2 GPa. Water, but not fluorcarbon, is able to enter the channels in the cordierite structure. Large variations in the lattice constants resulted from changing the pressure medium used. A previously supposed discontinuity of the b lattice constant at nearly 0.3 GPa could not be established by the measurements taken so that there is no evidence for a phase transition at this pressure. Possibly the observed tilting of two tetrahedra against each other in this structure could have led to this misinterpretion. When water, but not fluorcarbon, is used as a pressure medium at 2.3 GPa, an additional electron density peak, presumably a water oxygen atom, appears in the channels. The water prevents the channels from shrinking and fixes their width at a value comparable to that of a naturally hydrated cordierite. In one of the silicate-tetrahedra the Si-O bond lengths are compressed almost 1 percent (2.3 GPa). This process may initiate a phase transition at higher pressures.