Stability and structure of a new high-pressure silicate, Na_1.8 Ca_1.1Si₆O₁₄

Preliminary phase relations for the bulk composition Na₂O.CaO.5SiO₂ were experimentally determined at 8-16 GPa and 950-2300°C, using a split-sphere anvil apparatus (USSA-2000). The dominant phase at pressures between 8 and 14 GPa was a new compound with the ideal formula Na₂CaSi₆O₁₄. Two independent sets of X-ray diffraction data were collected on different crystals from the same batch: one with a conventional diffractometer and the other using imaging plates and a synchrotron X-ray source. The compound crystallizes in space group P321 with a = 7.903(2) and c = 4.595(1) Å. Its structure consists of layers in (001); one is centered at z = 0 and contains the (Ca,Na) and ^[6]Si sites. The other, centered at z = 1/2 , is a sheet of corner-linked ^[4]Si tetrahedra composed of open-branched crankshaft vierer chains linked by single tetrahedra. -from Authors