STABILITY AND STRUCTURE OF A NEW HIGH-PRESSURE SILICATE, NA1.8CA1.1SI6O14

Preliminary phase relations for the bulk composition Na2O . CaO . 5SiO (2) were experimentally determined at 8-16 GPa and 950-2300 degrees 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 fo rmula Na2CaSi6O14; microprobe analyses suggest vacancies that cannot b e resolved by X-ray data, resulting in a composition better expressed as Na1.8Ca1.1Si6O14. Single crystals suitable for structure determinat ion were synthesized at 14 GPa and 1900 degrees C. Two independent set s of X-ray diffraction data were collected on different crystals from the same batch: one with a conventional diffractometer and the other u sing imaging plates and a synchrotron X-ray source. The compound cryst allizes in space group P321 with a = 7.903(2) and c = 4.595(1) Angstro m. Its structure consists of layers in (001); one is centered at z = 0 and contains the (Ca,Na) and Si-[6] sites. The other, centered at z = 1/2, is a sheet of corner-linked Si-[4] tetrahedra composed of open-b ranched crankshaft vierer chains linked by single tetrahedra. Although the SiO6 octahedra are close to regular, with Si-O = 1.789(2) Angstro m, the two symmetry-independent SiO4 tetrahedra are distorted as a res ult of underbonding between O atoms bonding outside the sheet, with Si 1-O = 1.557(2) and 1.638(2) Angstrom, and Si2-O = 1.619(1) and 1.648(2 ) Angstrom. Half-normal probability plots show the positional paramete rs derived from the two data sets to be statistically equivalent.