Structure and stability of the high-pressure phase, Ca3TiSi2(Al,Ti,Si)(3)O-14

The pressure and compositional dependence of clement partitioning were exam ined in the pseudoternary section anorthite-titanite-perovskite of the syst em CaO-Al2O3-TiO2-SiO2. Preliminary phase relations in that section were ex perimentally determined at pressures from 1 atm to 1.5 GPa and at temperatu res from 1300 to 1500 degrees C, using a double-ellipsoid mirror furnace an d a piston cylinder apparatus. Further experiments were carried out with tr ace element extended compositions. All run products represent near-equilibr ium parageneses consisting of crystals with their coexisting melt; the pair ing is desired for element partitioning studies. Al pressures above 1.0 GPa a new Ti-rich compound, having the ideal formula Ca3TiSi2(Al,Ti,Si)(3)O-14 , appears as the dominant phase in this paragenesis. The composition and homogeneity of the phases were checked with electron mi croprobe and scanning electron microscopy. Determination of the structure o f this compound was performed with a conventional four circle diffractomete r on a single crystal, cut from a larger specimen of the equilibrium parage nesis. The compound crystallizes in space group P321 with cell dimensions a = 7.943(1) Angstrom and c = 4.930(1) Angstrom. Its structure consists of l ayers stacked parallel to (001) such that sheets of two types of corner-lin ked tetrahedra (T1 = Si; T2 = Al, Ti, Si) at, = 1/2 alternate with sheets c entered at z = 0 containing Ti-[6]- and irregular eightfold-coordinated Ca- sites. A statistical distribution for Al, Si, and Ti atoms at one of the tw o tetrahedral sites in the crystal structure was confirmed by X-ray diffrac tion data. The Ti-octahedron is distorted, with Ti-O = 1.952(3) Angstrom, a nd O-Ti-O bond angles between 85.5 degrees and 102.4 degrees, which results in an angle bend of the vertex O atoms of 167.6 degrees.