Complete substitution of Si for Ti in titanite Ca(Ti1-xSix)viSiivO5

• Published in: The American mineralogist 1998-12, Vol.83 (11-12), p.1168-1175
• Main Authors: Knoche, R, Angel, Ross J, Seifert, F, Fliervoet, T. F
• Format: Article
• Language: English
• Subjects: crystal chemistry; crystal structure; experimental studies; Experiments; Geophysics; high pressure; high temperature; laboratory studies; mantle; metals; Mineralogy; Minerals; nesosilicates; orthosilicates; P-T conditions; phase equilibria; pressure; related to mantle; silicates; silicon; substitution; synthesis; temperature; titanite; titanite group; titanium
• ISSN: 0003-004X; 1945-3027
• DOI: 10.2138/am-1998-11-1204

Phase relations on the join CaTiSiO5-CaSi2O5 were determined at 1350°C over the pressure range 3.5-12 GPa by a combination of synthesis and reversal experiments in a piston cylinder and a multi-anvil press. Titanite-like phases were recovered from all experiments in this pressure range. At 3.5 GPa the maximum SiVI content of titanite is 3.0±0.6 mol%, whereas bulk compositions with higher Si content yield a mixture of titanite solid solution plus coesite and walstromite-structured CaSiO3. The maximum SiVI content of the titanite increases with pressure to 21±2 mol% at 7 GPa and 46±2 mol% at 7.5 GPa. At pressures of 8.5 to 12 GPa all intermediate compositions yield a single titanite phase. X-ray and TEM analysis show that these have the A2/a symmetry of the titanite aristotype. The variations of the room-pressure unit-cell parameters of the A2/a phases with composition can be described by the equations a [Å] = 7.040(9)-0.492(15) X(SiVI); b [Å] = 8.713(7)-0.316(11) X(SiVI); c [Å] = 6.564(4)-0.220(7) X(SiVI); β [°] = 113.721(6) - 0.537(12) X2(SiVI); V (cell) [Å3] = 367.8(9)-47.5(1.6) X(SiVI). For the CaSi2O5 composition the recovered material has I1̄ symmetry but is known to transform back to A2/a titanite structure at 0.2 GPa at room temperature. Similarly, with increasing pressure the P21/a CaTiSiO5 transforms to A2/a symmetry at 3.6 GPa at room temperature. The conclusion is that at 1350°C and pressures in excess of 8.5 GPa there is complete solid solution between CaTiSiO5 and CaSi2O5 based upon the isovalent exchange of Si for Ti in the octahedral site of the A2/a structure. Rietveld structure analysis of intermediate compositions reveals no evidence for ordering or intermediate phases. Preliminary experiments at pressures between 13.5 GPa and 16 GPa yielded mixtures of titanite solid solution plus perovskite and stishovite. From these data and information on the phase relations for the CaSiO3-CaTiO3 join the topology of the phase relations between ∼3 and ∼13 GPa in the central part of the CaO-TiO2-SiO2 ternary have been deduced.