MNSI2O5 WITH THE TITANITE STRUCTURE - A NEW HIGH-PRESSURE PHASE IN THE MNO-SIO2 BINARY

Single crystals of MnSiO(SiO4) with the titanite structure together wi th MnSiO3 clinopyroxene were synthesized from a MnO-SiO2 oxide mixture at 1000 degrees C and 9.2 GPa in a multi-anvil press. The crystal str ucture of MnSi2O5 [space group C2/C, a = 6.332(1) Angstrom, b = 8.161( 1) Angstrom, c = 6.583(1) Angstrom, beta = 114.459(3)degrees, and V = 309.66 Angstrom(3)] was refined at room temperature from single-crysta l X-ray data to R1 = 2.23%. The monoclinic MnSi2O5 phase has the titan ite aristotype structure and is similar to the monoclinic Ca-analogue CaSi2O5. Si occurs in compressed octahedral coordination, replacing Ti in titanite, and in tetrahedral coordination as an orthosilicate grou p. Mn has a distorted sevenfold coordination with MnO distances betwee n 2.086 and 2.365 Angstrom.