THE TEMPERATURE-DEPENDENT P2(1) C-C2/C PHASE-TRANSITION IN THE CLINOPYROXENE KANOITE MNMG[SI2O6] - A SINGLE-CRYSTAL X-RAY AND OPTICAL STUDY/

The crystal structures and optical properties of kanoite, MnMg[Si2O6], from Bani Hamid (United Arab Emirates) were investigated as a functio n of temperature. Kanoite was found to undergo a phase transition from space group P2(1)/c to C2/c at approx. 240 degrees C. The temperature dependence of the cell dimensions shows considerable deviations from linearity at the phase transition. The h+k = odd X-ray reflections of the P2(1)/c phase remain sharp but decrease in intensity with increasi ng temperature and disappear at the phase transition. Single-crystal X -ray structure refinements were performed at 200 degrees C (space grou p P2(1)/c, a = 9.758(3), b = 8.897(1), c = 5.283(1) Angstrom, beta = 1 08.74(2)degrees, V = 434.3 Angstrom(3)) and at 270 degrees C (space gr oup C2/c, a = 9.796(5), b = 8.880(8), c = 5.315(3) Angstrom, beta = 10 9.26(4)degrees, V = 436.5 Angstrom(3)). The structures of low (P2(1)/c ) and high (C2/c) kanoite are isotypic with the low-Ca clinopyroxenes (P2(1)/c) and their high-temperature C2/c modifications, respectively. The two symmetry-independent [Si2O6](4-) chains of the P2(1)/c phase become equivalent in the C2/c phase. The M2 site (enriched in Mn) of t he low-temperature phase is irregularly seven-fold coordinated (5+2), whereas it is six-fold coordinated (4+2) in the high-temperature phase . Temperature-dependent optical properties of kanoite were calculated on the basis of the point dipole model, leading to non-linear birefrin gence at the phase transition. A non-linear change of retardation due to the phase transition was monitored with a polarising microscope equ ipped with a heating stage. A two-phase field, related to chemical inh omogeneity, was found over a temperature range of approximately 30 deg rees C.