Original paper
The temperature-dependent P21/c - C2/c phase transition in the clinopyroxene kanoite MnMg[Si2O6]: a single-crystal X-ray and optical study
Arlt, Thilo; Armbruster, Thomas
European Journal of Mineralogy Volume 9 Number 5 (1997), p. 953 - 964
40 references
published: Sep 24, 1997
manuscript accepted: Mar 19, 1997
manuscript received: Aug 1, 1996
DOI: 10.1127/ejm/9/5/0953
Abstract
Abstract The crystal structures and optical properties of kanoite, MnMg[Si2O6] from Bani Hamid (United Arab Emirates) were investigated as a function of temperature. Kanoite was found to undergo a phase transition from space group P21/c to C2/c at approx. 240°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 P21/c phase remain sharp but decrease in intensity with increasing temperature and disappear at the phase transition. Single-crystal X-ray structure refinements were performed at 200°C (space group P21/c, a = 9.758(3), b = 8.897(1), c = 5.283(1) Å, ß = 108.74(2)°, V= 434.3 Å3) and at 270°C (space group Cllc, a = 9.796(5), b = 8.880(8), c = 5.315(3) Å, ß = 109.26(4)°, V = 436.5 Å3). The structures of low (P2,/c) and high {Cllc) kanoite are isotypic with the low-Ca clinopyroxenes (P21/c) and their high-temperature C2/c modifications, respectively. The two symmetry-independent [Si2O6]4- chains of the P21/c phase become equivalent in the C2/c phase. The M2 site (enriched in Mn) of the 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 birefringence at the phase transition. A non-linear change of retardation due to the phase transition was monitored with a polarising microscope equipped with a heating stage. A two-phase field, related to chemical inhomogeneity, was found over a temperature range of approximately 30°C
Keywords
kanoite • clinopyroxenes • crystal structure • phase transition • optical properties • electronic polarisabilities.