Abrupt high/low-transition in flux-grown Mg-cordierite single crystals with hour-glass structure
European Journal of Mineralogy Volume 3 Number 5 (1991), p. 809 - 818
published: Oct 2, 1991
manuscript accepted: Mar 20, 1991
manuscript received: Nov 12, 1990
ArtNo. ESP147050305003, Price: 29.00 €
Abstract Mg-cordierite single crystals with a composition Mg1.98Al4.08Si4.95O18 were grown from a Pb0/V2O5- flux according to the method of Lee & Pentecost (1976). They exhibit an optically visible hour-glass structure with a distinctly higher face-specific Pb concentration up to 0.83 weight % (= 0.02 Pb per formula unit) on the (0001) basal faces. Determination of the distortion index by Weissenberg photographs shows the crystals to be hexagonal (Δ = 0.0) without any deviation from the space group P6/mcc. Nevertheless, the optic axial angle 2Vx varies between 0° and 20° in different parts of one single crystal, and features reminiscent of twinning perpendicular to (0001), combined with undulatory extinction can be observed. Heating of the crystals at 1290°C for 10 hours leads to the disappearence of the hour-glass structure and of the undulatory extinction in the now orthorhombic crystals, while 2Vx rises to a homogeneous value of nearly 90°. During heating at 1290°C, the flux-grown single crystals remained hexagonal up to durations of 3.5 hours ; after 4 hours a rather abrupt change to the stable orthorhombic form (Δ~0.20) was noted. Thus, at this temperature, the total heating time necessary to transform the metastable hexagonal single crystal into twinned orthorhombic ones is much less, by a factor of 5, than for intergrown polycrystalline aggregates of Mg-cordierite. In fact, the high/low transition itself occurs within less than 30 minutes, whereas polycrystalline aggregates tend to produce intermediate structural states over considerable periods of time.