Origin of twist in 'gwindel' quartz crystals from the Alps: a transmission electron microscopy study
Cordier, Patrick; Heidelbach, Florian
European Journal of Mineralogy Volume 25 Number 2 (2013), p. 145 - 153
published: Mar 1, 2013
Open Access (paper can be downloaded for free)
Two twisted quartz crystals (gwindels) from the Alps were studied by transmission electron microscopy and large-angle convergent-beam electron diffraction (LACBED) to elucidate the twist mechanism. We found that torsion results from a distribution of straight screw geometrically necessary b = 1/3 〈2110〉 dislocations with a dislocation density of the order of 1010 m−2. This dislocation microstructure induces an Eshelby twist which can account for the observed twist of a few degrees per centimetre. This microstructure suggests that twisted gwindel quartz forms by the spiral growth mechanism which enables crystal growth under small driving forces.