Plagioclase twin associations from the basic volcanic rocks of the Kamchatka, Russia: growth conditions and formation mechanisms
Punin, Yurii O.; Shtukenberg, Alexander G.; Smetannikova, Olga G.; Amelin, Konstantin S.
European Journal of Mineralogy Volume 22 Number 1 (2010), p. 139 - 145
published: Feb 1, 2010
ArtNo. ESP147052201013, Price: 29.00 €
The plagioclase twin associations found as crystal-lapilli in tephra and megacrysts in high-alumina basalt erupted from volcanoes of the Klyuchevskoy group (Kamchatka, Russia) reveal at least eight twin laws (Albite, Carlsbad, Albite-Carlsbad, Pericline, Baveno-d, -l, Prismatic-d, -l, and Periclinic twins), whose development and hierarchy can be described with the triad theory of L.A. Vardanianz. There are two genetic types of twins: (1) post-growth twins resulted from the thermal stress (Pericline twins and small twins of Albite-Carlsbad triad) and (2) growth twins (large twins of Albite-Carlsbad triad, Baveno and Prismatic twins), whose origin is explained by the action of the autodeformation mechanism. This mechanism implies deformation or stress-induced nucleation of small twin subunits in the subsurface layer of the growing crystal. The high level of stress is provided by the lattice mismatch on the zoning boundaries. Once nucleated, twin subunits grow independently of the original crystal that is favoured by the high growth-rate anisotropy.