International
Tables for Crystallography Volume D Physical properties of crystals Edited by A. Authier © International Union of Crystallography 2006 |
International Tables for Crystallography (2006). Vol. D. ch. 3.3, p. 410
Section 3.3.6.11. Plagioclase twins
a
Institut für Kristallographie, Rheinisch–Westfälische Technische Hochschule, D-52056 Aachen, Germany, and bMineralogisch-Petrologisches Institut, Universität Bonn, D-53113 Bonn, Germany |
From the point of view of the relationship between pseudosymmetry and twinning, triclinic crystals are of particular interest. Classical mineralogical examples are the plagioclase feldspars with the `albite' and `pericline' twin laws of triclinic (crystal class ) albite NaAlSi3O8 and anorthite CaAl2Si2O8 (also microcline, triclinic KAlSi3O8), which all exhibit strong pseudosymmetries to the monoclinic feldspar structure of sanidine. Microcline undergoes a very sluggish monoclinic–triclinic phase transformation involving Si/Al ordering from sanidine to microcline, whereas albite experiences a quick, displacive transformation from monoclinic monalbite to triclinic albite.
The composite symmetries of these twins can be formulated as follows:
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Both twin laws resemble closely the monoclinic pseudosymmetry in two slightly different but distinct fashions: each twin law uses one rational twin element from , the other one is irrational. The two frameworks of twin symmetry are inclined with respect to each other by about , corresponding to the angle between b (direct lattice) and (reciprocal lattice).
Both twins occur as growth and transformation twins: they appear together in the characteristic lamellar `transformation microclines'.