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

Th. Hahna* and H. Klapperb

a Institut für Kristallographie, Rheinisch–Westfälische Technische Hochschule, D-52056 Aachen, Germany, and bMineralogisch-Petrologisches Institut, Universität Bonn, D-53113 Bonn, Germany
Correspondence e-mail:  hahn@xtal.rwth-aachen.de

3.3.6.11. Plagioclase twins

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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 [{\bar 1}]) 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:

  • Albite law : reflection twin on (010); composition plane (010) rational (Fig. 3.3.6.11[link], Table 3.3.6.5[link]). [{\cal K}_A = 2'/m'({\bar 1})] with rational [m'\parallel(010).]

    Table 3.3.6.5 | top | pdf |
    Plagioclase: albite and pericline twins

    [{\cal H}] [k \times{\cal H}] (albite) [k \times{\cal H}] (pericline)
    1 [m\parallel (010)] rational [2\parallel [010]] rational
    [{\bar 1}] [2\perp (010)] irrational [m\perp [010]] irrational
    [Figure 3.3.6.11]

    Figure 3.3.6.11 | top | pdf |

    Polysynthetic albite twin aggregate of triclinic feldspar, twin reflection and composition plane (010).

  • Pericline law : twofold rotation twin along [010]; composition plane irrational [\parallel[010]]: `rhombic section' (Fig. 3.3.6.12[link], Table 3.3.6.5[link]). [{\cal K}_P = 2'/m'({\bar 1})] with rational [2'\parallel[010].]

    [Figure 3.3.6.12]

    Figure 3.3.6.12 | top | pdf |

    Pericline twin of triclinic feldspar. Twofold twin axis [010]. (a) Twin with rational composition plane (001), exhibiting clearly the misfit (exaggerated) of the two adjacent (001) contact planes, as indicated by the crossing of lines [{\bf a}] and [{\bf a}']. (b) The same (exaggerated) twin as in (a) but with irrational boundary along the `rhombic section': fitting of contact planes from both sides ([{\bf a}] and [{\bf a}'] coincide and form a flat ridge). (c) Sketch of a real pericline twin with irrational interface (`rhombic section') containing the twin axis.

Both twin laws resemble closely the monoclinic pseudosymmetry [2/m] in two slightly different but distinct fashions: each twin law [{\cal K}] uses one rational twin element from [2/m], the other one is irrational. The two frameworks of twin symmetry [ 2'/m'] are inclined with respect to each other by about [4^\circ], corresponding to the angle between b (direct lattice) and [b^\ast] (reciprocal lattice).

Both twins occur as growth and transformation twins: they appear together in the characteristic lamellar `transformation microclines'.








































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