On the origin of inhomogeneity of anomalous birefringence in mixed crystals: an example of alums |
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| Authors: | A G Shtukenberg Yu O Punin E Haegele H Klapper |
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| Institution: | Department of Crystallography, St. Petersburg State University, Universitetskaya nab. 7/9, 199034 St. Petersburg, Russian Federation?e-mail: sasha@as3607.spb.edu, RU
Mineralogisch-Petrologisches Institut, University of Bonn, Poppelsdorfer Schlo? 53115 Bonn, Germany, DE
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| Abstract: | Optical anomalies (deviations of the symmetry of optical properties from the ideal symmetry of the crystal) occur in many
minerals and synthetic compounds and have been under investigation since the last century. An important feature of optically
anomalous mixed crystals is a high degree of optical inhomogeneity, whereas the optical patterns of mixed crystals without
anomalies are usually rather uniform. This work is devoted to the study of this phenomenon. As a model object we have chosen
mixed alum crystals, which were known for their anomalous birefringence and which revealed the following types of optical
inhomogeneities: (1) sector zoning; (2) concentric zoning; (3) subsector zoning; (4) stripes normal to growth front.
The inhomogeneity of anomalous birefringence of mixed crystals of alums can be explained by superposition of several effects:
mismatch strain, strain along dislocations and growth ordering of isomorphous components. Optical inhomogeneities due to the
sector zoning of crystals and their dislocation structure arise even under stationary growth conditions and stationary micromorphology
of the growing face. Both variable growth conditions and the relief of the growing face strongly intensify the optical inhomogeneity
due to three interrelated factors: (1) a significantly inhomogeneous mismatch strain; (2) a variable degree of ordering of
isomorphous components due to the compositional inhomogeneities; (3) different degrees of ordering of isomorphous atoms caused
by different orientations, heights and velocities of growth steps. These effects lead to the formation of subsector zoning
and zoning superimposed on the optical sector zoning. These optical structures are crossed by birefringent stripes arising
from dislocations.
Received: 29 March 2000 / Accepted: 11 March 2001 |
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| Keywords: | Optical anomalies Alum Dissymmetrization Mismatch strain |
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