Cross-twinning in a natural spinel from Sri Lanka |
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| Authors: | Rosa Anna Fregola Eugenio Scandale |
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| Institution: | (1) Dipartimento Geomineralogico, Università di Bari, via E. Orabona 4, 70125 Bari, Italy |
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| Abstract: | A modified cross-twinning growth mechanism is put forward to explain the anomalous morphology of a spinel multiple-twin from Sri Lanka, flattened crosswise the twin
planes. Cross-twinning in spinel was found also in other specimens from Pegu (Myanmar), and the results were published in
a previous paper. This particular type of twinning is derived from the combination of cyclic
twinning with lamellar
twinning, so that these samples may be thought of as partial fivelings (cubic cyclic {111} twins with five components sharing a common <110> pseudo-fivefold axis). In the present paper, the sample
from Sri Lanka has been suitably cut with the aim of focusing the study on the cross-twinning region. The transformation matrices
that link the orientation states of each couple of twin components have been determined by means of White Beam Synchrotron
Radiation Topography. They showed that the specimen is made up of four twin components (A, B, C and D), with three twin planes:
 and  They also showed that the cross-twinned individuals (B and D) actually are not twinned to each other, and that a simple crystallographic
relationship holds between them. X-ray diffraction topography by conventional source allowed to image the crossing-region
and to determine that the cross-twinned individuals are in contact through a semi-coherent boundary, with twinning dislocations contributing to relieve the coherency strains. Electron probe microanalyses with wave dispersive spectroscopy showed that
the chemical composition is almost homogeneous, at least within the spatial resolution limit of this technique. The similar
growth features observed in the spinel sample from Sri Lanka and in those from Myanmar are interpreted as growth marks, indicators of a similar origin: in both cases they are found in impure dolomitic marbles. In particular, the specimen from
Sri Lanka results from the interaction of thermal and metasomatic effects due to contact metamorphism. An unusual stepped
morphology of the  face close to the  twin boundary, possibly due to corrosion and re-growth processes acted preferentially at a re-entrant corner by metasomatic
fluids, is interpreted as indicator of a metasomatic event that succeeded to the crystal growth, the latter occurred by thermal
effect.
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| Keywords: | Cross-twinning Spinel X-ray topography Synchrotron |
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