Physics and Chemistry of Minerals

, Volume 34, Issue 8, pp 529–541 | Cite as

Cross-twinning in a natural spinel from Sri Lanka

Original Paper

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 cyclictwinning with lamellartwinning, 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: \((\bar{1}11)_{\rm A/B}, (1\bar{1}\bar{1})_{\rm B/C}\) and \((\bar{1}\bar{1}\bar{1})_{\rm AC/D}.\) 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 \((1\bar{1}0)_{\rm C}\) face close to the \((\bar{1}\bar{1}\bar{1})_{\rm C/D}\) 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.

Keywords

Cross-twinning Spinel X-ray topography Synchrotron 

Notes

Acknowledgements

For the use of the synchrotron radiation facilities, the authors wish to thank: B. Capelle (Université Paris VI, France) and C. Rinaudo (Università di Alessandria, Italy) at LURE (Paris); J. Baruchel and J. Hartwig at ESRF (Grenoble). R. Carampin (CNR, Padova, Italy) is thanked for his assistance during microprobe analyses. The studied specimen was supplied by A. Monno (Dipartimento Geomineralogico Università di Bari, Italy). Financial support was given by: LURE (project number DK 901-02); ESRF (experiment number CH-1356); MIUR (Roma, PRIN 2005 project ‘Minerals to materials: crystal chemistry, microstructures, modularity, modulations’). Two anonymous referees are acknowledged.

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Copyright information

© Springer-Verlag 2007

Authors and Affiliations

  1. 1.Dipartimento GeomineralogicoUniversità di BariBariItaly

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