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Plastic deformations in kyanites by tectonometamorphic processes: a single-crystal X-ray diffraction study

¹ Dipartimento di Scienze della Terra, Universita' degli Studi di Milano, Via Mangiagalli, 34, I-20133 Milano, Italy
² CNR-Istituto per la Dinamica dei Processi Ambientali, Milano, Italy

^* E-mail: diego.gatta{at}unimi.it

The crystal chemistry and crystal structure of natural kyanite crystals from the Eclogitic Micaschists Complex of the Sesia-Lanzo Zone, Western Italian Alps, have been investigated by means of optical microscopy, wavelength dispersive X-ray microanalysis, and single-crystal X-ray diffraction. The association of kyanite + garnet + phengitic-mica + chloritoid suggests that the eclogite-facies stages occurred at P 2.1 GPa and T 650°C. Kyanite grains are large (cm-sized) porphyroblasts grown dynamically during one of the deformational events related to the subduction of the Austroalpine continental crust. Under the polarizing microscope, kyanite grains show almost homogeneous cores, whereas rims are sometimes symplectitic aggregates of quartz and kyanite, confirming at least two stages of growth most likely related to the multistage deformational history of these rocks. Chemical analysis shows that Fe³⁺ is the major substituting cation for Al³⁺, ranging between 0.038 and 0.067 a.p.f.u.

The single-crystal X-ray diffraction investigation of the kyanites shows severely textured patterns on the (h0l)^*-plane. Such evidence is not observed in the unwarped diffraction patterns on (0kl)^* and (hk0)^*. The most significant difference between the structural parameters refined in this study, with respect to those of previously published unstrained gem-quality crystals, concerns the displacement parameters. The anisotropic displacement ellipsoids of all the atomic sites are significantly larger than those previously described, and systematically oriented with the largest elliptical section almost perpendicular to [010]. The larger ellipsoids in the kyanite crystal investigated here reflect the displacement of the centre of gravity of the electron distribution, rather than an anomalous atomic thermal motion. The magnitude and orientation of the displacement parameters and the textured/strained diffraction pattern may be the result of two combined effects: (1) that the kyanite crystals are actually composed of several blocks; (2) the crystals are affected by a pervasive residual strain, as a result of tectonometamorphic plastic deformations and re-crystallization.

KEYWORDS: kyanite, Al₂SiO₅, single-crystal X-ray diffraction, residual strain, plastic deformation, tectonometamorphic processes