Electron microscopy (SEM and TEM) of unusual olivine (Fo = 88) crystals, with perfect (010) cleavages, from Chalk Hills, Salem, Tamil Nadu has been carried out. SEM studies reveal the effect of compressive stress. Microstructures by TEM showed the abundance of curved dislocations with jogs, kinks and dipoles, indicative of the dominant climb, characteristic of high temperature deformation. The stacking fault fringes observed in olivine are due to mechanical weakening caused by nonstructural chemical constituents. The evidences for this come from fiuid-microinclusions observed and higher amount of K, Na, Rb and Sr in the olivine. Hydrolytic weakening through dislocation glide motion, assisted by H2O containing incompatible elements, may break the metal-oxygen bonds. This leads to perfect parting of crystallographic planes under deformational stress, during solid emplacement of the dunite. The easily cleavable planes are those with the largest interplanar spacings.
Volume 128 | Issue 8
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