Analytical transmission electron microscopy of oscillatory zoned grandite garnets

Oscillatory zoned grandite garnets from skarns of the Oslo rift region (sou thern Norway) have been studied using analytical transmission electron micr oscopy (ATEM). Our results show lamellae with a width varying from about 10 nm to more than 500 nm in both periodic and non-periodic sequences, which represent oscillatory zoning at a very fine scale. The compositions of adja cent lamellae alternate bimodally from X-and = 0.55-0.75 to X-and = 0.95-1. 0. High resolution (HRTEM) images show that the compositional interfaces ar e sharp and coherent. Oscillatory zoning at the nm-scale puts constrains on the origin of oscillatory zoning in grandites and underlines the dynamic n ature of the crystal growth process. The influence of growth rate is discus sed against the background of local growth dynamics and external forcing at different length scales. This study emphasises the importance of adequate spatial resolution in analysing chemical variations in grandite garnets and shows that commonly applied methods (e.g. optical microscopy. backscattere d electron imaging and electron or ion microprobe) are not sufficient to de tect 'real' zonation patterns. Finally, proposed hypotheses on the origin o f birefringence of grandite garnets with intermediate compositions are revi ewed and discussed on the basis of the observed microstructure.