RESUMO
Crystal defects and chemical reactions occurring at scales beyond the resolution of light microscopes have major effects on the chemical and physical properties of rocks and minerals. High-resolution imaging, diffraction, and chemical analysis in the transmission electron microscope have become important methods for exploring mineral defect structures and reaction mechanisms and for studying the distribution of phases resulting from reactions. These techniques have shown that structural disorder is common in some rock-forming minerals but rare in others. They have also established mechanisms by which many reactions occur at the atomic cluster scale. These data thus provide an atomistic basis for understanding the kinetics of geological reactions. Furthermore, apparent major-element, minor-element, and trace-element chemistry of minerals can be influenced by submicroscopic inclusions or intergrowths, which commonly form as products of solid-state reactions.
RESUMO
Exsolution between orthorhombic and monoclinic amphibole has been postulated for many years on the basis of crystal-chemical and structural differences. Transmission and analytical electron microscope examination of calium-rich gedrite specimens from southwestern New Hampshire has now revealed evidence for exsolution of calcic clinoamphibole (hornblende) from ferromagnesian orthoamphibole. Analytical electron microscopy data suggests that calcium has a low solubility limit in the orthoamphibole structure. The hornblende lamellae range from only a few unit cells in thickness to about 80 nanometers. The formation of the calcic amphibole lamellae resulted from heterogeneous nucleation and growth along pre-existing (100) stacking faults. Thus, the deformation processes producing the stacking faults played a key role in preparing the sites for exsolution.
