High-voltage, high-resolution lattice images of dolomite

Abstract

Dolomite, CaMg(CO3)2, which belongs to the rhombohedral group of carbonates, is an important mineral commonly found in sedimentary and metamorphic environments. Ideally, it has an ordered trigonal structure (R3) with planes containing atoms of only Ca, or only Mg, alternating and being interleaved with the planes containing the carbonate groups. All these planes are perpendicular to the 3-fold axis. As the microstructural deformation observed in many natural dolomites of low and medium metamorphic grade could provide an insight into the geological history, attention has recently been directed to electron microscope studies1–4 of the mineral. Unfortunately, at a typical accelerating voltage of 100 kV, dolomite is highly susceptible to damage by the electron beam, whereas, at a voltage of 1MV, little electron damage is noticeable1. We report here observations of dolomite with the Cambridge University 600-kV high-resolution electron microscope5,6 Although electron beam damage is still observed at the intermediate operating voltages of this microscope, it has been possible, with care, to resolve directly details of crystal lattices in two dimensions. Thus, direct confirmation has been obtained of postulated models for the common types of stacking faults observed in dolomite.