Stacking faults in dolomite

Abstract

DOLOMITE, an important rock-forming mineral, is a mixed carbonate possessing trigonal symmetry (R), with alternate layers of Mg and Ca atoms separated by planes containing the carbonate groups. Its ordered structure offers the possibility of extended planar faults of low energy and Barber¹ has shown evidence for stacking disorder in dolomite of high metamorphic grade. There is no previous evidence, however, for the creation of extended faults as a direct result of deformation processes. We have prepared single crystals, approximately 25 mm² in cross section and 10 mm long, and tested them in compression at temperatures up to 800 °C while subjected to a confining pressure of either 3 or 7 kbar. Specimens were mostly oriented to give a large resolved shear stress on the basal planes while minimising the stress for mechanical twinning on the f ≡ {012} planes. (We use four-digit indices for planes and directions referred to the structural hexagonal unit cell, with c = 16.01 Å and a = 4.81 Å.) It has been shown (optically)² that slip on (0001) and twinning on {012} are common modes of deformation on dolomite. Using TEM methods, Barber¹ has shown that slip on {012} can also occur in dolomite. The current observations were also made by high voltage transmission electron microscopy, using sections which were thinned first mechanically and then by ion bombardment.