FORMATION OF HOUR-GLASS STRUCTURE IN AUGITE

The common 'swallow-tailed', sometimes skeletal augite crystals are found in the fine-grained ground- mass of many rock types, and it is suggested that rapid crystallization alone accounts for their formation. This type of hour-glass structure has been called 'quench hour-glass'. The hour- glass structures of larger augite crystals of porphyritic and coarse-grained rocks are commonly described as hour-glass 'zoning'. They are thought to have formed by a process involving adsorption of impurities on a particular crystal face so as to impede growth of these faces, producing an initialskeleton of hour-glass form, which is completed by later crystallization of augite richer in FeO, Na//2O, TiO//2, and Al//2O//3. This hypothesis also explains the patchy zoning of other augite crystals, casting doubt on some petrogenetic interpretations of such zones are core zones.