Formation of the hour-glass structure in augite

SUMMARY. A study of augite in over three hundred thin sections of mainly alkalic rocks permits the distinction of two main types of hour-glass tructure. The common 'swallow-tailed', sometimes skeletal augite crystals are found in the fine-grained groundmass ofmany rock types, and it is sug-gested that rapid crystallization alone accounts for their formation. Hence, this type of hour-glass structure has been called 'quench our-glass'. The hour-glass tructures of larger augite crystals of porphyritic and coarse-grained rocks are commonly described as hour-glass 'zoning', as they resttlt primarily from compositional differences between the different sectors. These were formed under conditions of relatively slower cooling than the 'quench our-glass', and thus cannot be explained in the same way. They are thought o have formed by a process involving adsorption of impurities on a particular crystal face so as to impede growth of these faces, producing an initial skeleton of hour-glass form, which is completed by later crystallization of augite richer in FeO, Na20, TiO2, and Al~O3. This hypothesis also explains the patchy zoning of other augite crystals, casting doubt on some petrogenetic interpretations of such zones as core zones. THE present study developed from a more general investigation of volcanic rocks of the Comores Archipelago, rocks that are generally rich in clinopyroxene showin