Formation of the hour-glass structure in augite

Abstract

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 structure. The common ‘swallow-tailed’, sometimes skeletal augite crystals are found in the fine-grained groundmass of many rock types, and it is suggested that rapid crystallization alone accounts for their formation. Hence, 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’, as they result primarily from compositional differences between the different sectors. These were formed under conditions of relatively slower cooling than the ‘quench hour-glass’, and thus cannot be explained in the same way. 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 initial skeleton of hour-glass form, which is completed by later crystallization of augite richer in FeO, Na 2 O, TiO 2 , and Al 2 O 3 . This hypothesis also explains the patchy zoning of other augite crystals, casting doubt on some petrogenetic interpretations of such zones as core zones.