On hour-glass sector zoning in clinopyroxene

SUMMARY. The observations ofhour-glass sector zoning from four selected clinopyroxenes in under-saturated lavas best fit a model in which the sector zoning is formed due to a structural requirement in a growth direction ormal to {IOO} where a high concentration f silicate units, required to build the silicate chains, is unsatisfied. In these sectors AI substitutes for Si, the charge balance being main-tained by octahedral substitutions, ofwhich Ti contributes to the deeper colour of the sectors. The presence of concentric zoning only reflects achange in temperature and a change in the composition of the liquid during crystallization ofthe pyroxene. HOUR-GLASS sector zoning has been observed in leucite-bearing volcanic rocks and four clinopyroxene crystals have been examined in detail: A Titanaugite phenocryst in lava, Bufumbira, Uganda; B Titanaugite phenocryst in lava, New South Wales, Australia; c Aluminous augite crystal ining a cavity in lava, Vulsinian volcanoes, Italy