Summary: The observations of hour-glass sector zoning from four selected clinopyroxenes in undersaturated lavas best fit a model in which the sector zoning is formed due to a structural requirement in a growth direction normal to {100} where a high concentration of silicate units, required to build the silicate chains, is unsatisfied. In these sectors Al substitutes for Si, the charge balance being maintained by octahedral substitutions, of which Ti contributes to the deeper colour of the sectors. The presence of concentric zoning only reflects a change in temperature and a change in the composition of the liquid during crystallization of the pyroxene.