title: Helicoidal crack propagation in aluminous orthoamphiboles.
abstract: Sodie and alluolminous orthoamphiboles from metamorphosed volcanic rocks develop helicoidal fractures upon cooling, due to elastic strain gradients radical to the c-axis. This condition may arise, fromradial composition gradients involving subastitution of tetrahedral silicon by aluminum in the double chain structure or from intergranular boundary constraints with adjacent quartz.
authors.forename: J H
authors.initials: JH
authors.lastname: Stout