Helicoidal crack propagation in aluminous orthoamphiboles.
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.
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