Helicoidal Crack Propagation in Aluminous Orthoamphiboles

Science  19 Jul 1974:
Vol. 185, Issue 4147, pp. 251-253
DOI: 10.1126/science.185.4147.251


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.