Summary

Itacolumites (flexible sandstones) display a bimodal stress-strain behaviour resulting from an unusual inter-digitated grain-to-grain fabric. The sandstone can move freely over a limited arc, then locks, and displays only elastic behaviour. Grain contacts are all open about 5 μm, thereby permitting some free movement. Each quartz grain is interdigitated with its surrounding neighbours, therefore the specimen has appreciable fabric cohesion despite a lack of true mineral cement. Fabric cohesion, as opposed to cementitious cohesion, is strongly stress dependent, and itacolumites can be expected to have highly curvilinear failure criteria.

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