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helicitic structure Sigmoidal trails of mineral inclusions in porphyroblasts formed during metamorphic recrystallization. The sigmoidal inclusion trails may represent remnants of a pre-existing fold-fabric preserved as the growing crystal enclosed the fabric, leaving untouched those minerals it did not require to provide its chemical components. Sigmoidal inclusion trails may also represent relative movement between the growing porphyroblast and the groundmass or external fabric. Thus, if the porphyroblast grows under the influence of shear stress, it rotates during growth, and may include relicts of any parallel external fabric as an S-shaped internal inclusion trail.