DEFORMATION CONSTRAINED GROWTH
Under deformational constraint the crystals in veins or pressure shadows grow in the direction of maximum principal stretch of the infinitesimal strain ellipsoid. The crystals grow as fibres, quite unlike their natural crystal habit. The material constituting the fibres is of a mineral species foreign to the rock since the rock can fracture under given PT conditions but does not contain any minerals which can undergo pressure solution. The active surface is the crystal fibre tip to which the new material is more or less continuously added by grain boundary migration and diffusion. The crystals are not in optical continuity with those in the host rock. If the growth is around a porphyroblast, this too is towards the pophyroblast and not away from it. Typical examples are of the growth around crinoid stems and framboidal aggregates of pyrite. The photograph is from the webpage of Dr Koehn.