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ANTITAXIAL 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
bottom one of antitaxial (away from the axis) calcite is after Jessel and
Bons and the upper is of crystals of clinozoisite from the Proterozoic
rocks near Barwaha in Central India.
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