STRETCHED CRYSTAL GROWTH

Stretched Crystal growth: This is another type of growth in which the crystals are stretched either subperpendicular to the wall rock or at large angle. The crystals undergo fracturing due to stretching and the mechanism is similar to that of the crack seal. Above is the photograph of a vein with stretched crystal growth of quartz developed within an extensional crack in a basic dyke intruded into phyllites and graywackes of Mahakoshal group rocks near Barmhan in Central India. The growths of this type can also be observed in the pressure shadows of minerals such as pyrite or the framboidal aggregates of pyrite or on sides of rigid crinoid stems. These were studied by Durney in the Helvetic Alps and he could deduce the emplacement history of nappes in the external Swiss Alps based on this study. He showed that Morcles nappe, the Wildhorn nappe, the Diableretes nappe and the extrahelvetic nappes had a progressively changing incremental strain directions and showed lower strain progressively in that order but were emplaced in just the opposed sequence.  For example the following photograph of antitaxial growth around framboidal pyrite from British Columbia.