The development of vein and rock textures by tensile strain crystallization

Abstract

The characteristics of fibrous veins of gypsum and other minerals are described and it is concluded that the growth of the fibres kept pace with the dilation of the veins because fluid pressures were in excess of lithostatic pressures but less than the combined lithostatic pressures and tensile strengths of the fibres. Crystallization in the regions of greatest tensile strain in the fibres would be promoted by the greater probability of atoms being accommodated in the crystal structure. The free energy associated with the crystal dislocations which are likely to be concentrated in these regions would contribute to the crystallization process and allow it to proceed at very low levels of supersaturation. Crystallization during tensile strain explains the common occurrence of strain effects such as undulose extinction and dislocation planes in fibrous crystals. It is suggested that polycrystalline aggregates may deform under triaxial stress by pressure solution of the grains on planes approximately perpendicular to the direction of greatest stress and simultaneous crystallization of elongate crystals in the direction of maximum extension from the points of concentrated tensile strain. Tensile strain crystallization may be an important mechanism in the development of penetrative preferred orientations in metamorphic rocks.