Matrix of the Diamond | Matrix Diamonds Page 26



	14 THE MATRIX OF THE DIAMOND The olivine alters by decomposition either into serpentine, or into tremolite, or into a bastitic fibrous mineral. The alteration into serpentine proceeds in the usual manner, beginning at the outer edge and in cracks in the crystal, and gradually penetrating deeper, as new cracks are formed, until the change is complete. A homogeneous nearly isotropic serpentine is the final result. Fibrous serpentine or chrysotile also forms around the olivine grains, and in this case the chrysotile fibres stand more or less at right angles to the original faces of the crystal. The chrysotile is much more highly doubly refracting than the compact serpentine, which latter, when most dense, is often nearly isotropic. Another serpentinous mineral is faintly pleo-chroic in pale shades of green, and is, perhaps, more nearly related to bastite. Tremolite (fig. 6) in the form of asbestos occurs as a secondary mineral pseudomorphic after olivine. It frequently happens that while serpen tinisation begins at the outside of a crystal, fibrous tremolite begins growing within, finally forming a mass of asbestiform fibres surrounded by a zone of green serpentine. These asbestos fibres often grow partly parallel to the vertical axis, and partly parallel to the domes of the olivine. They are distinguished from other minerals by their high colour in polarised light, and by vertical fibres separated by partings or cleavages like those which divide sillimanite, and by an obliquity of extinction at about 15°. They are perfectly colourless and non-pleochroic. A fibre may be compact in the centre or at one end, and at the other end may be fringed out into fine hair-like asbestos. In the second figure (7) two olivine crystals have grown side by side, their axes parallel. A thin green rim of serpentine surrounds each crystal. A round grain of olivine remains in each, and crystals of tremolite surround it. Between the tremolite and the serpentine is a pseudomorphic mass of talcose substances, in which lie rutile needles parallel to the faces of the crystal.