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.
