THE MATRIX OF THE DIAMOND 39
Zeolites occur
in cavities and cracks as a product of infiltration and decomposition.
They are very rare in the hard rock here described, but nearer the
surface of the mine they occur abundantly in the decomposed blue
ground, sometimes forming rock dike masses. The principal zeolite is
natrolite. Fibres of natrolite were seen in thin sections radiating
inwards, with a more or less tufted arrangement, from the outer edge of
a mass of calcite.
Chalcedony was observed in one instance filling a microscopic cavity in the rock.
Cyanite.—A microscopic
mineral occurs in minute quantity in the peridotite, whose exact nature
is uncertain, but whose optical characters seem to agree more closely
with cyanite than with any other known mineral. It is particularly
interesting, as forming usually a zone around enclosures of shale and
other rocks, as if a contact mineral. It usually forms a fine compact
crystalline zone, but sometimes the separate crystals are large enough
to be separately studied under the microscope. It occurs in fibrous
masses resembling cyanite or actinolite, and has the following
properties : a high index of refraction, about equal to that of
pyroxene; the double refraction low, being lower than in actinolite. It
is very faintly pleochroic from pale blue to faint green, or, more
accurately, pale lavender (corresponding to a) to pale
greenish-lavender (corresponding to t). The fibrous structure is well
marked in this mineral. It has two cleavages, one parallel to the
fibres, and one at right angles to them, the latter often causing a
separation, as in apatite. The crystals seem to be brittle, being often
broken, as is so common in silli-manite. It has a large angle of
extinction ( = 32°—35°). It is unattacked by acid. All these characters
seem to be those of cyanite. It would be curious, however, to find
cyanite forming contact zones around enclosures in a peridotite.
