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38 THE MATRIX OF THE DIAMOND
tonalite, but seems not to have been previously observed in peridotite or serpentine.
Tourmaline, in
strongiy pleoehroic short prisms, is a rare but interesting constituent
of the rock, being especially abundant around certain inclusions of
shale.1 Its pleochroism changes from brown to light blue.
This and other minerals in small quantities are best detected after
treating the thin section with hydrochloric acid.
Sphene, or
titanite, was rarely observed, only a few irregular grains being
noticed. The finely fibrous form of sphene as a secondary mineral,
known as leucoxene, also rarely occurs.
Tremolite has already been described, also rutile, as secondary minerals.
Serpentine and serpentinous minerals, as already stated, form a large part of the rock, resulting from the decomposition of olivine.
Talc.—Very
minute scales of this mineral were noticed in the ground-mass, and, as
a contact or alteration mineral, around certain enclosures. It was
highly refracting, had a wavy sheen, high colour, and parallel
extinction.
Calcite is
abundantly present in this rock. Diopside can be seen directly altering
into this mineral, but the quantity of diopside is so small that we
must look elsewhere for the large mass of calcite that penetrates the
rock. It is possible that some readily decomposed lime-bearing mineral,
like melilite, originally furnished it; but it may be that, as Cohen2 has suggested, the lime has liltered in from without. Calcite is said frequently to form a crust around the diamonds.
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1 It will be remembered that tourmaline is very often a product of contact metamorphism in an aluminous rock.—T. G. B.
2 Neues Jahrb. Beil.
Bd. v. 1887, p. 195. Cohen here holds, in opposition to Moulle and
others, that the ' calc-tuff ' encrusting the ' blue ground ' at the
diamond mines is not derived from the diamantiferous ground, but is a
later deposit from brackish water during a depression in the
Pleistocene Period.
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