THE MATRIX OF THE DIAMOND 29
region of Syssersk, Urals, imbedded in serpentine.1 Other small garnets, again, of bluish-green colour, giving a chrome reaction, may belong to the variety named ' ouvarovite.'
The
pyropes are about as abundant as in the garnet-bearing serpentines, and
it is probable that many of these serpentines have been derived from a
peridotite of similar nature to that now under consideration. These
serpentines are related not merely in the occurrence of garnet, but
also in the association of olivine, bronzite, diallage and sometimes
perovskite, and even diamond.
Perovskite.—Perovskite
is an abundant and characteristic mineral in the rock under
consideration. It occurs in crystals, in crystalline aggregates, and
in combination with ilmenite. It was isolated with the garnets from the
other constituents by treatment with sulphuric and hydrofluoric acids,
and submitted to blowpipe tests. Most of the mineral thus isolated was
in the form of brownish-black cubical crystals, with octahedral
truncations (fig. 19). These crystals were almost all of a nearly
uniform size. In the thin sections the perovskite appeared as
reddish-brown crystals or aggregates, deep yellow when thin, but
sometimes nearly opaque, scattered abundantly through the ground-mass.
In their deep colour and very high index of refraction they resembled
the rutile grains of the crystalline schists, but were readily
distinguished from that mineral or from zircon by their remarkably low
power of double refraction. The colour in polarised light seldom rose
above grey of the first order, and was sometimes as high as white of
the first order. The characteristic optical properties therefore are an
index of refraction nearly as
