|
THE MATRIX OF THE DIAMOND 11
The
last two minerals being of identical colour with the rest of the rock
are not so conspicuous as the first two. Scattered through the whole
rock are a large number of angular fragments of altered black shale.
These are often so abundant as to give the rock a brecciated
appearance-The rock takes a good polish, and, on such polished
surfaces, the olivine is clearly seen to be the predominating mineral,
and to occur in porphyritic crystals, lying in a ground-mass of
serpentine. Traces of fluidal structure are also seen in polished
specimens. The rock resembles, externally, certain dark picrites, like
those of Tringenstein or Schriesheim, yet even its general appearance
is different from that of any other known rock.1
The rock from Kentucky has the same characters, though containing less numerous foreign fragments.
We
now proceed to the more exact description of this reck. We will
consider (1) the minerals of which it is composed, (2) its chemical
composition, (3) its structure, (4) its geological characters and
significance.
(1) Constituent Minerals
The following minerals occur in the Kimberley rock, many of them being detected only under the microscope.
Olivine, forming the larger portion of the rock, often quite fresh.
Enstatite, chrome-diopside, smaragdite and bastite, often in fine green plates or crystals.
Biotite, a very prominent ingredient.
Garnet, common in bright red grains.
Perovskite, abundant in microscopic crystals.
Magnetite, chromite, Ilmenite, picotite, common under the microscope.
Apatite, epidote, orthite, tremolite, tourmaline, rutile, sphene, leucoxene (scarce and minute).
1
So far as my knowledge goes, this remark, though written almost ten
years ago, still holds good. A general idea of the aspect of the rock
may be obtained from Plate I.- T. G. B.
|
||
|
|
||