THE KIMBERLY DIAMOND FIELDS AND THEIR VOLCANIC ORIGIN
The elementary substance carbon occurs in three forms, i. e.,
_charcoal_, _graphite_, and the _diamond_. The commonest form of
carbon is to be found in charcoal, as well as in bituminous coal,
anthracite coal, and _lignite_. Graphite, also known as _plumbago_,
or _black lead_, is the substance you have seen so often in the lead
of pencils. The diamond, as every one knows, is the highly prized
precious stone that sparkles so brightly in the light, and is so hard
that it is capable of scratching almost any other substance.
Diamonds are found in various parts of the world. We are now
interested in them, however, only as they occur in certain parts of
the world, as in the great Kimberly diamond fields in Southern Africa.
Dr. Max Bauer in his book on precious stones says that the discovery
of diamonds in South Africa was made by a traveller named O'Reilly,
who, in 1867, saw a child sitting in the house of a Boer named Jacobs,
playing with a shining stone. Jacob's farm was a short distance south
of the Orange River near Hopetown. This stone proved to be a diamond
weighing some twenty-one and three-tenths carats and was afterwards
sold for $2,500. The incident led to the discovery and consequent
development of the Kimberly diamond fields.
Without going into a description of the different deposits in which
diamonds are found, it will suffice to say that in the Kimberly
district the diamonds occur distributed through the materials that
fill peculiar funnel-shaped depressions called _pipes_ which extend
vertically downward to unknown depths. The rock that fills a pipe
consists of an entirely different material from that in which the pipe
occurs. The upper extremity of the pipe is generally slightly elevated
above the general surface for a few yards. The pipes vary in diameter
from twenty to 750 yards, diameters of from 200 to 300 yards being
quiet common.
In 1892, the diamond-bearing material found in the pipes of the
Kimberly mines had been excavated vertically downwards a distance of
1,261 feet, without any signs of its being exhausted.
Now, the materials which fill the pipe of the great Kimberly mine are
practically the same in all the mines in the neighborhood. At the
upper part of the pipe the materials show the action of weathering by
exposure to the air. Here the ground is of a yellowish color. Below,
the materials have a blue color.
According to Bauer the diamond-bearing material that fills the upper
part of the pipe consists of a soft, sandy material of a light yellow
color, known to diamond miners as _yellow ground_, or _yellow stuff_.
In the case of the Kimberly mine, the yellow ground has a thickness of
about sixty feet. Below it the material has a blue color and is known
as the _blue ground_. This latter material possesses the character
of a volcanic _tuff_, which is a hardened clay. It is of a green or
bluish green color and has the appearance of dried mud that holds
or binds together numerous irregular, tough, and sometimes rounded
fragments of a green or bluish black serpentine.
The diamonds are found near the surface in the yellow ground, as
well as downwards through the blue ground. It was at one time thought
that most of the diamonds existed in the yellow ground, and that they
would soon disappear entirely at short distances below where the blue
ground began. Under this belief some of the most valuable claims
changed hands at prices far below their true value. It was soon found,
however, that large and valuable stones existed in the blue ground,
and, indeed, this ground has never been mined to a depth below where
valuable diamonds appear.
The diamonds occur in very small quantities spread through the yellow
and blue grounds. The following statement by Bauer will show this:
"A striking illustration of their sparing occurrence is
furnished by the fact that in the richest part of the
richest mine, namely, in the Kimberly mine, they constitute
only one part in 2,000,000, or 0.00005% of the blue ground.
In other mines the proportion is still lower, namely, one
part in 40,000,000, a yield which corresponds to five
carats per cubic yard of rock."
Of course, you will desire by this time to know the manner in which
the pipes of the diamond mines of South Africa have become filled with
the diamond-bearing rocks, and particularly what diamonds have to do
with a book on volcanoes and earthquakes.
Dr. Emil Cohen, who has made a study of these regions, regards the
pipes as volcanic vents or chimneys, and that the materials filling
the pipes have been brought up from below by volcanic forces. He says:
"I consider that the diamantiferous ground is a product
of volcanic action, and was probably erupted at a
comparatively low temperature in the form of an ash
saturated with water and comparable to the materials
ejected by a mud volcano. Subsequently new minerals were
formed in the mass, consequent on alterations induced in
the upper part by exposure to atmospheric agencies, and in
the lower by the presence of water. Each of the crater-like
basins, or, perhaps, more correctly, funnels, in which
alone diamonds are now found, was at one time the outlet
of an active volcano which became filled up, partly with
the products of eruption and partly with ejected material
which fell back from the sides of the crater intermingled
with various foreign substances, such as small pebbles,
or organic remains of local origin, all of which became
imbedded in the volcanic tuff. The substance of the tuff
was probably mainly derived from deep-seated crystalline
rocks, of which isolated remains are now to be found,
and which are similar to those which now crop out at the
surface, only at a considerable distance from the diamond
fields. These crystalline rocks from which the diamonds
probably took their origin, were pulverized and forced
up into the pipes by the action of volcanic forces, and
imbedded in this eruptive material, these diamonds either
in perfect crystals or in fragments are now found."
So far as the volcanic origin of the diamonds of the Kimberly diamond
fields is concerned, Cohen's theory has been generally accepted with
the following modifications: that the pipes were not filled by a
single volcanic eruption, but by successive eruptions, and that in
the case of the Kimberly mine, the pipes contain the results of as
many as fifteen successive eruptions. There has, however, been another
and more important modification proposed to Cohen's theory, which is
far more probable. It will be noticed that Cohen's theory regards the
action of the volcanic eruption as only serving to bring fragments
of a deep-seated mother rock that contained the diamonds up from
below with the material that fills the pipe. Now, Prof. Carvill Lewis
proposes the following very important change in Cohen's theory: that
the blue ground does not consist of fragmentary material or tuff, but
was forced up from below in the pipe in a molten mass and consolidated
on cooling. In other words, the blue ground is filled with an ordinary
igneous rock that was solidified in place in the vent or pipe.
In the great Kimberly mines the surface of the pipe is divided into
numerous separate claims, each consisting of a small square lot. There
are so many of these claims in the Kimberly mine that its surface is
honey-combed by numerous square pits. The work is done largely by
native Kaffirs employed there since the '70's. As the material was
removed from the pit, the adjoining claims were separated from each
other by high vertical walls.
At a later date, in order to remove the material and separate the
lots, high staging provided with ropes and hauling machinery was
erected. The number of these ropes is now so great that the mine has
the appearance of a huge cobweb.
A very extensive series of investigations has been made at a
comparatively recent date by Prof. Henri Moissan of France on various
chemical products that are obtained under the influence of the high
temperatures of the electric furnace. When a powerful electric current
is caused to pass through a highly refractory material, that is to
say, a material difficult to fuse, such as carbon, it raises it to an
extremely high temperature. A still higher temperature can be obtained
by causing a powerful current to flow between two carbon rods that
are first brought into contact, and then gradually separated from
each other, just as they are in the ordinary arc lights employed for
lighting the streets of our cities. In the latter way a temperature
that is estimated as high as 3,500° C. (6,332° F.), can be readily
obtained. Under these very high temperatures some very curious
chemical products have been obtained in electric furnaces. These
furnaces consist of small chambers made of highly refractory materials
closely surrounding the incandescent carbon, or the carbon voltaic
arc. Among some of the most curious of these products are artificially
produced diamonds.
Moissan, however, was not the first to produce diamonds artificially.
As soon as Lavoisier had experimentally shown that the chemical
composition of the diamond and carbon are the same, efforts were made
to convert charcoal into diamonds, and Despretz, as early as 1849,
by means of the combined influence of a powerful burning glass, the
oxyhydrogen blowpipe, and the carbon voltaic arc obtained a very high
temperature. He claims by this temperature to have been able to change
carbon into a few microscopic diamonds. It is quite possible, in the
light of later investigations, that Despretz may have been mistaken
in his belief that he had actually produced diamonds; but whether
this be so or not, he was certainly one of the pioneers in this early
transformation of charcoal.
Theoretically, all that would be required in order to change the
non-crystalline form of carbon into the diamond, would be to
subject the carbon to a temperature sufficiently high to fuse it
and then permit it slowly to crystallize. Could this be done, there
should be no trouble in transforming any amount of coal into any
equal amount of diamonds. But the transformation is by no means as
simple as might be supposed. It is not that the temperature of the
carbon cannot be raised to its point of fusion, but that as soon
as a certain temperature has been reached, the carbon, instead of
fusing or melting, is suddenly volatilized or turned into vapor.
There is no doubt that this is done. Thousands of feet of carbon
rods are volatilized every night in the arc lamps of our cities,
but the trouble is that this carbon vapor so formed, when cooled,
or condensed, is not converted into the exceedingly hard, clear,
crystalline diamond, but into the soft, dull black graphite or
plumbago.
Now the process adopted by Moissan in order to cause volatilized
carbon, or carbon vapor, to condense in the form of crystalline
diamonds was practically as follows: he placed pieces of pure carbon
inside a very strong steel tube, such, for example, as would be formed
by boring a short cylindrical hole in a piece of strong thick steel,
and placing a small quantity of carbon inside the tube so formed.
Closing the open end of the tube by means of a tightly fitting screw
plug, he volatilized the carbon inside the tube. The steel, tube, and
plug formed an electric furnace, for, as soon as he passed an electric
current through it, the temperature at once became high enough to
volatilize the carbon.
Under these circumstances the carbon vapor was subjected to great
pressure owing to the limited space in which it was liberated. As soon
as this mass of dense vapor had been formed, he seized the steel tube
with a pair of furnace tongs, and plunged it below the surface of cold
water in a bucket.
Of course, as the hot tube suddenly chilled, there was a great
shrinking in the walls of the furnace, so that the already compressed
carbon vapor was subjected to a still greater pressure which possibly
liquified it. Of that, however, we cannot speak definitely. This,
however, can safely be asserted, that when the tube was broken open
a confused mass of small crystals was found inside, some of which,
on examination with the microscope, were found to consist of small
crystals of two forms of diamonds, namely, the black diamond, or
carbonado, and the regular crystallized diamond.
Moissan made a great number of experiments for producing diamonds
in this way, and succeeded in forming some very beautiful, though
microscopic, diamonds.
What may be said to characterize especially Moissan's experiments was
the comparatively great number of diamonds, so small as to be scarcely
distinguishable under the microscope. The high temperature to which
the materials inside the tube were exposed resulted in the production
of numerous minute crystals of different minerals. In order to get rid
of as many of these as possible Moissan adopted the plan of subjecting
the material to the action of powerful solvents, such as sulphuric
acid, aqua regia, or a mixture of sulphuric and nitric acid, and
hydrofluoric acid. These acids destroyed most of the minute crystals
of other minerals, but left the minute crystals of diamonds unaffected.
Now it will be observed that the theory proposed by Prof. Carvill
Lewis as to the probable origin of the diamonds of the Kimberly mines
bears a wonderfully close resemblance to the method adopted by Moissan
for the production of artificial diamonds, since it supposes the
diamonds to have been formed by the sudden cooling or chilling within
the pipe of various molten materials brought up from great depths by
the volcanic forces. If this be true, then besides the comparatively
large crystallized and perfect diamonds found in the blue ground
of the Kimberly mines, there should also be found large quantities
of microscopic diamonds, just as are found in Moissan's electric
furnaces, in which he produced artificial diamonds.
Moissan, considering this, obtained a specimen of the blue ground
from the Kimberly diamond pipe and on subjecting it to the action of
the different solvents before named, found in the mass that was left
undissolved a great number of microscopic diamonds. It would seem,
therefore, that there is no reasonable doubt but that the Kimberly
diamond fields had their diamonds produced by the sudden chilling in
the volcanic pipes of molten materials brought from great depths by
the force of volcanic eruption.