PHYSICAL PROPERTIES.
B--CLEAVAGE.
By cleavage is meant the manner in which minerals separate or split off
with regularity. The difference between a break or fracture and a
"cleave," is that the former may be anywhere throughout the substance of
the broken body, with an extremely remote chance of another fracture
being identical in form, whereas in the latter, when a body is
"cleaved," the fractured part is more readily severed, and usually takes
a similar if not an actually identical form in the divided surface of
each piece severed. Thus we find a piece of wood may be "broken" or
"chopped" when fractured across the grain, no two fractured edges being
alike; but, strictly speaking, we only "cleave" wood when we "split" it
with the grain, or, in scientific language, along the line of cleavage,
and then we find many pieces with their divided surfaces identical. So
that when wood is "broken," or "chopped," we obtain pieces of any width
or thickness, with no manner of regularity of fracture, but when
"cleaved," we obtain strips which are often perfectly parallel, that is,
of equal thickness throughout their whole length, and of such uniformity
of surface that it is difficult or even impossible to distinguish one
strip from another. Advantage is taken of these lines of cleavage to
procure long and extremely thin even strips from trees of the willow
variety for such trades as basket-making.
The same effect is seen in house-coal, which may easily be split the way
of the grain (on the lines of cleavage), but is much more difficult and
requires greater force to break across the grain. Rocks also show
distinct lines of cleavage, and are more readily split one way than
another, the line of cleavage or stratum of break being at any angle and
not necessarily parallel to its bed. A striking example of this is seen
in slate, which may be split in plates, or laminae, with great facility,
though this property is the result of the pressure to which the rock has
been for ages subjected, which has caused a change in the molecules,
rather than by "cleavage" as the term is strictly understood, and as
existing in minerals. Mica is also another example of laminated
cleavage, for given care, and a thin, fine knife to divide the plates,
this mineral may be "cleaved" to such remarkably thin sheets as to be
unable to sustain the most delicate touch without shattering.
These are well-known examples of simple cleavage, in one definite
direction, though in many instances there are several forms and
directions of cleavage, but even in these there is generally one part or
line in and on which cleavage will take place much more readily than on
the others, these planes or lines also showing different properties and
angular characters, which, no matter how much fractured, always remain
the same. It is this "cleavage" which causes a crystal to reproduce
itself exactly, as explained in the last chapter, showing its parent
form, shape and characteristics with microscopic perfection, but more
and more in miniature as its size is reduced.
This may clearly be seen by taking a very small quantity of such a
substance as chlorate of potash. If a crystal of this is examined under
a magnifying glass till its crystalline form and structure are familiar,
and it is then placed in a test-tube and gently heated, cleavage will at
once be evident. With a little crackling, the chlorate splits itself
into many crystals along its chief lines of cleavage (called the
cleavage planes), every one of which crystals showing under the
microscope the identical form and characteristics of the larger crystal
from which it came.
The cleavage of minerals must, therefore, be considered as a part of
their crystalline structure, since this is caused by cleavage, so that
both cleavage and crystalline structure should be considered together.
Thus we see that given an unchangeable crystal with cleavage planes
evident, it is possible easily to reproduce the same form over and over
again by splitting, whereas by simply breaking, the form of the crystal
would be lost; just as a rhomb of Iceland spar might be sawn or broken
across the middle and its form lost, although this would really be more
apparent than real, since it would be an alteration in the mass and not
in the shape of each individual crystal. And given further cleavage, by
time or a sudden breaking down, even the mass, as mass, would eventually
become split into smaller but perfect rhombs.
Much skill is, therefore, required in cutting and fashioning a precious
stone, otherwise the gem may be ruined at the onset, for it will only
divide along its lines of cleavage, and any mistake in deciding upon
these, would "break," not "split" the stone, and destroy the beauty of
its crystalline structure. An example of this was specially seen in the
great Cullinan diamond, the splitting of which was perhaps the most
thrilling moment in the history of precious stones.[A] The value of the
enormous crystal was almost beyond computation, but it had a flaw in the
centre, and in order to cut out this flaw it was necessary to divide the
stone into two pieces. The planes of cleavage were worked out, the
diamond was sawn a little, when the operator, acknowledged to be the
greatest living expert, inserted a knife in the saw-mark, and with the
second blow of a steel rod, the marvellous stone parted precisely as
intended, cutting the flaw exactly in two, leaving half of it on the
outside of each divided portion. The slightest miscalculation would have
meant enormous loss, if not ruin, to the stone, but the greatest feat
the world has ever known in the splitting of a priceless diamond was
accomplished successfully by this skilful expert in an Amsterdam
workroom in February, 1908. Some idea of the risk involved may be
gathered from the fact that this stone, the largest ever discovered, in
the rough weighed nearly 3,254 carats, its value being almost anything
one cared to state--incalculable.
[Footnote A: The hammer and knife used in cutting the diamond, the two
largest pieces of which are now called "The Stars of Africa," together
with a model of the great uncut stone, are in the Tower of London
amongst the Regalia.]
These cleavage planes help considerably in the bringing of the stone to
shape, for in a broad sense, a finished cut stone may be said to be in
the form in which its cleavages bring it. Particularly is this seen in
the diamond "brilliant," which plainly evidences the four cleavage
planes. These cleavage planes and their number are a simple means of
identification of precious stones, though those possessing distinct and
ready cleavages are extremely liable to "start" or "split" on these
planes by extremes of heat and cold, accidental blows, sudden shocks and
the like.
In stones possessing certain crystalline structure, the cleavage planes
are the readiest, often the only, means of identification, especially
when the stones are chemically coloured to imitate a more valuable
stone. In such cases the cleavage of one stone is often of paramount
importance in testing the cleavage of another, as is seen in the
perfection of the cleavage planes of calcite, which is used in the
polariscope.
It sometimes happens, however, that false conditions arise, such as in
substances which are of no form or shape, and are in all respects and
directions without regular structure and show no crystallisation even in
the minutest particles; these are called amorphous. Such a condition
sometimes enters wholly or partially into the crystalline structure, and
the mineral loses its true form, possessing instead the form of
crystals, but without a crystalline structure. It is then called a
pseudomorph, which is a term applied to any mineral which, instead of
having the form it should possess, shows the form of something which has
altered its structure completely, and then disappeared. For instance:
very often, in a certain cavity, fluorspar has existed originally, but,
through some chemical means, has been slowly changed to quartz, so
that, as crystals cannot be changed in shape, we find quartz
existing--undeniably quartz--yet possessing the crystals of fluorspar;
therefore the quartz becomes a pseudomorph, the condition being an
example of what is termed pseudomorphism. The actual cause of this
curious chemical change or substitution is not known with certainty, but
it is interesting to note the conditions in which such changes do occur.
It is found that in some cases, the matrix of a certain shaped crystal
may, after the crystal is dissolved or taken away, become filled by some
other and foreign substance, perhaps in liquid form; or a crystalline
substance may become coated or "invested" by another foreign substance,
which thus takes its shape; or actual chemical change takes place by
means of an incoming substance which slowly alters the original
substance, so that eventually each is false and both become
pseudomorphs. This curious change often takes place with precious
stones, as well as with other minerals, and to such an extent that it
sometimes becomes difficult to say what the stone ought really to be
called.
Pseudomorphs are, however, comparatively easy of isolation and
detection, being more or less rounded in their crystalline form, instead
of having sharp, well-defined angles and edges; their surfaces also are
not good. These stones are of little value, except in the specially
curious examples, when they become rare more by reason of their
curiosity than by their utility as gems.
Some also show cleavage planes of two or more systems, and others show
a crystalline structure comprised of several systems. Thus calcspar is
in the 2nd, or hexagonal, whilst aragonite is in the 4th, the rhombic,
system, yet both are the same substance, viz.:--carbonate of lime. Such
a condition is called dimorphism; those minerals which crystallise in
three systems are said to be trimorphous. Those in a number of systems
are polymorphous, and of these sulphur may be taken as an example, since
it possesses thirty or more modifications of its crystalline structure,
though some authorities eliminate nearly all these, and, since it is
most frequently in either the 4th (rhombic) or the 5th (monoclinic)
systems, consider it as an example of dimorphism, rather than
polymorphism.
These varieties of cleavage affect the character, beauty and usefulness
of the stone to a remarkable extent, and at the same time form a means
of ready and certain identification and classification.