similar
to those furnished by the Youndegin iron: his observation, however,
has been forgotten, and is without record in modem meteoric
literature. The crystals—of the sine, number, and completeness of
which Haid-inger makes no mention—were obtained by him from a nodule of
grapihte which had dropped out of the Arva meteoric iron, and chiefly
from a study of their form he inferred that they were pseudomorphous
after iron pyrites; Even yet no iron pyrites, crystallized or massive,
has been found in a meteorite, the meteoric sulphide of iron being, not
the bisulphide, but the protosulphide: further, Gustav Rose, after
examination of the crystals, expressed the opinion that the replacement
of the edges of the cubes was suggestive rather of holosymmetry than of
hemisymmetry, an interpretation which would exclude iron pyrites as a
possible antecedent mineral.
The
Youndegin graphitic crystals support the view entertained by Rose: The
existence of the dodecahedron face, of which there is goniornetrical
proof, is of itself quite sufficient to show that the crystalline form
is distinct from that of iron pyrites.
The
iron pyrites theory being discarded, and the fact being recognized that
no mineral constituent of meteorites has yet been found which
crystallizes in forms similar to those of the graphitic crystals, there
naturally arises 4 feeling of doubt as to the correctness of the view
according to which'they are of pseudomorphic origin, and thus a
question as to whether they may not possibly be a third allotropic
condition of crystallized carbon presenting the general characters of
graphite, but a crystalline form frequent in the diamond. Bischof
denies the possibility of explaining the pseudomorphism of terrestrial
minerals by any other process than the slow action of water, of which
there is no evidence in meteorites; and though it would be unsafe to
argue that only in this way could meteoric pseudomorphs be produced,
there is sufficient difficulty in their explanation to demand strong
evidence before pseudomorphism of the graphitic crystals is granted,
more especially when we have regard to the fact that no other graphitic
pseudomorph has yet been established either in meteoric or in
terrestrial minerals.
Examination
of the Youndegin crystals under the microscope shows that some of them
are hollow, and appear to be built up of successive cubical shells: on
several of the crystals there are globular growths covering a large
part of a cube-face, and occasionally the globule is broken, and is
seen to be merely a thin, now empty, shell, of which the bottom is the
face of the cube. The crystals are easily frangible, and no cleavages
were observed; they appear to be quite homogenous in their material.
Although
some of these characters suggest a. pseudomorphic origin of the
crystalline form, it cannot be said that they prove it. Soth' of the
recognized crystalline forms of carbon, graphite and diamond have
tbrig*"been standing difficulties for the crystallographer. As already
pointed out, the crystals of graphite are rarely more than mere tables,
of which there is a controversy as to the crystalline system ; those of
the diamond are often so different in their geometrical characters from
the crystals of every other known substance, that it cannot be
satisfactorily determined whether they are to be referred to a
holosymmetric or to a hemisymmetric type.
Hollow
and skeleton crystals are often the result of a hurried crystallization
as is so well seen in the artificial crystals of bismuth and of common
salt. The diamond, too, when in cubes, has faces more uneven than those
of the Youndegin crystals, and shows usually the same replacement of
its edges T?y rounded faces of tetrakishexahedra.
It
thus might be argued with some force that the Youndegin crystals have
been the result of a hurried crystallization of carbon, and ,that,
while striving to reach a dignity which has been assigned to cubes of
diamond, they have been overtaken by misfortune and come out in cubes
of the less horto ured mineral, graphite. The obtuse, almost flat,
square pyramid seen on some of the eube-faces, the hollow globular
growths, the occasional parallelism »f the group-
