THE CRYSTAL SPACE-LATTICE AND ITS MOLECULAR UNIT CELL. THE 230
POINT-SYSTEMS OF HOMOGENEOUS CRYSTAL STRUCTURE.
The interval between the morphotropic work described in the last chapter
and the present time has been remarkable for the completion of the
geometrical and mathematical investigation, and the successful
identification, of all the possible types of homogeneous structures
possessing the essential attributes of crystals. It has now been
definitely established that there are 230 such types of homogeneous
structures possible, and the whole of them conform to the conditions of
symmetry of one or other of the thirty-two classes of crystals. This
fact is now thoroughly agreed upon by all the authorities who have made
the subject their special study, and may truly be considered as
fundamental.
There has long been a consensus of opinion that the crystal edifice is
built up of structural units which can be likened to the bricks or stone
blocks of the builder, but which in the case of the crystal are so small
as to be invisible even under the highest power of the microscope. The
conceptions of their nature, however, have been almost as numerous as
the investigators themselves, everyone who has thought over the subject
forming his own particular ideas concerning them. We have had the
“Molécules intégrantes” of Haüy, the “Polyhédres” of Bravais, the
“Fundamentalbereich” of Schönflies, the “Parallelohedra” of von Fedorow,
and the fourteen-walled cell, the “Tetrakaidecahedron” of Lord Kelvin,
and again the “Polyhedra” of Pope and Barlow. Ideas have thus been
extremely fertile, and indeed almost every variety of speculation has
been indulged in as to the shape and nature of the unit of the structure
which can exhibit such remarkable evidences of organisation and such
extraordinary optical and other physical properties as those of a
crystal.
There is one inherent difficulty, however, which renders all such
speculations more or less chimerical, until we know very much more as to
the structure of the chemical atom, and the organisation of the
corpuscles composing it. Such speculations, however, are deeply
interesting, and the difficulty alluded to accounts largely for the
great variety of conception possible. It is this, that the matter of the
molecules, and again that of the atoms composing them, is not
necessarily, nor even probably, continuous and in contact throughout,
but that on the contrary the space which may legitimately be assigned to
the unit of the structure is partly void. How much of this unit space is
matter and how much is unoccupied, and how the one is related to the
other as regards its position or distribution in space, we have yet no
means of knowing, although there are signs that the day is not far
distant when we shall know at least something concerning it. The recent
brilliant work of Sir J. J. Thomson and his school of physicists has
rendered it clear that the chemical atom is composed of cycles of
electronic corpuscles, the orbital motions of which determine its
boundaries.
In this condition of our knowledge obviously the only safe course is
to consider each atom of the chemical molecule as occupying a “sphere
of influence,” within the limits of which the material parts of the
atom, the corpuscles in organised motion, are confined. The
“Fundamentalbereich” of Schönflies and the “Sphere of Influence” of
Barlow, are the conceptions which in all probability have the greatest
value in the present state of our knowledge, and if we adopt the
latter we shall not be committing ourselves to anything more than the
experimental facts fully warrant.
It may be quite definitely stated, however, that there is a considerable
amount of experimental evidence that the unit of the space-lattice of
the crystal structure is certainly not more complex than the chemical
molecule, the idea of an aggregation of chemical molecules to form a
“physical molecule” acting as a structural unit having proved to be a
misleading myth.
Fortunately, however, there is no necessity whatever to introduce the
subject of the actual shape of the unit, and the greatest progress has
been effected by disregarding it altogether, and agreeing to the
representation of the unit by a point. This leads us at once to perceive
the importance of the brilliant work of the geometricians, who have now
completed their theory of the homogeneous partitioning of space into
point-systems possible to crystals, the structural units of the latter
being regarded as points. The investigations extend from those of
Frankenheim in the year 1830 to the finishing touches given by Barlow in
1894, and prominently standing forth as those of the greatest
contributors to the subject, besides the two investigators just
mentioned, are the names of Bravais, Sohncke, Schönflies and von
Fedorow.
Bravais, perfecting the work of his predecessor Frankenheim, made us
acquainted with the fourteen fundamentally important space-lattices, or
same-ways orientated arrangements of points. If we regard each chemical
molecule as represented by a point, disregarding the separate atoms of
which it is composed, then these fourteen space-lattices represent the
possible arrangements of the molecules in the crystal in all the simpler
cases; three of these lattices have cubic symmetry; the tetragonal,
hexagonal, trigonal, rhombic and monoclinic systems claim two
space-lattices each; while one space-lattice conforms to the lack of
symmetry of the triclinic system.
The fourteen space-lattices of Bravais thus represent the arrangement of
the chemical molecules in the crystal, and determine the systematic
symmetry. The points being taken absolutely analogously in all the
molecules, and the whole assemblage being homogeneous, that is, such
that the environment about any one spot is the same as about every
other, the arrangement is obviously a same-ways orientated one, the
molecules being all arranged parallel-wise to each other.
[Illustration:
FIG. 59.—Triclinic Space-Lattice.
]
But the fact that the structure is that of a space-lattice also causes
the crystal to obey the law of rational indices. To enable us to see how
this comes about it is only necessary to regard a space-lattice. In Fig.
59 is represented the general form of space-lattice, that which
corresponds to triclinic symmetry. It is obviously built up of
parallelepipeda, the edges of which are proportional to the lengths of
the three triclinic axes, and their mutual inclinations are those of the
latter. As we may take our representative point anywhere in the
molecule, so long as the position chosen is the same for all the
molecules of the assemblage, we may imagine the points occupying the
centres of the parallelepipeda instead of the corners if we choose, for
that would only be equivalent to moving the whole space-lattice slightly
parallel to itself. Hence, each cell may be regarded as the habitat of
the chemical molecule.
Now the faces of the crystal parallel to each two of the three sets of
parallel lines forming the space-lattice will be the three pairs of
axial-plane faces, and any fourth face inclined to them must be got by
removing parallelepipedal blocks in step-wise fashion, precisely like
bricks, as already shown in Fig. 12 (page 28) in Chapter III., in order
to illustrate the step by step removal of Haüy’s unit blocks. It will
readily be seen that if one more cell be removed from each row than from
the row below it, the line of contact touching the projecting corner of
the last block of each row will be inclined more steeply than if two
more cells were removed from each row. Moreover, the angle varies
considerably between the two cases, and if three blocks are removed at a
time the angle gets very small indeed. Hence, there cannot be many such
planes possible, and we see at once why the indices of the faces
developed on a crystal are composed of low whole numbers and why the
forms are so relatively few in number. Owing to the minuteness of a
chemical molecule, all the irregularities of such a surface are
submicroscopic, and the general effect to the eye is that of a smooth
plane surface.
The space-lattice arrangement of the molecules in the crystal structure
thus causes the crystal to follow the law of rational indices, by
limiting and restricting the number of possible facial forms which can
be developed. It also determines which one of the seven systems of
symmetry or styles of crystal architecture the crystal shall adopt. It
does not determine the details of the architecture, however, that is, to
which of the thirty-two classes it shall conform, this not being the
function of the molecular arrangement but of the atomic arrangement that
is, of the arrangement of the cluster of atoms which form the molecule,
and this leads us to the next step in the unravelling of the internal
structure of crystals.
The credit of this next stage of further progress is due to Sohncke,
whose long labours resulted in the discrimination and description of
sixty-five “Regular Point-Systems,” homogeneous assemblages of points
symmetrically and identically arranged about axes of symmetry, which are
sometimes screw axes, that is, axes about which the points are spirally
distributed. Sohncke’s point-systems express the number of ways in which
symmetrical repetition can occur. Moreover, the points may always be
grouped in sets or clusters, the centres of gravity of which form a
Bravais space-lattice.
This latter fact is of great interest, for it means that Sohncke’s
points may represent the chemical atoms, and that the stereometric
arrangement of the atoms in the molecule is that which produces the
point-system and determines the crystal class, while the whole cluster
of atoms forming the molecule furnishes, as above stated, the
representative point of the space-lattice.
This, however, is not the whole story, for the sixty-five Sohnckian
regular point-systems only account for twenty-one of the thirty-two
crystal classes, the remaining eleven being those of lower than full
holohedral systematic symmetry, and which are characterised by showing
complementary right and left-handed forms. In other words, they exhibit
two varieties, on one of which faces of low symmetry are developed on
the right, while on the other symmetrically complementary faces are
developed on the left; that is, these little faces modify on the right
and left respectively the solid angles formed by those faces of the
crystal which are common to both the holohedral class of the system and
to the lower symmetry class in question. In some cases, moreover, these
two complementary forms are known to exist alone, without the presence
of faces common to both the holohedral class and the class of lower
symmetry. The two varieties of the crystals are the mirror images of
each other, being related as a right-hand glove is to a left-hand one.
Further, the crystals of these eleven classes very frequently exhibit
the power of rotating the plane of polarised light to the right or to
the left, and complementarily in the cases of the two varieties of any
one substance, corresponding to the complementariness of the two crystal
forms. The converse is even more absolute, for no optically active
crystal has yet been discovered which does not belong to one or other of
these eleven classes of lower than holohedral symmetry.
The final step of accounting for the structure of these highly
interesting eleven classes of crystals was taken simultaneously by a
German, Schönflies, a Russian, von Fedorow, and an Englishman, Barlow,
who quite independently and by totally different lines of reasoning and
of geometrical illustration showed that they were entirely accounted for
by the introduction of a new element of symmetry, that of mirror-image
repetition, or “enantiomorphous similarity” as distinguished from
“identical similarity.” These three investigators all united in finally
concluding that when the definition of symmetrical repetition is thus
broadened to include enantiomorphous similarity, 165 further
point-systems are admitted, and the whole 230 point-systems then account
for the whole of the thirty-two classes of crystals.
Schönflies’ simple definition of the nature of the structure is that
every molecule is surrounded by the rest collectively in like manner,
when likeness may be either identity or mirror-image resemblance. Von
Fedorow finds the extra 165 types to be comprised in “double systems”
consisting of two “analogous systems” which are the mirror images of
each other. Barlow proceeds to find in how many ways the two
mirror-image forms can be combined together, there being in general
three distinct modes of duplication, including the insertion of one
inside the other. He also shows that all homologous points in a
structure of the type of one of these additional 165 point-systems
together form one of the 65 Sohnckian point-systems, the structure being
capable of the same rotations or translations, technically known as
“coincidence movements” (movements which bring the structure to exhibit
the same appearance as at first), as those which are characteristic of
that point-system.
This fascinating subject of mirror-image symmetry, and the optical
activity connected with it, will be reverted to and the latter explained
in Chapter XI.
We have thus seen how satisfactorily the geometrical theory of the
homogeneous partitioning of space has been worked out, and how admirably
it agrees with our preliminary supposition that a crystal is a
homogeneous structure. The fact that the 230 homogeneous point-systems
all fall into and distribute themselves among the thirty-two classes of
crystals, the symmetry of which has also now been fully established,
affords undeniable proof that as regards this branch of the subject
something like finality and clearness of vision has now been arrived at.