Occasionally, only half the faces of the hexakis-octahedron are developed, namely, those occupying alternate octants. The hemihedral form so derived is that of the hexakis-tetrahedron, shown in Fig. K. The faces of this form, which is of rare occurrence, are always curved, smooth, and bright. If the symmetry of the diamond is really tetrahedral-hemihedral, the complete hexakis-octahedron may be regarded as a combination of two hexakis-tetrahedral forms, but the faces in adjacent octants would then have different surface characters, and this has not hitherto been observed.
A regular or twin intergrowth between two hemihedral crystals frequently takes place and results in the production of a holohedral form. The twin intergrowth of two hexakis-tetrahedra gives rise to the twinned crystal, shown in Fig. L, in which for the sake of clearness the edges are represented as straight instead of curved lines. The two crystals interpenetrate at right angles, and the sharp corners, a (Fig. K) of one individual project from the obtuse corners, b, of the other, the faces of the two interpenetrating individuals thus forming re-entrant angles. The sharp projecting corners of such a group are not always present, usually being truncated, as is indicated in the figure, the truncating faces of each individual belong to the tetrahedron, and are never curved but always perfectly plane. The truncation, shown in Fig. L, is only slight, while that of Fig. M, is more pronounced.
On observing the figures it will be seen that these grooves are striated in the direction of their length. The size of the grooves depends on the degree to which the corners of the hexakis-tetrahedra are truncated by the faces of the tetrahedra. When the truncation is a maximum the grooves will be completely absent, but an octahedron of diamond in which such re-entrant grooves are not to be seen is a rarity. An octahedron of diamond with the sharp edges of the geometrical form must be considered to be the same as shown in Fig. M and in which the truncation has quite obliterated the grooves, in other words, it is the limiting form of such twinned crystals.
This twin intergrowth is simpler when the two individuals are tetrahedra instead of hexakis-tetrahedra, as in the case considered above. Such a twin-crystal is shown in Fig. P, where the projecting portions, removed by truncation, of the two interpenetrating tetrahedra are represented by dotted lines. Here the grooves are quite straight, and of the same width throughout, and they do not show the nick in the middle as in the previous twinned form, in which also the grooves widen out away from this nick.
This simpler twinned form, consisting merely of two interpenetrating tetrahedra, is, however, of very rare occurrence in diamond. On the other hand, the form consisting of two interpenetrating hexakis-tetrahedra, as shown in Fig. M, is very characteristic of diamond, and is of frequent occurrence. This figure has therefore been drawn again in Fig. N, the dotted lines having been omitted and the characteristic markings on the faces inserted. The small faces of the hexakis-tetrahedra, which form the re-entrant grooves due to the twinning, are always somewhat curved and exhibit a delicate striation in the direction of their length. A slightly different form of such an interpenetrating twin of octahedral habit is shown in Fig. O, this also is a frequently observed form of diamond crystal. Here the edges of the octahedron have, in place of grooves, two small planes meeting at a very obtuse angle in a short edge at the middle of, and perpendicular to, the octahedral edge, and away from this short edge formed by their mutual intersection they gradually widen out.
The twinned forms just described (Fig. M, Fig. N, Fig. O, Fig. P) are very characteristic of diamond, and they constitute the octahedral or Indian type. Crystals of this kind are sometimes known in the trade as "points."
It has already been pointed out above that while the faces of the rhombic dodecahedron and of the hexakis-octahedron show a convex curvature, those of the octahedron are plane and even. The octahedral faces are, however, characterized by the presence of striations and pits, both of which are repeated on the surface with definite regularity and have definite orientation.
The triangular pits are regular pyramidal depressions, of which the bases are equilateral triangles. They are usually small and often only to be seen distinctly under the microscope. The pyramidal faces inside the pits are finely striated and may terminate in the apex of the pyramid, as shown at Fig. A, or they may not extend to such a depth into the interior of the crystal, the apex of the pyramid being then truncated by a triangular face parallel to the face of the octahedron, as at a, Fig. Q. Sometimes on this inner face there is a smaller pyramidal depression as at c, Fig. Q. These depressions are of the same general character as those produced on the octahedral faces of a diamond during its combustion, but while the corners of the pits of natural origin are adjacent to the octahedral edges (Fig. N, Fig. O, Fig. Q), this position is occupied by the sides of the pits produced by etching (Fig. R), thus the two positions are the reverse of each other. The pits occur singly or in large numbers, and the striations may or may not be also present on the same face (Fig. O, Fig. N).
Beside, the twin-crystals formed by the interpenetration of two hemihedral crystals, illustrated in Fig. L to Fig. P, diamond presents still another type of twin-crystal, which is illustrated in Fig. G to Fig. I. Here two octahedral or rhombic dodecahedral crystals are united together along a face of the octahedron. Fig. G, shows two octahedra symmetrically united in this manner, the two individuals having one octahedral face in common. This kind of twin-growth is frequent in diamond but still more so in the mineral spinel, so that the law which governs this kind of twinning is referred to as the spinel twin-law. At the line of junction of the two individuals, three re-entrant angles alternate with the same number of salient angles. These spinel twins of diamond, which are known to the diamond-cutters of Amsterdam as "naadsteenen" (suture-stones), are very frequently flattened in a direction perpendicular to the common octahedral plane, they are, indeed, sometimes reduced to mere thin plates, but the faces and edges always have the surface characters described above.
Rafal Swiecki, geological engineer email contact
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