![]() At a more advanced stage of growth these bodies react Substances and in glassy rocks (obsidian and pitchstone), or mayīe obtained artificially from a solution of sulphur in carbonĭisulphide rendered viscous by the addition of Canada-balsam. They are met with in imperfectly crystallized Owing to unfavourable conditions of growth, are known asĬrystallites ( q.v.). Incipient crystals, the development of which has been arrested Result, which are sometimes characteristic of certain substances, When crystals are aggregated together, and so interfere withĮach other’s growth, special structures and external shapes often May be reduced to the same fundamental form. Calcite ( q.v.) affords a goodĮxample of a substance crystallizing in widely different habits,īut all crystals are referable to the same type of symmetry and Mass or from a solution and in the latter case the solvent mayīe of different kinds and contain other substances in solution, The material may have crystallized from a fused In different crystals of the same substance, these differencesĭepending largely on the conditions under which the growth has The actual form, or “habit,” of crystals may vary widely two tetragonal prisms ( a and m,) two tetragonalīipyramids ( e and p), and one ditetragonal bipyramid ( x, withĬrystal of Zircon (clinographic drawings and plans). This crystal is a combination of five simpleįorms, viz. 88) is placed at the side for comparison,Īnd the parallelism of the edges between correspondingįaces will be noticed. Shows the actual shape of a crystal of zircon from Ceylon the Have not been developed at all, is illustrated in fig. An extreme case, where several of the planes Irregularities of growth that it can only be determined by measurement ![]() The symmetry of actual crystals is sometimes so obscured by In the direction of an edge or diagonal of the cube. A remarkable case of irregularĭevelopment is presented by the mineral cuprite, which is oftenįound as well-developed octahedra but in the variety knownĪs chalcotrichite it occurs as a matted aggregate of delicate hairs,Įach of which is an individual crystal enormously elongated Irregularity of development, they still are regular (cubic) octahedra Yet crystallographically they are just as symmetricalĪs the ideally developed form, and, however much their ![]() Longer symmetrical with respect to the axes and planes of symmetry, Althoughįrom a geometrical point of view these figures are no The same and the faces have the same inclinations to the axesĪnd planes of symmetry as in the equably developed form. The angles (70° 32′ or 109° 28′) between the faces also remain Though here there are additional edges not present in fig. Which the faces intersect have the same directions as before, It will be noticed in these figures that the edges in Two misshapen or distorted octahedra are represented in figs.Ĩ5 and 86 the former is elongated in the direction of one of theĮdges of the octahedron, and the latter is flattened parallel to one Only whenĪ crystal is freely suspended in the mother-liquid and materialįor growth is supplied at the same rate on all sides does an equably To some other solid it cannot grow in that direction. One part than on another for instance, if the crystal be attached Grows that there will be a more rapid deposition of material on The crystal is so placed with respect to the liquid in which it Shown in the figures given above, in which similar facesĪre all represented as of equal size. Only rarely do actual crystals present the symmetrical appearance ( h) Irregularities of Growth of Crystals Character of Faces. Many of the so-called “optical anomalies” of crystals may beĮxplained by this pseudo-symmetric twinning. stannite, phillipsite, &c.) belonging to other systems. Pseudo-cubic forms often result by the complex twinning ofĬrystals ( e.g. May, by twinning, give rise to pseudo-hexagonal forms: and Or again, a substanceĬrystallizing in, say, the orthorhombic system ( e.g. Group with apparently the same degree of symmetry as the Hemimorphite) are often united in twinned position to produce a Hemihedral or hemimorphic crystals ( e.g. Known as “mimetic-twins or pseudo-symmetric twins.” Two Which apparently display a higher degree of symmetry than thatĪctually possessed by the substance. ( a), Glide-plane).Īnother curious result of twinning is the production of forms In calcite and some other substances this lamellar twinning may be produced artificially by pressure (see below, Sect. of corundum or pyroxene) may be readily broken in this direction, which is thus a “plane of parting,” often closely resembling a true cleavage in character. When often repeated on the same plane, the twinning is said to be “polysynthetic,” and gives rise to a laminated structure in the crystal.
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