Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/387

Rh MINERALOGY 309 edges, or with the longer or the shorter diagonal of its faces. In the first case, seen in garnet (fig. 226), there is a passage into the six- faced octahedron ; the second results from an oscillatory combination of the dodecahedron with the regular octahedron, as in magnetite ; and the last with the cube, as in aplome. Rhombohedrons of chabasite are often striated parallel to the Fig. 226. Fig. 227. terminal edges (fig. 227), indicating an oscillatory combination be tween the primary faces and a secondary plane which bevels these edges. Striation of faces is more frequent in the rhombohedral system than in any of the others. Horizontal striae are of almost invariable occurrence in the prismatic faces of quartz, whatever be the form of the crystal. The oscil lation hero has taken place between the pyramidal and the prismatic faces (figs. 228, 229, 230). During its growth there seems to have been a con tinued effort to complete the crystal by the assumption of Fig. 228. Fig. 229. Fig. 230. the terminal planes, which effort was intermittently over come by a preponderating one to continue the deposition of matter along its main axis. Quartz crystals, from these alternate efforts, often taper to a point, without having any regular pyramidal face. Tho lateral planes of prisms of tourmaline are very frequently convex, owing to oscillation between several lateral faces. In all such cases the interfacial angles cannot be determined, as they are lost in the rounding. The striations on the lateral faces of foliated minerals are merely the edges of laminae. Examples : mica and gypsum. Cavern- 2. Cavernous Crystals. Crystals not un frequently occur with a cus faces, deep pyramidal depression occupying the place of each plane, as is often observed in common salt, galena (fig. 21), and sulphur. In the solution of crystals through atmospheric exposure, an approach to the same form is sometimes obtained, owing to the fact that the centres of the faces yield sooner than the edges and angles. Crystals of redruthite are often thus cavernous. Sometimes octahedrons occur with a triangular cavity, in place of each face (fig. 22). The same is met with in other forms. Curved 3. Curved Surfaces. Curved surfaces sometimes result from the surfaces, oscillatory combination already noticed. Oti:ers result from a curvature in the laminae constituting the crystal. Crystals of diamond have convex faces, and are sometimes almost spheres. This mode of curvature, in which all the faces are equally convex, is less common than that in which a convex surface is opposite and parallel to a corresponding concave surface. Rhombohedrons of spathic iron and pearl spar are usually thus curved, as is shown in fig. 231. The saddle-shaped crystals of the same mineral (fig. Fig- 231. Fig. 232. 232) are remarkable instances of several reversed curvatures in the same face. A singular curvature is shown in fig. 233, of calcite. The conical crystals of brown zinc blende, and the lenticular and conical crystals of gypsum, are other examples. Crystals of quartz are sometimes curved and twisted. When this takes place in the left-handed and right-handed crystals, the twist is to the right or left according as the crystal is right- or left-handed. The surfaces of crystals are frequently far from flat, on account of fracture, with dislocation of the several fragments, occasioned by motion in the enclosing rock, the material of which is forced, or it may be transfused, into the rents. The tourmalines and beryls (fig. Dislo- 234) which occur in granitic dykes are very subject to this, the frag- cated ments being often bent as well as displaced. A more or less sinixii- crystals, taneous effort in the crystallization of two substances may produce a structure with the external form of one, the interior of which exhibits imbedded crystals of the other, more or less perfect in their development. In pegmatite or graphic granite, rude crystals of felspar contain skeleton forms of quartz, of which generally only one side of the prism and two of the pyramid occur, forming a rude lettering. Similar hollow quartz forms occur imbedded in garnet, radiating from its centre (fig. 235), and roughening its surface from protrusion, without distorting its form. Totally imbedded micro scopic crystals, &quot;microliths,&quot; arc, as in the latter cases, chemi- Fig. 233. Fig. 235. Fig. 234. Fig. 236. cally non-assimilable. These are frequently arranged in layers in the including crystal, as in augite and leucite. When there is a certain amount of chemical resemblance there may occur a definiteness in the arrangement ; and if the enclosed substance crystallizes in a system differing from that of the mineral which includes it, the angles of the latter are more or less distorted. This is the case in &quot;microcline,&quot; where the intrusion of a plagioclastic felspar causes some departure from the rectangularity of orthoclase. Foreign amorphous matter caught up or attaching itself to the surface: of a crystal, during the process of its growth, causes lines of feeble cohesion, as in the case of capped crystals of quartz. Here an occasional selectiveness in the sets of faces to which the foreign matter adheres seems to indicate that it has been to some extent under the influence of a polarity in its adhesion. Something of the same kind seems to have influenced the arrangement of the quartz grains caught up during the formation of the crystal of garnet shown in fig. 236. The perfect modelling of rock crystals is, however, but little interfered with by the almost numberless substances which they contain. Aggregation of Crystals. Crystalline aggregates which pass into amorphous masses may, in their more marked or perfect form, be assigned to an imperfect twinning. Crystals are often grouped in linear series, as in native Regular copper and silver, and thus constitute long threads or re- aggre- ticulations. In clustered crystals those adjoining each other gates&amp;gt; are generally parallel in position, and are united by a plane parallel to one of the principal sections, or to planes of common occurrence. Senarmont mentions a union in galena, XVI. - 47