Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/386

Rh 368 MINERALOGY crystal or needle, as happens in red copper and pyrites. Crystals of acicular pyrites occur at the Newton-Stewart lead-mine. An octahedron flattened parallel to two of its faces is reduced to a tabular crystal (fig. 210). If lengthened in the same direction, it takes the form in fig. 211 ; or if it is still further lengthened, to the obliteration of two opposite octa hedral faces, it becomes an acute rhombohedron (same figure). Of octa- When an octa hedron, hedron is extend ed in the direction of a line between two opposite Of do. deca hedron. Of tra- pezo- hedron. it has the general form of a rectangular octahedron ; and still further ex- / tended, as in fig. / 212, it is changed / to a rhombic k~/ ^ prism with dihe- // dral summits. The figure repre- 211. Fig. 212. sents this prism lying on its acute edge (spinel, fluor, magnetite). The dodecahedron when lengthened in the direction of the up right axis becomes a square prism with pyramidal summits (fig. 213); and when shortened along the same axis it is reduced to a square octahedron with truncated basal angles (fig. 214). Both Fig. 213. Fig. 214. Fig. 215. these forms are modifications of the square prism; the first mods of distortion is common in garnet, rendering it liable to be con sidered zircon ; the second is seen in aplome, when it might be taken Fig. 216. Fig. 217. for stannite. &quot;When the first of these forms is flattened, as in fig. 215 it resembles a form of stilbite. When a dodecahedron again is lengthened along a diagonal between the obtuse solid angles, it becomes a six-sided prism with trihedral summits, as in fig. 216; and when shortened in the same direction, it becomes a rhombohedron which has its six acute angles truncated (fig. 217). In the first case, a crystal of green garnet or uwarowite would resemble dioptase ; in the latter, colourless garnet would resemble calcite. The trapezohedron is exceedingly subject to distortions which fre quently disguise it much. When elongated in the direction of the upright Fig- 218. Fig. 219. axs it becomes a double eight-sided pyramid with four-sided summits (fig. 218) ; a further elongation along the same axis would result in the obliteration of these summit faces, and in the produc tion of a perfect double octagonal pyramid (fig. 219). The first of these distortions is exceedingly common in analcime and not Fig. 220. Fig. 221. uncommon in garnet ; the latter rarely occurs in analcime. Length ened along an octahedral axis it becomes fig. 220 ; shortened along the same it becomes fig. 221. Both are seen in analcime. Fig. 223. Fig. 222. When the tetrakishexahedron is lengthened along a single octa- Of tetra- hedral axis it assumes the form of fig. 222 ; still further elongated, kishexa- with obliteration of one half of its planes, it becomes a scalene do- hedron. decahedron, resembling the &quot;dog tooth&quot; form of calcite (fig. 223). Fig. 224 is a hemihedron of this form, produced by shortening along an octahedral axis, with obliteration of all the planes which do not touch the poles of that axis. In the case of modi fied crystals of this system the distortions are more complex. Fig. 225 represents a crystal of cinnamon- stone from Aberdeen- shire ; it is a combina tion of the dodecahe dron and the trapezo hedron. Only four dodecahedral faces re main (d), and those Fig. 224. Fig. 225. of the trapezohedron (?;) are of unequal size. It may be best understood by regarding it as fig. 218 with the four vertical faces of fig. 213 ; so that it combines the distortions of both of these figures. Crystals of diamond are very frequently distorted, though gene rally through curvatures of their faces. Imperfections in tlie Surfaces of Crystals. Of these the most important are striae, caverns, and curvatures. 1. Striated Surfaces. The parallel furrows on the surfaces of Imperfeo- crystals are called strife, and such surfaces are said to be striated, tion of Each ridge on a striated surface is enclosed by two narrow planes, surfaces. These planes often correspond in position to a secondary or to the primary planes of the crystal, and we may suppose these ridges to have been formed by repeated oscillation in the operation of those causes which give rise, when acting uninterruptedly, to larger planes. By this means the surfaces of a crystal are marked in parallel lines with a succession of narrow planes, meeting at angles alternately re-entering and salient, and constituting the ricfges referred to. This combination of different planes in the formation of a surface has been termed an oscillation of faces. Cubes of pyrites are generally striated in such a way that the Striation. stride on adjacent faces are at right angles to one another. These lines are parallel to the intersections of the primary faces with the planes of the pentagonal dodecahedron, which is the most common form of pyrites ; and they have evidently resulted from an oscilla tion between the primary and this secondary form. The rhombic dodecahedron is often striated parallel either with the