Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/388

Rh 370 MINERALOGY parallel to the octahedral faces, as common ; and he also describes an instance where the union was parallel to the plane 3f. The positions of crystals on the supporting rock seem at first to be without any regularity. But by closer inspection we detect even here the same law of harmony that governs the formation of the simple and compound crystal. The various positions assumed correspond generally with the more common kinds of composition iu twin crystals. This regularity is not always manifest on account of the unevenness of the surface on which they rest. In general, however, on glancing over a surface covered with crystals, a reflexion from one face will be accompanied with reflexions from the corresponding face in each of the other crystals, showing that the crystals are similarly positioned throughout. This tendency to parallelism in the positions of associated crystals is apparent even in crystalline aggregates. In granite, for example, which is composed of felspar, quartz, and mica, the felspar crystallizations have usually a common position ; that is, the corresponding extremities lie in the same direction, or nearly so. On this account granite is cleavable in one direction more easily than in others, and this direction is that of the perfect cleavage plane of the felspar ; the second less perfect cleavage of the felspar permits of fracture of the rock nearly at right angles to the first ; but, as there is no such third cleavage in the felspar, the workman, in fashioning the blocks of granite for paving stones, is compelled to chip or dress them off in the third direction. Parallel- The dominant action of polarity may, moreover, give a parallel ism in position to the main axes of different minerals belonging to the crystal same system, when crystallizing in association, and even to those growth, which belong to different systems. Fig. 237 is an illustration of the first of such cases, where a crystal of zircon is implanted into a crystal of xenotime, and has its main axis identically in the same line. As illustrations of the latter a parallel position of the axes of crystals of different systems there are records of such association in crystals of cyanite and staurolite, of muscovite and haughtonite, of albitc and orthoclase. The same has been observed between crystals of rutile and specular iron, the crystals of rutile in this case having the vertical .,_ axis in the direction of a lateral axis *& of the specular iron. Haidinger has observed pyroxene and horn blende crystals associated in parallel positions. A prism of calcite terminating in the planes g (fig. 106) has been observed, in which each plane was covered with small crystals of quartz all lying symmetrically, with their pyramids pointing towards the summit of the calcite crystal. When one mineral is changed into another, a polarity of accretion is still often seen to have domi nated in the arrangement. In a crystal of calcite which had been changed into a number of minute crystals of aragonite, the main axes of the latter all lay in the direction of the main axis of the original crystal of calcite. Irregular Irregular Aggregation of Crystals. Besides the regular unions now described, crystals are often aggregated in peculiar ways, to which no fixed laws can be assigned. Thus some crystals, apparently simple, are composed of concen tric crusts or shells, which may be removed one after the other, always leaving a smaller crystal like a kernel, with smooth distinct faces. Some specimens of quartz from Beeralston in Devonshire consist apparently of hollow hexagonal pyramids placed one within another. Other minerals, as fluor-spar, apatite, idocrase, heavy spar, and calc-spar, disclose a similar structure by bands of dif ferent colours. A growth rendered intermittent through the deposition of a thin layer of foreign matter is thus developed. Many large crystals, again, appear like an aggregate of numerous small crystals, partly of the same partly of different forms. Thus some octahedrons of fluor-spar from Schlaggenwald arc made up of small dark violet-blue cubes, whose projecting angles give a drusy character to the faces of the larger form. Such polysynthetic crystals, as they may be called, are very common in calc-spar. Varieties Forms of Crystalline Aggregates. Crystals have often of struc - been produced under conditions preventing the free de- ture in velopment of their forms ; and, according to the direction of the axis in which the development has been checked, they may be divided into &quot;columnar&quot; and &quot;lamellar&quot; arrangements. The columnar structure is made up of a more or less fibrous arrangement; and this may be supposed to have accrued from the simultaneous growth of a multitude of crystals from a single or gates. aggre- from closely adjacent centres of support, so that, while the crystals were free to elongate themselves in the direction of their main axis, their increase was restrained laterally, by their impact upon one another. When the surfaces of support are level, or consist of the opposing sides of a vein, the columns or fibres, frequently exceedingly delicate, are parallel, and not unfrtquently they then have a silky lustre. In the latter of the above circumstances the fibres are disposed transversely to the vein. Examples : gypsum, chrysotile, satin-spar. When the surface of support is rough, or has angular projections, the fibres radiate from certain of these in all directions, producing, in a thin vein, a starlike form, whence the arrangement is called &quot;stellular.&quot; Example : wavellite. When this takes place in an open cavity, producing brush-like forms, they are termed &quot;radiant.&quot; Examples : autimonite, needlestone. When the points of divergent growth are so positioned that the radiating groups interlace with one another, the structure is said to be &quot;reticulated, &quot;from its resemblance to a net. Example : tremolite. When individual members of such fibrous structure project above the general surface with acuminated extremities, they are said to be &quot;acicular&quot;; when the protruding columns are of uniform thickness they are termed &quot;bacillary,&quot; or rod-like. Such terms as straight, curved, twisted-columnar, diverging, or confused-fibrous explain themselves. Such fibrous arrangements as the above may occur imbedded centrally in a rock mass, which had been the magma out of which they were formed; or they may line the inner surface of cavities, filled originally either with water or aqueous vapour. These modes of occurrence have been distinguished by Moris as crystal groups and druses. The former includes all unions of im bedded crystals round a central nucleus; the latter those of crystals of simultaneous or regularly successive growth on a common support. In the first case, there may be spheroidal, ellipsoidal, cocks comb, or other forms, frequently seen in marcasite, pyrite, and gypsum. In the second, spheroidal forms are less rare, but are seen in the case of several of the fibrous zeolites. In such cases surfaces more or less rough are coated, and diminished in angularity, through the hemispherical forms produced by the radiation of a multitude of fibres. Certain imitative outlines thus result from the successive deposition of layers of these crystals. These forms or uniting masses are termed &quot;globular&quot; when nearly spherical, &quot;botry- oidal &quot; when like bunches of grapes, &quot; reniform &quot; or kidney-shaped when the spheres are larger, more confluent, and less distinct, and &quot;mammillated&quot; when the masses are nearer to hemispheres. Mesolite occurs in globular forms; prehnite in botryoidal; haematite and chalcedony in reniform; and siderite and calamine in mammillated. In all the above cases the transverse fracture of such structures dis closes the fibrous arrangement of the parts ; but, if the growth has been intermittent, lines of deposit, concentric with the central nucleus of each sphere, arc evidenced by layers of distinct colours. Fracture or separation frequently takes place, also, along such lines. In such drusy cavities termed &quot;geodes&quot; when they are circular after a certain number of such lines of deposit, grouped arrange ments which have somewhat more of free crystalline development may assume other imitative forms in which there is a certain dependence on the crystallographic character of the mineral con cerned. There are thus produced coralloidul or coral-like groups, fruticose or cauliflower-like groups, capillary or hair-like, and fili form or thread-like or wire-like forms. Often these groups expand in several directions, and produce arborescent, dendritic, plumose, mossy, dentiform, or other forms. Such are common among the native metals ; as gold, silver, and copper. Mesolite is very frequently plumose. A drusy crust&quot; is the term applied to a thin rough layer of crystals, which invests either a large crystal or the surface of some other body lodged in the interior of cavities. In the lamellar structure a development along the main axis would appear to have been checked, and the crystallographic force to have expended itself laterally; though this is not the invariable habit of a species under all circumstances, as exemplified by baryte. This structure consists of flat crystals, plates, or leaves. It is termed &quot;tabular&quot; when the plates are of uniform thickness, &quot;lenticular&quot; when they are thinner on the edges, &quot; wedge-shaped&quot; when sharp on one edge, &quot;scaly&quot; when the plates are thin and small, &quot; foliaceous&quot; when larger and easily separable; &quot; micaceous 1 is also used to describe this kind of structure. It may also be curved lamellar and straight lamellar. Wollastonite, when flat lamellar, is called tabular spar ; gypsum is frequently lenticular, talc scaly. Lamellar minerals when radiating from a centre often form fan- shaped, wheel -like, almond-shaped, comb-like, and other groups. In the granular structure, the force of crystallization has been exerting itself along all the axes ; but, from the multiplicity of crystallizing centres, there has been such mutual interference that no single individuals have been able to assume perfect or even characteristic forms. The particles in a granular structure differ much in size. When coarse, the mineral is described as coarsely granular ; when fine, finely granular ; if not distinguishable by the naked eye, the structure is termed impalpable. Examples of the first may be observed in granular carbonate of lime, of the