Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/384

Rh 366 MINERALOGY aragonite, cerussite, mispickel, and marcasite. In aragonite the crystals are partly interpenetrating, and partly merely in juxta position, as in fig. 184, where the individuals are formed by the Fig. 182. combination ooP(M), ooPoo(A), Pao(k). Fig. 183. In fig. 185 several crystals of the same combination form a series with parallel planes of union, the inner members of which are often so shortened that they form mere films, which appear as striae on the faces Poo and ooPoo of the twin. In fig. 186 four crystals, each of the combination ooP, 2Poo, having united in inclined planes, form a circular group, which returns into Fig. 184. Fig. 185. Fig. 186. itself. Cerussite occurs in similar groups, building up a composite hexagonal crystal. It also occurs in stellate twins of two or three individuals, as in figs. 158, 159. Similar stellate combinations are also common in chrysoberyl. In staurolite, individuals of the pris matic combination ooP, ooPoo, OP combine, either as in fig. 144 by a face of the brachydome. having their chief axes almost at right angles, or as in fig. 140 by a face of the brachypyramid fPf, the chief axes and the brachypinacoids (o) of each of the crystals meeting at an angle of about / 60. This mineral, which is very frequently twinned, also forms combinations with the axes parallel (fig. 187). Oblique ^ n the oblique prismatic system, twins are by prismatic no means so frequent as hemitropes. Twins of twins. interpenetration with parallel axes, but the one turned as regards the other round a vertical axis, are common in orthoclase (figs. 188, 189). Such crystalB are termed right-handed (fig. 188) and left-handed (fig. 189), according to the side of the crystal which has been turned. In this mineral hemitropes occur around an axis normal to M, to P, and to n (fig. 529) ; double twins of the last two are common (fig. 530). Harmotome and phil- lipsite form first hemi- Twinsof tropes, and then twins hemi- of these, which are ar- tropes. ranged sometimes as crosses and sometimes as double crosses (fig. 190). In hemitropes of gypsum the two halves are united by a face parallel to the ortho- diagonal section, as in fig. 161, where the two halves have united so regularly that the faces F, P form only one plane. In a similar 187. Fig. 188. Fig. 189. manner the two halves of the augite crystal represented in fig. 130 are in fig. 191 united so perfectly and symmetrically that the line of junction cannot be observed on the clinopinacoid. The two hemipyramids P(s) (like - P (I) in the gypsum crystal) form at one end of the crystal a re-entering, at the other a salient angle. Fig. 190. Fig. 191. Hornblende (fig. 192) and wolfram exhibit a similar appearance. This results in the imparting a pseudo-hemimorphism to certain oblique prismatic twins, which is well seen in the twins of sphene (figs. 193 and 58 9), and in exalting the charac teristic appearance of true hemimorphs, as seen in the twin of acmite (fig. 194). In other cases the individuals partially penetrate each other in the direction of the orthodiagonal. This mode of union is not uncommon in gypsum, and is very frequent in orthoclase. Two crystals Fig. 192. Fig. 193. of the latter of the combination (ooPoo), ooP, OP, 2Poo, as in fig. 131, are often pushed into each other, as shown in fig. 195. In the anorthic system some twin formations are of great import- Anorth ance, e.g., as a means of distinguishing the triclinic from the mono- twins. clinic species of felspar. In one variety the twin axis is the normal to the brachydiagonal chief section. But in the anorthic felspars this section is not perpendicular to the base, and conse quently the two bases form on one side a re-entering, on the other a salient angle ; whereas in the oblique prismatic felspars (where the brachydiagonal chief section corresponds to the clinodiagonal) no twin crystals can be produced in conformity to this law, and the two bases fall in one plane. Fig. 194. Fig. 195. Albite and oligoclase very often exhibit such twins as in figs. 196, 197, where the very obtuse angles formed by the faces of OP, or P and P (as well as those of P o, or x and x), are a very characteristic appearance, marking out this mineral at once as a triclinic species. Usually the twin formation is repeated, three or more crystals being combined, when those in the centre are reduced to mere plates. When very numerous, the surfaces P and x are covered with fine striae, often only perceptible with a microscope. A second law ob served in triclinic felspars, particularly in albite and labradorite,