MINERALOGY 361 Hemi- pyra- midal Prisms. with the macrodome, or ooP2, P, Poo. The above figures are very common forms of barytes, figs. 125 and 126 being both composed of the pinacoid OP, a brachydome, and a macrodome, with sign OP(c), Poo (f), Poo (d). The variation in aspect arises from the pre dominance of different faces; and fig. 127 consists of the macrodome Poo , the prism ooP(gr), and the pinacoid OP. Oblique V. The Oblique Prismatic System. This system is char- prismatic acterized by three unequal axes, two of which intersect system. ^^ O tj ier at an oblique angle, and are cut by the third at right angles. One of the oblique axes is chosen as the chief axis, and the other axes are then distinguished as the orthodiagonal (right-angled) and clinodiagonal (oblique- angled). The same terms are applied to the chief sections, and the name of the system refers to the fact that these two planes form with the base two right angles and one oblique angle C. The forms of this system approach very near to those of the right prismatic series, but the inclination of the axis, even when almost a right angle, gives them a peculiar character, by which they are character, always readily distinguished. Each pyramid thus separates into two altogether independent forms or hemipyramids. Three varieties of prism also occur vertical, inclined, and nori- zontal with faces parallel to the chief axis, the clinodiagonal, or the orthodiagonal. The horizontal prisms, like the pyramids, sepa rate into two independent partial forms named hemiprisms or hemi- domes. The inclined prisms are often designated clinodomes, the term prism being restricted to the vertical forms. Ortho- pinacoids and clinopinacoids are also distinguished, from their position in relation to the axes. The monoclinic pyra mids (fig. 128) are bounded by eight scalene triangles of two kinds, four and four only being similar. Their lateral edges lie all in one plane, and the similar triangles are placed in pairs on the clmodiagonal polar edges. The two pairs in the acute angle between the orthodiagonal and pyramids, basal sections are designated the positive hemipyramid, whilst the two pairs in the obtuse angles of the same sections form together the negative hemipyramid. But, as these hemipyramids are wholly independent of each other, they are rarely observed combined. More frequently each occurs alone, and then forms a prism-like figure, with faces parallel to the polar edges, and open at the extremi ties. Hence, like all prisms, they can only appear in combination with other forms. The vertical prisms are bounded by four equal faces parallel to the principal axis, and the cross section is a rhombus; the clinodomes have a similar form and section; whilst the hori zontal prisms or domes have unequal faces, and their section is a rhomboid. The mode of derivation of these forms closely resembles that of the rhombic series. A complete double pyramid is assumed as the fundamental form, and designated P, in order to express the two portions of which it consists. Its dimensions are given when the proportion of its axes a: b: c and the angular inclination of the oblique axes C, which is also the inclination of the orthodiagonal section to the base, are known. The fundamental series of forms is OP .... mP . . . . P . . . . wP . . . . oo P, from each of whose members, by changing the dimensions of the other axes, new forms may be again derived. Thus from wP, by multiplying the orthodiagonal by any number n, a series of orthopyramids??iP?i is "produced, with the ortho- domes mPoo as limiting forms. The clinodiagonal produces a similar series of clinopyramids mP , with the limiting clinodome 7?iP c oo always completely formed, and therefore without the signs attached. From ooP arise orthoprisms ooP"n and the ortho- pinacoid ooPoo, and clinoprisms cx>T> e n and the clinopinacoid ooP oo. In these signs the o or c attached to the P indicates that the orthodiagonal (o) or clinodiagonal (c) axis has been multiplied. Formerly the latter forms were enclosed in brackets, thus (mPoo ) = TOP 00. The combinations of this system may be easily understood from their resemblance to those of the right prismatic, the chief difficulty being in the occurrence of partial forms, which, however, closely resemble the hemihedral forms of the previous systems. A few examples only need therefore be given. Fig. 128. + and - Fig. 129 represents a very common form of gypsum crystals, ooP oo , (P), oo P(/), P(J). The most common form of augite is repre sented in fig. 130, with the sign ooP(J/) , ooPoo (r) , ooP oo (/) , P( s ). Fig. 131. Fig. 132. Fig. 131 is a crystal of common felspar or orthoclase, composed of the clinopinacoid ooP oo (JJ/), the prism ooP(T), the basal pinacoid OP(P), and the hemidomes 2Poo (y); to which, in fig. 132 of the same mineral, the hemipyramid P(o) and the clinodome 2P c oo (n) are added. VI. Anorthic or Triclinic System. This is the least Anorthic regular system, and departs the most widely, indeed almost system, absolutely, from symmetry of form. The axes are all unequal, and inclined at angles none of which are right angles, so that, to determine any crystal, or series of forms, the proportion of the axes a : b : c, and also their angles, or those of the inclination of the chief sections, must be known. As in the previous systems, one axis is chosen as the principal axis, and the two others distinguished as the macrodiagonal and brachy diagonal axes. In consequence of the oblique position of the principal sections, this system consists entirely of partial forms wholly independent of each other, and each composed only of two parallel faces. The complete pyramid is thus broken up into four distinct quarter-pyramids, and the prism into two hemiprisms. Each of these partial forms is thus nothing more than a pair of parallel planes, and the various forms consequently mere individual faces. This circumstance renders many triclinic crystals very unsymmetrical in appearance. Triclinic pyramids (fig. 133) are bounded by eight triangles whose lateral edges lie in one plane. They are equal and parallel two and two to each other, each pair forming, as just stated, a tetartopyramid or open form, only limited by combination with other forms, or, as we may sup pose, by the chief sections. The prisms are again either vertical or inclined; the latter are named domes, and their section is always rhomboidal. In deriv ing the forms, the fundamental pyramid is placed upright with its brachydiagonal axis to the spectator, and the partial forms designated, the two upper by P and P , the two lower by ,P and P,, as in the figure. The further derivation now follows as in the right prismatic system, with the modifica tions already mentioned. Some combinations of this system, as the series exhibited by most of the felspars, approach very near to the oblique prismatic system; whilst others, as cyanose and axinite, show great incom-
XVI. - 46Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/379
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