Crystallography

(-log'rah-fe.) (Gr. krystal los, and graph ein, to write.) Almost all solid chemical compounds, when slowly formed, a ume a regular shape, bounded by plane surfaces. The science of C. treats of the laws by which these surfaces are disposed one to the other. Crystals are a umed to po e certain axes, and the form is determined by the relation which the plane surfaces bear to these axes. Although the forms in which bodies crystallize are almost infinitely varied, it has been found that they maybe cla ified into seven crystallographic systems. These are briefly as follows: 1. The regular Cubic or Monometric System. The simplest forms are the cube or hexahedron (A, Fig. 230), which has 6 square faces or planes, and the3 equal axes (printed in bold lines) terminate in the centre of the square faces. Ex. of substances which crystallize in this form: gold, silver, platinum, copper, common salt,fluor-spar. The Octahedron (B), which has 8 faces, all of which are equilateral triangles, and 12 edges, with 6 angles, each of which has 4 faces. The terminations of the axes are in the angles of the crystals. Ex. Diamond, alum, sal-ammonia. The Rhombic Dodecahedron (C), which has 12 faces, and is derived from the cube and octahedron. Ex. Garnet.-2. The QuadA B D C E F H G K M N Fig. 230.- PRINCIPAL FORMS OF CRYSTALLIZATION. ra tic or Dimetric System. These crystals are symmetrical about 3axes, which are rectangular, but only 2 of equal length, the 3d being different. To this belong the Right square prism (D), in which the lateral axes terminate in the centre of each side-face; and the Right square-based octahedron (E), which resembles 2 pyramids placed base to base, and having 8 faces, which | form isosceles triangles. Ex. of crystals of this system: Sulphate of nickel and tungstate of lead.-3. The Hexagonal or Rhombohedral System, in which the crystals po e 4 axes, 3 being equal in length, situated in one plane, and inclined 60° to one another, and a principal axis at right angles to the plane of the former. To this belong the Regular six-sided prism (F), and the Rhombohedron (G). Ex. of this system: Quartz, beryl, arsenic, antimony, and apatite.-4. The Rhombic or Trimetric System. These crystals have 3 rectangular axes, all of different lengths. The Right rhombic prism (H), and the Right rhombic-based octahedron (K), are forms included in this system. Ex. Sulphur, topaz, and sulphate of potash.-5. The Oblique Prismatic or Monoclinic. These have 3 axes, which may be all of unequal lengths, 2 of which are placed at right angles to each other, while the 3d axis is so inclined as to be perpendicular to one of the two axes, or oblique to the other. To this belong the Oblique rhombic prism (M), and the Oblique rhombic based octahedron (N). Ex. of this system: Borax, sulphate of soda, and carbonate of soda.- 6. The Doubly oblique Prismatic or Triclinic System, in which the 3 axes are all inclined obliquely, and of unequal length. The forms are very irregular, which render them very puzzling to make out satisfactorily. Ex. Nitrate of bismuth and sulphate of copper.-7. The Diclinic System, in which there are 2 axes at right angles, and a 3d oblique to the plane of these two, the primary form being a symmetrical eight-sided pyramid.- The angles of crystals are measured by an instrument called a Goniometer. One of the most frequently used is the reflecting G. of Wollaston, which consists of a divided circle graduated to degrees and subdivided with a vernier. On the axis is an arrangement for supporting the crystal. A distant object is viewed, reflected in one of the faces of the crystal, and the vernier is brought to zero. The circle carrying the crystal is then turned until the same object is reflected from another face of the crystal, when the angle formed by the two faces can be read off on the circle. [s. 249]