# Articles from Kürschner's Pocket Dictionary of Conversations

#### Crystal

[Mineral] bounded by flat surfaces. Differentiation: 6 systems. The basic form of a crystal system is that whose surfaces pass through the end points of the axes.

1) Tesseral system: Based on 3 axes of equal length that are perpendicular to each other. Plenotesseral (holohedral) systems refer to those in which the three axes have perfect symmetry. The basic shape of the octahedron (Fig. 1 of Plate XIX) is bounded by 8 equilateral triangles. The rhombic dodecahedron (Fig. 3) is bounded by 12 equal rhombi. The hexahedron (cube) (Fig. 2) is bounded by 6 equal squares. Tetrakis Hexaeder (Fig. 4) begr. v. 24 gleichschenklig. Dreieck. Gest. ist e. Würf. a. dess. Seiten j.e. 4seitige Pyramide afgesetzt ist. Triakisoktaeder (Fig. 5) (Pyramidenokt.), begr. v. 24 gleichschenkl. Dreiecken. Octahedron with an attached side. 3-sided pyramid; Icosahedra (Fig. 6) based on 24 deltoids; Hexakisoctahedron (Fig. 7) (tetracontaoctahedron) based on 48 unequal-sided triangles. Each complete form is divisible into 2 geometrically equal halves, which are different in position (positive, negative). The tetrahedral half is obtained by extending the faces of the alternating octants. It breaks down: octahedron into 2 tetrahedra (Figs. 8 and 9), triakisoctahedron into 2 deltoid dodecahedra (Fig. 11), the icositetrahedron into 2 trigondodecahedra (Fig. 10). The hexakisoctahedron into 2 hexakistetrahedra (Fig. 12). Dodecahedral halves a.d tetrakishexahedron u. hexakisoktahedron dch expansion z. their intersection: pentagondodecahedron (Fig. 13), dyakisoktahedron (Fig. 14). Combinations. Cryst. in which faces of the system occur, e.g. when the corners of the cube are truncated by faces with the position of the octahedron or the 6 corners of the tetrahedron are replaced by tetrahedral faces (Fig. 15/18).

2) Tetragonal system based on 3 perpendicular axes, 2 of equal length. Pyramid of 8 isosceles triangles (Fig. 19). Prism with square cross-section (the square boundary surface is called basic pinacoid) (Fig. 20). Ditetragonal pyr. arises when instead of the 8 triangles, 2 occur. Analogous to the ditegial prism (Fig. 21, 22). Deuteropyr. and deuteroprism arise when the sides of the base of the pyr. or the cross-section of the prism are not through the end points of the axes, but rather parallel to them (Figs. 23, 24). Hemispherical figures: tetrahedron sphenoid (half of the tetrahedron pyramid) (Fig. 25).

3) Rhomb. S. bezog. a. 3 ungleich. u. einand. senkr. Achs. D.e. wird as Hauptachse gewählt, dann v.d. and. d. läng. Makroachse, d. kürz. Brachyachse grundgest. Pyr (Fg 28) Prism (Fig 29). Macrodoma prism with 4 faces parallel to the macroaxis, terminated on both sides by faces parallel to the brachyaxis (brachypinacoid) (Fig. 30). Brachydoma that in the plane of the brachyaxis (Fig. 31).

4) Monoclinic. S around a. 3 uneven axes. 2 lower oblique angle inclined, 2 and. perpendicular E.d. both oblique main axes. 2. oblique Klinoachse, d. 3. af both perpendicular Orthoachs. Grundgest. Pyr. begr. v. 8 uneven triangle (Fig. 34) Prisma (Fig. 35) Klinodoma (4 z. Kl. achs. par. Fläch. (36) Orthodoma (Fig. 37). 5) Triklinisch. S. bez. a. 3 u. schief. Wink. gem. Achs. Eine Hptachse d. beiden and. Makro-, Brachyachs. Subject Pyr. (Fig. 39) the rest analogous to the rhombic system.

6) Hexagonal. See 3 equivalent axes, which intersect at 60° and the 4 larger or smaller angles on the two opposite sides (main axis). Basal hexagonal pyr. with a regular hexagonal (Fig. 41). Prism (Fig. 42). The dihexagonal form corresponds to that of the rhombic system. Deuteropyr. and Deuteroprism. e., against the ordinary Pyr. by 30° used. position. surfaces intersect not the axis, but parallel d. alternating surfaces of the hex. pyr. extended given rhombohedron (Fig. 45). B. 6 equal rhombohedra. The dihex. pyr. gives a. half. given scalene scalene. B. 12 unequal triangles bounded by twins, resulting from the lawful intergrowth of crystal individuals. Special features of the Karlsbad twins, caused by intergrowth: 2 monoclinic prisms (feldspar), Kr Druse and Druse: crystalline [mineral] of the same kind, incomplete.

#### Mineral

Inorganic, solid, rare liquid homogeneous natural body, neither through the life process of organic beings nor through human arbitrariness. Entry: Elements (metals, amines) ores (sulfides, oxides, sulfides; sulf. again in gravel, luster, blends), stones (sclerites, felsites, zeolites, phyllites, steatites), haloids, psytogenids.