somewhat translucent; density greater than 4, harder than the point of a graver's tool; fusible with difficulty to a black, magnetic enamel; decomposed by boiling sulphuric acid. Pyrophyllite (Silicate and Hydrate of Alumina).-Contains sometimes a little Magnesia or Iron oxides; is found in right prisms with a rhombic base, cleavable and elongated parallel to the base; assumes, before the blow-pipe, considerable volume, spreading out like a fan without completely fusing; yields water in a closed tube, and is but slightly attacked by sulphuric acid. It has a greasy lustre, a white or greenish-white color; soft; yields an unctuous powder; exhibits the flexibility and want of elasticity which distinguishes Talc. The Pagodite of China, and the Parophite of Canada, are substances equally talcose in their nature-being Silicates of Alumina with more or less Magnesia. TALC AND STEATITE. (Hydrated Silicate of Magnesia). Silica about 62%; Magnesia, 32% to 33%; water 48%. 1. Crystallized Talc. - Hexagonal laminae derived from a right prism with a rhombic base, with angles-as in the Micas-600 and 1200; greasy to the touch; non-elastic; greenish-white, silver-white, or a leek-green color. The base is cleavable and has a pearly lustre. Powder or streak is white; hardness 1. 2. Massive Talc Steatite.—In granulated masses ; in aggregated laminæ fibers; radiated; schistose. Before the blow-pipe, the laminated Talcs exfoliate more or less; they fuse with difficulty upon the edges to a white enamel; yield a little water in the closed tube; not attacked by acids. AMPHIBOLES. All the species of this group crystallize in oblique prisms which cleave parallel to their faces (mm) under an angle of 1240 11' to 124° 30'. Generally the crystal is modified on its lateral edges, and terminated by the base, which latter is modified upon its posterior edges in such manner that its form resembles a hexagonal prism, bearing a rhombohedral summit, similar to that of Tourmaline; but the measure of the angle of the three terminal facets serves to distinguish the form of the crystal. Tremolite (Magnesium and Calcium Silicate).-Hardness, 5 to 6. Specific gravity, 2.9 to 3. Colorless, white, applegreen, or gray; often fibrous with a silky lustre. It fuses easily with a slight ebullition to a white semi-transparent glass, not attacked by acids. The Jade of China has a greasy lustre; exhibits a scaly fracture; is of a whitish color, more or less greenish; and is a compact variety of Tremolite. It is Actinolite (Iron, Magnesium and Calcium Silicates).Isomorphous with the preceding. Density, 3 to 3.2. found in greenish masses, radiated in Mica schists. Hornblende.-Composition, analogous to the preceding, but containing Alumina. Black when in large masses, but deep green or brown when in thin laminæ. One variety, Pargasite, is of an olive-green or greenish-black, and sometimes of a deep blue. The powder of Hornblende is a greenish or brownish gray. Hornblende presents itself quite frequently, in rocks, in fibrous crystals which have the appearance of wood-charcoal, but in which may be perceived the two cleavage-planes which have an inclination of 124o, and present a brilliant vitreous lustre when turned successively toward the light. In those rocks in which it appears as an essential element, Syenites, Diorites, Andesites, etc., its powder is greenish-gray. In Basalts, where it appears as an accidental element, the powder is generally brownish. Hornblende fuses to a black enamel, and is attacked by acids when it contains much iron. PYROXENES. (Bi-Silicate of Magnesia, or its isomorphs). Diopside (Calcium and Magnesium Silicates). - Colorless, white, pale green, olive-green, grayish-green. Hedenbergite (Calcium and Iron Silicates).- Fuses to a black mass more or less Magnetic. Fig. 8. Form: monoclinic prism; (mm = 87° 5′); cleavage difficult, following the faces m. Figs. 8 and 9 show the ordinary forms of Augite. Fig. 10 shows the ordinary form of Hornblende. It also shows by the ellipses drawn on the faces of the crystal, the position of the axes of thermic conductibility of Hornblende. If a little melted tallow be smeared upon the faces of these crystals, it will, in cooling, exhibit the figures represented in the diagrams. The elliptical figure on the face g', in the Amphiboles, has its longer axis inclined only 40 or 50 to the vertical edge. Upon the base, the longer axis is directed to the right and left, while in the Pyroxenes the longer axis is at right angles to this direction. Fig. 9. Augite and Hornblende are easy to distinguish, when they are crystallized. In the granular varieties, Hornblende is always laminated, slightly fibrous, and the two cleavages, with their angle of 1240, may be perceived. The grains of forming paste in Porphyry, they may be recognized in some cases by means of a magnifying glass. In lamina thin enough to be transparent, the microscope enables one to recognize the angle b b of Augite, or the fibrous laminæ of Hornblende. If such laminæ be seen through a crystal of Iceland spar, Amphibole exhibits two images whose colors vary from yellow to brown, or green, Fig. 11. or violet, according to the faces which are presented to the plate. Augite, under the same conditions, exhibits greens and yellows, and their combinations. Smaragdite. An assemblage of crystalline laminæ of Amphibole and Pyroxene, alternately placed; of beautiful green color, and more or less clear. Olivine, or Peridot (Iron and Magnesium Silicates). - A right prism with a rhombic base. Hardness, 7; density, 3.4; conchoidal fracture; color, greenish-yellow to olive-green. The variety so abundant in certain basalts is granular, and of a more or less deep green color; often in the very ferruginous varieties, the colors are a dirty red or brown. Olivine is infusible before the blow-pipe; reduced to a fine powder, it forms a gelatinous mass with acids. Enstatite (Magnesium Silicate with a little Iron).-Cleavage parallel to the faces of a right prism with a rhombic base, whose angle is about 930. Only fusible with great difficulty before the blow-pipe; has a density of 3.9; pearly lustre, somewhat fibrous aspect; of a grayish, yellowish or greenish-white color; the powder is quite gray. Not attacked by hydrochloric acid. Bronzite.-Differs from the above only by qualitative composition, containing a certain amount of iron; it has a brighter lustre and is almost metallic upon the cleavage-face. It is of a bright yellow, brown or olive-green. Not attacked by acids. Hypersthene.-Contains as much iron as magnesia. It is distinguished by a lively red-brown color with a coppery lustre on the cleavage-face; it fuses to an opaque greenish-gray glass. Diaclasite. Bronzite slightly hydrated, yellowish or greenish-gray; hardness, 3. 5; fuses easily to a brownish-green enamel; not acted upon by acids. Diallage (Calcium and Magnesium Silicate).-A variety of Pyroxene diopside. It differs from the related minerals by its unique and perfect cleavage parallel to the face h1, of which the lustre is bright, sometimes pearly, often metallic, bearing some resemblance to the Micas. Diallage is yellow, yellowish or greenish-gray, or brown; is feebly translucent at the edges. Hardness, about 4. Density, 3.75. Is not attacked by acids. Is fusible with difficulty before the blow-pipe. Cordierite or Dichroite (Aluminum, Calcium, and Magnesium Sili cates). It crystallizes in six-sided prisms, derived from the right prism with a rhombic base. Hardness 7.5. Density 2.6. Fusible with difficulty upon the edges; not easily attacked by acids; has a vitreous fracture; when seen by transmitted light, it is azure blue in one direction, and grayish or yellowish in the other. Fig. 12. |