CHAPTER XIII.

METALLIC ROCKS.

Sulphides and Sulpho-Arsenides.

HEATED on charcoal with carbonate of soda, they yield sodium sulphide which blackens lead paper or silver foil. Heated in the oxidizing flame, they yield sulphurous acids.

150. Galena.-Lead Sulphide (Pb S). Dominant forms: cube, octahedron, and a combination of these two; sometimes the facets of a rhombododecahedron. It has three rectangular cleavages; a metallic lustre. Color: steel gray; powder: grayish-black. It is easily fused. Heated on charcoal it gives a lead globule and yellow coating. With Potassic bisulphate it yields sulphuretted Hydrogen.

Varieties; Laminated; foliated; granular; compact.

151. Blende.-Zinc Sulphide (Zn? S). Dominant forms : rhomboidal dodecahedron, tetrahedron. It has a laminated, fib

rous fracture; almost a metallic lustre in the black varieties. Always resinous, analogous to wax. Colors: various; generally, yellow or brown, often black; powder: gray, inclining to brown. Heated on charcoal, it shines brightly, giving a coating, yellow while hot,

white when cold. It is colored green by nitrate of Cobalt; is soluble with difficulty in nitric acid; and only fusible on its edges.

Pyrites.(FeS2). Ferric Sulphide. Easily reduced to

152. Cubical Pyrites.-Of a bright yellow color, metallic lustre. Exhibits hemihedral forms. The cubes often have striæ parallel to the edges and perpendicular to the striæ of each adjacent face (Fig. 35). Powder, a brownish-black. Strikes fire with steel.

Fig. 35.

153. Marcasite. Of a yellow or pale bronze color, slightly inclined to greenish or white. It crystallizes in orthorhombic prisms, terminated by octahedrons or domes. It has the same composition as the preceding. It is altered to Limonite, sometimes to Ferric carbonate. 154. Copper Pyrites.- Chalco-Pyrites. Of a greenishyellow color frequently showing blue and purple tints; powder of a greenish-black color. Wet with hydrochloric acid it colors the flame green. Fusible to a magnetic globule. With carbonate of soda, it yields a copper bead. Soluble in nitric acid, the solution turning blue with ammonia. It is often a mixture of different sulphides of Copper and Iron. All these sulphides give copper reactions before the blow-pipe or with acids.

Fig. 36.

Metallic Oxides.-The metal can be easily determined by heating on charcoal or by ordinary

reactions.

155. Magnetite.-(Fe,O,) Magnetic oxide of Iron. Forms: octahedron, dodecahedron and a combination of the two. It is quite black in color and reduces to a black powder. Is soluble in boiling hydrochloric acid. Accessory minerals: Chlorite; Chromic Iron; Garnet; Pyrites; Calcite.

It exists in stratiform masses in Gneiss and crystalline Schists (Sweden and Norway). In the Ural mountains, large masses are found with the Melaphyres.

156. Titanic Iron.-Two forms: one cubical, the other rhombohedric, found usually in blackish-brown grains. Feebly

it gives a powder which is magnetic. Slowly soluble in hydrochloric acid, the solution giving reactions for Iron. Dominant forms: rhombohedron of 86° combined with a dihexahedron, with bases of the hexagonal prism,

or flat and lamellar following these bases.

Density 5.3; hardness about 6. Sometimes the crystal takes lenticular forms or is reduced to scales (Micaceous Iron ore).

Color: black with a bright metallic lustre. The fibrous varieties establish the passage between the preceding forms and the earthy ores.

Earthy varieties have a bright red or brownish-red color; a hardness and density both less than the crystalline forms. They are often mixed with Silica, or are mixed as powder with clays (Ochre). They give the red color to Jaspers, Silica, Sandstone, Marls, and other rocks. The oolitic varieties of a deep or brownish-red color are mixtures of Hematite, Limonite, and a little clay under the form of spherical or lenticular grains, having a density of about 3.

158. Itabirite.A granular aggregate of Micaceous Iron and Quartz, having a schistose texture. Hematite presents it

self in small scales, isolated, or reunited in a continuous layer which separates equally thin layers of grayish Quartz. Accessory minerals: Magnetite, Talc, Chlorite, Native Gold. These rocks are developed to a great extent in Brazil, alternating with Quartzites and Itacolumites. They are worked for gold at Villarica.

159. Limonite.-Ferric hydrate. In the closed tube it gives water and becomes red. On the coal, it is reduced to magnetic ferrous oxide. It is soluble in hydrochloric acid, depositing

sand and clay.

Varieties Compact L. has frequently a brilliant surface, with a texture somewhat fibrous in rounded nodules in the form of stalactites; some varieties are porous, cellular, or scoriaceous.

Earthy L. of a brownish-yellow or other yellow color, generally mixed with clay and spotted, but reduces to a yellow powder. It is disseminated through a great number of rocks which it colors. It is generally mixed with Silicates and Phosphates. Sometimes this kind takes globular forms, the globules exhibiting concentric layers. One interesting variety, though very impure, is the Limonite of the marshes. It has a greasy resinous lustre, slightly resembling pitch, is compact, somewhat crystalline, often cellular (cloisonnée). It is a mixture of Ferric oxide, Manganic oxide, Silica, Phosphorus, and perhaps Ferric Silicate.

Tapanhoacanga.—Conglomerate composed of fragments of Magnetite, Limonite, Hematite, Quartzite, and Itacolumite cemented by oxide of Iron.

160. Siderite.-Spathic Iron ore, Ferrous Carbonate (Fe Co1). Isomorphous with Calcite. Density 3.9. Hardness 4. Cleavable in rhombohedrons of 107°. Varieties: (1) Spathic Iron has a yellow or yellowish-gray color with a pearly lustre on a fresh surface. It is generally altered, and becomes reddish-brown or blackish by reason of change to Ferrous oxide. Before the blowpipe it gives a brown or black powder which is magnetic. It is slowly soluble in acids, the solutions giving Iron reactions. It

is found in the Silurian at Styria and it forms masses in the Erzberg. It is found with the Calcites of Zechstein.

(2) Spherosiderite.-Rounded concretions with an earthy fracture, generally flat; gray or brown, often containing the remains of fishes, saurians, and prints of leaves and crystals of sulphites or sulphates. Soluble in acids, depositing clay. It is found in nodules in coal beds and in the Lias. It is often mixed with Silica and Manganese.

(3) Black Band.—A name given in England to this last kind containing a mixture of Carbon; about 10%.

(4) Oolitic Iron Ore. - Generally, altered Limonite. 161. Manganese Oxides.-Manganite. Hydrated Manganic Oxide. Orthorhombic prism; generally, splintery or fibrous; yields water in the open tube; brown powder. Gives a green bead with carbonate of soda.

162. Pyrolusite.- Manganic dioxide. Differs from preceding only by the angles of the primitive prism, but the powder is black. Often earthy or compact.

163. Psilomelane.—Found in mammillary masses with tubercular excrescences, sometimes in stalactite form. Composed of Oxide of Manganese with Baryta and water. Colors: Salt of Phosphorus bead, violet; and Carbonate of Soda, green.