Elements of mineralogy, crystallography and blowpipe analysis from a practical standpoint .. . Fig. Magnetite in Schist, Geikie. Port Henry, Kemp. more rarely with the angles modified by the tetragonal trisoctahe-dron tf = rt : 3a : 3«, Fig. 397. THE IRON MINERALS. 203 Twinning parallel to an octahedral face occurs, sometimes shownby striations upon the octahedral faces, as in Fig. 399. Physical Characters. H., to Sp. gr., to Lustre, metallic to submetallic. Opaque. Color and Streak, black. Tenacity, brittle. Strongly attracted by magnet and sometimes itself a magnet(lo
Elements of mineralogy, crystallography and blowpipe analysis from a practical standpoint .. . Fig. Magnetite in Schist, Geikie. Port Henry, Kemp. more rarely with the angles modified by the tetragonal trisoctahe-dron tf = rt : 3a : 3«, Fig. 397. THE IRON MINERALS. 203 Twinning parallel to an octahedral face occurs, sometimes shownby striations upon the octahedral faces, as in Fig. 399. Physical Characters. H., to Sp. gr., to Lustre, metallic to submetallic. Opaque. Color and Streak, black. Tenacity, brittle. Strongly attracted by magnet and sometimes itself a magnet(lodestone). Breaks parallel to octahedron. Before Blowpipe, Etc.—Fusible with difficulty in the reduc-ing flame. Soluble in powder in hydrochloric but not in nitricacid. Similar Species.—No other black mineral is strongly attractedby the magnet. Remarks.—Magnetite occurs chiefly in crystalline metamorphic rocks and in erup-tive rocks partly derived from silicates containing iron. It is little altered by expo-sure but organic matter reduces it to ferrous oxide which by oxidation becomes hema-tite
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