▄Usual Appearance in Sections▄: Colorless patches, incrustations or veins, also at times with sphærulitic structure (hyalite), showing interference cross, of negative character, between crossed nicols. Often shows anomalous double refraction due to strains. The refractive index is very low (1.46) so that the surface of the opal appears rough. REMARKS: Found as a secondary mineral in many acid volcanic rocks, rhyolite, trachyte, andesite, etc., and also in basic basalts. H., 5.5 to 6.5. Sp. gr., 2.2. LIMONITE. AMORPHOUS. COMPOSITION: Fe_{2}(OH)_{6}, Fe_{2}O_{3}, frequently quite impure. ▄Usual Appearance in Sections▄: Brownish and opaque, in very thin sections may be translucent. REMARKS: Limonite is essentially a decomposition product, often forming pseudomorphs after ferruginous silicates or halos about the iron ores. PYRITE, Pyrites. ISOMETRIC. COMPOSITION: FeS_{2}. ▄Usual Appearance in Sections▄: Cubes, pyritohedrons, combinations of these forms; or in irregular grains. Outline of cross-sections generally square. _Opaque_, and by reflected light, bright yellow, with strong metallic lustre. Alters very easily to the oxides of iron (rust). REMARKS: May be present in all kinds of rocks, igneous, metamorphic and sedimentary. Not noticeably acted on by hydrochloric acid. H., 6 to 6.5. Sp. gr., 4.9 to 5.2. PYRRHOTITE, Magnetic Pyrites. COMPOSITION: FeS. Distinguished from pyrite by being practically always in irregular masses and not in crystals, and by bronze yellow color with reflected light. Found in basic eruptive rocks, more rarely in schists. MAGNETITE, Magnetic Iron Ore. ISOMETRIC. COMPOSITION: Fe_{3}O_{4}, often contains Ti. ▄Usual Appearance in Sections▄: Grains and crystals (generally octahedra), Fig. 33 B. Skeleton crystals frequent in highly ferruginous eruptive rocks. _Twinning._—Common, according to _Spinel_ law. [Illustration: FIG. 33.—_A_, Zircon crystals (isolated from granite) in balsam, showing high relief. _B_, Magnetite crystals. _C_, Ilmenite, showing partial decomposition to _leucoxene_ along crystallographic directions. ] _Opaque_, and by reflected light, bluish-black, with strong metallic lustre. ▄Distinguished from▄: HEMATITE, CHROMITE, ILMENITE and GRAPHITE, by being easily separated from powdered rock by weak magnet.