REMARKS: Very widely distributed in eruptive rocks and crystalline schists. In the eruptive rocks magnetite belongs to the oldest secretions from the magma, immediately followed by chrysolite, biotite, hornblende, augite, etc.; hence often appears as inclusions in these and other minerals. Magnetite grains may form with other substances pseudomorphs after hornblende, biotite, hypersthene, etc. Such pseudomorphs appear to be caused by “resorption.” Magnetite is strongly magnetic and soluble in hydrochloric acid. H., 5.5 to 6.5. Sp. gr., 4.9 to 5.2. CHROMITE. ISOMETRIC. COMPOSITION: FeCr_{2}O_{4}. ▄Usual Appearance in Sections▄: Octahedral crystals, grains and in the olivine rocks sometimes in dense aggregates. May be surrounded by green, pleochroic halo of chrome ochre. _Opaque_, and by reflected light, brownish-black to black, with general absence of metallic lustre. Usually translucent and brownish on the edges (by transmitted light), with a very rough surface due to high index of refraction (_n_ = 2.1). ▄Distinguished from▄: (_a_) MAGNETITE by brownish-black to black color and general absence of metallic lustre (by reflected light) and by grains being usually translucent and brownish on the edges (by transmitted light). (_b_) SPINEL (Picotite), see under Spinel. REMARKS: Common in crystalline rocks, rich in magnesia, and in serpentine. Chromite is not acted on by acids, is non-magnetic and gives chromium bead test. H., 5.5. Sp. gr., 4.3 to 5.6. SPINEL. ISOTROPIC. ISOMETRIC. COMPOSITION: Mg(AlO_{2})_{2}. Pleonaste (Fe, Mg spinel), Picotite (Cr spinel). ▄Usual Appearance in Sections▄: Octahedral crystals and twins (after spinel law), less often in grains. Fracture cracks. Always optically normal and never decomposed in rocks. Usually colorless or dark green (pleonaste) to brown (picotite). The refractive index is high (_n_ = 1.72, spinel proper, to 2.00, chrome spinel), hence the _relief_ is marked and the surface rough. ▄Distinguished from▄: (_a_) GARNET when colorless by octahedral shape of crystals (garnet forms being 110 and 211), when brown (picotite) from melanite garnet by common zonal coloration of the latter, but may require chemical tests. Furthermore spinel may have green color and is never decomposed. (_b_) PEROVSKITE by the lower index of refraction and the absence of reaction for Ti. (_c_) CHROMITE chemically or by density or hardness. REMARKS: Found in gneiss, granulite, lherzolite and in regions of contact metamorphism and secondary bedding formations (picotite), olivine-basalt and serpentine. Spinels are insoluble in hydrochloric acid. H., 8. Sp. gr., 3.5 to 4.1. GARNET. ISOTROPIC. ISOMETRIC.