20. Chlorite.—This is, properly, the name of a group of highly basic minerals of very variable composition, but they are all essentially hydrous silicates of aluminum, magnesium, and iron; and the average composition of the most abundant species, prochlorite, is as follows: silica (SiO₂), 30; alumina (Al₂O₃), 18; magnesia (MgO), 15; protoxide of iron (FeO), 26; and water (H₂O), 11; = 100. The chlorites crystallize in several different systems, but in all there is a highly perfect cleavage in one direction, giving, as in talc, a foliated structure with flexible but inelastic laminæ. The cleavage scales, however, are sometimes minute, and the structure massive or granular. Hardness of prochlorite, 1-2; between talc and serpentine. Sp. gr., 2.78-2.96. All the chlorites have a pearly to vitreous lustre. Color usually some shade of green; in prochlorite a dark or blackish green, darker than serpentine, as that is darker than talc. Streak, a lighter, whitish green. Less unctuous than talc, but more so than serpentine. The chlorites are produced very commonly, but not generally, by the alteration of basic anhydrous silicates, like augite and hornblende. Specimen 26. 21. Hydro-mica.—This, too, is properly the name of a group of minerals; but for geological purposes they may be regarded as one species. Taking a general view of the composition, these are simply the anhydrous or ordinary micas, which we have already studied, with from 5 to 10 per cent. of water added. In crystallization and structure they are essentially mica-like. Although not distinctly softer than the common micas, they are lighter, always more unctuous and slippery, and usually of a greenish color. The micaceous structure with _elastic_ laminæ serves to distinguish the hydro-micas from other hydrous silicates. 22. Glauconite.—Hydrous silicate of aluminum, iron, and potassium: silica (SiO₂), 50; alumina (Al₂O₃), protoxide of iron (FeO), and potash (K₂O), together, 41; and water (H₂O), 9; = 100. Amorphous, forming rounded and generally loose grains, which often have a microscopic organic nucleus. It is dull and earthy, like chalk, and always soft, green, and light, but not particularly unctuous. Glauconite is the principal, often the sole, constituent of the rock greensand, which occurs abundantly in the newer geological formations, and is now forming in the deep water of the Gulf of Mexico and along our Atlantic sea-board. Specimen 27. This completes our list of minerals occurring chiefly as _essential_ constituents of rocks; and following are three of the more common and important minerals occurring chiefly as _accessory_, rarely as essential, rock-constituents.