Hornblende—an amphibole; name sometimes used as a synonym for amphibole; sometimes to designate a variety under amphibole (q.v.).
Hyalite—a variety of silica in the opal condition; clear and colorless like glass, consisting of globular concretions or crusts.
Hypersthene—one of the pyroxenes; a ferromagnesian silicate; orthorhombic; H. 5–6; luster somewhat pearly on cleavage; surface often iridescent; dark brownish green, grayish, or greenish black and brown; a frequent constituent of crystalline rocks.
Iceland spar—a form of transparent calcite (q.v.).
Ilmenite—menaccanite; a titanium iron oxide; rhombohedral; resembles hematite; luster submetallic; iron-black; powder black or brownish red; occurs frequently in crystalline rocks associated with magnetite.
Iron pyrites—pyrite (q.v.).
Kaolin—kaolinite; essentially a hydrous aluminum silicate; usually in clay-like or earthy form; white or grayish white; often tinged with impurities; commonly arises from decomposition of aluminous silicates, especially the feldspars; basis of pottery and china.
Labradorite—a plagioclase feldspar; essentially an aluminum-calcium-sodium silicate; composition intermediate between that of albite and anorthite; triclinic; H. 6; luster pearly or vitreous, gray, brown, or greenish; sometimes colorless or white; frequently shows play of colors; important constituent of various crystalline rocks, especially of the basic class; usually associated with a pyroxene or amphibole.
Lepidolite—lithia mica; essentially like muscovite (q.v.) except that potash is replaced by lithia.
Leucite—essentially an aluminum-potassium silicate, allied to the feldspars; H. 5–6; luster vitreous, white, ash-gray, or smoke-gray; occurs in certain volcanic rocks, particularly lavas of Vesuvius.