Limonite—brown hematite, ocher;—a hydrous iron oxide; commonly earthy; also concretionary, stalactitic, botryoidal, and mamillary, with fibrous structure; H. 5–5.5; luster silky, sometimes submetallic, but commonly dull and earthy; brown, ocherous yellow; streak and powder yellowish brown; constitutes ocher, bog-ore, ironstone, etc.; is the chief source of the yellow color of soils and rocks; arises from the alteration of other iron ores.
Magnesite—magnesium carbonate; rhombohedral; white, yellowish, grayish white to brown; fibrous, earthy, or massive; found in altered magnesium rocks.
Magnetite—magnetic iron ore; iron oxide, Fe3O4; octahedral or dodecahedral; strongly magnetic; H. 5.5–6.5; abounds in igneous and metamorphic rocks.
Marcasite—white iron pyrites; iron sulphide; same composition as pyrite, which it closely resembles; H. 6–6.5; luster metallic, pale gray, bronze, or yellow; prone to decomposition; disseminated through various rocks, particularly plastic clays containing organic matter.
Martite—iron sesquioxide; originally magnetite, which by oxidation has assumed the composition of hematite.
Mica—the type of an important group of rock-forming minerals well known for their perfect cleavage into thin elastic laminæ; among the leading varieties are the common potassium mica (muscovite), the sodium mica (paragonite), the lithium mica (lepidolite), the magnesium-iron mica (biotite), the magnesium mica (phlogopite), and the iron-potash mica (lepidomelane).
Menaccanite—ilmenite; titanium iron ore (q.v.).
Microcline—a triclinic feldspar, closely resembling orthoclase in appearance and having the same composition.
Muscovite—common or potash mica; essentially an aluminum-potassium silicate; H. 2–2.5; monoclinic; remarkable for its basal cleavage; splits easily into exceedingly thin, flexible, elastic laminæ; luster vitreous, more or less pearly or silky; colorless or variously tinged brown, green, or violet; a common mineral in crystalline rocks, particularly in the granites or gneisses.
Nephelite—nepheline; essentially an aluminum-sodium silicate with potash; allied to the soda-feldspars; hexagonal; usually in thick prisms; H. 5.5–6; luster vitreous to greasy, white or yellowish, varying to greenish, bluish, and red; occurs in volcanic rocks; the variety elæolite characterizes the elæolite syenite.