464 GEOLOGY. 



monly 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 vol- 

 canic rocks, particularly lavas of Vesuvius. 



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, FcgO^; octahedral or dodecahe- 

 dral; 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, particu- 

 larly plastic clays containing organic matter. 



' Martite — iron sesquioxide ; originally magnetite, which by oxidation has as- 

 sumed the composition of hematite. 



Mica — the type of an important group of rock-forming minerals well known for 

 their perfect cleavage into thin elastic laminae; 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 laminae; 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 elaeolite characterizes the elaeolite 

 syenite. 



