MATERIALS OF THE EARTH 79 



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 mag- 

 nesium rocks. 



Magnetite. Magnetic iron ore; iron oxide, Fe 3 O 4 ; octahedral or dodeca- 

 hedral; 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. 



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). 



Microcline. A triclinic feldspar, closely resembling orthoclase in appear- 

 ance 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 lamina?; 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. 



Oligoclase. A plagioclase feldspar; essentially an aluminum-calcium- 

 sodium silicate' which may be regarded as a mixture of albite and a small 

 amount of anorthite; triclinic; luster vitreous, pearly, or waxy; whitish 

 grading into greenish and reddish; H. 6-7; common in crystalline rocks. 



Orthoclase. A potash feldspar; essentially a potassium-aluminum 

 silicate; varying by the replacement of the potassium by sodium and less 

 frequently by other substitutions ; monoclinic ; occurring in distinct crystals 

 and also in cryptocrystalline forms; cleavage planes perfect with pearly 

 luster on cleavage surface; white, gray, and flesh-red, occasionally varying 

 to greenish white and bright green; H. 6-6.5; difficultly fusible; sanidine a 

 glassy variety; a very common mineral, especially in the granites, syenites, 

 and gneisses. 



Olivine. Chrysolite (q. v.). 



Petroleum. Naphtha; a native mineral oil; a hydrocarbon, commonly 

 believed to arise from organic matter, both animal and vegetable, but held 

 by some to be due to deep-seated chemical and thermal action. 



Plagioclase. A general term embracing the triclinic feldspars whose two 

 cleavages are oblique to each other; embracing albite, oligoclase, andesine, 

 labradorite, and anorthite (q. v.). 



Plumbago. Graphite (q. v.). 



