MINERALS OF IGNEOUS ROCKS 255 



Amphiboles and pyroxenes. Complex silicates, usually containing iron, lime, 

 and magnesia. H. 5-6; Sp. gr. 2.8-3.6. The amphiboles and pyroxenes occur 

 chiefly in igneous and metamorphic rocks, in some of which they are the most 

 abundant dark colored constituents. 



Th.-y an- hard minerals, that is, can be scratched by steel with difficulty. 

 Tin- commoner ones arc black, greenish black or brown. Hornblende is the most 

 important of the amphiboles, and augite of the pyroxenes. These minerals re- 

 semble eachother rather closely, and in very small crystals it is in some cases diffi- 

 cult to distinguish them. The most notable difference is in the cleavage. Horn- 

 blende cleaves in two directions at an angle of 124 (and 56) from each other, while 

 in augite the two cleavage directions are nearly perpendicular to eachother. 

 Most hornblende has a jet-like luster, while augite is more likely to be dull, and 

 is likely to be coated with rust on weathered surfaces. Another amphibole of 

 importance is actinolite, which occurs only in metamorphic rocks and is easily 

 recognized by its long, slender, needle-like crystals, which have a diamond-shaped 

 cross-section and are usually bright green in color. With the exception of bronzite, 

 which is distinguished by its brown color, the other important pyroxenes cannot 

 be distinguished readily from augite. 



The micas. Complex hydrated silicates. H. 2; Sp. gr. 2.76-3. The common 

 micas occur in igneous and metamorphic rocks and to a small extent as minute 

 flakes in some sandstones and shales. The commonest micas are Muscovite, which 

 is white, greenish or yellowish brown, and biotite, which is dark brown or black. 

 They are very soft, that is they can be scratched with the thumb-nail, and have a 

 conspicuously good cleavage so good that they may be split into sheets thinner 

 than the thinnest paper. These thin sheets or flakes are very elastic and tough. 

 In metamorphic rocks the plates are roughly parallel, and this results in the char- 

 acteristic schistose appearance of many such rocks. Microscopic flakes of mica, 

 arranged parallel to one another are responsible for the cleavage of slates. The 

 only simple means of distinguishing between muscovite and biotite is the color. 



Olivine. (FeMg) 2 SiO 4 ; H. 6.5-7; Sp. gr. 3.3-3.5. Olivine occurs in the more 

 basic igneous rocks, that is, those that are comparatively low in silica, especially 

 those that are rich in ferromagnesian minerals. Olivine also occurs in meta- 

 morphic rocks, especially metamorphosed dolomitic limestones. It is a very hard 

 mineral, and has conchoidal fracture. In color it is yellowish green, varying 

 somewhat according to the amount of iron present. It is usually transparent, and 

 has a vitreous luster. It commonly occurs in granular aggregates which contain 

 some pyroxene, but is found also in crystals in some dark colored igneous rocks. 

 A variety used as a gem is called peridot. Chrysolite is another name applied to 

 divine. 

 The iron oxides. 



Hematite, Fe 2 O 8 ; H. 5.5-6.5; Sp. gr. 4.8-5.4. 



Limonite, 2Fe 2 O 3 ,3H 2 O; H. 5-5.5; Sp. gr. 3.6-4. 



Magnetite, Fe 3 O 4 ; H. 6; Sp. gr. 5.18. 



If* niiitilc l is the most important of the iron ores. It occurs (i) in sedimentary 

 rocks, in some cases being the cementing material in sandstones, as in many of the 

 red sandstones; (2) in igneous rocks as the result of weathering of the iron minerals; 

 (3) in veins, and (4) in contact metamorphic deposits. It is mostly hard, but 



1 Turgite is here included with hematite. 



