58 OUR PHYSICAL WORLD 



All such are primary minerals; that is, they are formed directly 

 in the cooling of molten materials. Such primary minerals con- 

 tain no water. In addition there is a series of minerals, also 

 silicates, that contain water. The former are the anhydrous 

 minerals, the later the hydrous. These secondary minerals are 

 the result of alterations of the primary ones through the addi- 

 tion of water and other chemical changes. Among the important 

 primary anhydrous silicates are the pyroxenes, the amphiboles, 

 olivine. The hydrous silicates include mica, kaolin, the chlorites, 

 serpentine, talc. 



The pyroxene group includes hypersthene, diopside, common 

 pyroxene, augite, and aegirite, all similar in physical properties 

 but differing in the relative amounts of magnesium, iron, calcium, 

 sodium, and aluminium that combine with the silicic acids to 

 form the mineral. Pyroxene and augite are the commonest 

 and may be taken as typical. Both consist of silicates of calcium, 

 magnesium and iron, the latter containing aluminium also. The 

 pyroxenes are dark green in color, the augite, black. They are 

 quite hard, 5-6. The fracture is uneven, but they cleave fairly 

 well in two planes that are so nearly at right angles to each other 

 that they appear such except on very careful measurements. 

 They crystallize usually in short, thick crystals that are eight- 

 sided prisms, the ends capped with four-sided pyramids, which, 

 however, are commonly very imperfect, frequently reduced to 

 two faces. 



The amphiboles or hornblendes include also a series of min- 

 erals which are so much alike that for our purposes we may de- 

 scribe common hornblende as typical. If the beginner can dis- 

 tinguish it in the rocks it will be sufficient. Hornblende looks on 

 casual inspection much like pyroxene. It is green to black, has 

 a hardness of 5-6, and occurs in the same dark igneous or meta- 

 morphic rocks. However, it has a highly perfect cleavage in 

 two directions, the cleavage planes meeting at angles of 55 

 or 125; pyroxene, it will be recalled, cleaves at right angles and 

 not very perfectly. The crystals are long and slender, as a rule, 



