ALTERATION OF MONOCLINIC AMPHIBOLES. 285 



rocks, especially those of igneous origin. Not infrequently it is also an 

 abundant constituent in the metamorphosed sedimentary rocks. The schists 

 in which the hornblende is the chief constituent, whether of aqueous or 

 igneous origin, are generally known as amphibolites. In many other scfhsts 

 and gneisses which are chloritie and micaceous it is an important constitu- 

 ent, As a metamorphic mineral hornblende has been noted as derived from 

 almandite, augite, melanite, and pyrope. 



Grlaucophane occurs abundantly in certain of the amphibole-schists, 

 especially those which are derived from the debris of basic rocks which were 

 originally rich in sodium. Naturally, such rocks have a somewhat limited 

 occurrence; but where they do occur abundantly, as in the Coast Ranges 

 of California, glaucophane is also very abundant — in fact, is the chief con- 

 stituent of some of the schists, so that they may properly be called 

 glaucophane-schists. 



Riebeckite occurs in some eruptive rocks which are rich in sodium and 

 iron, and also in metamorphosed rocks of both sedimentary and igneous 

 origin. Like glaucophane, it may locally occur abundantly, but is not a 

 widespread mineral. 



Arfvedsonite, a soda-amphibole, very naturally occurs in the soda- 

 bearing igneous rocks, especially in elseolite-syenites and nepheline-syenites. 



Alterations. — The' minerals of the monoclinic amphibole group, of such a 

 wide variety of composition and extensive occurrence, have naturally a 

 large number of alteration products. The more common of these are talc, 

 serpentine, bastite, chlorite, epidote, and biotite. These are frequently 

 accompanied by more or less magnetite, hematite, and limonite. In some 

 cases the amphiboles alter into the zeolites, pinite, and chabazite. 



Taking up the individual minerals, tremolite is most frequently trans- 

 formed into talc (orthorhombic or monoclinic; sp. gr. 2.75), which may be 

 accompanied by calcite (rhombohedral ; sp. gr. 2.7135). Actinolite com- 

 monly alters into talc or serpentine (monoclinic; sp. gr. 2.575), often 

 with the simultaneous formation of calcite, quartz (rhombohedral; sp. gr. 

 2.6535), and iron oxide. Cummingtonite commonly alters to bastite (ortho- 

 rhombic; sp. gr. 2.6). The standard text-books do not describe the altera- 

 tions of griinerite, although it is believed to alter to the iron oxides. 

 Hornblende under weathering conditions ordinarily changes to chlorite 

 (monoclinic; sp. gr. 2.71-2.725), which is often accompanied by epidote 



