MICA GROUP. 339 



Biotite is hydrogen-potassium-magnesium-aluminum silicate, a part of 

 the magnesium frequently being* replaced by iron. 



occurrence. — Biotite is an original chief constituent of many of the igneous 

 rocks, both plutonic and volcanic, and ranging from those which are acid 

 to those which are basic. It is a very abundant secondary constituent 

 in the slates, schists, and gneisses, developing on a great scale in the 

 metamorphosed rocks, both igneous and sedimentary. As a secondary 

 constituent it seems usually not to be derived from a single mineral, as is 

 frequently the case with muscovite, but is produced from material furnished 

 by two or more minerals. For instance, it is frequently a reaction product 

 between magnetite and other minerals, the magnetite furnishing the iron 

 for the biotite, the other constituents being derived from such minerals as 

 the pyroxenes, amphiboles, and feldspars. A frequent case is the formation 

 of biotite from the pyroxenes, feldspars, and magnetite. The feldspars and 

 feldspathoids frequently furnish the potassia, parts of the alumina, and silica. 

 The pyroxenes and amphiboles frequently furnish a part of the magnesia, 

 alumina, and silica. Dolomite is often a source of the magnesia. The 

 oxides and carbonate of iron are the most frequent sources of this element. 



In summary, as a metamorphic mineral, biotite is derived from anortho- 

 clase,#augite, hornblende, microcline, orthoclase, and the scapolites. 



Alterations. — -Perhaps the most frequent alterations of biotite are to 

 hydrobiotite (probably monoclinic; sp. gr. 2.90, average of biotite) and to 

 chlorite (monoclinic; sp. gr. 2.80). It also alters into epidote (monoclinic; 

 sp. gr. 3.25-3.50); rarely it alters into hypersthene (orthorhombic; sp. gr. 

 3.40-3.50) and sillimanite (orthorhombic; sp. gr. 3.23-3.24); and in some 

 cases it apparently alters into serpentine (monoclinic; sp. gr. 2.575). Its 

 alteration into the above minerals may be accompanied by the separation 

 of quartz (rhombohedral; sp. gr. 2.6535), and if the biotite be ferriferous, 

 by the formation of magnetite (isometric; sp. gr. 5.174), or other iron oxide. 



The alteration of biotite into serpentine probably requires the simulta- 

 neous production of kaolin (monoclinic; sp. gr. 2.615) and gibbsite (mono- 

 clinic; sp. gr. 2.35). Supposing that all the mag'nesium of normal biotite 



