302 



micas. The name biotite is often used by petrographical writers as almost 

 synonymous with black mica, but it must be remembered that some 

 magnesian micas are colourless and that some of the dark-coloured micas 

 contain only small quantities of magnesia. Much work will have to be 

 done before a satisfactory classification of the rock-forming micas is 

 established. The black-micas of the present group of rocks occur for the 

 most part in hexagonal plates. Less frequently as plates with irregular 

 boundaries. As a rule they are nearly uniaxial. The optical axial plane 

 lies in the plane of symmetry that is parallel to two of the sides of the 

 hexagon and the double refraction is negative. The principal cleavage is 

 approximately at right angles to the acute bisectrix so that these characters 

 can be easily determined in a cleavage flake. The variety known as 

 lepidomelane is characterized by a high percentage of ferric oxide. 

 Dr. HEDDLE has proposed the term Haughtonite for a variety containing a 

 small percentage of magnesia (To to 1477 p. c., mean 9'07 p. c.) and a large 

 percentage of ferrous oxide (14-01 to 18-70, mean 17 > 22).< 1 > These varieties 

 are probably the most common forms of black-mica in the acid igneous 

 rocks. A very interesting form of black mica has been described by 

 Professor SANDBERGER^ under the name of proto-lithionit. It is stated by 

 him to be a constituent of many granites, including those of Cornwall and 

 the Mourne Mts. It is biaxial with a small optic axial angle. It melts 

 easily before the blow-pipe and colours the flame a purplish-red. It is 

 easily decomposed by warm hydrochloric acid. Specimens from different 

 localities, when examined qualitatively, showed the presence of titanic acid, 

 stannic acid, arsenic, copper, bismuth and uranium, with traces of tungstic 

 and boracic acids. A specimen analysed by Dr. SHROEDER contained 3'386 

 p. c. of lithia, 8*514 of potash and less than 1 p. c. of magnesia. In colour 

 it varies from black to brown. Thin flakes are brown by transmitted light. 

 This mica differs from zinnwaldite in colour and mode of occurrence. The 

 latter mineral is found only in veins and druses, the former occurs as a 

 constituent of many granites. Protolithionite is of considerable importance 

 as a possible source of the ores found in many mineral veins. 



The following analyses will give some idea of the variability in the 

 composition of the black-micas in British rocks. 



(1) Min. Mag., Vol. III., 1879, p. 81. 



(2) Untersuohungen iiber Erzgango. Wiii-zburg. 1885, p. 167. 



