Ill 



obviously been long exposed to the action of atmosplieric 

 agents. On such specimens, on the other hand, as have not 

 been subject to atmospheric influence, the crystal tables are 

 sharply developed. 



The complete combination of the forms occurring on these 

 crystals is shown by Fig. 5, Plate VI. When the third pinacoid 

 с is not a cleavage face, but the original crystal face, it gives 

 a very sharply defined and simple reflection. The lateral faces, 

 on the contrary, are, as usual with the micas, so deeply striated 

 that accurate measurements can only in rare cases be obtained 

 from them. However, the values found show clearly enough 

 that the axial ratios and the forms of this mica agree very 

 closely with those of biotite. 



Referred to the axial ratios calculated for biotite from the 

 measurements of G. vom Rath M, viz. 



a:b:c = 0,57735 : 1 : 3,27432; /? = 90°, 



the forms observed on the zinnwaldite from Narsarsuk have 

 the following symbols: 



с ={00l}, Z) = {010}, Я= {20l}, ^- = {017}, (? = {Г. I.U}, 

 2={Г14}, ^ = {ril}, >7={22l}. 



The determination of these forms is based on the following 

 measured angles: 



Measured Calculated 



(201) : (001) = 85° 38' 85° 8' ' 



(017) : (001) = 24° 41' 25° 4' 



(T. 1. 11) : (001) = 30° 37' 30° 38' 



(Г14) : (00 1) = 58° 50' 58° 35' 



(ni) : (001) = 81° 40' 81° 40' 



(221) : (001) = 85° 44' 85° 38' 



(010) : (001) = 90° 90° 



^) Pogg. Ann. ErganzungsLd. VI, 1874, p. 366. 



