112 



In most cases the crystals form nearly regular hexagonal tables, 

 the faces of the second pinacoid being of about the same size 

 as those of the prisms of the fourth order. Sometimes, however, 

 the second pinacoid is totally absent, so that only the prismatic 

 forms remain as the lateral boundary of the crystals. Such 

 crystals have a rhombic outline with the a- and й -axes as 

 diagonals. The plane angles on the third pinacoid are 55° 

 and 125°. 



Of the forms enumerated above only the following have 

 been previously observed on zinnwaldile from other localities: 

 c, 6, R^ and II. 



The form g has been observed before on biotite. Conse- 

 quently the forms .^^ ^^ ^^^^ ^ 



are new for zinnwaldite as well as for the mica group in general. 

 The zinnwaldite crystals from Narsarsuk are very often 

 subject to twin formation. This follows the common law: the 

 twinning plane = the third pinacoid ; the twinning axis, about 

 which one individual is revolved 60°, is a normal to this 

 pinacoid. This twinning is often repeated, so that a large 

 number of individuals, each in twin position with reference to 

 its neighbour, unite so as to form a thick table or a column 

 prolonged in the direction of their common с -axis. Each se- 

 parate twin lamella in these aggregations is often very thin, 

 and the twinning is then generally shown by a strong horizontal 

 striation of the lateral faces. This striation is due partly to the 

 salient and re-entrant angles which the lateral faces of con- 

 tiguous lamellae make with each other, partly and especially to 

 the circumstance that the single lamellae in twin position do 

 not fully cover each other, the angles of their outline not being 

 precisely 60° and 120°. If the separate individuals have the 

 form of rhombic tables (when the second pinacoid is absent), 

 the twins will bear a certain resemblance to the corresponding 

 formations of eudidymite. 



