100 



are turned about 10° from that direction, to the right or to 

 the left on the two opposite faces of c{000l). This turning 

 is distinctly shown on Fig. 3 where the directions of the edges 

 (OOOl) : (1012) are clearly represented by the rays of the per- 

 cussion figure. From this 

 phenomenon it may be seen 

 that this gyrolite belongs 

 to a tetartohedral class, 

 either the trapezohedral or 

 the rhombohedral, and from 

 the above named fact that 

 the figures on the two basal 

 planes are orientated in 

 absolutely opposite direc- 

 tions one may conclude 

 that the mineral belongs 

 to the latter of these 

 classes. 



Physical properties. The cleavage along c{000l} is very 

 strong so that gyrolite in that respect takes a place between 

 gypsum and mica; the cleavage folia are almost perfectly ine- 

 lastic and break very easily into pieces of quite irregular form; 

 in that respect there is a small difference between the mineral 

 from Niakornat and other forms of gyrolite which are, as a 

 rule, somewhat flexible. A fine percussion figure can readily 

 be made with the apparatus described by K. J. V. Steenstrüp^) 

 and consists of a six-rayed star, a part of which is seen on 

 figure 3. The lustre of c{000l} is strong pearly, of the other 

 faces vitreous. The colour is on the fresh crystals grayish or 

 silvery white, on the somewhat altered crystals yellowish white. 

 The purest crystals are perfectly transparent, but mostly they 

 are somewhat dull and, seen under the microscope, traversed 

 by numerous stripes which are curved and pass in all directions 



Fig. 3. Gyrolite, l^ia.koTna.t Etching figures; 

 the sides of the triangles are turned about 10" 

 from the directions of the rays of the per- 

 cussion figure. — The enlargment 1:120. 



1) Geol. Foren. Förh., Stockholm, 10, 1888, p. 113. 



