T. L. Walker — Percussion Figures of Mica. 



Akt. II. — Observations on Percussion Figures on Cleav- 

 age plates of Mica ; by T. L. Walker, Leipsic. 



Reusch"^ discovered that when a sharp-pointed punch is 

 placed on a thin plate of mica and struck a sharp blow with a 

 small hammer, a six-rayed star is formed on the mica plate. 

 He also observed that one of the three lines forming the star 

 is parallel to the clinopinacoid. The other two lines he 

 described as parallel to the prismatic edges of the six-sided 

 cleavage fragment of mica. These observations were con- 

 firmed and the investigation extended by Bauer,f who recom- 

 mended the use of a large, somewhat blunted needle instead of 

 the blunter instrument used by Reusch. He also emphasized 

 the importance of the nature of the foundation on which the 

 mica rests when the needle is struck. Reusch recommended a 

 glass plate covered with a thin sheet of vulcanite. The accom- 

 panying figure represents a basal section of a mica crystal and 

 indicates the directions of the different rays of the percussion 

 figure. 



00 YOO 



These figures are fully described in all the larger text-books 

 on Mineralogy, and although they all agree in asserting that 

 two of the lines forming the star are parallel to the prismatic 

 edges of the basal section, I find as a result of a large number 

 of measurements that this is seldom if ever the case. 



Since the prismatic faces of the micas form an angle vary- 

 ing only a few minutes from 120°, we would expect that all 

 the rays of the percussion figure would form angles very 

 nearly 60°. By measuring the angles in percussion figures of 

 different micas great variations were observed. The following 

 table shows measurements for about twenty specimens repre- 

 senting all the commoner species of mica and from widely 



^ Reusch, Ber. Ak. Berlin, 428, July 9th, 1868. 



f Fogg. Ann., 1869; also Zeitsch d. d. g. Gesellschaft, 1874. 



