Hidden and Pratt — Twinned Crystals of Zircon. 325 



Besides this type the Henderson County zircon exhibits a 

 series of twin crystals in which the twinning planes are parallel 

 to pyramids of the first order. Five new twinning planes 

 have been identified by us. Of these, four are parallel to the 

 pyramidal faces, p, 111 ; d, 553 ; v, 221 and u, 331, and are 

 3, 4, 5 and 6 respectively ; the fifth 



figs. 



15 



represented by 



parallel to the pyramid (f> (774).* 



The twin crystals are usually well developed and doubly 

 terminated, the faces being somewhat vicinal but with sharp 

 edges, thus enabling the faces and twinning planes to be 



readily identified by means of the contact goniometer. The 

 measurements were very satisfactory and close to the cal- 

 culated angles, as shown in the following table : 



Twinning 

 planes. 



JP OH, 1) 



d (553, f) 

 <P ("4, |) 

 v (221, 2) 

 u (331, 3) 







Measured angles m ~ m- 





Calculated 

 angle. 

 95° 40' 

 112 50 

 115 30 

 122 12 

 139 35 



95 c 

 112 

 116 

 121 

 138 



10'; 

 10 ; 

 30 ; 

 30 ; 

 139 c 



96° 30' 95° Average 95° 



113 20'; 111 40' " 112 



116 : 115° 30'; 115° 15' " 115 



122 30 " 122 



10'; 140 ; 139° 30' " 139 



33' 

 23 

 49 



10 



The re-entrant angle formed by m^?n over the twinning 

 plane is the same as the angle of divergence of c ^ c. These 



prismatic edges being very sharp, the re-entrant angle could be 

 measured very accurately with the contact goniometer, the 

 measured angles agreeing within half a degree of the calculated 

 angles. 



The minerals associated with the zircon are the following : 

 Pyrite, in cubes partially changed into limonite ; fluorite, pale 



* Dana's Mineralogy, sixth edition, 1892, p. 482. 



