642 S. KOZU 



a) With slight elongation along the axis c, as seen in Fig. 10. 



b) With slight elongation along the axis a, as seen in Fig. 11. 

 The prevailing habits are the first and third types; the second 



and the b type of the fourth are not rare; the a type of the fourth 

 is the scarcest. 



Twinning according to the Carlsbad, Manebach, albite, and 

 pericline laws has been observed. There are two or more different 

 types in combination. The albite and pericline types occur poly- 

 synthetically, while the Carlsbad type occurs in combination with 

 one or both of these. The Manebach is only found in the tabular 

 crystal parallel to P, mostly combined with pericline twinning. 

 The specific gravity measured by the Westphal's balance in 

 Thoulet's solution is 2.759. 



Optical characters. — Extinction angles on P(ooi) and M(oio), 

 measured on cleavage pieces, are — 36 54/ and — 35 24/, respec- 

 tively. The measurement of the mean index of refraction was 

 made approximately by means of Wright's solution. The solution 

 corresponding to the mean refraction of the mineral was deter- 

 mined on the Abbe total-reflectometer. The result is « y = 1.5785. 

 The measurement of orientation of the optic axis B was made by 

 the Becke method, 1 with single screw micrometer ocular. The 

 values p and r of the axis B were measured on three thin slices 

 parallel to P(ooi). The results are as follows: 



(1) On +P(ooi) 



Micrometer \ p=-6o° 44.8' Micrometer] p = +28° 12.6' 



parallel ( r= 0.405 diagonal \ r= 0.337 



(2) On +P(ooi) 



Micrometer ( p= — 6o° Micrometer ( p=-f 25 5' 



parallel \ r= 0.346 diagonal \ r= 0.318 



(3) On -P(ooi) 



Micrometer j p= + 55° 28.5' Micrometer ( p= —20° n' 



parallel \ r= 0.354 diagonal \ r= 0.354 



1 Becke, "Bestimmung kalkreicher Plagioklase durch die Interferenzbilder von 

 Zwillingen," Tschermaks Min. Mitth., 1895, Bd. 14, s. 415-42. 



