No. 2385. CRYSTALLOGRAPHIC STUDY OF DATOLITE— SHANNON. 523 



I (263) occurred on crystal 41 as a somewhat rounded face yielding 

 a close group of signals. The face is shown in figure 18. It gave 

 the angles: 



(263) Measured < P = 28°55 / p = 70° 15' 



Calculated <p = 27° 37' p - 70° 45' 



A= 1° 18' A = 0° 30' 



This form fits normally in the zone as shown below, and since it is 

 an entirely probable form it may be considered established. The 

 same form occurred on crystal B 7 as a narrow face between Q (1.10.2) 

 and If (T22). This occurrence of the form yielded a somewhat 

 better signal, giving the angles: 



(263) Measured <p = 27° 57' p = 70° 44' 

 A = 0° 20' A = 0° 01' 



(295) occurred as one of a small group of rare forms on crystal 41 

 as shown in figure 18. The face of 295, though small, gave a sharp 

 though faint signal, furnishing the angles : 



(295) Measured <p=1S°54' p = 67° 00' 

 Calculated *» = 19° 15' p = 67° 29' 



A = 0° 21' A= 0° 29' 



8 (296) occurred on crystal 41 as a small face, as shown in figure 

 18, which gave a distinct signal, although, owing to the minuteness 

 of the face, the signal was too faint to illuminate the cross hairs. 

 The angles on this crystal were: 



(296) Measured <p=19° 39' p = 65°32' 

 Calculated <p = 19° 14' p = 63° 34' 



A= 0° 25' A = 1° 58' 



The same form occurred on crystal 10 as a narrow face between 

 i (T23) and b (010), as shown in figure 19. This gave a somewhat 

 brighter and more accurately measurable signal than the last, yielding 

 the following angles: 



(296) Measured <p= 18° 50' p = 63°54' v 



A= 0° 24' A= 0° 20' 



The measurements of the two crystals are averaged in Table 5. 



p (2.10.5) occurred on crystal 41, as shown in figure 18, as a minute 

 face giving a signal too dim to be accurately centered. The angles 

 are: 



