No. 2385. CRYSTALLOGRAPHIC STUDY OF DATOLITE— SHANNON. 487 



orientation according to the preference of the worker. The investi- 

 gators using the 2-circle goniometer have been compelled to choose 

 Goldschmidt's orientation or undertake the alternative of recalculat- 

 ing long tables of angles and of converting all forms to indices in 

 terms of the Dana orientation. This has been done by the writer, 

 and the tables, which represent a considerable expenditure of time 

 and labor, are here reproduced for the benefit of future workers who 

 may prefer the Dana position. Thus in table 1 is given a list of all 

 known forms described for datolite, with the letters assigned them 

 and the equivalent indices and symbols in the two orientations, 

 together with references to the publication from which the writer 

 obtained the data on the form. In table 2 the same list of forms is 

 given with the angles calculated for all forms in the Dana position. 

 For convenience in comparing measured angles this table is arranged 

 by ascending <p and positive and negative forms are segregated. 

 Also in table 4 is given a list of the new forms added by the present 

 investigation, with equivalent indices and symbols, while in table 6, 

 for the convenience of those preferring the Goldschmidt position, the 

 new forms are listed with their calculated angles in the Goldschmidt 

 orientation. For calculating angles in Dana's orientation the follow- 

 ing mathematical constants were used: 



a= .63446 lg. a= 9.80243 h= 1.0000 



c= 1.26574 lg. c= 10.10243 e= .0025 



p = 1.995 lg. p = 10.29994 lg. ^= 10.19762 



# =1.226 lg. £,= 10.10232 



1-89° 51' ! g ^ = 1 = !- e= 1= 7.41797 



180° -0 J lg. sin. fi\ lg. cos. juJ 



Owing to the similarity in angles that exists between the two zones 

 which are taken as prismatic in the two orientations, it is very easy 

 to inadvertently interchange these. The very close similarity is 

 shown by the following comparison of angles: 



ra(110)/\m'"(ll0)=64°47' #(012) A g'(012) =64° 40'. 



«(320)A e"'(320)=45°51' <(013)A *'(0l3) =45° 45'. 



A(210)A A"'(2l0)=35 o 12' a (014) A <r'(0l4) =35° 07'. 



o(120) A o' (T20)=76°29' w x (011)A 777 x (OlT) =76° 37'. 



In addition to this common cause of confusion is the fact that the 

 mineral approximates so very closely to orthorhombic symmetry 

 (j8 = 89° 51£') that the difference in angle between corresponding 

 positive and negative forms is very small, and it is consequently 

 possible to rotate crystals 180° from true position, especially if the 

 angles measured are not very exact and the crystal is simple in com- 



