398 C. Palache — JVote on Crystal Form of Benitoite. 



Art. XXX Y. — Note on Crystal Form of Benitoite; by C. 



Palache. 



The symmetry class to which the crystals of the interesting 

 new mineral benitoite belong conld not be definitely established 

 by the forms hitherto observed upon them. Rogers* assigns it 

 to one of two classes, the trigonal-bipyramidal (Class 19 of 

 Groth) or the ditrigonal-bipyramidal (Class 22 of Groth) with 

 probabilities favoring the latter. Neither of these classes has 

 hitherto had a representative among crystals. 



Crystals of benitoite recently acquired by the Harvard 

 Mineral Cabinet present a new form which establishes the 

 correctness of Rogers' assumption of its ditrigonal-bipyramidal 

 symmetry. The form is the second order pyramid (2241); it 

 occurs on all the crystals on the specimen with distinct but 

 small faces which uniformly present a dull luster in striking 

 contrast to that of the trigonal pyramids. The lateral edge 

 between these second order pyramid faces is in most cases 

 truncated by the second order prism with similarly dull faces. 

 These two forms, characteristic for class 22, would appear as 

 trigonal forms in class 19 ; the fact that when a face of either 

 occurs, others are present of like quality and with six-fold 

 repetition, seems to determine the type positively. Although 

 dull the new faces gave distinct reflections on the goniometer. 

 The average of six excellent readings, 0001 to 2211, was 71° 15'. 



The average of ten excellent readings, 0001 to 1011, was 

 40° 12', a value just half way between those obtained by Rogers 

 (40° 10') and Lauderbach (10° 14 7 ). Calculated from this value 

 a : c=l : 0'7319 and ^> =^4879. Calculated from this axial 

 ratio the angle 0001 to 2241 is 71° 10'. The list of forms for 

 benitoite is then as follows: 



Prisms : m (1010), /* (0110), 

 a (1120). 



Pinacoid : c (0001). 



Trigonal pyramids: p(1011), 

 7T (0111), e (0112). j,. 



Second order pyramid : x 

 (2241). j 



The figure shows a tj^pical 

 combination. 



Octahedrite, a mineral not before recorded from the benitoite 

 locality, was observed on part of the specimen from which 

 natrolite' had been wholly removed by solution in hydrochloric 

 acid. It appears in groups of pale brown crystals, combinations 

 of unit pyramid and base ; the crystals are small and present 

 facetted and curved faces so that they could not be measured 

 but chemical tests showed the presence of titanic oxide alone. 



Harvard Mineralogical Laboratory, March, 1909. 

 * Science, xxviii, 616, 1908. 



