Vol. 5] 



Louderback 



■Benitoite. 



373 



Louderback. 



Palaehe. 



e(0001) 



o(ll20) 



m(01TO) 



m(10T0) 



tt(OITI) 



p(ion) 



e(01I2) 

 x (2241) 



Hlawatsch. 



Rogers. 

 (0001) 



c(0001) 

 a(1110) 

 m(10T0) 

 ^(OITO) 

 P(10T1) 

 tt(OITI) 

 r(10T2) 

 d(224l) 



c(0001) 

 a (1120) 

 m(01T0) 

 3f(10T0) 

 p(01Tl) 

 P(10T1) 

 r(01T2) 

 d(224l) 



(01T0) 

 (10TO) 

 (01T1) 

 (10T1) 

 (01T2) 



Hlawatsch gives also Z>(2243) as dull faces on one crystal, 

 and also s(112l) and a(3.T9.16.12) . These are reported as 

 "unsiehere Flaehen," and of the two latter he says, p. 296, "es 

 konnen leicht Abformungen von den begleitenden Neptunit 

 Kristallen gewesen sein." This is easily possible as neptunite 

 has the stronger crystallizing force and benitoite is often found 

 molded against or around it. The writer's form of doubtful index 

 x=( 10.1.9.10) is not reported by the others. 



The fundamental form of benitoite. The three authors cited 

 above agree in selecting /x (Louderback) as the positive unit 

 pyramid and their positive forms correspond to the writer's 

 negative forms and vice versa. AVhile they do not discuss the 

 point, they were apparently led to the selection by the fact that 

 this form is usually developed at this locality in broader faces 

 than the complementary pyramid. The designation of positive 

 unit form ought to be applied whenever possible to the physically 

 most fundamental pyramid. It is well known that the relative 

 size of faces is a very variable matter and commonly determined 

 by the character of the solution from which the crystal separates. 

 Calcite is an excellent example. The cleavage rhombohedron is 

 very appropriately taken as the positive unit form but other 

 rhombohedra both positive and negative are often developed in 

 larger faces and the fundamental rhombohedron is frequently not 

 present among the growth planes at all. Furthermore negative 

 rhombohedra may dominate the positive even to their complete 

 exclusion. The peculiar symmetry of the trigonal pyramids is 

 such that a cleavage if present would be of no value in dis- 

 crimination, for m 1 is parallel to ^ ; n 2 || m 5 , etcetera. 25 



25 Numbers superscript refer to sextants counted clockwise; a bar 

 below signifies a lower dodecant, the upper one being unmarked, as used 

 by Goldschmidt. 



