Vol. 5] 



Loudcrback. — Bcnitoite. 



341 



needles of greenish or bluish amphibole, etc., and recalling in 

 its general nature the " Eisennatrolith" of Norway. 



Following this is the more or less altered wall-rock, which 

 along the central part of the deposit is made up largely of bluish 

 or greenish amphibole in minute prisms or needles or irregular 

 tangled mats. It often has a rather porous and irregular text- 

 ure, as the result of considerable leaching of the original rock 

 substance. 



THE MINEEAL8 OF THE DEPOSIT. 



BENITOITE. 



Crystallography. 



Benitoite crystallizes in the trigonal division of the hexagonal 

 system and, as will be more fully shown below, it belongs to the 

 twenty-second or ditrigonal-bipyramidal group of Groth, the tri- 

 gonotype group of Dana — the first actual example of this type 

 of symmetry. This is the highest symmetry group of the tri- 

 gonal division — too high to exhibit rhombohedra which are so 

 characteristic of this division that it is often called the rhom- 

 bohedral division or system. 



The axial ratio c : a is 0.7344, determined as the average of 

 27 direct measurements of the angle between p (1011) and c 

 (0001) with the two-circle goniometer. 



The detailed data obtained from 7 crystals are as follows : 



40° 14' — 2 good, 1 fair. 

 16J' — 1 good, 1 poor. 



18' — 4 excellent, 3 good, 7 fair, 2 poor. 

 19' — 1 excellent, 1 poor. 

 21' — 1 good, 1 fair. 

 22' —2 fair. 



Giving weights of 4, 3, 2, and 1 for excellent, good, fair, and 

 poor reflections, respectively, the average is 40°17:94; a simple 

 average, all readings being given same weight, gives 40° 18 : 04. 

 The closeness to the value for apatite (Dana, 0.7346 5 ) is strik- 

 ing, but the symmetry is different and the relationship other- 

 wise not apparent. 



s Baumhauer in Zeit. fur Kryst., XVIII (1891), p. 40, has collected the 

 various values for the axial ratio of apatite and gives values from 0.7294 

 to 0.7353. 



