378 



BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



with the chloritoid are variable amounts of a green chlorite and 

 colorless muscovite. The chlorite is described below. 



In thin section under the microscope the chloritoid appears to be 

 the earliest mineral of the aggregate and this has been shattered and 

 the chlorite and muscovite have filled the fractures. 



Optically the chloritoid is well characterized by its intense pleo- 

 chroism. It is biaxial positive with 2V small to medium dispersion 

 r > v very strong, crossed dispersion strong. The refractive indices 

 are approximately a = 1.720, 0= 1.722, 7 = 1.731, birefringence =0.011. 

 The optical orientation is X = b, Zac=18° to 21°. Pleochroism: 

 X = colorless to pale greenish-brown, Y = indigo blue, Z = pale 

 brownish-green. Absorption Y > Z > X. 



The material was separated from the associated chlorite and mus- 

 covite for analysis by means of heavy solutions and the electro- 

 magnet. This was analyzed but the analysis was not extremely 

 accurate since it was found that the sample contained about 3 per 

 cent of cobaltiferous arsenopyrite present as the very finely dis- 

 seminated dust-like grains and not discovered until the analysis was 

 well advanced. After correcting as well as possible for this arseno- 

 pyrite, the analysis, recalculated to 100 per cent, gave the following 

 results and ratios: 



Analysis {corrected) and ratios of chloritoid 

 (E. V. Shannon, analyst) 



Silica (S1O2) 



Alumina (AI2O3)-- 

 Ferric iron (FejOs) 

 Ferrous iron (FeO) 



Lime (CaO) 



Magnesia (MgO)_. 

 Water (H 2 0) 



Total__ 



100.00 



The analysis approximates rather closely to the simple formula 

 FeO.Al 2 3 .Si0 2 .H 2 0. 



It is evident that the chloritoid must be present in large amount 

 at the Idaho locality and, in view of the rarity of the mineral, speci- 

 mens should be of considerable interest to collectors. 



THE CHLORITE GROUP 



The chlorites include a large number of named species whose exact 

 relations to one another are not well understood. It is one of the 

 least known of the natural mineral groups and it is often very diffi- 

 cult to decide where, among the several species, to classify a given 

 specimen, even when a complete analysis and optical measurements 

 have been obtained. The minerals of the group are hydrous silicates 



