ART. 23 DURANGO MINERALOGY AND GEOLOGY FOSHAG 13 



hexagonal edge under crossed nicols. This extinction is very vari- 

 able, the same zone extinguishing hetcrogenoously and varying from 

 8° to over 30°. The prismatic crystals are likewise zoned, the outer 

 shell peeling off easily leaving a firm glassy center. Their indices 

 of refraction are higher than normal apatite €=1.648, 57=1.053. 

 They effervesce slowly with acid and give a microchemical test for 

 chlorine and hence may be considered as an isomorphous mixture of 

 chlorapatite and dahllite. 



The platy dahllite has been found as a very fine grained mass with 

 a somewhat silvery luster coating martite crystals. The prismatic 

 type occurs as isolated crystals seldom over 1 millimeter in size scat- 

 tered over goethite that in turn coats the martite crystals. 



Unknown phospluite. — Associated with the colorless apatite in the 

 vuus of martite is a mineral occurring in rose red to lavender 

 botryoidal masses with a finely drusy surface (92156). This min- 

 eral gives strong microchemical reactions for lime and phosphorus 

 and weak reactions for iron. Its amount, liowever, is insufficient 

 for a complete chemical investigation. The mineral has the follow^ing 

 optical properties: Biaxial, optically negative with a large axial 

 angle. Dispersion strong, p greater than v. a = 1.568, /?=1.5Y8, 

 7 = 1.582. Color pale to bright rose, the tint being more intense at the 

 center of the radiated groups. It is pleochroic in shades of rose 

 in thick grains only. 



Augite. — Augite is a common constituent of the altered wall rock 

 about the iron ores, where it forms reticulated to radiated masses 

 in the clayey altered latite (92147) or forms a pure augite rock 

 (92148). Tlie color varies from bright grass green to a dark green- 

 ish black. In some of the old workings on the north slope of the 

 hill crystals of augite were found in abundance associated with clear 

 yellow apatite, coating the fragment of breccia. These crystals are 

 prismatic (92159) and the larger ones exceed 1 centimeter in length. 

 Measurement on the two circle goniometer shows the following faces 

 to be present c (001), m (110), g (210), / (310), a (100), h (010), 

 and sometimes e (Oil) . The crystals are nearly all twinned, and Fig- 

 ure 3 shows their general habit. 



In color these crystals are dark greenish black but in thin sec- 

 tions or in small grains under the microscope they have a clear grass 

 to emerald green color. Their pleochroism is weak. The optic axial 

 angle is about 60° and the extinction angle about 40°. The indices 

 of refraction are a= 1.700, /8=1,708, y= 1.727. An analysis made in 

 the laboratory of the United States National Museum upon selected 

 crystals that had first been washed with h3'drochloric acid to remove 

 .some slight stains of limonite gave the followinji results : 



