12 PROCEEDINGS OF THE NATION" AL MUSEUM vol.74 



found with augite in the cracks of the brecciated rock where it coats 

 the pebbles of the breccia. Very often they are broken off and lie 

 in a cement of silica. 



Phenacite and topaz have been reported from Cerro Mercado by 

 Wiedner and his reference to these minerals has been passed down 

 to the present time. The phenacite crystals have been reported as 

 numerous on the north side of the hill and are described as yellow in 

 color, consisting of hexagonal prisms terminated by a six-sided pyra- 

 mid. Wiedner described the crystals as being somewhat altered be- 

 cause they did not have the superior hardness of phenacite although 

 they still retained their natural luster and brightness. Later Wied- 

 ner calls these crystals apatite. The topaz is described as occuring 

 in prismatic crystals of a straw-yellow color. Ail these associations 

 are common for the yellow apatites and search by a number of ob- 

 servers have failed to reveal any true phenacite or topaz. 



A second type of apatite is frequently found in the vugs of the 

 martite ore at Penascos de la Industria (92151). This apatite is 

 colorless or white and is characteristically in radiated groups of small 

 crystals. They are of a later generation than the yellow apatite for 

 in many cases the latter are coated w^ith goethite and this in turn 

 supports the colorless apatite. These later crystals differ in habit 

 from the yellow ones and occur in three forms. The commonest is 

 long needlelike crystals with a steeply curving pj^ramidal zone so that 

 they are barrel shaped. Others are long needles terminated by steep 

 but rounded faces of a pyramid similar to the " berylloid " of beryl. 

 A third form are short prisms terminated by the unit pyramid or by 

 the base. 



The clear yellow crystals of apatite are normal fluorapatite in their 

 properties. They are uniaxial, optically negative with e= 1.630, vj= 

 1.633. The acicular needles are likewise apatite although their 

 indices of refraction are somewhat lower than those of the yellow 

 ones. They give a weak reaction for carbon dioxide and carry only a 

 trace of chlorine. Their indices of refraction e= 1.626, ct= 1.629, are 

 somewhat low for normal apatite, due to the small carbon dioxide 

 content. 



DahlUte. — At Penascos de la Industria there occurs sparingly in 

 the vugs of martite small hexagonal plates or prismatic crystals, 

 usually not more than 1 millimeter in length, consisting of the simple 

 prism and unit pyramid (92150). These tabular crj^stals effervesce 

 with hydrochloric acid and are always divided into six weakly bire- 

 fracting segments which in turn are again divided into a variable 

 number of concentric zones. Their indices of refraction are £=1.609, 

 t!T= 1.626. The sectors are biaxial and negative, with a small but 

 variable axial angle and they show inclined extinction with the outer 



