Rogers — Dahllite (Podolite) from Tonopah, JVevada. 477 



to assume that oxygen replaces fluorine and the carbonate 

 radical on account of the small amounts of these constituents. 

 As the carbonate-phosphate molecule is present to the extent 

 of at least half, the mineral should be called dahllite (or 

 podolite). 



The optical properties of the mineral are also interesting. 

 The crystals are hexagonal tabular in habit as represented in 

 figure 1. The interior of the crystals is almost opaque white, 

 while the exterior is subtransparent. The central portion of 



Fig. 1. Fig. 2. 



Dahllite (Podolite) from Tonopah, Nevada. 



the crystals, including a narrow zone of the subtransparent 

 part (black area of fig. 2), is dark between crossed nicols, while 

 the remainder of the subtransparent exterior is double refract- 

 ing and divided into six sectors. These sectors extinguish in 

 opposite pairs at an angle of 7° or 8° with the edge as 

 indicated in ^.g. 2 and give negative biaxial interference figures 

 in convergent light. 



The hexagonal prism is either {6170} (or {1670}) with axial 

 plane parallel to {1010} or_it is ] 1010 J with axial plane 

 parallel to J6170} (or {1670}), for the Jheoretical angle 

 (1010 A 6170) is 7° 35'. If the prism is { 6170}, as seems prob- 

 able, the crystals have the symmetry of the hexagonal 

 pyramidal or apatite class. 



In the podolite described by Tschirwinsky* the biaxial sec- 

 tors extend to the center of the crystal. The question arises 

 as to whether all the Tonopah mineral, or only the exterior, is 

 dahllite (podolite). This can not be definitely settled as the 

 mineral contains some fluorine and also an excess of oxygen, 

 but probably the exterior of the crystals more nearly approaches 

 dahllite (podolite) than the interior. 



The formula for podolite established by Tschirwinskyf is 

 3Ca 3 (P0 4 ) 2 .CaC0 3 . SchallerJ gives good arguments for con- 

 sidering podolite and dahllite as identical. The name dahllite 

 given by Brogger and Backstromg has priority. 



* Centralblat. Mineral., etc., 1907, pp. 279-283. f Loc. cit. 



X Loc. cit. 



§ Of v. Akad. Stockh., xlv, 493, 1888 ; Dana System, 6th ed., p. 866. 



