C. Palache and II. E. Merwin — Alamosite. 401 



parallel to plane of symmetry. Refraction and double refrac- 

 tion high but not determined. 



Chemical Composition : — A small amount of the mineral 

 was picked out under the microscope as free as possible from 

 adhering quartz and other substances contained in the ore. 

 The absence of cerussite was proved by lack of effervescence 

 in nitric acid. The mineral fuses at 3 to a greenish yellow 

 bead, colorless when cold ; it is easily reduced on charcoal to 

 a lead button and is soluble in nitric acid with strong o-elatini- 

 zation. 



The analysis by H. E. Merwin gave the following result : 



Per cent 



Si0 o 21-11 



PbO 78*13 



CaO trace 



FeO . -09 



Residue* from PbO . '53 



Insol. residue, quartz -08 





Per cent 



Mol. Eatio 



for PbSiO: 



•348 



21-32 



•351 



78-68 



99'94 100-00 



* This residue was lost after weighing and was therefore not determined. 



The molecular ratio of PbO to Si0 2 is almost exactly 1 : 1, 

 indicating for the mineral the formula PbSi0 3 . 



There seems little doubt after considering its characteristics 

 that alamosite should be classified with wollastonite. That it 

 is to be regarded as isomorphous with that mineral is, however, 

 open to question. In favor of this interpretation are (1) sim- 

 ilarity of chemical type and behavior with acids ; (2) identity 

 of crystal system and habit ; (3) close approximation in the 

 values of the angle /3 and in the lengths of the c-axes ; (4) 

 similarity of optical orientation. Opposed to the assumption 

 of isomorphism are (1) difference in lengths of the a-axes ; 

 (2) difference of cleavage. 



The case is analogous to the relation of anglesite and anhy- 

 drite where, however, isomorphism is less strongly indicated 

 than in the present pair of minerals. 



Alamosite is very similar in appearance to barysilite but 

 may be readily distinguished from it by its optical characters. 



Harvard University, March, 1909. 



