McDonnell and Smith — Lead-Chlor Arsenate. 145 



Compounds prepared in this way from saturated sodium 

 chloride solutions generally contained a small amount of 

 sodium, replacing, apparently, an equivalent amount of lead. 



The following physical properties of the artificial mimetite, 

 prepared as just described, were determined : 



Color. — In bulk this material had a slight yellow tint, but 

 the color varied with the different preparations, which suggests 

 that it is probably due to traces of impurities. 



Specific Gravity. — 7"15 at 15° C. (determined by the use of 

 a 10 co specific gravity bottle with 4 grams of material and 

 water as the medium). This agrees with the value for the 

 natural mineral, which according to Dana* varies from 7'0 to 

 7-25. 



Optical Properties.^ — Parallel extinction, apparently uni- 

 axial (if biaxial the axial angle is very small) ; approximate 

 refractive indices, e = 2 - 13, and a> = 2*16, and therefore 

 optically negative. These observations and the general form 

 of the crystals indicate that they belong to the hexagonal sys- 

 tem. 



It will be noted that the artificial mimetite which we have 

 prepared is, in so far as we have been ahle to determine from 

 the examination of a number of preparations, uniaxial. Natu- 

 ral mimetite, the occurrences of winch, as stated by Clarke,;}: 

 indicate formation by hydrochemical reactions, is generally 

 biaxial. Specimens from Johanngeorgenstadt, examined by 

 Bertrand,§ had an axial angle in air of 64°. Jannettazjj found 

 an axial angle of 39° in air on a specimen from the same local- 

 ity. A variety of uniaxial mimetite has been reported by 

 Jeremejew.*!" The optical properties of the artificial mimetite 

 prepared by Lechartier, Michel and Weinschenk are not 

 recorded. The anomalous optical behaviour of natural mime- 

 tite may be due to the presence of other chlor arsenates such as 

 we have here briefly referred to. We have not been able, 

 however, to prepare any of the higher chlor arsenates in 

 sufficiently large crystals to determine their optical properties. 



Insecticide & Fungicide Laboratory, 



Bureau of Chemistry, Washington, D. C. 



* System of Mineralogy, 6th ed., p. 772, 1892. 



f Determinations made by F. E. Wright, Geophysical Laboratory, Car- 

 negie Institution of Washington. 



fU. S. Geological Survey Bull. No. 616. p. 682, 1916. 



£ Bull. Soc. Min. de France, iv, 36, 1881 ; v, 254, 1882. 



ft Ibid., iv, 39,1881. 



ITVerh. Russ. min. Ges., (2), xxii, 179, 312, 332, 1886; through Zeitschr. 

 Kryst. Min., xiii, 193, 1888. 



