254 merwin: azurite and alamosite 



2V by using /3 = 1.758. The resulting value is 2V = 67? Thus 

 the crystals from three localities are optically similar or quite 

 identical. 



Alamosite, PbSiOs. In the original description of alamosite^ 

 the only optical property definitely described was that the optic 

 plane is parallel to the plane of symmetry and the cleavage. 

 The results of a further microscopical study of a few fragments 

 of the original mineral are here presented. 



a and 7 for Na-light were measured, with a probable error of 

 ±0.001, in standard mixtures of piperine, and antimony and 

 arsenic iodides, and also in solutions of arsenic trisulphide in 

 methylene iodide. jS was computed a = 1.947, I3 = 1.961, y = 1.968. 

 A measurement of 7 — a on a cleavage fragment in Na-light gave 

 the value 0.023. Dispersion of the optic axes is very strong, 

 but the inclined dispersion of the bisectrices is too weak to be 

 measm-ed with the microscope. A few grains embedded in a 

 medium with refractive index near /3 were found suitable for 

 measuring the angle between the acute bisectrix and an optic 

 axis. By observing the direction of greater refraction in these 

 grains the negative optical character of the mineral, which had 

 not been satisfactorily determined on account of the strong 

 dispersion, was established. 2V for Li-Hght=58°; for Na- 

 light =65°; for blue light (X = 425mm) =78°— all" within ±5? 



Two small pieces were fused on a strip of platinum, and the 

 refractive index of the glass found by the method used for the 

 crystals. Most of the fragments of the crushed glass from both 

 pieces were alike, n = 1.906. For a very few fragments n = 1.910 

 to 1.920. Possibly a few specks of cerrusite were adhering to the 

 crystal grains. 



2 Palache and Merwin, Amer. Journ. Sci. IV. 27: 399. 1909. 



