418 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 41. 



As the value 0.032 is more accurate than the value 0.030, obtained 

 by measuring the indices directly, the values of y and a are each 

 changed one unit so as to bring their diJfference in accord with the 

 more accurately determined value of the birefringence. The values 

 of y and a are therefore taken as 1.578 and 1.546, respectively. 



The value for /? was found by measuring the difference of refrac- 

 tion (/?—«') in basal sections of the nuneral. A thin section of the 

 crystalline aggregate furnished basal and brachypinacoid sections 

 of the same thickness, so that the value (/5— or) could be determined 

 by noting the interference colors (blue first order) on the basal sec- 

 tions and comparing them \vith the colors on sections showing the 

 maximum birefringence. The value (/? — «) was thus foimd to be 

 0.010. Therefore /?= 1.556. 



A section of a crystal that seemed to be nearly parallel to a (100), 

 showed a very large axial angle and a difference of refraction of about 

 0.020, which, for a section parallel to a (100); should be 0.022. 



Summarizing the optical properties as determined, we have: 



Axial plane || h (010). Bxa 1 c (001). Positive. 



0'= 1.546, /?= 1.556, 7-= 1.578 



(r-«') = 0.032, (;--/?) =0.022, (/?-«') = 0.010. 



3 



71 = 



1.560. 



STRUCTURE OF CRYSTALS. 



An additional feature of considerable interest is the structure of 

 these crystals. Although the material is well crystallized and the 

 individual crystals sharply bounded by plane 

 and brilliant surfaces, the interior of the crys- 

 tals is in most cases crowded with a mass of 

 small particles of the same material irregularly 

 bounded and not in parallel position with the 

 crystal itself. 



In some crystals these particles are entirely 

 absent; in others they are sparsely scattered 

 tlirough the crystal. Still others contain a 

 vast number of these particles, generally ag- 

 gregated close together. These aggregates are 

 either irregular in shape and occupy the center 

 of the crystal or else they are arranged in a reg- 

 ular fashion patterned after the shape of the 

 crystal itself. Such a regular grouping of these 

 particles is shown in figure 1, which illustrates 

 fiNAcoiD 6{oio>is DRAWN IN also the tabukr habit of the variscite crystals. 

 In figure 1 the brachypmacoid h (010) is 

 drawn in front, in order to better show the mode of distribution of 

 the particles. These particles are generally irregular in their shape. 



Fia. 1.— Variscite crystal, 



SHOWING regular GROUPING 

 OF SMALL PARTICLES — 6 •{010}, 



m{110}, e{012}. The brachy- 



