388 JOURNAL OF THE WASHINGTON ACADEMY OV SCIENCES VOL. U, NO. 16 



habit and frequently twinned. Shepard^ described and figured 

 five crystals of lazulite from the Georgia locality, and stated that a 

 twin (his fig. 5) "is by far the most abundant form equalling in fre- 

 quency all the others combined." 



The mineral is opaque, of azure blue color when fresh, has uneven 

 fracture, indistinct cleavage, and vitreous luster. Blow-pipe tests 

 gave the usual reactions characteristic of the mineral. The lazulite 

 is frequently intergrown with greenish to colorless columnar cyanite 

 which is partly altered to muscovite (damourite), and is less often 

 intergrown with coarse white quartz. Small grains of red rutile 

 and of colorless quartz, especially the former, form frequent inclusions 

 in the blue lazulite. The cyanite weathers yellow-brown in color and, 

 like the lazulite, also contains inclusions of red rutile and colorless 

 quartz. Similar red grains of rutile are also frequent in the quartzite. 

 Weathered surfaces of the lazuli te-bearing portions of the itacolumite 

 are generally rough from the more resistant coarse lazulite and cyanite 

 standing in relief. Scales of colorless mica are more or less conspicu- 

 ous in weathered specimens of the rock. 



When weathered, the lazulite is of lighter or paler blue color, some- 

 times almost entirely white or colorless, but is frequently spotted 

 or mottled white and blue. Microscopically, thin sections show al- 

 teration of some of the lazulite along the periphery and fractures, 

 into minute scales and fibers of a light gray nearly white substance, 

 sometimes stained with iron oxide, having high refraction and double 

 refraction, which probably can be referred to hydrargillite. 



Under the microscope thin sections of the lazulite show inclusions 

 of rutile and quartz, and occasionally cyanite and muscovite, all of 

 which are common associates in hand specimens of the lazulite. 



The lazulite is light blue in thin section and distinctly pleochroic, 

 with Z = Y > X. It is optically negative (-), with 2V large. 



The dispersion is slight, p < V. The values for the indices of 

 refraction and the birefringence determined by Dr. E. S. Larsen on 

 specimens of the lazulite yielding the analysis given below are as follows : 



a = 1.604, nearly colorless 



/3 = 1.633, deep blue 



7 = 1.642, deep blue 



T-« = 0.038 



A chemical analysis of the lazulite yielding the above optical data 

 is given in table 1. 

 ^ C. U. Shepard. Op. cit. 



