332 



BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



There is no known mineral species from which it may not readily 

 be distinguished optically. Its birefringence and twinning suggest 

 albite, but its refractive indices are much higher. Its relief, lack 

 of color, and cleavage in thin section might at first glance lead to 

 its being mistaken for a colorless pyroxene, but its refringence and 

 birefringence are lower and its extinction angle is less than in all 

 pyroxenes except aegirite. It is believed that custerite is a mineral 

 of rare occurrence, for its optical properties are so distinctive that 

 it would scarcely have been overlooked. In the hand specimen, 

 however, it is so unpromising in appearance that thin sections of it 

 may never have been cut. The mineral should be looked for in 

 fluorine-bearing contact zones, apparently in the border phases of 

 metamorphism. 



When heated gently in the closed tube custerite turns a transitory 

 yellow and phosphoresces with a golden yellow light. As seen in a 

 darkened room the color of the glow is like that of a deep colored 

 golden beryl. On increasing the heat the phosphorescence is de- 

 stroyed and water is given off. The mineral does not decrepitate. 

 A white ring due to fluorine is obtained by heating the mineral at a 

 temperature sufficient to melt the glass tube. In the blowpipe 

 flame custerite fuses with difficulty to an opaque white enamel. 



The mineral is very readily decomposed by acids, gelatinous silica 

 separating so quickly when the powdered mineral is treated with 

 hydrochloric acid (HC1) as to form a stiff coherent mass. The 

 separated gelatinous silica floats around in an excess of acid the solu- 

 tion itself not gelatinizing on further boiling. Custerite therefore 

 does not "gelatinize' ' like natrolite. 



The chemical analyses showed that water, fluorine, silica, and 

 lime were the essential constituents, the small amounts of iron and 

 magnesium present being probably due to magnetite and diopside, 

 respectively. Fairly fresh material suitable for analysis was avail- 

 able in portions only of 0.25 to 0.75 gram. The analytical results 

 are shown in the following table: 



Analysis and Ratios of Custerite 

 (W. T. Schaller, analyst) 



