Butler and Schaller — Thaumasite. 133 



fibrous character. It is readily recognized in the field as an 

 unusual mineral by its silky luster and lightness (specific grav- 

 ity 1-84). Under the microscope it is seen to be composed of 

 minute slender prisms, none of which showed terminal faces. 

 Microscopically it is most readily distinguished by its low 

 index of refraction and rather high double refraction ; o)=l-507, 

 e=l-468, as determined by Levy and Lacroix. The indices 

 of refraction for the Beaver County mineral were approxi- 

 mately determined by immersion in solutions of known index 

 as <» = 1 , 500 + , e = l - 464 + . The mineral extinguishes parallel 

 to the elongation cf the prisms and is probably hexagonal, as 

 it has been found to be in the previously described occur- 

 rences. Crystals suitable for measurement and exact deter- 

 mination of the refractive indices were not found. 



Chemical Composition. 



A determination of the density of the thaumasite by means 

 of the Joly balance gave the value 1"84. A second determi- 

 nation, using the Thoulet solution and small fragments of the 

 mineral, gave the value l - 85. These figures. are slightly lower 

 than those found by Penfield (1 # 88) on the New Jersey thau- 

 masite. 



The analyses of the Utah thaumasite, with the ratios 

 deduced therefrom, are shown below. 



Analyses and ratios of thaumasite, Utah. 



SiO, 



... 10-14 



•169 

 •156 



1-06 

 •98 



1 



SO„ 



... 12-60 



1 



co 2 



... 6-98 



•159 



1-00 



1 



CaO 



... 26-81 



•479 



3-00 



3 



H a O 



... 42-97 



2-387 



14-95 



15 



(Al,Fe),0,-~ 



•20 



•002 j 

 •006 \ 



•05 





MgO 



•23 







Alk.,P,0 8 ... 



. . . trace 









99-93 



The ratios agree very closely with the established formula 

 3CaO.Si0 2 .S0 3 .CO„.15H 2 and the mineral is very pure, only 

 a trace of some foreign matter, probably a silicate, being 

 present. 



A comparison of the analyses of the mineral from Utah, 

 from New Jersey and from Sweden (the average of the three 

 original analyses given by Lindstrom), strikingly shows the 

 uniform composition of this rare mineral. 



Am. Jour. Sci. —Fourth Series, Vol. XXXI, No. 182.— February, 1911. 



