352 



BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



with 2V = 52°. The indices of refraction are a = 1.482, 0=1.485, 

 7 = 1-489, y — a =--0.007. The acute bisectrix (Z) is perpendicular 

 to the 6(010) face, while the extinction, measured from what is taken 

 as the c crystallographic axis as determined by the trace of the 

 bounding prism faces of the plates, averages 35°, the optic plane 

 being nearly parallel (measured 5°-8°) to the trace of one of the 

 terminal clinodomes, either £(101) or s(T01). The crystals, as seen 

 between crossed nicols are frequently not entirely simple being divided 

 by irregular sutures into areas which differ slightly in extinction 

 angle. Thus the crystal measured and figured in the drawing (fig. 

 110) was found to be divided, vertically, by a straight line, on one 

 side of which the extinction, measured from vertical cleavage lines, 

 was 38 H° while on the other side it was 44J^° and a wedge entered at 

 the termination of the crystal in which the extinction was 33°, the 

 latter being about the normal extinction angle of homogeneous 

 crystals. In addition to this peculiarity the crystals frequently show 

 a very narrow outer border of a material of decidedly higher index of 

 refraction. The crystals of this heulandite which were measured did 

 not give very satisfactory angles, mainly because of the poor de- 

 velopment, bulging, or parallel growth in the vertical zone rendering 

 accurate orientation on the goniometer very difficult. The measure- 

 ments are given in detail in the original paper above cited, for the 

 crystals illustrated in Figures 109 and 110. The agreement is suf- 

 ficiently close to show the essential crystallographic identity of the 

 mineral with heulandite. 



The material was anlyzed, the material for analysis being selected 

 small red nodules and the analyzed powder was shown to be essen- 

 tially pure and homogeneous by optical study. The refractive 

 indices given above are for the analyzed powder. The analysis gave 

 the following results and ratios: 



Analysis and ratios of heulandite from Challis 

 (E. V. Shannon, analyst) 



Constituent 



Ratios 



Silica (Si0 2 ) 



Alumina (AI2O3). 



Lime (CaO) 



Magnesia (MgO) 

 Potash (K 2 0).... 



Soda (Na 2 0) 



Water (H 2 0) 



Total. 



1.014 0.145X7 1.00X7 

 .142 .142X1 .98X1 

 .064) 

 .0071 

 .0101 

 .072j 

 .787 .157X5 1.09X5 



153X1 1.05X1 



This analysis gives the formula (Ca,K 2 ,Na 2 )O.Al 2 3 .7Si0 2 .5H 2 0. 

 This differs from the formula for heulandite given by Dana in con- 

 taining one more molecule of silica. The replacement of lime by 

 alkalies has furthermore progressed to the point where the combined 



