56 Field Columbian Museum — Geology, Vol. III. 



of the lamellae is such as to bisect the above-noted cleavage angle 

 of 53 . Single cleavage cracks || with the lamellae are also occa- 

 sionally seen. 



The interpretation of this structure is difficult, but the following 

 may be suggested: The cleavage cracks at angles of 53 ° are those of 

 prisms of which the crystal groups are composed. These prisms have 

 the symbol 540, corresponding to an angle of 52 44'. This is a new 

 form for zoisite. The twins of which these prisms are made up are 

 formed on c as the twinning axis and the twinning plane is some 

 highly inclined brachy-dome such as e (061). 



The lustre of fragments of the mineral is vitreous and the frac- 

 ture sub-conchoidal. Hardness 6.5 and specific gravity, determined 

 with a chemical balance, 3.32. The mineral fuses B. B. at 3 with in- 

 tumescence, to a brownish enamel and is only slightly attacked by 

 hydrochloric acid. Qualitative tests showed it to be essentially a 

 hydrous calcium aluminum silicate, from which the water could be 

 driven off only by strong ignition. Quantitative analysis by Mr. H. 

 W. Nichols gave the following result: 



Ratio 



SiO, 38.15 3.02 



ALP, 29.50 \ 



FeA 4.60 f I51 



MuO 0.55 



CaO 22.71 / 



MgO 0.63 \ 2 ° 5 



' H 2 3.76 1. 00 



K *° I tr 



Na,0 f tn 



99.90 



These ratios lead to the formula H 4 Ca 4 Al 6 Si 6 O a ,, which is 

 that usually accepted for zoisite with the addition of one molecule 

 of water. For the determination of the water both of Penfield's 

 methods* were employed. By the first method, that of heating in a 

 blast lamp, 1.81% of water was obtained. The mineral did not fuse. 

 By the second method, which consists in heating the tube containing the 

 assay in an oven of fire-brick lined with charcoal, an additional percent- 

 age of water amounting to 1.95% was obtained. Under this treatment 

 the mineral fused completely. The close similarity between the per- 

 centages of water obtained by the two methods, each corresponding to 

 one molecule, suggests that the molecules may be differently com- 

 bined. Thus one may be united with aluminum and the other with 



* Amer. Jour. Sci. 1894, 3rd ser. Vol. XLVIII, pp. 30-37 



