STUDIES IN HYDROTHERMAL ALTERATION 



193 



nate solutions, it was deemed advisable to try a mixture of the two 

 solutions to see if the traces of fluoride present would modify the 

 results in any way (Table V). The. feldspars in Nos. 20, 21, and 22 



TABLE V 



were not visibly attacked and the decanted solutions gave no pre- 

 cipitate on standing. No. 23 showed crystals of analcite. No. 24 

 showed some needles with parallel, or <— 



nearly parallel, extinction and pos- 

 sibly some isotropic forms, though 

 these were not positively identified. 

 No. 25 contained many needles like 

 those in No. 24 with no analcite. 

 These needles have an extinction 

 angle of less than 2 ; index in the 

 direction of elongation is 1 . 490, and 

 at right angles to this 1 . 517; elonga- 

 tion is negative. The crystals after 

 drying at no° C. yield no water in a 

 closed tube. Good terminations at 

 both ends are common. No. 25 also 

 contained beautifully twinned crystals, illustrated in the accom 

 panying sketch (Fig. 7) made with a camera lucida. The crystals 



Fig. 7. — Twinned crystals pro- 

 duced by experiment No. 25. 



