100 Cross and Eakins — New occurrence of Ptilolite. 



fact, however, that a comparatively high temperature was 

 found necessary for the total expulsion of the water, would 

 render it easy to admit the possible basicity of part of it in 

 case other considerations made such an admission advisable in 

 order to construct a rational formula for the mineral. 



In mordenite no attempt seems to have been made to frac- 

 tionate the water, except to determine the loss (3"6 per cent) at 

 100°, so here we have no evidence as to whether it may be 

 basic in part or not. 



The fact that in the new ptilolite we have direct evidence 

 that a definite portion of the water may be basic, and that no 

 good ground against such an assumption exists in the analysis 

 of the original mineral, makes it necessary to carefully con- 

 sider this point in any attempt to theorize upon the structure 

 of the ptilolite molecule. Prof. Clarke has kindly written a 

 note on the theoretical formula of ptilolite and of the allied 

 mordenite, which appears in the succeeding pages. 



In view of their similar mode of occurrence, of their identi- 

 cal physical properties, of their exact chemical agreement 

 except as to water, and of the considerations concerning the 

 water presented above, there can be no question but that these 

 two downy minerals belong to one and the same species — 

 ptilolite. 



In comparing the analyses of mordenite and the two ptilo- 

 lites a great variation in the amounts of lime, soda and potash 

 is noticeable. In mordenite the molecular ratio is CaO : K,0 : 

 ]STa 2 = 1:1:1; but no simple ratio exists in ptilolite. 



The Green Mountain mineral is relatively poor in soda, the 

 Silver Cliff mineral in potash, and the mordenite in lime, facts 

 which seem very strange when we consider the rocks in which 

 the minerals occur. Mordenite occurs in a basalt, but is richest 

 in potash and poorest in lime ; the original ptilolite occurs in 

 an andesite of medium composition and is poorest in soda ; and 

 the new mineral occurs in a devitrified pitchstone and is poorest 

 in potash. The last instance is most remarkable, for the pitch- 

 stones of the region about are usually rich in potash and con- 

 tain but very small amounts of lime. Analysis of the greenish 

 felsitic mass in which the ptilolite occurs shows, however, that 

 the entire rock has in this case undergone a decomposition 

 which brings it to a constitution similar to that of ptilolite. 

 The lime, at least, of this ptilolite was not derived from the 

 surrounding rock, unless secondarily. The analysis of the 

 rock containing the ptilolite of Silver Cliff is as follows 

 (Analyst, L. G. Eakins) : 



