Cross and Eahins — New occurrence of Ptilolite. 99 



Ptilolite, Green Mt, Colo. Mordenite (L. V. Pirsson) 



Si0 2 .... 



70-35 



10-06 



A1 2 3 ... 



11-90 



1-00 



CaO .... 



3-87 ) 





K 2 .... 



2-83 V 



•96 



Na 2 ... 



0-77 ) 





H 2 0...._ 



10-18 



4-86 



99-90 



SiO„.._. 



66-40 



9-88 



A1 2 3 ... 



Fe„0 3 ... 



11-17 ) 



0-57 f 



1-00 



CaO .... 



1-94 "} 





MgO... 

 K 2 0.... 



0-17 1 



3-58 { 



roi 



Na„0... 



2-27 J 





H o 0.... 



13-31 



6-60 



99-41 



The empirical formula derived from the analysis of the new 

 ptilolite is: R'Al 2 Si 10 O 24 +6f-II 2 O, which it will be noticed is 

 the same as that of mordenite, but as Pirsson has already 

 pointed out (loc. cit.), the two minerals are physically very 

 unlike. 



The two ptilolites themselves are entirely similar both in 

 their manner of occurrence and in their physical properties. 

 In chemical composition the only difference is that the new 

 mineral contains 8 26 per cent more water than the old, with a 

 proportional reduction of the other constituents ; the molecular 

 proportions of silica and bases being the same in both occur- 

 rences. 



This excess of water may possibly be accounted for by the 

 different conditions under which the two minerals were anal- 

 yzed. The original analysis was made in Denver, the present 

 one in "Washington, that is, one in a very dry, the other in a 

 very damp climate. Both minerals were air dried, and as both 

 evidently hold a considerable part of the water very loosely, it 

 seems not unreasonable to suggest that the dry Denver atmo- 

 sphere may have had sufficient desiccating power to remove 

 part of the water from the mineral analyzed there. In fact, 

 on account of this atmospheric difference, water determinations 

 in the two minerals, especially at low and medium tempera- 

 tures would hardly be directly comparable, since it is now 

 recognized that at a given temperature the relative humidity 

 of the surrounding atmosphere may markedly influence not 

 merely the rate, but also the total amount, of loss on drying. 



The fractional determinations in the new ptilolite show that 

 3*10 per cent of the water is stable at 300° C, so that this may 

 be basic. In the original material no such line of demarkation 

 in the water was detected, although it was thought then that 

 sufficient care was used, but one important factor was not then 

 so well recognized as it is now, namely, the element of time in 

 drying minerals of this type, and it may be that time enough 

 was not given at each temperature before increasing it. The 



Am. Jour. Sci.— Third Series, Yol. XLIV, No. 260.— August, 1892. 



7 



