438 Foote and Bradley— Chemical Composition of Analcite. 



one Land, and silica and water on the other, show simple con- 

 stant ratios, but there is no ratio either simple or constant 

 between alumina (or soda) and silica (or water). The case 

 appears to be very similar indeed to that of nephelite,* in 

 which the ratio of soda to alumina was simple while their ratio 

 to silica varied largely. The results can be accounted for very 

 simply by assuming in analcite that the excess of water and 

 silica over the amount corresponding to the simple ratios is 

 due to solid solution. The formula of analcite can then be 

 written JSTa 2 Al 2 (Si0 3 ) 4 .2H 2 0.«H 2 Si 2 5 . The point must be 

 emphasized that this does not mean that the compound H 2 Si 2 & 

 as such is necessarily present in solid solution. The real com- 

 ponents, for instance, may be Na 2 Al 2 (Si0 3 ) 4 .2H 2 and JSTa 2 A] 2 

 Si 6 16 .3H 2 0. Other possible components may also be chosen. 

 There is no possible way, at the present time, of deciding defi- 

 nitely in regard to the actual components, and furthermore, 

 this seems unnecessary. The facts all point to a case of solid 

 solution of the unusual type found in nephelite wmere there 

 appear to be isomorphous relations existing between chemical 

 compounds of very different type, and these facts are suffici- 

 ently well expressed in the formula as it has been written 

 above. 



In deriving the formula, we have used only the eight ratios 

 given in Table IV, six of which are calculated from our own 

 analyses. As shown in Table I, a number of analyses have 

 been made in which the silica ratio is considerably below the 

 ratio number 4, and these cannot be represented by the formula 

 given above. We were unable to obtain any specimens .of 

 analcite giving these low ratios and it is impossible to judge of 

 the accuracy of the analyses and the quality of the material 

 used in cases where these low ratios have been found. Until 

 it can be demonstrated that such cases actually occur in pure 

 analcites, we prefer to leave the formula as given. If the 

 occurrence of the low ratios is proved, a slight change in the 

 components chosen will take account of it. 



It is interesting to note that the water in analcitef as well as 

 most of the other zeolites:}; does not behave like ordinary water 

 of ciwstallization. The vapor pressure at a given temperature 

 does not remain constant as the substance is dehydrated as, for 

 instance, the vapor of Glauber's salt does, but it continually 

 diminishes, behaving in this respect like the vapor pressure of 

 an amorphous hydroxide such as silicic acid. For this reason, 

 zeolites do not lose a definite number of molecules of water by 

 heating at a certain temperature or through a certain range of 



* Loc. cit. 



fFriedel, Bull. Soc. Min., xix, 363, 1896; xxii, 5, 1899. 



JTammarm, Zeitschr. phys. Chern., xxvii, 323, 1898. 



