320 Weed and Pirsson — Missourite, a new Leucite 



This yields approximately the formula (K2Ca)Al2Si30,o.2H20, 

 which is exactly that of a natrolite lSra2A.l2Si3Ojo.2H2O, in which 

 potash and lime have replaced soda. The ratio of CaO : K^O 

 is 1 : 2'91 or almost exactly 1 : 3, and the theoretical composi- 

 tion of such a compound (KgCa Al8Sij204o.8tl20) is given above 

 in the column to the right, and it can be seen that the agree- 

 ment with the amounts obtained is moderately close. If, on the 

 other hand, we assume that the potash yielded by the analysis 

 belongs to leucite and consider it the isometric mineral, then the 

 soda and lime would belong to a mesolite-like mineral, but in that 

 case the agreement of the ratios is very poor and the water 

 entirely too high. The material also floated at a specific gravity 

 of 2*30 and was thrown down below this, which should have 

 excluded leucite, if present in the proportion the amount of 

 potash would indicate. It is reasonable to suppose also that 

 the zeolitization of leucite would yield a potassic compound and 

 not a sodium one. Taking into consideration the mathemati- 

 cal chances against the improbability of the above ratios being 

 accidental and the natural chemical possibility of a potash 

 molecule similar to natrolite, it is not unreasonable to infer 

 that we have a potash zeolite of the natrolite type in this rock. 



In thin section this zeolite is seen as small feathery particles 

 of low birefraction running in narrow bands around the Jeucites 

 and along fractures ; it evidently attacks the mineral from the 

 outer surfaces. In places where it has grown into considerable 

 areas, the areas, while they extinguish as units, are seen to be 

 composed of a curious grouping of two substances in winding, 

 interlaced, vermicular forms almost exactly like micrographic 

 intergrowths of quartz and feldspar, but excessively fine. Of 

 these two substances one is birefractive, the other isotropic, 

 and from what has already been said it seems probable that they 

 are a mixture of the potash zeolite with analcite. 



Occasional separate isotropic grains also occur, which do not 

 ghow the cross-banded twinning of the leucite, and these are 

 supposed to be also of analcite. 



Chemical composition. — A mass analysis of the rock has 

 been made for the U. S. Geological Survey laboratory by Mr. 

 E. B. Hurl hurt of the Sheffield Scientific School, which gave 

 the following results (average of two) ; 



