8 



PKOCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 64 



the Colorado ptilolite (Ca,K2,Na2)O.Al2O3.10SiO2.7H2O, and 1 

 molecule of silica and § molecule of water less than the formula 

 commonly given for mordenite, based upon Pirsson's Wyoming 

 material (Ca,K2,Na2)O.Al2O3.10SiO2.6§H2O. Boggild^ has recently 

 concluded that flokite is identical with ptilolite, while Walker ^ 

 suggests that ptilolite and mordenite are probably identical. Schaller, 

 in the paper above referred to, however, has shown that mordenite 

 and flokite are identical, but that mordenite, which is monoclinic, 

 is distinct, chemically as well as crystallographically, from ptilolite 

 which is orthorhombic. The material from Wyoming described as 

 mordenite by Pirsson, which is distinct from either mordenite or 

 ptilolite (clinoptilolite, Schaller), was instrumental in confusing the 

 species mordenite. The best available data on the refractive indices 

 of these minerals is assembled below: 



Refractive indices of mordenite and related minerals. 



Mineral. 



Mordenite 



Do 



Do 



Do 



Do 



Ptilolite.... 

 Mordenite. 



Do 



Flokite 



Locality. 



Idaho 



Idaho (1). 

 Idaho (2) . 

 Idaho (3). 



_-.do(4)-. 



Colorado. 



Nova 

 Scotia. 

 ._-do 



Iceland _ . 



Authority. 



Koch 



Ross 



._-do 



...do 



...do 



Larsen 



..-do »..- 



Ross * 



CalHsen.. 



Refractive indices. 



1.475 1.477 1.478 



1.470 1.475 1.475 

 1.472 1.475 1.476 

 1.473 



1.473 



1.476 1.480 1.480 

 1.473 



1.471 1.475 



1.472 1.474 



Extinction. 



Undetermined. 

 IncUned. 



Do. 

 Not determin- 

 able. 



Do. 

 Parallel. 



Not determin- 

 able. 



Do. 

 Inclined. 



1 Specimen of How's original material from Morden, Nova Scotia, owned by Col. W. A. Roebling. The 

 value given originally (1 465) in Larsen's tables (Bull. U. S. Geol. Surv. No. 079, p. 113, 1921) has been found 

 to be an error. 



' Specimen of recent collection by Walker and Parsons from Hall's Harbor, Kings County, Nova Scotia 

 (Cat. U. S. N. M. 94, 552). Data determined by Ross and not pre\iously published. 



Two fibrous minerals, one orthorhombic with parallel extinction, 

 and the other monoclinic, with inclined extinction, are represented 

 by ptilolite and mordenite which differ one molecule in silica and one 

 molecule in water. These minerals are very similar, however, both 

 in composition and properties, and very careful work is necessary to 

 distinguish between them. It may be mentioned, in passing, that 

 erionite has the same formula type, the same habit, and similar op- 

 tical properties, although the refractive indices are much lower. 

 This mineral has the formula (Ca,K2,Na2)O.Al203.6Si02.6H20. Miss 

 Callisen made flokite monoclinic on the basis of a constant extinc- 

 tion inclination of 5° on (010) combined with twinning on (100). 

 These observations Boggild set aside as probably due to inexact ob- 



' O. B. Boggild. Dansk. Videnskab. Selskab. Mathemat.-Phys., vol. 4, No. 8, p. 19, 1922. 

 » Thos. L. Walker. Contributions to Canadian Mineralogy, 1922. Univ. Toronto Geol. Series, No. 14, 

 p. 59, 1922. 



