L. V. Pirsson — Mordenite. 235 



lated for this formula and 



also present How's 



analysis and 



theory. 







Mean f. 



Theory. How f. 



Theory. 



Si0 2 66-40 



65-72 68-40 



66-73 



Al 2 3 (Fe 2 3 ) u - 7i 



12-53 12-77 



12-74 



CaO (MoO) 2-11 



2-27 3-46 



4-62 



K„0 3-58 



3-82 







Na 2 , 2-27 



2-52 2-35 



2-56 



H„0 13-31 



13-14 13-02 



13-35 



Totals, 99-41 100-00 100-00 100-00 



If, instead of accepting How's formula we take the ratio of 

 the bases to the silica as found by my analysis, it will be seen 

 that they give with remarkable exactness RO : Al 2 O s : Si0 2 : H 2 

 as 1:1:10: 6-f. The formula becomes in this case (^K 2 0, 

 £Na a O, iCaO) A] 2 3 (SiO a ) 10 6fH 2 0. This becomes in gen- 

 eral 3RAl 2 Si 10 O 24 +20H 2 O, the three Rs being replaced by a 

 molecule each of potash, soda and lime. 



In 1886, under the name of ptilolite, Cross and Eakins* de- 

 scribed a new zeolite from Jefferson Co., Colorado, which, 

 like the mordenite, occurs as a secondary formation in a basic 

 lava. As the result of their investigations they assigned to the 

 mineral the general formula RAl 2 Si 10 O24+5H 2 O, in which R 

 consisted of lime, potash and soda, not however in any simple 

 ratio. The similarity of these two formulae is very striking, 

 and it seems evident that the two minerals belong to the same 

 group of zeolites, the ratio of bases and silica being the same 

 in each, the chief difference being that the ptilolite contains 

 one-quarter less of water. In the crystal form and optical 

 properties, however, the two zeolites are entirely unlike. 



While this formula for mordenite confirms the work of 

 Cross and Eakins in the existence of these very acid hydrous 

 silicates, which can no longer remain doubtful, and the theo- 

 retical percentages calculated for it agree with very great 

 closeness with the given analysis, it will be best, however, to 

 retain the composition given by How on account of its greater 

 simplicity and because it differs but slightly from the above. 



In the symmetry of its crystal form mordenite is monoclinic 

 and also isomorphous with heulandite. The crystal habit is 

 shown in the figure and is remarkably similar to heulandite 

 from Jones' Falls, near Baltimore, Md. The only forms 



observed were e, 001, O; h, 010, i-l; I, 450, i-f ; t, 201, -2-1, 

 and s, 201, 2-i. The measurements were made on a Fuess 

 goniometer, using the d ocular of Websky. From the small 

 size of the crystals and poor reflections, owing to dullness of 



* This Journal, vol. xxxii, pp. 117, 1886. 



