113 



interesting of these minerals seems to be tlie centrallassite^) 

 which in physical properties viz. laminar structure and strong 

 pearly lustre, seems to be identical with the gyrolite and in 

 respect to specific gravity (= 2Ч5 — 2-46) and chemical com- 

 position directly to form a transition between the two forms 

 of gyrolite. Most of the analyses, to the sure, contain more 

 Si02 than the gyrolite, but one of them has only 54'72 p. ct. 

 SiO^ and perhaps the other may be impure from some free 

 silica as is assumed to be the case with the variety of cen- 

 trallassite named cyanolite; in other respects {Al.-^O^ 1 — 2 p. ct. 

 and H^O c. Il"5 p. ct.) the mineral will be seen to take a 

 position between the gyrolite from Ni ak ornat and the other 

 gyrolites. 



Another mineral which is very nearly related to gyrolite is 

 tobermorite'^); in physical respects it is so fine grained an 

 aggregate that cleavage etc. cannot be observed; the specific 

 gravity is 2"423. The analyses show that the amount of SiOo 

 (46'61 — 46*62 p. ct.) is smaller than in gyrolite, but in respect 

 to CaO, AlryOr^ and H.^O it closely resembles centrallassite. 



Okenite does not stand very far from gyrolite in com- 

 position, but here the physical properties show that the two 

 minerals cannot be very nearly related to each other. 



The rest of the calcium zeolites, viz. the chalcomorphite, 

 the plombierite, the louisite and the zonotlite, differ in com- 

 position very much from gyrolite; chalcomorphite'^) is found as 

 very small hexagonal crystals with the axis с = 1"9091 (it is 

 almost the same as in gyrolite); as the mineral is, however, 

 holohedric and the cleavage parallel to the base is only de- 

 scribed as "distinct" it is most probable that the similarity 

 of the axial ratios must be considered as quite accidental. 



1) How: Ed. n. phil. Journ. 10, 1859, p. 84 and Phil. Mag. 1, 1876, p. 128. 



-) Heddle: Min. Mag. 4, 1890, p. 119. 



3) С V. Rath: Pogg. Ann. 1873: Erg.-Bd. 6, p. 376. 



