ZEOLITE GROUP. 329 



In short, the above reactions are probably as unsatisfactory as any 

 that have been written, because the text-books do not record what minerals 

 accompany the talc, chlorite, and muscovite as a result of the transforma- 

 tion of the staurolite. It is certain that in each case some other minerals 

 must be produced. 



ZEOLITE GROUP. 



The zeolites are a great group of hydrous silicates about which there 

 seems to be no consensus of opinion as to the species in the group, as to 

 the composition of the species, or as to their classification. Since Groth 

 and Clarke are among the latest authors to discuss this group, their 

 formulas are used, Groth's being placed first and Clarke's second when he 

 differs from Groth. Groth's formulas are put into an empirical form, and 

 the subordinate constituents which may replace the chief bases are omitted. 

 The differences between the formulae given and Dana's also are pointed 

 out. The important rock-making zeolites are as follows, rang*ed from basic 

 to acid: 



THOMSONITE, HYDROJiEPHELITE, NATROLITE, MESOLITE, SUOLECITE, AXALCITE, APOPHTLLITE, EPISTI1B1TE, 

 HEULANDITE, STILBITE, PHILLIPS1TE. HARMOTOME, GISMOJfDITE, CHABAZITE, GMELINITE, AND 



LAUMONTITE. 



Thomsonite: 



( CaNa 2 ) Al 2 Si 2 8 . 2iH 2 ( Dana agrees with Groth. ) 



CasAl 6 Si 6 24 .7H 2 



Orthorhombic. 



Sp. gr. 2.3-2.4. 



Hydroaephelite: 



HNa 2 Al 3 Si 3 Oi 2 .3H 2 

 HNa 2 Al 3 Si 3 12 .3H 2 

 Hexagonal. 

 Sp. gr. 2.263. 



Natrolite: 



Na,Al 2 Si s O 10 .2H 2 O (Dana agrees with Groth.) 



Orthorhombic. 



Sp. gr. 2.20-2.25. 



MesolUe: 



H 2 Na.,CaAl 4 Si 6 21 .4H 2 0. (Dana makes all water that of hydration. ) 

 H 8 Na 2 CaAl 4 Si 6 24 . H 2 0. 

 Monoclinie and triclinic. 

 Sp. gr. 2.29. 



