MINEEALOGY; WITH A CLASSIFICATION OF SILICATES. 



55 



§ 60. The sig'iiificauce of this tribe of amorphous hydrous silicates in mineralogy will 

 be more apparent when we come to study the corresponding tribes among the other two 

 suborders of silicates, and still further among the non-silicated oxyds. In each of these 

 we lind a group of compounds which, although occasionally assuming crystalline form in 

 parallel tribes, require for their crystallization conditions which are not always present. 

 The particular silicate of magnesia which constitutes serpentine, though occasionally crys- 



VL 



Tribe b. Opuiîoid. 



Species. 



Serpentine. - 

 Eetinalite. 

 Deweylite. 



Genthite. - - 

 Aphrodite. 



Cerolite. - - 



Chrysocolla. - 



Spadaite. - - 

 Kensselaerite. 



Sepiolite. - - 



Glauconite. - 



FOKMCLA. 



(rag^sijo; + 2aq - - - 



(mg;;Sij)o; T 2Jaq - - - 



(mg;Si3)05 + 3aq - - - 



(ni2si3)Oj + 3aq - - - 



(mgiSij)oj + -4aq - - - 



(mgiSiaK+liaq - - - 



(cu,sij)Oj + 2aq - - - 



mg,si,,)o„ + 4aq - - - 



(mg,siio)o,, + laq - - - - - 1 15 -90 



(nig,si;;)û, + laq - - - 



Ei;m.\i:ks. 



Perhaps a compact ph ylloid. 

 See § 41. 



tallizing in hydrous forms, as in thermophyllite and chrysotile, appears incapable of 

 forming an anhydrous species like the more and the less basic silicates of the same base 

 which readily crystallize, such as chrysolite and enstatite. Hence we often find the hydrous 

 colloid, serpentine, still associated with the one or the other of these, into a mixture of 

 which it is resolved when its dehydration and fusion are effected by heat. 



Tribe 6. — Zeolitoid. 



§ "70. The sixth tribe in orir system, being the first in the suborder of the protopersili- 

 cates, has been designated Zeolitoid for the reason that it includes, and is chiefly represented 

 by that large family of silicates familiarly known as zeolites, which have been aptly described 

 as hydrated feldspars. These are double silicates of a protoxyd base and alumina, the 

 atomic ratio between the two being 1 : o, and the protoxyds essentially lime and alkalies, 

 occasionally with baryta and strontia, — magnesia being for the most part absent or found 

 only in traces. The proportion of silica varies from that of thomsonite, which gives the 

 ratios 1 : 3 : 4, to stilbite and related species with 1:3:12. The water is also subject to 

 great variations, and is held with different degrees of force — some species, such as laumout- 

 ite and chabazite, parting with a portion in dry air at ordinary or slightly elevated 

 temperatures, while others are much more stable. The intumescence before the blowpipe- 

 flame, which is characteristic of many species of this family, and which suggested the 

 name of " zeolite," would seem to indicate that a partial melting of these takes place before 

 the complete expulsion of water, or in other words, that the silicate fuses in its water of 

 crystallization. The zeolites are attacked by acids with pectisatiou, and are but little 



