so DE. THOMAS STERRY HUNT ON THE 



and magnesian alumina-silicates, so conspicuous in the rocks, is au important and unsolved 

 problem. 



Artificial zeolitic compounds, like the soda-mesolito formed by Way, with the ratio, 

 R : al = 1 : 3, may, as we have seen, exchange their alkaline base for lime or magnesia, but 

 for the silicates in question, in which this ratio is 1 : 2, or 1 : 1, or 2 : 1, the correspond- 

 ing silicates of alumina and alkalies are as yet unknown to chemistry, being soluble, and 

 probably unstable and uncrystallizable. Analogy, however, as well as the modes of 

 occurrence of these calcareous and magnesian silicates, would lead us to expect the 

 the production of such alkaline double silicates, under certain conditions, in solution, and 

 we are not without evidence of the occurrence of such compounds. The soluble alkaline 

 extract from the decomposition of an aluminous glass, in Daubrée's experiment, holding in 

 solution both silica and alumina, gave, if the data are exact, the oxygen-ratio for 

 E : al : Si ^ 3 : 1 : 4. We have also, in Friedel and Sarrasin's experiment, the separation of 

 analcite from a like solution, which retained both silica and alumina in solution. 

 Researches in this direction will probably make known to us the conditions under which 

 such residual solutions may be prodiiced, containing alkaliuo-aluminous silicates with 

 the ratios corresponding to epidote, garnet, biotite, i^hlogopite and the chlorites. 



§ 101. Magnesian silicates corresponding to the zeolitic and feldspar group are rare, and 

 known to us only through the artificial compound of Way, the species iolite, esmarkite and 

 fahlunite, and the partially magnesian zeolites, picrothomsonite and picranalcite. Chaba- 

 zite, when finely pvUverized, according to Eichhorn, exchanges a portion of its lime for 

 potash when digested with a potassium salt, bu.t is very slightly attacked by a solution of 

 magnesian chlorid."" The more silicic of these zeolites are apparently indifferent to such 

 substitirtions and, as we have seen, phillipsite is formed in sea-water. We should, how- 

 evei", exjîect the more basic of the calcareo-aluminous silicates, with the ratios, R : al = 1 : 1 

 or 2 : 1, to be very susceptible to replacement by magnesia. Bunsen has shown that 

 palagonite, a hydrous silicate of this class (§ Q^J, foot-note) with a large proportion of 

 calcareous base, decomposes even a solution of ferrous sulphate, which removes its lime, 

 and it would doubtless decompose in a like manner magnesian salts. I have long since 

 shown that an artificial hydrous silicate of lime readily decomposes a solution of 

 magnesium-chlorid, with the production of calcium-chlorid and a magnesian silicate ; 

 a result in accordance with the earlier observations of Bischof on the power of solutions of 

 silicate of lime to decompose magnesian salts."' 



§ 102. While on one side of what we may call the normal type of alumina-protoxyd 

 silicates, with the ratio, R : al ^ 1 : 3, as seen in the group of the feldspars and the zeolites, 

 we have those with an excess of protoxyds, including scapolites, epidote, garnet, biotite, 

 phlogopite, and the chlorites ; there is another series of aluminous silicates in which the 

 proportion of protoxyds falls below this normal ratio, and still another series in which 

 protoxyd-bases are absent. Of the latter we need only name the anhydrous species, 

 andalusite, fibrolite and cyanite, and the hydrous species, pyrophyllite, pholerite, and 

 kaolinite, with the amorphous halloysite, a more highly hydrated and colloidal form of the 

 kaolin-silicate. The aluminous protoxyd-silicates with a diminished proportion of alkali, 



"'Cited by S. AV. Johnson, Amer. Jour. ScL, 1859, xxviii, 74. 

 "' Hunt, Cliem. and Geol. Essays, p. 122, 



