Constitution of the Natural Silicates. 311 



showing it to be an impure enstatite, the impurities being anal- 

 ogous to those of the original mineral. In the last case the 

 splitting up seems to have been practically complete ; in the 

 chrysotile it went fully half w T ay, while in the ordinary serpen- 

 tine it barely began. The conclusion is obvious. When ser- 

 pentine is simply dehydrated, the molecule Mg 3 Si 2 7 , decom- 

 posable by acids, remains ; and this on further heating splits 

 up in accordance with DuabreVs observations. The salt 

 Mg 3 Si 2 7 corresponds to certain well-known silicic ethers, and 

 is probably a definite compound. 



Now, bearing upon the constitution of serpentine, we have 

 several lines of evidence. First, its empirical formula, 

 H 4 Mg 3 Si 2 9 is well-known, and in this all the water is con- 

 stitutional. Second, upon dehydration it yields the salt 

 Hg 3 Si 2 7 . Third, a part of the magnesia is less stably com- 

 bined than the remainder of the base, and is presumably pres- 

 ent as — Mg — OH. If one atom of magnesium, or one-third 

 of the total, is thus combined, the excess of one atom of 

 oxygen over the normal orthosilicate ratio in the formula is 

 accounted for, and the conditions imposed by our experimental 

 results are satisfied. 



Taking all these considerations into view it seems highly 

 probable that the constitutional formula of serpentine may be 

 written Mg 2 (Si0 4 ) 2 H 3 (MgOH). In ultimate structural form 

 this may be interpreted in several ways, any one of which will 

 admit of a linking together of the two orthosilicic groups, 

 after dehydration, with elimination of one oxygen atom, to 

 form the acid group Si 2 7 . Between the several possible 

 structures, however, we are not as yet prepared to decide, and 

 further investigation covering other hydro-magnesia silicates is 

 necessary. Bearing in mind the very common derivation of 

 serpentine from olivine, and the obvious relations of both 

 species to chondrodite, the following formulse are highly sug- 

 gestive ; the first one representing olivine with double its 

 simplest expression : 



Mg 4 Si 2 Cv Olivine. 



Mg 3 Si 2 O a (MgF) 2 Chondrodite. 



Mg 2 Si 2 8 H 3 (MgOH) Serpentine. 



In the chondrodite formula of course, the group -Mg-O-Mg- 

 to a certain extent replaces the two univalent MgF groups. 

 Although these formulae are not absolutely proven, they are 

 at least highly probable ; and they conform perfectly to the 

 general working hypothesis that the more complex silicates 

 are substitution derivatives of normal salts. 



Am. Jour. Sci.— Third Series, Vol. XL, No. 238.— Oct., 1890. 

 20 



