CHAPTER VII. 
THE META AND DIMETASILICATES. 
Although the metasilicates appear at first sight to be extremely sim- 
ple, they are actually quite difficult to iuterpret. It is easy enough to 
deduce their empirical formulae, and to write them afterwards in struc- 
tural terms; but this is not sufficient. The structural formulas must 
express all known relations in the case of each species, and in attempt- 
ing to satisfy the established conditions the difficulties begin to appear. 
In the first place, metasilicic acid itself is defectively known, and no 
ether of the form R 2 SiG 3 has yet been certainly obtained. Troost and 
Hautfeuille's ether (C 2 H 5 ) 8 Si 4 Oi2 suggests the possibility that meta- 
silicic acid, like metaphosphoric acid, may polymerize, but an attempt 
to draw general conclusions on so important a supposition from one 
datum only would be most unwise. The possibility of polymeric acids, 
however, must be recognized. 
Again, as we have repeatedly seen, a mineral may be apparently a 
metasilicate and yet really a mixture of ortho- and trisilicates. Even 
a basic trisilicate can have seemingly metasilicate ratios. All of these 
considerations complicate the identification and study of the true meta- 
silicates to such an extent that only provisional conclusions can be 
drawn from the data now on hand. 
A crystallized silicate of sodium, Na2Si0 3 .8H 2 0, is well known. A 
solution of this salt added to a solution of calcium chloride precipi- 
tates a compound which, dried over sulphuric acid, has, according to 
my own observations, the composition 0a 2 Si 2 O 6 .5H 2 O. This, minus the 
water, is analogous to the mineral wollastonite, from which another 
mineral, pectolite, is derived. If wollastonite, instead of the formula 
Ca2Si 2 6 , be given the formula 0a 3 Si 3 O 9 , it may be compared structur- 
ally with pectolite, as follows: 
Wollastonite. 
Pectolite. 
/Si0 3x 
Ca<; 
Si0 3 — Na 
Ca< 
>SiO, >Ca 
>SiO, 
Ca/ 
Ca/ 
X SiO/ 
\Si0 3 — II 
It must be remembered that the molecular weights of the inorganic 
silicates are not known, but only assumed; and the problem suggested 
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