CHAPTER VI. 
THE DIORTHOSILICATES. 
Although the existence of the sexbasic acid H 6 Si 2 7 has been well 
established by the preparation of its ethers, its metallic salts are little 
known and uncertain. I have already shown, in the case of serpentine, 
that a mineral may be apparently a diorthosilicate, and yet equally 
well explainable otherwise; and what is true in that instance may be 
true in others. For the following species the diorthosilicate formuhe 
seem to be the best and simplest, even though they are not wholly free 
from objection. They fit existing evidence, but are not absolutely con- 
clusive. 
The typical member of this group of minerals is the hexagonal lead 
silicate, barysilite, Pb 3 Si 2 Q 7 . The artificial compound from the slags of 
Bonneterre, Mo., described by Dana and Penfield, is near this, and may 
have the composition Pb 2 CaSi>0 7 . The tetragonal ganomalite is 
another similar compound, approximating to 3Pb 3 Si 2 7 -|-2Ca 3 Si 2 7 , 
although the latest analysis agrees rather better with Pb 3 Si 2 7 -f-Ca 2 Si0 4 . 
Two other orthorhombic species, kentrolite and melanotekite, maybe 
regarded as basic salts derived from barysilite, thus : 
Barysilite Pb 2 Si 2 7 Pb 
Melanotekite Pb 2 Si 2 7 (Fe /// 0) 2 
Kentrolite Pb 2 Si 2 7 (Mn"'0) 2 
The group of zeolitic calcium silicates, okenite, gyrolite, and apophyl- 
lite, are unquestionably related to one another, and are best represented 
as salts of H 6 Si 2 7 . In nature, gyrolite may be derived from apophyl- 
lite, and apophyllite also from gyrolite, while Doelter has generated 
apophyllite from okenite by artificial means.* First, by heating oken- 
ite with potassium silicate and water to 200°, crystals of apophyllite 
were obtained. Secondly, by heating okenite with aluminum chloride, 
sodium carbonate, and carbonated water together at 220°, apophyllite, 
analcite, and chabazite were produced. The most satisfactory general 
formulae for the three species are these : 
Okenite. Gyrolite. Apophyllite. 
/Si 2 7 .H 5 /Si,0 7 .CaH 3 /Si 2 7 .H 4 (CaOH) 
Ca< Ca< Ca< 
>Si 2 7 .CaH 2 >Si,0 7 .H 4 >Si 2 7 .H 4 
Ca< Oa< Oa< 
x Si 2 7 .H 5 \Si 2 7 .CaH 3 \Si 2 O 7 .H 4 (0aOH) 
* Neues Jahrb., 1890, 1, p. 118. 
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