84 THE CONSTITUTION OF THE SILICATES. [bull. 122. 
* The agreement is as close as could be reasonably expected. The 
replacement of a little magnesia by a little water in the original altera- 
tion product.accounts for the discrepancies. 
Two other aluminous silicates possibly belong in this group. One, 
barylite, is near 
.Si 2 7 .AlBa 
Al-Si 2 7 .AlBa 
\si 2 : .AlBa 
which requires 
SiO, 35.19 
A1 2 3 19.94 
BaO 44.87 
100. 00 
The other silicate, sphenoclase, is approximately Al 2 (Si 2 O 7 ) 3 0a 6 ; which 
may be analogous to barylite in structure, or written as a calcium 
salt similar in type to okenite. Both barylite and sphenoclase, how- 
ever, are uncertain, and their relations are not definitely known. The 
species glauconite, pholidolite, and celadonite, all of doubtful composi- 
tion, seem also to have diorthosilicate ratios. 
To the rare mineral rowlandite, in accordance with Hillebrand's 
analysis,* a diorthosilicate formula belongs, viz : 
Y— F 
II 
y Si 2 7 =Y 
Fe< 
\Si 2 7 =Y 
II 
Y— F 
which expresses the composition of the species very closely. 
* Bull. U. S. Geol. Survey No. 113, p. 45. 
