clarke] OKTHOSILICATES OF TETRAD BASES. 79 
Even the complex inosandrite reduces to the same general type 
agreeing very closely with 
OH /F 
Si0 4 HlHNa(Ce0 2 H 2 ) / SiO,=CaH 
1R< v ' + 4 R < 
\* Si0 4 =HNa(Ce0 2 H 2 ) \^ SiO =CaH 
\Si0 4 =HNa(Ce0 2 H 2 ) \SiO=CaH 
in which E,=Oe lv : Zr : Ti :: 1 : 2 : 2. For each of these species the 
published analyses agree well with the composition calculated from 
these formulae. 
There is still another group of titan iferous silicates, represented by 
sphene, keilhauite, and tscheffkinite, which seems to be unconformable 
with the foregoing scheme of interpretation. Sphene, the typical 
member of the group, has the empirical formula 0aTiSiO 5 , for which 
two distinct structures have been proposed. One regards the mineral 
as the calcium salt of an acid, H 2 TiSiO^, analogous to H 2 Si 2 5 , while 
the other treats it as a basic orthosilicate, 
Ca=Si0 4 =TiO. 
A careful study of the recorded analyses of sphene leads me to prefer 
the latter expression, for the actual ratios vary in a way which indicates 
a replacement, sometimes of Ca and sometimes of TiO by other bases. 
On the meta formula, only the calcium should be replaceable. This 
variability of ratio is well shown by some of the varieties of sphene, 
such as grothite, alshedite, and eucolite-titanite, but the data are not 
absolutely conclusive. If, however, sphene is a basic orthosilicate, it 
should be placed in the preceding chapter with datolite and the other 
calcium salts. On the other hand, the acid character of the titanium is 
suggested by the remarkable hydration derivative of sphene, xanthi- 
tane, of which the composition is approximately represented by the 
formula Al=Ti 2 7 ^H 3 . 
For tscheffkinite, as shown by Eakins 7 analyses, the composition is 
approximately (FeCa) 3 Ce 6 Ti i Si 6 03 2 , but the constitution of the mineral 
is very doubtful. Keilhauite appears to be like sphene, with Ca or TiO 
replaced by R'"OH or R^'O^g, but the analyses are widely discordant. 
The orthorhombic guarinite is generally assumed to be isomeric with 
sphene, although there is but one incomplete analysis from which to 
calculate. 
Some light may possibly be shed upon the sphene groups of mineral 
by a study of the columbo-titanates a3schynite and polymignite. The 
composition of aeschyuite is very well represented by a mixture of 
molecules 
2 O=Ti=TiO 4 =0a 
2 (0bO 3 ) 2 =Ti=TiO 4 =Fe 
3 (0bO 3 ) 2 =Th=TiO 4 =(CeO) 2 
