78 THE CONSTITUTION OF THE SILICATES. [bull. 125. 
derived from an original ThSi0 4 by hydration ; and that that ThSi0 4 
was isomorphous with zircon. The nearest approach to the type is 
found in orangite, which may perhaps be regarded as a thorium cyrto- 
lite, or thorium malacone. Yttrialite approximates to the composition 
Th(Si 3 8 ) 4 Fe" 2 (YO) 8 
and in steenstrupine a thorium silicate exists which may be Th(Sr0 4 ) 4 
Ca 4 Na,. Further discussion of these minerals at present would be 
fruitless. The data are wholly inadequate. 
To the titanium silicates astrophyllite, johnstrupite, and rinkite simi- 
lar formulae to the foregoing are assignable. Indeed, this has already 
been done for astrophyllite by Brogger, who writes the formula Ti(Si0 4 ) 4 
R" 4 RV This seems to be the dominant molecule in astrophyllite, 
which, however, varies in composition. To the Colorado mineral we 
may more precisely give the formula 
SiO=FeH OH 
3 Ti ^SiO =FeK 1 rp/siO 4 =FeH 
\Si0 4 =FeNa \SiO=FeH 
SiO=FeH SiO=FeH 
which requires 
SiO, 34.30 
TiO, 12.20 
FeO 41.16 
K 2 5.36 
Na 2 3.55 
H 2 3.43 
100. 00 
Some iron is replaced by manganese, and ferric iron, perhaps as 
=Fe— OH, is also present. In the iluoriferous astrophyllites the fluor- 
ine should replace hydroxyl. 
Johnstrupite and rinkite are both fluoriferous, and both contain 
earths of the cerium groups. In johnstrupite, a little Zr0 2 , Th0 2 , and 
0eO 2 replace some Ti0 2 . For johnstrupite the expression 
SiO 4 £ECa(0eF 2 ) 
/siO~CaXa 
Ti< 
\Si0 4 ^CaNa 
SiO 4 =0aH 
agrees well with the ratios given by analysis. In rinkite we have, with 
great probability, the mixture 
SiO^CaNa SiO=Ti-F 
/SiO =CaKa / SiO 4 IZ0a(CeF 2 ) 
3 Tl \Si0 4 ^CaNa + 2 Ti ^SiO 4 =Ca(0eF 2 ) 
SiO=CaNa SiO 4 ~Ca(0eF 
