100 THE CONSTITUTION OF THE SILICATES. [bull. 125. 
Langbanite. — A silicate of manganese and iron containing antimony. 
Allied on crystallographic grounds to the oxides hematite and ilmenite. 
Uranophane. — Composition represented by the empirical formula 
0aU 2 Si 2 O u .6H 2 O. 
Thaumasite. — 0aSiO 3 ,CaSO 4 ,CaCO 3 ,15H 2 Q. To the molecular consti- 
tution of this extraordinary substance there is as yet no tangible clew. 
A structural formula can be assigned to it, but would be meaningless. 
Neotocite. — Hydrous silicates of manganese and iron, of variable 
composition. Several distinct compounds are probably covered by this 
name. 
A multitude of other silicates have been described as species, but 
without, as yet, securing full recognition. Some of these are doubtless 
mixtures, or impure varieties of well known minerals j while others may 
be ultimately established as good and definite compounds. A discus- 
sion of the records, without experimental investigation of the various 
minerals, would have very uncertain value. I therefore omit these 
doubtful species from consideration. 
