48 THE CONSTITUTION OF THE SILICATES. [bull. 125. 
accompanying clintonite micas, and of which the formation is rendered 
intelligible by the formulae. Whether chloritoid belongs here or not is 
doubtful, but if it does its formula may be written 
/°>Ee 
Al— O 
\siO=H.A10H 
Ottrelite may possibly be essentially the trisilicate equivalent of 
chloritoid, 
/°>Fe 
Al-O 
\si 3 8 =H.A10H 
and Damour's venasquite approaches this composition. Variations in 
composition in ottrelite correspond to mixtures of this molecule with 
small quantities of chloritoid. Venasquite, however, is also reducible 
to the normal mica type, and agrees sharply with 
2 Al 3 (SiG 4 ) 3 H 3 + 
3 Al 2 (Si0 4 ) 3 Fe 2 H 2 
It may be that ottrelite and its varieties represent the first stage 
intermediate between micas and vermiculites ; the alkalies of mica hav- 
ing been replaced by water, all W becoming H. 
Although fluorine is found in micas of all classes, it is chiefly charac- 
teristic of lepidolite, zinnwaldite, and phlogopite, and is generally con- 
nected with a low oxygen ratio. Indeed, the extreme end of the 
lepidolite series, polylithionite, is entirely trisilicate in character, and 
its existence fully justifies the assumption that trisilicates occur in 
other micas. Furthermore, polylithionite furnishes a clew to the entire 
lepidolite or lithia mica series, and its formula is easily written 
5 Al-F 4-1 Al-Si 3 8 =Na 2 K 
\si 3 8 =Li 3 \si 3 9 =Na 2 K 
the dominant molecule being akin to clintonite in general constitution. 
Ordinary lepidolite contains the same molecule, mingled with molecules 
of the muscovite type, and an average example is represented by 
.F .F .SiO=KH 2 
2 Al-F +1 Al-F +2 Al— SK) 4 =A1 
\si 3 8 =Li 3 \Si 3 8 =K 3 ^SiO ~Ai 
In cryophyllite we have similar molecules, plus molecules of an iron 
biotite, Al 2 (Si0 4 ) 3 Fe 2 H 2 . Cryophyllite is associated most intimately 
with an iron mica of that general character, and lepidolite, similarly, 
is always accompanied by muscovite. In zinnwaldite the molecule 
Al-F 
\si 3 8 =FeLi 
