Rock-Salt. Sodium chloride, NaCl.
Rutile. Titanium dioxide, TiO2, crystallised in the tetragonal system. Anatase has the same composition, and is tetragonal, but has different fundamental angles.
Serpentine. Hydrous magnesium iron silicate, H4(Mg, Fe)3Si2O9.
Siderite. Iron carbonate, FeCO3.
Sillimanite. See [Andalusite].
Talc. Hydrous magnesium silicate, H2Mg3(SiO3)4.
Tourmaline. A borosilicate of aluminium with various other elements, R′9Al3(BOH)2Si4O19. R represents H, Na, Al, Mg, Fe.
Tridymite. Silica, SiO2, crystallised in doubly refracting six-sided plates. Its specific gravity is 2·3, that of Quartz being 2·65.
Wollastonite. See [Pyroxenes].
Zeolites. A series of hydrous aluminium silicates, with potassium, sodium, calcium, and sometimes barium.