A white solid, SnO2, insoluble in water; tetrahedral; r.d. 6.95; m.p. 1127°C; sublimes between 1800°C and 1900°C. Tin(IV) oxide is trimorphic: the common form, which occurs naturally as the ore cassiterite, has a rutile lattice but hexagonal and rhombic forms are also known. There are also two so-called dihydrates, SnO2.2H2O, known as α- and β-stannic acid. These are essentially tin hydroxides. Tin(IV) oxide is amphoteric, dissolving in molten alkalis to form stannates; in the presence of sulphur, thiostannates are produced.