AbstractThis article provides data on cristobalite and tridymite from the Arsenatnaya active fumarole of the Tolbachik volcano, Kamchatka, Russia. The minerals occur in associations with fumarolic sylvite, sanidine, cassiterite, hematite, pseudobrookite, johillerite, tilasite, and badalovite. Fumarolic cristobalite is tetragonal (α-modification); the unit-cell parameters for one sample are: а = 4.975 (7) Å, с = 6.944 (13) Å, and V = 171.89 Å3. There are two types of tridymite, that is, monoclinic (MC) and orthoorthorhombic (PO-10), in the Arsenatnaya fumarole. The unit-cell parameters of these tridymite modifications are: a = 18.553 (5) Å, b = 5.006 (1) Å, с = 25.952 (10) Å, β = 117.68 (2)°, V = 2134.3 (11) Å3 (MC); a = 9.941 (2) Å, b = 17.165 (4) Å, с = 82.362 (18) Å, and V = 14053.4 (29) Å3 (PO-10). Mineral assemblages of cristobalite and tridymite indicate high-temperature formation conditions of these minerals not lower 450–500°С with considerable participation of HCl and HF in process of basalt alteration by fumarolic gas. The surrounding basalt was a source of silicon. This element was probably transported in the form of SiX4, where X = F, Cl.