This study investigates the conversion of biotite, a subgroup of clay minerals, into photocatalysts through heat treatment with CaCl2. The resulting reaction products were analyzed in terms of their composition, structure, optical properties, and photocatalytic activity against CrVI and salicylic acid (SA). Biotite and CaCl2 mixtures could be heated to 600 °C while retaining the biotite crystal structure, whereas heating to 700 °C resulted in transformation to octahedral wadalite crystals. The band gap of the wadalite obtained after heat treatment was approximately 3.10 eV. The photocatalytic reduction rate per unit surface area increased markedly with increasing heat-treatment temperature, and the CrVI reduction and SA degradation rates of the sample calcined at 800 °C were approximately 18 and 9 times greater, respectively, than those of the sample calcined at 500 °C. Leaching tests of the reaction products revealed that the elution of Ca2+ and Cl− was particularly significant. Even the samples that retained the biotite structure after heat treatment displayed some photocatalytic activity, indicating that this method may be applicable to preparing photocatalysts from mica and related minerals with similar structures. Display omitted •Photocatalytic clays were obtained by easy heat treatment of photo-inert biotite.•The heated biotite demonstrated photocatalytic redox reactions towards CrVI and salicylic acid.•Biotite, a ubiquitous material, could be used as an advanced redox material.