Hydroxyapophyllite occurs as a coating of multiply twinned crystals in the Mofjellet mine near Mo i Rana, northern Norway. The composition corresponds to (K0.94Na0.06)Ca4.10(Si7.98Al0.02)-020(OH)1.18·7.89H2O. It is white to nearly colourless, nonfluorescent. H = 4½. The measured density is 2.375(5) g/cm3, the calculated density 2.385 g/cm3. The unit cell dimensions are a= 8.968(0) Å, c= 15.869 (13) Å and V= 1276.5 Å3. DTA and TG curves are given. Dehydration occurs in two steps, at 285°C and 410°C with respectively 2.5 percent and 14.4 percent weight loss. It is assumed that recrystallization occurs between the two weight loss steps. Recrystallization and formation of CaSiO3 (wollastonite) occurs at 915°C.