Two specimens of alluaudite-group minerals, badalovite NaNaMg(Mg,Fe³⁺)₂(AsO₄)₃ and calciojohillerite NaCaMgMg₂(AsO₄)₃, were taken on the Second scoria cone of the Great fissure Tolbachik eruption, Kamchatka Peninsula, Russia. The crystal structures of these monoclinic minerals have been refined by single-crystal X-ray diffraction. The thermal expansion of badalovite and calciojohillerite has been studied by high-temperature powder X-ray diffraction. The electrical conductivity was evaluated by impedance spectroscopy. The thermal expansion of these minerals is similar: maximum expansion is close to the longer diagonal of the parallelogram ac, minimum expansion is close to the shorter diagonal of the parallelogram ac. Thermal deformation of the ac plane is described in terms of the theory of shear deformation of monoclinic crystals. The results of shear deformations calculation and its consideration with thermal expansion are given. The shear deformations were calculated using bisectors of the ac parallelogram at different temperatures. The axes of shear deformation are located in the directions of the mutually perpendicular bisectors of ac parallelogram. The electrical conductivity of badalovite is ~ 3–4 times greater than for calciojohillerite in the studied temperature range (350–600 °C).