Dobrovolskyite, Na4Ca(SO4)3, is a new sulfate mineral from the Great Tolbachik fissure eruption, Kamchatka peninsula, Russia. It occurs as aggregates of tabular crystals up to 1–2 mm in maximum dimension, with abundant gas inclusions. The empirical formula calculated on the basis of O = 12 is (Na3.90K0.10)Σ4(Ca0.45Mg0.16Cu0.12Na0.10)Σ0.83S3.08O12. The crystal structure of dobrovolskyite was determined using single-crystal X-ray diffraction data as: trigonal, R3, a = 15.7223(2), c = 22.0160(5) Å, V = 4713.1(2) Å3, Z = 18 and R1 = 0.072. The Mohs’ hardness is 3.5. The mineral is uniaxial (+), with ω = 1.489(2) and ɛ = 1.491(2) (λ = 589 nm). The seven strongest lines of the powder X-ray diffraction pattern [d, Å (I, %)(hkl)] are: 11.58(40)(101); 5.79(22)(202); 4.54(18)(030); 3.86(88)(033); 3.67(32)(006); 2.855(50)(306); and 2.682(100)(330). The mineral is named in honour of Prof. Dr. Vladimir Vitalievich Dolivo-Dobrovolsky (1927–2009), one of the leading Russian scientists in the field of petrology, crystal optics and crystal chemistry. The crystal structure of dobrovolskyite can be described as composed of three symmetrically independent rods running parallel to the c axis. The rods consist of six octahedral–tetrahedral [Na(SO4)6]11– or [Ca(SO4)6]10– clusters of central octahedra sharing common corners with six adjacent SO4 tetrahedra. Alternatively, the crystal structure of the mineral can be described as a 12-layer ABACABACABAC eutactic array of Na+ and Ca2+ cations, and vacancies with disordered (SO4) tetrahedra in interstices. Dobrovolskyite and similar minerals probably formed upon cooling of a high-temperature phase with disordered cation and anion arrangements.

Petrovite, Na10CaCu2(SO4)8, is a new sulfate mineral discovered on the Second scoria cone of the Great Tolbachik fissure eruption. The mineral occurs as globular aggregates of tabular crystals up to 0.2 mm in maximal dimension, generally with gaseous inclusions. The empirical formula calculated on the basis of O = 32 is Na6(Na1.80K0.20)Σ2Na(Ca0.82Na0.06Mg0.02)Σ0.90(Cu1.84Mg0.16)Σ2(Na0.52□0.48)Σ1S8.12O32. The crystal-chemical formula is CuNa6−2xCax(SO4)4, which, for x ≈ 0.5, results in the idealised formula Na10CaCu2(SO4)8. The crystal structure of petrovite was determined using single-crystal X-ray diffraction data; the space group is P21/c, a = 12.6346(8), b = 9.0760(6), c = 12.7560(8) Å, β = 108.75(9)°, V = 1385.1(3) Å3, Z = 2 and R1 = 0.051. There are one Cu and six Na sites, one of which is also occupied by the essential amount of Ca. The Cu atom forms five Cu–O bonds in the range 1.980–2.180 Å and two long bonds ≈ 2.9 Å resulting in the formation of the CuO7 polyhedra, which share corners with SO4 tetrahedra to form isolated [Cu2(SO4)8]12− clusters. The clusters are surrounded by Na sites, which provide their linkage into a three-dimensional framework. The Mohs’ hardness is 4. The mineral is biaxial (+), with α = 1.498(3), βcalc = 1.500, γ = 1.516(3) and 2V = 20(10) (λ = 589 nm). The seven strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 7.21(27)(110); 6.25(38)(102); 4.47(31)(212); 3.95(21)(30$\bar{2}$); 3.85(17)(121); 3.70(36)(202); and 3.65(34)(22$\bar{1}$). The mineral is named in honour of Prof Dr Tomas Georgievich Petrov (b. 1931) for his contributions to mineralogy and crystallography and, in particular, for the development of technology for the industrial fabrication of jewellery malachite.