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Natisite, a natural tetragonal (P4/nmm) modification of Na2TiSiO5 = Na2TiO(SiO4) is an abundant indicator mineral of relatively low-temperature (no higher than 300–350°C) peralkaline (hyperagpaitic) hydrothermal assemblages in the Lovozero and Khibiny alkaline plutons, Kola Peninsula, Russia. The proportion of natisite in hypersodic (with Na K) hydrothermal veins in Lovozero and in the hydrothermally altered zones of potassium-rich pegmatites in Khibiny is up to 30 and 10 vol %, respectively; i.e., natisite can be a rock-forming mineral and the major Ti concentrator in the rock. In Lovozero, natisite is a primary mineral crystallized from hydrothermal solution, whereas in Khibiny, natisite is secondary: it predominantly occurs as pseudomorphs after earlier Ti minerals, namely, ilmenite, titanite, aenigmatite, lamprophyllite and rinkite. The chemical composition of natisite is rather stable, close to Na2TiSiO5. A very bright blue to bluish white fluorescence in shortwave ultraviolet light (λ = 245 nm) is an important identification feature of natisite. The crystal structure of natisite was first solved for a natural sample (Mt. Rasvumchorr, Khibiny), R1 = 1.97%. A comparative study of natisite and other titanosilicates with tetragonal pyramids TiO5 revealed a distinct negative correlation between the length of an essentially covalent Ti–O bond in the titanyl group and the frequency of its stretching vibrations in the IR spectrum. [ABSTRACT FROM AUTHOR]

The crystal structure of lemanskiite is determined for the first time (R = 0.019) and the mineral is redefined. Its chemical formula, crystal system, space group and unit-cell parameters are revised. The powder X-ray diffraction pattern of lemanskiite has been correctly indexed; the IR spectrum of an impurity-free sample has been obtained. It has been shown that the mineral is not a dimorph of lavendulan NaCaCu5(AsO4)4Cl · 5H2O and differs from the latter by a lower H2O content. The studied sample was from the oxidation zone of the Perseverancia deposit, Guanako, Antofagasta, Chile. It empirical formula is Na0.98(Ca0.98Sr0.03)Σ1.01Cu5.07As3.97O15.97Cl1.03 · 3H2O; the idealized formula is NaCaCu5(AsO4)4Cl · 3H2O. The mineral is monoclinic, P21/m, a = 9.250(2), b = 10.0058(10), с = 10.0412(17) Å, β = 97.37(3)°, V = 921.7(3) Å3, Z =2. Lemanskiite represents a new structure type in the lavendulan group. Its structure is based on heteropolyhedral layers composed of clusters consisting of four edge-shared distorted Cu-centered tetragonal pyramids joined with eight AsO4 tetrahedra, which are also linked with a Cu-centered plane squares not involved in the clusters. Interlayer edge-shared Na-centered trigonal prisms and Ca sevenfold polyhedra are linked with the heteropolyhedral layers from both sides. [ABSTRACT FROM AUTHOR]