Variations in the chemical composition of lamprophyllite-group minerals and the crystal structure of fluorine-rich barytolamprophyllite from new peralkaline dyke

The variations in the chemical composition of lamprophyllite-group minerals from a peralkaline dyke of the Mokhnatye Roga area (Kandalaksha region, Kola Peninsula), which are crystallized during the entire period of dyke formation and form several generations, have been investigated. The early generations differ in a steadily high fluorine content, while the later ones exhibit reduced amount of fluorine, impurity elements, and sodium, with a simultaneous increase in the potassium content. The crystal structure of fluorine- rich barytolamprophyllite (potentially a new representative of the lamprophyllite group, differing by the predominance of fluorine in the anion X site) has been analyzed by single crystal X-ray diffraction. This mineral is found to have a monoclinic unit cell with the following parameters: a = 19.5219(8) , b = 7.0915(2) , c = 5.3925(2) , beta = 96.628(3)A degrees, and sp. gr. C2/m. The structure is refined to R = 5.73% in the anisotropic approximation of the atomic displacement parameters using 3668I > 2 sigma(I). The idealized formula (Z = 2) is (Ba,Sr)(2)[Na(Na,Fe)(2)(Ti,Mg)F-2][Ti-2(Si2O7)(2)O-2].