Structure of the new mineral paratsepinite-Na and its place in the labuntsovite group

X-ray diffraction study of high-strontium Ti-enadkevichite showed that it is a new mineral, which was given the name paratsepinite-Na, sp. gr. C2/m, a double value of the parameter c, and a different distribution of the non-framework cations in comparison with tsepinite-Na. In particular, strontium is located not only in the hexagonal window of the small channel, but also in three positions of the large channel. The characteristics of the mosaic blocks constituting the crystal are studied. It is assumed that polysynthetic twinning in these crystals and many other representatives of this family is associated with the phase transformation occurring in this hydrothermal mineral during its cooling after formation. The comparison of all the three structures of the vuoriyarvite subgroups of the labuntsovite group allows us to explain the disability of vuoriyarvite to absorb strontium from the aqueous solution by its smaller unit-cell volume and higher framework charge in comparison with those of tsepinites.