LOCAL STRUCTURE OF Ba- AND F-DOMINANT MINERALS OF THE LAMPROPHYLLITE STRUCTURAL FAMILY (BASED ON VIBRATIONAL SPECTROSCOPY DATA)

To study the features of the local structure of minerals in the lamprophyllite group, vibrational spectroscopy methods (IR and Raman) were used to investigate holotype fluorine-dominant representatives of this group (fluorobarytolamprophyllite, lileyite, and emmerichite), as well as the related mineral schüllerite. It has been shown that an increase in iron content at the L-site (L = Ti, Fe³⁺) with a coordination number of 5 is accompanied by elongation of the titanyl bond Ti=O and a decrease in the frequency of symmetric stretching vibrations of Ti=O–M1. In the case of lileyite, the decrease in the frequency of these vibrations may also be associated with partial substitution of Na at the M1-site by Ca. It has been established that the most intense band of apical Si–O stretching vibrations in the Raman spectrum of lileyite, unlike the spectra of other minerals, is split into components at 867 and 898 cm⁻¹. The most probable cause of this splitting is the mixed occupancy of the adjacent M2-site, where 55% of the occupants are divalent cations (predominantly Ca²⁺ with admixtures of Fe²⁺ and Mn²⁺).