X-ray diffraction and spectroscopic study of wiluite: implications for the vesuvianite-group nomenclature

Abstract

Two wiluite samples from the Wiluy River, Yakutia, Russia have been investigated by means of single-crystal and powder X-ray diffraction, electron microprobe analysis, 1H, 27Al, 11B, and 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR), thermogravimetric analysis (DSC/TGA), X-ray photoelectron spectroscopy (XPS) at the Si2p, Ca2p, Al2p, Mg1s, B1s and Fe2p core levels, 57Fe Mossbauer spectroscopy, infrared (IR) spectroscopy and optical measurements. The crystal structures have been refined in the P4/nnc space group [a = 15.7027(3), c = 11.7008(3) A, V = 2885.1(1) A3 for 1 and a = 15.6950(2), c = 11.6787(4) A, V = 2876.9(1) A3 for 2] to R 1 = 0.022 and R 1 = 0.021, respectively. In the crystal structure of wiluite, five-coordinated Y1 site is predominantly occupied by Mg. IR spectra of wiluite substantially different from those of vesuvianite, in particular, by the presence of additional bands in the range 1080‒1415 cm−1, which correspond to symmetric B‒O stretching vibrations of the BO 3 3− and BO 4 5− groups. According to the MAS NMR data, tetrahedrally coordinated T1 site is occupied by B3+ with minor amounts of Al3+. The general formula of wiluite can be written as follows (Z = 2): Ca19Mg(Al,Mg,Fe,Ti,Mn)12(B,Al,◻)5(Si2O7)4(SiO4)10(O,OH)9O2–3. The diversity of vesuvianite-group minerals is largely determined by the population of the Y1 sites. However, wiluite is characterized by the presence of additional T1 and T2 sites and should be considered as special among other vesuvianite-group minerals. The reasonability of subdivision of the wiluite subgroup within the vesuvianite group is discussed.