title: Towards a revisitation of vesuvianite-group nomenclature: the crystal structure of Ti-rich vesuvianite from Alchuri, Shigar Valley, Pakistan.
abstract: Vesuvianite containing 5.85 wt% TiO2from an Alpine-cleft-type assemblage outcropped near Alchuri, Shigar Valley, Northern Areas, Pakistan, has been investigated by means of electron microprobe analyses, gas-chromatographic analysis of H2O, X-ray powder diffraction, single-crystal X-ray structure refinement,27Al NMR,57Fe Mössbauer spectroscopy, IR spectroscopy and optical measurements. Tetragonal unit-cell parameters are: a = 15.5326 (2), c = 11.8040 (2) Å, space group P4/nnc. The structure was refined to final R1= 0.031, wR2= 0.057 for 11247 I > 2σ(I). A general crystal-chemical formula of studied sample can be written as follows (Z = 2):[8-9](Ca17.1Na0.9)[8]Ca1.0[5](Fe2+0.44Fe3+0.34Mg0.22)[6](Al3.59Mg0.41)[6](Al4.03Ti2.20Fe3+1.37Fe2+0.40) (Si18O68) [(OH)5.84O2.83F1.33]. The octahedral site Y2 is Al-dominant and does not contain transition elements. Another octahedral site Y3 is also Al-dominant and contains Fe2+, Fe3+and Ti. The site Y1 is split into Y1a and Y1b predominantly occupied by Fe2+and Fe3+, respectively. The role of the Y1 site in the diversity of vesuvianite-group minerals is discussed.
authors.affiliation: Department of Crystallography, St Petersburg State University, University Embankment 7/9, St Petersburg 199034, Russian Federation.
authors.forename: Sergey M
authors.initials: SM
authors.lastname: Aksenov
authors.affiliation: Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka 142432, Russian Federation.
authors.forename: Nikita V
authors.initials: NV
authors.lastname: Chukanov
authors.affiliation: Faculty of Physics, Moscow State University, Vorobievy Gory, Moscow 119991, Russian Federation.
authors.forename: Vyacheslav S
authors.initials: VS
authors.lastname: Rusakov
authors.affiliation: Department of Crystallography, St Petersburg State University, University Embankment 7/9, St Petersburg 199034, Russian Federation.
authors.forename: Taras L
authors.initials: TL
authors.lastname: Panikorovskii
authors.affiliation: Institute of Crystallography of Federal Scientific Research Centre `Crystallography and Photonics', Russian Academy of Sciences, Leninskiy Prospekt 59, Moscow 119333, Russian Federation.
authors.forename: Ramiza K
authors.initials: RK
authors.lastname: Rastsvetaeva
authors.affiliation: Kazan Federal University, Kremlyovskaya Street 18, Kazan 420008, Russian Federation.
authors.forename: Ramil R
authors.initials: RR
authors.lastname: Gainov
authors.affiliation: Kazan Federal University, Kremlyovskaya Street 18, Kazan 420008, Russian Federation.
authors.forename: Farit G
authors.initials: FG
authors.lastname: Vagizov
authors.affiliation: Institute of Organoelement Compounds RAS, Vavilova Street 59, Moscow 119991, Russian Federation.
authors.forename: Konstantin A
authors.initials: KA
authors.lastname: Lyssenko
authors.affiliation: Fersman Mineralogical Museum, Russian Academy of Sciences, Leninskiy Prospekt 8-2, Moscow 119071, Russian Federation.
authors.forename: Dmitry I
authors.initials: DI
authors.lastname: Belakovskiy
mesh_terms: D002849:Chromatography, Gas; D018360:Crystallography, X-Ray; D007501:Iron / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D008903:Minerals / Q000032:analysis / Q000737:chemistry*; D010154:Pakistan; D011208:Powders / Q000737:chemistry; D013055:Spectrophotometry, Infrared; D015204:Spectroscopy, Mossbauer; D014025:Titanium / Q000737:chemistry; D014961:X-Ray Diffraction
chemical_list: D008903:Minerals; D011208:Powders; C009495:titanium dioxide; D014025:Titanium; D007501:Iron
keywords: 27Al NMR NMR spectroscopy; IR spectroscopy; Mössbauer spectroscopy; crystal chemistry; vesuvianite