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Fluorine-rich vesuvianite from Borborema province, Brazil

Investigated crystals were collected from a skarn deposit with molybdenite and F-apophyllite in paragenesis in Sítio São Gonçalo, Santa Luzia, Borborema Province, state of Paraiba, Brazil (coordinates: S 06º54.916´ W 036º50.478´). During this investigation calcite, pyrite, zircone and molybdenite were noticed. Crushing of samples releases H2S. There is a record of vesuvianite crystals from this province in literature [1]. Crystals are well developed, but placed as fragments in breccia and often zoned. Several habits can be distinguished: shorter prismatic where {; ; ; ; 001}; ; ; ; is dominant form and are lavender-grey in colour ; elongated with smaller {; ; ; ; 001}; ; ; ; which are Ipê yellow. Purple – orange crystal’s habit is in between the shorter prismatic and elongated habits. Colourless or almost colourless crystals are minute and covered with calcite crystals. They are probably the latest. Goniometric measurements on 8 samples exhibit following 30 forms: {; ; 001}; ; and {; ; 110}; ; as dominant forms, {; ; 100}; ; , {; ; 450}; ; , {; ; 320}; ; , {; ; 210}; ; , {; ; 310}; ; , {; ; 410}; ; , {; ; 510}; ; , {; ; 102}; ; , {; ; 203}; ; , {; ; 101}; ; , {; ; 201}; ; , {; ; 1.1.10}; ; , {; ; 113}; ; , {; ; 111}; ; , {; ; 443}; ; , {; ; 221}; ; , {; ; 331}; ; , {; ; 10.10.3}; ; , {; ; 551}; ; , {; ; 313}; ; , {; ; 414}; ; , {; ; 312}; ; , {; ; 211}; ; , {; ; 311}; ; , {; ; 511}; ; , {; ; 223}; ; , {; ; 421}; ; and {; ; 955}; ; ; ; which appears very rarely on vesuvianite crystals. Axis ratio calculations yield a:c = 1:0.5373 which is very similar to ratio given by Goldschmidt (a:c=1:0.5376 ; [2]). X-ray powder diffraction data of investigated samples show following unit cell parameters: for lavender-grey - a = 15.512(2) Å, c = 11.786(3) Å, V = 2835.8(7) Å3 ; for Ipê yellow - a = 15.598(1) Å, c = 11.784(2) Å, V = 2833.9(5) Å3. Annealing at 1025°C results in decomposition to gehlenite and wollastonite type structures with the remnants of vesuvianite structure. Due to annealing the following loss of water occurs: for lavender-grey 2.39 wt. % ; for Ipê yellow 3.09 wt. % ; Microprobe analysis performed on a lavender-grey sample, calculated on the basis of Ca+Na = 19.00 apfu, yield following formula: (Ca18.86Na0.14)SUM=19.00(Al10.30Mg1.23Fe1.34Ti0.16Mn0.04)SUM=13.08(SiO4)10(Si2O7)4(OH4.75F3.38 O3.10)SUM=11.23. According to [3], a mineral can be classified as a fluorvesuvianite if containing F> 4.5 apfu. Thus, this sample is a fluorine-rich vesuvianite.

fluorine; vesuvianite; morphology; Borborema province; Brazil

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