- Biagion, Cristian, et al.
The crystal structure of turneaureite, Ca<sub>5</sub>(AsO<sub>4</sub>)<sub>3</sub>Cl, the arsenate analog of chlorapatite and its relationships with the arsenate apatites johnbaumite and svabite
Ingår i: American Mineralogist. - 0003-004X .- 1945-3027. ; 102, s. 1981-1986
- <p>The crystal structure of turneaureite, ideally Ca<sub>5</sub>(AsO<sub>4</sub>)<sub>3</sub>Cl, was studied using a specimen from the Brattfors mine, Nordmark, Värmland, Sweden, by means of single-crystal X-ray diffraction data. The structure was refinedto R1 = 0.017 on the basis of 716 unique reflectios with F<sub>o</sub> > 4σ(F<sub>o</sub>) in the <em>P</em>6<sub>3</sub>/<em>m</em> space group, with unit-cell parameters <em>a</em> = 9.9218(3), <em>c </em>= 6.8638(2) Å, V = 585.16(4) Å<sup>3</sup>. The chemical composition of the sample, determined by electron-microprobe analysis, is (in wt%; average of 10 spot analyses): SO<sub>3</sub> 0.22, P<sub>2</sub>O<sub>5</sub> 0.20, V<sub>2</sub>O<sub>5</sub> 0.01, As<sub>2</sub>O<sub>5</sub> 51.76, SiO<sub>2</sub> 0.06, CaO 41.39, MnO 1.89, SrO 0.12, BaO 0.52, PbO 0.10, Na2O 0.02, F 0.32, Cl 2.56, H<sub>2</sub>O<sub>calc</sub> 0.58, O(≡F+Cl) –0.71, total 99.04. On the basis of 13 anions per formula unit, the empirical formula corresponds to (Ca<sub>4.82</sub>Mn<sub>0.17</sub>Ba<sub>0.02</sub>Sr<sub>0.01</sub>)∑<sub>5.02</sub> (As<sub>2.94</sub>P<sub>0.02</sub>S<sub>0.02</sub>Si<sub>0.01</sub>)<sub>∑2.99</sub>O<sub>12</sub>[Cl<sub>0.47</sub>(OH)<sub>0.42</sub>F<sub>0.11</sub>]<sub>∑1.00</sub>.Turneaureite is topologically similar to the other members of the apatite supergroup: columns of face-sharing M1 polyhedra running along c are connected through TO<sub>4</sub> tetrahedra with channels hosting M2 cations and X anions. Owing to its particular chemical composition, the studied turneaureite can be considered as a ternary calcium arsenate apatite; consequently it has several partially filledanion sites within the anion columns. Polarized single-crystal FTIR spectra of the studied sample indicate stronger hydrogen bonding and less diverse short-range atom arrangements around (OH) groups in turneaureite as compared to the related minerals johnbaumite and svabite. An accurate knowledge of the atomic arrangement of this apatite-remediation mineral represents an improvement in our understanding of minerals able to sequester and stabilize heavy metals such as arsenic in polluted areas.</p>