A contribution to the mineralogy of Sicily (Italy). - Kintoreite from the Tripi mine, Peloritani Mountains: Occurrence and crystal structure
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- Università di Pisa
A new occurrence of kintoreite, ideally PbFe 3(PO 4)(PO 3OH)(OH) 6, is described from the Tripi mine, Ali, Peloritani Mountains, northeastern Sicily, Italy. Kintoreite occurs as yellow equant crystals, up to 0.05 mm in size, on quartz, associated with another alunite-supergroup mineral currently under study. The studied grains of kintoreite are chemically zoned, showing the occurrence of two distinct domains. Electron microprobe analyses gave (in wt%): SO 35.47(27), P 2O 517.55(14), Al 2O 37.81(35), Fe 2O 323.81(44), PbO 35.11(41), H 2Ocalc 9.93, total 99.68 (darker domains) and SO 36.03(5), P 2O 516.66(23), Al 2O 34.22(2), Fe 2O 327.31(6), PbO 33.78(22), H 2O calc10.25, total 98.25 (brighter domains). On the basis of 14 O atoms per formula unit (apfu) and (P+S) = 2 apfu, the darker and brighter domains have chemical formula Pb 1.00(Fe 3+ 1.89A l0.97) Σ2.86[P 1.57S 0.43O 7.43(OH) 0.57][(OH) 5.58(H 2O) 0.42]Σ6.00 and Pb 0.98(Fe 3+ 2.21Al 0.53) Σ2.74[P 1.51S 0.49O 7.49(OH) 0.51][(OH) 5.18(H 2O) 0.82]Σ6.00, respectively. Single-crystal X-ray diffraction was performed on a grain extracted from the same sample. Unit-cell parameters are a = 7.2191(10), c = 16.834(3) A, V = 759.8(3) A 3, Z = 3; space group R-3m. The crystal structure was refined on the basis of 226 unique reflections with F o> 4σ(Fo) and 31 least-square parameters to R1 = 0.0415. Kintoreite is isotypic with other alunite-supergroup minerals. Micro-Raman spectroscopy suggests the possible occurrence of H 2O groups, supporting the possible protonation of (OH) groups as a charge-balance mechanism in this mineral. Kintoreite may play an environmental role in the Ali area, where previous authors detected high concentrations of potentially toxic elements in soils.