Minerals of the apatite supergroup have the general formula M12M23(TO4)3X. Within the apatite supergroup, the triad svabite – johnbaumite – turneaureite represents one of the rare cases in which three natural end-members are known corresponding to three different X– anions, i.e., svabite, Ca5(AsO4)3F, johnbaumite, Ca5(AsO4)3(OH), and turneaureite, Ca5(AsO4)3Cl. The only other known case is represented by the eponymous series of calcium phospates fluorapatite – hydroxylapatite – chlorapatite (Pasero et al., 2010). We present here the results of a combined chemical (electron microprobe) and structural (single crystal Xray diffraction) study of turneaureite, which follows similar studies carried out on johnbaumite (Biagioni & Pasero, 2013) and svabite (Biagioni et al., 2016). This allows us to make a comparative analysis of the crystal-chemical features of the three calcium arsenate minerals with the apatite structure. The studied sample of turneaureite comes from Nordmark, Värmland, Sweden. Electron microprobe analyses resulted in the following empirical formula: (Ca4.82Mn0.17Ba0.02Sr0.01)(As2.94P0.02S0.02Si0.01)O12 [Cl0.47(OH)0.42F0.11]. The crystal structure of turneaureite was refined in the space group P63/m to R1 = 0.0180, wR2 = 0.0475 for 716 independent reflections, with a = 9.9218(3), c = 6.8638(2) Å. The crystal structure of turneaureite was compared with those of johbaumite (sample from the Jakobsberg mine) and of svabite (sample from the Harstigen mine). The samples of all three minerals originate from three neighbouring Långban-type deposits in the county of Värmland. The geometry of the coordination polyhedra is similar in these three minerals of the savbite subgroup. M1Ca atoms are nine-fold coordinated in tri-augmented trigonal prisms, M2Ca2 atoms are sevenfold coordinated in pentagonal bipyramids, and TAs atoms are tetrahedrally coordinated. Also average bond distances are quite similar in the three structures (johnbaumite, svabite, turneaureite, in Å: <M1-O> = 2.586, 2.586, 2.579; <M2-O> = 2.480, 2.485, 2.529; <T-O> = 1.671, 1.674, 1.678). There is, however, a relevant difference in the z fractional coordinate of X– anionic position (0, 0, z). In turneaureite chlorine lies at z = 0.319, with an additional mixed-occupancy (OH/F/Cl) site at z = 1/4. In johnbaunite the hydroxyl group lies at z = 0.216, with no additional site. In svabite fluorine lies at z = 1/4, with an additional, mixed-occupancy (F/OH) site at z = 0.195. This results in different Ca2–X bond distances in the three minerals, in keeping with the variable nature of the X– anion.

Crystal structure of turneaureite and crystal-chemical relationships among the minerals of the svabite subgroup (apatite supergroup)

BIAGIONI, CRISTIAN;PASERO, MARCO
2016-01-01

Abstract

Minerals of the apatite supergroup have the general formula M12M23(TO4)3X. Within the apatite supergroup, the triad svabite – johnbaumite – turneaureite represents one of the rare cases in which three natural end-members are known corresponding to three different X– anions, i.e., svabite, Ca5(AsO4)3F, johnbaumite, Ca5(AsO4)3(OH), and turneaureite, Ca5(AsO4)3Cl. The only other known case is represented by the eponymous series of calcium phospates fluorapatite – hydroxylapatite – chlorapatite (Pasero et al., 2010). We present here the results of a combined chemical (electron microprobe) and structural (single crystal Xray diffraction) study of turneaureite, which follows similar studies carried out on johnbaumite (Biagioni & Pasero, 2013) and svabite (Biagioni et al., 2016). This allows us to make a comparative analysis of the crystal-chemical features of the three calcium arsenate minerals with the apatite structure. The studied sample of turneaureite comes from Nordmark, Värmland, Sweden. Electron microprobe analyses resulted in the following empirical formula: (Ca4.82Mn0.17Ba0.02Sr0.01)(As2.94P0.02S0.02Si0.01)O12 [Cl0.47(OH)0.42F0.11]. The crystal structure of turneaureite was refined in the space group P63/m to R1 = 0.0180, wR2 = 0.0475 for 716 independent reflections, with a = 9.9218(3), c = 6.8638(2) Å. The crystal structure of turneaureite was compared with those of johbaumite (sample from the Jakobsberg mine) and of svabite (sample from the Harstigen mine). The samples of all three minerals originate from three neighbouring Långban-type deposits in the county of Värmland. The geometry of the coordination polyhedra is similar in these three minerals of the savbite subgroup. M1Ca atoms are nine-fold coordinated in tri-augmented trigonal prisms, M2Ca2 atoms are sevenfold coordinated in pentagonal bipyramids, and TAs atoms are tetrahedrally coordinated. Also average bond distances are quite similar in the three structures (johnbaumite, svabite, turneaureite, in Å: = 2.586, 2.586, 2.579; = 2.480, 2.485, 2.529; = 1.671, 1.674, 1.678). There is, however, a relevant difference in the z fractional coordinate of X– anionic position (0, 0, z). In turneaureite chlorine lies at z = 0.319, with an additional mixed-occupancy (OH/F/Cl) site at z = 1/4. In johnbaunite the hydroxyl group lies at z = 0.216, with no additional site. In svabite fluorine lies at z = 1/4, with an additional, mixed-occupancy (F/OH) site at z = 0.195. This results in different Ca2–X bond distances in the three minerals, in keeping with the variable nature of the X– anion.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11568/833915
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