According to [1] metasideronatrite [Na2Fe3+(SO4)2(OH) _H2O] - the first dehydration product of sideronatrite [Na2Fe3+(SO4)2(OH)_3H2O] - is orthorhombic, space group Pbnm, with a = 7.3959(8), b = 16.0979(15), c = 7.1607(8)Å, V = 852.5(2) Å3, Z = 4. Following [2] sideronatrite may present two OD polytypes: sideronatrite-2O, P212121, a = 7.265, b = 7.120, and c = 20.522 Å, V = 1061.5 Å3; and sideronatrite-2M, P21/c, a = 7.265, b = 7.120, c = 20.828 Å, _ = 99.84°, V = 1061.5 Å3. Nevertheless, the monoclinic ordered polytype has not been described so far and the structural model is only based on OD theory [3]. An exploratory study of samples coming from the dumps of the Me_ica mines, Me_ica (Slovenia), revealed the presence of a new polytype of metasideronatrite, along with intermixed crystals of d’ansite-(Fe) (Na21Fe2+(SO4)10Cl3) and d’ansite-(Mg) (Na21Mg(SO4)10Cl3). A quick indexing of the cell results in a monoclinic symmetry with a = 7.416, b = 7.1440, c = 32.237 Å, _ = 96.57°, V = 1696.8 Å3, thus a sort of super cell of metasideronatrite. Careful inspection of the reciprocal space showed the presence of a twin [100](001) with a = 7.423(2), b = 7.151(3), c = 8.469(3) Å, _ = 109.06(4)°, V = 424.9(2) Å3. Intensity data was obtained in a thin flake (ca. 30 _m thick) showing no streaks along c* and the structure was solved with direct methods in the space group P21/m and refined by full matrix least-squares (R = 5.6% and RW 2 = 11.3% on 935 reflections with resolution of 0.77 Å-1 from both twin elements, twin ratio 0.458(3):0.542). The observed topology contains infinite [Fe3+(SO4)2(OH)]2– octahedral-tetrahedral chains of the type [M(TO4)2_] with a 7.15 Å repeat running parallel to the b axis. Columns of NaO5(H2O) polyhedra, running also parallel to b axis, complete corrugated (001) slabs, linked just by strong hydrogen bonds [Ow-H...O2 = 2.78(1) Å, Ow-H-O2 = 167.6(5)°]. This is in agreement with the observed {001} cleavage. The observed slabs and geometry of polyhedra match those observed in orthorhombic metasideronatrite by [1]. The main difference is the alternation of the slabs in the orthorhombic polytype upside up and upside down, where in the monoclinic polytype all the slabs point in the same direction. Thus the observed topology can be related to the theoretical natrosiderite-2M proposed by [2], by removal of two (H2O) groups. Incidentally, it is close related to the basic OD layer crystallographic parameters described by [2], i.e.: a = 7.265 and b = 7.120, and symmetry defined by the layer group P21/m, while the width c0 = 10.261 Å is reduced by the different staking sequence. Na-Na distance is 3.257(4) Å, slightly shorter than in the orthorhombic polytype. The finding of the monoclinic polytype of metasideronatrite supports the monoclinic model for sideronatrite and its formation by hydration of metasideronatrite-1M is under investigation. [1] Ventruti, G., Stasi, F., Scordari, F. (2010) Am. Mineral. 95, 329–334. [2] Scordari, F. & Ventruti, G. (2009) Am. Mineral. 94, 1679–1686. [3] Dornberger-Schiff, K. and Fichtner, K. (1972) Kristall und Technik, 7, 1035–1056.
Metasideronatrite-1M, Na2Fe(SO4)2(OH)∙H2O: a new polytype from Mežica (Slovenia).
CAMARA ARTIGAS, Fernando;
2012-01-01
Abstract
According to [1] metasideronatrite [Na2Fe3+(SO4)2(OH) _H2O] - the first dehydration product of sideronatrite [Na2Fe3+(SO4)2(OH)_3H2O] - is orthorhombic, space group Pbnm, with a = 7.3959(8), b = 16.0979(15), c = 7.1607(8)Å, V = 852.5(2) Å3, Z = 4. Following [2] sideronatrite may present two OD polytypes: sideronatrite-2O, P212121, a = 7.265, b = 7.120, and c = 20.522 Å, V = 1061.5 Å3; and sideronatrite-2M, P21/c, a = 7.265, b = 7.120, c = 20.828 Å, _ = 99.84°, V = 1061.5 Å3. Nevertheless, the monoclinic ordered polytype has not been described so far and the structural model is only based on OD theory [3]. An exploratory study of samples coming from the dumps of the Me_ica mines, Me_ica (Slovenia), revealed the presence of a new polytype of metasideronatrite, along with intermixed crystals of d’ansite-(Fe) (Na21Fe2+(SO4)10Cl3) and d’ansite-(Mg) (Na21Mg(SO4)10Cl3). A quick indexing of the cell results in a monoclinic symmetry with a = 7.416, b = 7.1440, c = 32.237 Å, _ = 96.57°, V = 1696.8 Å3, thus a sort of super cell of metasideronatrite. Careful inspection of the reciprocal space showed the presence of a twin [100](001) with a = 7.423(2), b = 7.151(3), c = 8.469(3) Å, _ = 109.06(4)°, V = 424.9(2) Å3. Intensity data was obtained in a thin flake (ca. 30 _m thick) showing no streaks along c* and the structure was solved with direct methods in the space group P21/m and refined by full matrix least-squares (R = 5.6% and RW 2 = 11.3% on 935 reflections with resolution of 0.77 Å-1 from both twin elements, twin ratio 0.458(3):0.542). The observed topology contains infinite [Fe3+(SO4)2(OH)]2– octahedral-tetrahedral chains of the type [M(TO4)2_] with a 7.15 Å repeat running parallel to the b axis. Columns of NaO5(H2O) polyhedra, running also parallel to b axis, complete corrugated (001) slabs, linked just by strong hydrogen bonds [Ow-H...O2 = 2.78(1) Å, Ow-H-O2 = 167.6(5)°]. This is in agreement with the observed {001} cleavage. The observed slabs and geometry of polyhedra match those observed in orthorhombic metasideronatrite by [1]. The main difference is the alternation of the slabs in the orthorhombic polytype upside up and upside down, where in the monoclinic polytype all the slabs point in the same direction. Thus the observed topology can be related to the theoretical natrosiderite-2M proposed by [2], by removal of two (H2O) groups. Incidentally, it is close related to the basic OD layer crystallographic parameters described by [2], i.e.: a = 7.265 and b = 7.120, and symmetry defined by the layer group P21/m, while the width c0 = 10.261 Å is reduced by the different staking sequence. Na-Na distance is 3.257(4) Å, slightly shorter than in the orthorhombic polytype. The finding of the monoclinic polytype of metasideronatrite supports the monoclinic model for sideronatrite and its formation by hydration of metasideronatrite-1M is under investigation. [1] Ventruti, G., Stasi, F., Scordari, F. (2010) Am. Mineral. 95, 329–334. [2] Scordari, F. & Ventruti, G. (2009) Am. Mineral. 94, 1679–1686. [3] Dornberger-Schiff, K. and Fichtner, K. (1972) Kristall und Technik, 7, 1035–1056.
I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
