Sideronatrite: OD structure of the orthorhombic polytype

[Marco E. Ciriotti]

Di prossima pubblicazione.

Referenza:
• Scordari, F. & Ventruti, G. (2009): Sideronatrite, Na2Fe(SO4)2(OH)3H2O: crystal structure of the orthorhombic polytype and OD character analysis. American Mineralogist, 94, (in press).

Qui un breve sommario da Plinius:

""Sideronatrite is a secondary hydrated sulfate of sodium and ferric iron occurring in nature as a result of pyrite alteration. For this reasons it is one of the environmental indicators of soil-water processes operating in specific landscapes, and, such as, an important marker of acid mine drainage pollution. This mineral is quoted in literature as orthorhombic with space group Pnnm or Pnn2 [1]. Scordari [2], analyzing a sample from desertic sulfate deposits of Northern Chile, first recognized the OD character of the structure on the basis of its peculiar diffraction pattern (coexistence of sharp spot for l = 2n and diffuse streaks along b* for l = 2n+1). The small dimensions and the extensive disorder shown by this
sample hampered to date a complete structural determination.
In this work, a crystal with very weak diffuse streaks proved to be suitable for a single crystal X-ray diffraction study. The crystal structure has been solved by direct methods and refined in the space group P212121 with a = 7.265(2), b = 20.522 (6), c = 7.120 (2) Å; V = 1061.5(5) Å3; Z = 4, by full matrix least-squares (R = 8.4% and RW = 9.3%) using 810 reflections with I > 3.0 σ(I).
Sideronatrite is characterized by infinite [Fe3+(SO4)2(OH)] octahedral-tetrahedral chains of the type [M(TO4)2φ] running parallel to the c axis. The same topology is also present in guildite [2, 3]. These chains are cross-linked by a chains system of corner-sharing Na distorted octahedra along c to form corrugated sheets parallel to the (010) plane. Adjacent sheets are hydrogen-bonded through water molecules. The weak hydrogen bonds, the easy coming in/out of two water molecules seem to be the main factors which affect the stability of the sideronatrite making easy and reversible the transformation of sideronatrite into metasideronatrite and vice versa.
The OD character will be discussed in the light of the structural results, i.e., the peculiar disorder and the building layer and the possible MDO polytypes.

[1] Strunz, H. & Nichel, E.H. (2001): Strunz mineralogical tables. Chemical structural mineral classification system, 9th ed. Schweizerbart’sche, Stuttgart, 870 p.
[2] Scordari, F. (1981): Tschermaks Mineral. Petrogr. Mitt., 28, 315-319.
[3] Wan, C., Ghose, S., Rossman, G.R. (1978): Am. Mineral., 63, 478-483.""

[Marco E. Ciriotti]

Referenza:
• Scordari, F. & Ventruti, G. (2009): Sideronatrite, Na2Fe(SO4)2(OH)·3H2O: crystal structure of the orthorhombic polytype and OD character analysis. American Mineralogist, 94, 1679-1686.

Abstract:
Sideronatrite, Na2Fe(SO4)2(OH)·3H2O, is a secondary hydrated sulfate occurring in desert areas as the result of pyrite alteration. It is one of the environmental indicators of soil-water processes operating in specific landscapes, and, as a consequence, an important marker of acid mine drainage pollution. Sideronatrite has been demonstrated from its peculiar diffraction pattern to belong to a family of OD structures formed by equivalent layers.
In this work, a crystal with weak diffuse streaks proved to be suitable for a single-crystal X-ray diffraction study. The crystal structure was solved by direct methods and refined by full matrix least-squares (R = 7.4% and RW = 8.0%) in the space group P212121 with a = 7.265(2), b = 20.522(6), c = 7.120(2) Å, V = 1061.5(5) Å3, and Z = 4, using 798 reflections with I > 3.0 (I).
Sideronatrite is characterized by infinite [Fe3+(SO4)2(OH)]2– octahedral-tetrahedral chains of the type [M(TO4)2] running parallel to the c axis. These chains are cross-linked by a columnar system of corner-sharing, Na-distorted octahedra along c to form corrugated sheets parallel to the (010) plane. Adjacent sheets are hydrogen-bonded through water molecules coordinated by Na atoms. The present results allow a complete description of the OD character of the structure, with the derivation of the OD groupoid and MDO polytypes.
Finally, chemical and structural relationships are taken into account to explain the possible paragenetic sequence concerning several sulfates associated with sideronatrite.