Metasideronatrite was obtained as the first dehydration product of sideronatrite, Na2Fe(SO4)2(OH)·3H2O, from Sierra Gorda, Chile. The crystal structure of metasideronatrite was solved by direct methods and refined by full-matrix least-squares to R = 0.039, using 574 independent reflections with I > 3.0σ(I). It 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. The crystal-chemical formula derived from this structural study is Na2Fe(SO4)2(OH)·H2O. The backbone of the structure is the same as that in sideronatrite: infinite [Fe3+(SO4)2(OH)]2− chains of interconnected octahedra and tetrahedra parallel to the c axis. These chains are linked primarily by Na atoms to build a 3-dimensional network of strong (Fe-O-S) and weak (Na-O) bonds. Another prominent feature of the structure is the arrangement of distorted (NaO5H2O) octahedra, which alternately share one edge and one face to form columns parallel to the [Fe3+(SO4)2(OH)] chains. Subsidiary intra-chain bonds are provided by H atoms belonging to OH groups shared by adjacent Fe octahedra, and to the unique water molecule shared between two adjacent (NaO5H2O) octahedra. At normal conditions of relative humidity (RH) and temperature (i.e., RH > 60% and T < 40 °C), metasideronatrite rehydrates rapidly to sideronatrite.

The structure solution has allowed us to: (1) investigate the strong relation between sideronatrite and metasideronitrite; (2) elucidate the mechanism involved in the transformation of metasideronatrite into the order/disorder (OD) structure of sideronatrite; and (3) get insight into the stability of this mineral from the valence-matching principle applied to the main structural unit [Fe3+(SO4)2(OH)]2 and Na+ interstitial species. The weak hydrogen bonds and the particular arrangement of the face-sharing adjacent [NaO5(H2O)] octahedra are the main factors affecting the stability of metasideronatrite.

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