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.