Sideronatrite - Na2Fe3+(SO4)2(OH)•3(H2O) - is a secondary iron-bearing, hydrous sulfate formed during oxidation of pre-existing sulfides. It is a typical sedimentary product in some soils and evaporatic deposits. Sideronatrite is formed in acid, sulfate-rich environments, and plays an important role in the acid drainage mobilization of metals and in the monitoring of water quality. The dehydration behavior of sideronatrite from Sierra Gorda (Chile) was investigated by in situ high-T FTIR spectroscopy. The structural variations induced by thermal treatment were monitored by FTIR using the bending (2) mode of the water molecule around 1650 cm-1, the stretching (3 and 1) modes around 3580 and 3420 cm-1, and the corresponding combinations modes in the wavenumber region 4000-6000 cm-1. The reaction sequence that describes the thermal decomposition of sideronatrite in the range RT-400°C , is the following: sideronatrite → metasideronatrite I → metasideronatrite II → Na3Fe(SO4)3 Na2Fe(SO4)2(OH)∙3H2O Na2Fe(SO4)2(OH)∙H2O Na2Fe(SO4)2(OH) Fe - Na - SO4 During the transformation sideronatrite → metasideronatrite I (Ventruti et al. 2010), only two interstitial water molecules, coordinated by the Na atoms, are lost, while the characteristic framework of the structure of sideronatrite, i.e. the [Fe3+(SO4)2(OH)]∞ chain, remains unchanged both in metasideronatrite I and in metasideronatrite II structures. The loss of OH- group corner-sharing between two adjacent Fe3+-octahedra in sideronatrite chain, involves the break down of the [Fe3+(SO4)2(OH)]∞ chains and the subsequent formation of an intermediate amorphous transitional phase from which the Na3Fe(SO4)3 compound will form later. The first and second reaction (i.e. sideronatrite to metasideronatrite I and metasideronatrite I to metasideronatrite II) are certainly topotactic, whereas the formation of Na3Fe(SO4)3 is a typical nucleation and growth reaction process which originates from metasideronatrite II decomposition through the following reaction: 6Na2Fe(SO4)2OH →4 Na3Fe(SO4)3 + Fe2O3+ 3H2O. The presence of Fe2O3 in the observed diffraction pattern is thus justified as a co-product of the decomposition process. In the recrystallization of Na3Fe(SO4)3 from the amorphous phase, the OH– group is replaced by a (SO4)2– group according to: OH– → (SO4)2– + Na+ to preserve the electro-neutrality of the structure. Such (SO4)2– group, together with two bridging (SO4)2– tetrahedral, already present in sideronatrite chain, provides a further linkage between two consecutive Fe3+-octahedra in Na3Fe(SO4)3 (Fig. 1), to build up infinite [Fe3+(SO4)3] chains, which extend along the c axis.
Ventruti, G., Bellatreccia, F., DELLA VENTURA, G., Scordari, F. (2011). In situ, high-T FTIR spectroscopy of sideronatrite: Na2Fe3+(SO4)2(OH)•3(H2O), 4, 322.