Crystal chemistry, Mössbauer spectroscopy, and thermodynamic properties of botryogen
Majzlan, Juraj; Plášil, Jakub; Dachs, Edgar; Benisek, Artur; Bender Koch, Christian
published: Apr 1, 2016
ArtNo. ESP154019302005, Price: 29.00 €
Botryogen, nominally MgFe(SO4)2 (OH) ˙ 7H2 O, is a mineral of some oxidation zones and gossans, missing in other ones. Here, we attempted to investigate the mineral by several methods; once sufficient data are collected for other (Mg)-Fe sulfates, the results of this work could answer the question as to what are the patterns of occurrence of botryogen. The sample from Nuevo Cuyo, Argentina, has the chemical composition (Mg0.773 Zn0.165 Mn0.047)Fe3+ 1.010(SO 4)2(OH) ˙ 7H2O, with molecular mass of 423.831 g˙mol–1. The parameters of the two doublets in the Mössbauer spectrum are: isomer shifts of 0.41 and 0.39 mm˙s–1, quadrupole splittings of 1.16 and 1.62 mm˙s–1, line widths of 0.26 and 0.24 mm˙s–1, and relative spectral areas of 54.6 and 45.4 %, respectively. The data indicate two different, well defined coordination sites in the sample for Fe 3+ high spin. The structure of botryogen has been refined to wR (all) = 0.0760. It consists of [Fe3+ (SO4)2 (OH)(H2 O)]2– chains that are cross-linked by Mg6 polyhedra. We have also deter- mined the positions of the H atoms. There are three transformer H2O groups and two non-transformer H2 O with three- and four-fold coordinated O atoms, respectively. Upon heating, the structure of botryogen is monotonously expanding up to ~ 100 °C and then it collapses into X-ray amorphous state. Using acid-solution calorimetry, we determined the enthalpy of formation of botryogen as –4488.4 ± 2.3 kJ˙mol–1. The heat capacity (C p) data show a weak anomaly, centered at ~4.5 K, possibly corresponding to a magnetic transition in the structure of botryogen. Surprisingly, C p of botryogen levels off at a value of 0.4 J˙mol–1 ˙K –1 at a temperature of ~2 K. There may be another phase transition and an associated C p anomaly at these low temperatures. Using relaxation calorimetry, we determined the standard entropy at T = 298.15 K as 501.3 ± 6.0 J˙mol –1 ˙K –1. The Gibbs free energy of formation of the botryogen (composition given above) is –19.0 ± 2.9 kJ˙mol–1. The equilibrium constant for the reaction (and the composition given above) Mga Znb Mnc Fed (SO4)2 (OH) ˙ 7H2 O + H+ = aMg2+ + bZn 2+ + cMn 2+ + dFe 3+ + 2SO4 2– + 8H2 O at T = 298.15 K, that is, the solubility product, is log K = –5.34 ± 0.64.