Crystal chemistry, Mossbauer spectroscopy, and thermodynamic properties of botryogen

Botryogen, nominally MgFe(SO4)(2)(OH)center dot 7H(2)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 (Mg0.773Zn0.165Mn0.047)Fe-1.010(3+)(SO4)(2)(OH)center dot 7H(2)O, with molecular mass of 423.831 g.mol(-1). The parameters of the two doublets in the Mossbauer 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 Fe3+ high spin. The structure of botryogen has been refined to wR (all) = 0.0760. It consists of [Fe3+(SO4)(2)(OH)(H2O)](2-) chains that are cross-linked by Mg phi(6) polyhedra. We have also determined the positions of the H atoms. There are three transformer H2O groups and two non-transformer H2O with three-and four-fold coordinated O atoms, respectively. Upon heating, the structure of botryogen is monotonously expanding up to similar to 100 degrees 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 similar to 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 similar to 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 -3819.0 +/- 2.9 kJ.mol(-1). The equilibrium constant for the reaction (and the composition given above) MgaZnbMncFed(SO4)(2)(OH)center dot 7H(2)O + H+ = aMg(2+) + bZn(2+) + cMn(2+) + dFe(3+) + 2SO(4)(2-) + 8H(2)O at T = 298.15 K, that is, the solubility product, is log K = -5.34 +/- 0.64.