Crystal structure, thermodynamic properties, and paragenesis of bukovskyite, Fe₂(AsO₄)(SO₄)(OH)·9H₂O

摘要

Bukovskyite is a relatively rare secondary ferric arsenate-sulfate. At the type locality near the municipality of Kutná Hora (Czech Republic), it is the main secondary mineral in the medieval dumps, where it occurs in enormous amounts and forms nodules of prodigious dimensions. We investigated the mineral bukovskyite and the type locality in detail to understand the abundance of the mineral at this locality. The crystal structure of bukovskyite was solved for bukovskyite crystals from Gro?voigtsberg (Germany) and found to be of the space group P 1 with a final R factor of 5.08% from 2403 reflections. The lattice parameters at room temperature are a = 7.549(1) ?, b = 10.305(1) ?, c = 10.914(2) ?, α = 115.136(3)°, β = 99.798(3)°, and γ = 92.864(3)°. The structure consists of octahedral-tetrahedral Fe-arsenate chains. Sulfate tetrahedra are bonded to the chains and free H_(2)O molecules via a complicated network of hydrogen bonds. Calorimetric measurements (acid-solution calorimetry atT = 298.15 K and relaxation calorimetry yielded heat capacities from T = 0.4 K to 300 K) gave an enthalpy of formation of -4742.4 ± 3.8 kJ·mol~(-1) and standard entropy of 615.2 ± 6.9 J·mol~(-1)·K~(-1). A combination of these values gives a Gibbs free energy of formation of -3968.9 ± 4.3 kJ·mol~(-1) and aqueous solubility product (log K ) of -30.627. Bukovskyite is metastable with respect to scorodite; if scorodite is not considered in the thermodynamic calculations, a stability field of bukovskyite appears at low pH and high sulfate and arsenate activity. Field observations showed that bukovskyite occurs in dumps where the space between the rock fragments is filled by clays. Bukovskyite crystallizes from Fe-As-S-rich gels that replace Si-Al gels. The exact mechanisms that control the entire process are not clear but will be the subject of further studies. We presume that the clays play an important role in creating microenvironments where the activity of the components needed for bukovskyite crystallization remains high for a long time. Bukovskyite is then an intermediate step in the conversion of the unstable gels to the stable assemblage of scorodite and iron sulfates.