Crystal structure, thermodynamic properties, and paragenesis of bukovskyite, Fe-2(AsO4)(SO4)(OH)center dot 9H(2)O

Warning

This publication doesn't include Faculty of Sports Studies. It includes Faculty of Science. Official publication website can be found on muni.cz.
Authors

MAJZLAN Juraj LAZIC Biljana ARMBRUSTER Thomas JOHNSON Michel B WHITE Mary Anne FISHER Robert Allan PLÁŠIL Jakub LOUN Jan ŠKODA Radek NOVÁK Milan

Year of publication 2012
Type Article in Periodical
Magazine / Source JOURNAL OF MINERALOGICAL AND PETROLOGICAL SCIENCES
MU Faculty or unit

Faculty of Science

Citation
Doi http://dx.doi.org/10.2465/jmps.110930
Field Geology and mineralogy
Keywords Bukovskyite; Crystal structure; Thermodynamics; Paragenesis
Description Bukovskyite is a relatively rare secondary ferric arsenate-sulfate. At the type locality near the municipality of Kutna 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 Grossvoigtsberg (Germany) and found to be of the space group P (I) over bar with a final R factor of 5.08% from 2403 reflections. The lattice parameters at room temperature are a = 7.549(1) angstrom, b = 10.305(1) angstrom, c = 10.914(2) angstrom, alpha = 115.136(3)degrees, beta = 99.798(3)degrees, and gamma = 92.864(3)degrees. The structure consists of octahedral-tetrahedral Fe-arsenate chains. Sulfate tetrahedra are bonded to the chains and free H2O molecules via a complicated network of hydrogen bonds. Calorimetric measurements (acid-solution calorimetry at T = 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(-l).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.
Related projects:

You are running an old browser version. We recommend updating your browser to its latest version.

More info