Crystal Chemistry and High-Temperature Behaviour of Ammonium Phases NH4MgCl3·6H2O and (NH4)2Fe3+Cl5·H2O from the Burned Dumps of the Chelyabinsk Coal Basin

Abstract

The technogenic mineral phases NH4MgCl3·6H2O and (NH4)2Fe3+Cl5·H2O from the burned dumps of the Chelyabinsk coal basin have been investigated by single-crystal X-ray diffraction, scanning electron microscopy and high-temperature powder X-ray diffraction. The NH4MgCl3·6H2O phase is monoclinic, space group C2/c, unit cell parameters a = 9.3091(9), b = 9.5353(7), c = 13.2941(12) Å, β = 90.089(8)° and V = 1180.05(18) Å3. The crystal structure of NH4MgCl3·6H2O was refined to R1 = 0.078 (wR2 = 0.185) on the basis of 1678 unique reflections. The (NH4)2Fe3+Cl5·H2O phase is orthorhombic, space group Pnma, unit cell parameters a = 13.725(2), b = 9.9365(16), c = 7.0370(11) Å and V = 959.7(3) Å3. The crystal structure of (NH4)2Fe3+Cl5·H2O was refined to R1 = 0.023 (wR2 = 0.066) on the basis of 2256 unique reflections. NH4MgCl3·6H2O is stable up to 90 °C and then transforms to the less hydrated phase isotypic to β-Rb(MnCl3)(H2O)2 (i.e., NH4MgCl3·2H2O), the latter phase being stable up to 150 °C. (NH4)2Fe3+Cl5·H2O is stable up to 120 °C and then transforms to an X-ray amorphous phase. Hydrogen bonds provide an important linkage between the main structural units and play the key role in determining structural stability and physical properties of the studied phases. The mineral phases NH4MgCl3·6H2O and (NH4)2Fe3+Cl5·H2O are isostructural with natural minerals novograblenovite and kremersite, respectively.

Novograblenovite from Radlin, Upper Silesia, Poland and its relation to ‘redikortsevite’

“…According to our data and those of Zolotarev et al (2019), "redikortsevite" was erroneously assigned to the orthorhombic system by Chesnokov et al (1988). The crystal structure of the synthetic analogue (Solans et al, 1983;Marsh, 1992) also indicates the monoclinic system to be correct for this mineral.…”

“…The weighted residual factor wR 2 presents a divergence of the refined crystal structure model from all recorded X-ray scattering intensities. In this study the discrepancy factor wR 2 = 5.7% is improved over twice with respect to Okrugin et al (2019), wR 2 =12.3% and 3 times compared to Zolotarev et al (2019), wR 2 =19.1%.…”

“…Okrugin et al (2019) is occupationally disordered with K in a ratio of 2:1. The crystal analyzed byZolotarev et al (2019) was non-merohedrally twinned. Both groups positioned the N atom on a two-fold symmetry axis which constrained also the positions of bonded H atoms.…”

“…This method was applied also byZolotarev et al (2019) for reexamination of Chesnokov's material. As discussed by these authors EDS analysis in this case, performed both on unpolished and polished surfaces, carries some uncertainty.…”

Infrared and Raman Spectroscopy of Ammoniovoltaite, (NH4)2Fe2+5Fe3+3Al(SO4)12(H2O)18

“…The mineralogy of fumaroles is in close relation to technogenic/natural phase formation at burning coal dumps under so-called "pseudofumarolic" conditions, for example, at the Chelyabinsk coal basin (Chelyabinsk, Russia) [10,11]. It is worth noting that numerous technogenic mineral-like phases representing Fe-Al sulphates have been identified at burning coal dumps [12].…”