Manganese Sturmanite from the N’Chwaning 2 Mine, Kalahari, South Africa

Abstract

The oxidation state of Mn is determined for the first time by iodine titrimetry, and the Mn⁴⁺/Mn³⁺ ratio (54/46%) is identified for Mn-sturmanite from the N’Chwaning 2 Mine (Kalahari manganese field, South Africa). The Mössbauer spectrum of sturmanite, which contains a single asymmetric Lorentz doublet with parameters ^RTIS_α-Fe = 0.31(1) mm/s, ^RTQS = 0.18(1) mm/s, FWHM_L = 0.55 mm/s, and FWHM_R = 0.33 mm/s, confirms the presence of Fe³⁺ only in the composition of the mineral. The process of its thermal transformation is studied using thermal analysis and IR spectroscopy: dehydration and decomposition of Ca polyhedra, dehydroxylation, decarbonatization, and complete decomposition of structure with the formation of anhydrite, hematite, and Ca borates occur up to 200, 600, 750, and 1000°C, respectively. The chemical formula of the studied sturmanite, which is calculated on the basis of our studies per eight cations, is as follows: Ca_5.94 \({\text{Fe}}_{{1.03}}^{{3 + }}{\text{Mn}}_{{0.54}}^{{4 + }}{\text{Mn}}_{{0.44}}^{{3 + }}\) Al_0.03Si_0.02[B(OH)₄]_1.00[B(OH)₃]_0.11[SO₄]_2.32[СО₃]_0.49(OH)₁₂⋅22.7H₂O. The unit cell parameters of the mineral are determined using X-ray powder diffraction: a = 11.148(3) Å, c = 21.830(9) Å, and V = 2349(2) ų. The unit cell parameters of minerals of the sturmanite–jouravskite series linearly decrease with an increasing mole amount of the jouravskite endmember. It is suggested that the studied mineral can be an intermediate member of the sturmanite–jouravskite series with heterovalent isomorphic substitution according to following schemes: Mn³⁺ and Mn⁴⁺ → Fe³⁺ in the cation octahedral site and [CO₃]^2– → [B(OH)₄]^– and [SO₄]^2– in the anion site.