Synthetic samples of krautite, Mn[AsO3(OH)]⋅H2O, koritnigite, Zn[AsO3(OH)]⋅H2O and cobaltkoritnigite, Co[AsO3(OH)]⋅H2O, were used for calorimetric experiments. For krautite and koritnigite, single-crystal X-ray diffraction was used to determine positions of all atoms, including the H atoms. These data allowed the hydrogen-bond network and the function of H2O molecules in these structures to be determined. The structural formulae are Mn4(H2[3]O)4[AsO3(OH)]4 and Zn4(H2[3]O)2[AsO3(OH)]4(H2[4]O)2, where [3]H2O and [4]H2O are the ‘transformer’ and ‘non-transformer’ H2O groups, respectively. Even though the principal features of these structures are identical, the details, especially those regarding the H2O groups, differ from one structure to another structure in this group. The solubility products (log Ksp) were determined from calorimetric data, that is, from the experimentally measured enthalpies of formation and entropies. They relate to the reaction M[AsO3(OH)]⋅H2O → M2+ + HAsO42– + H2O and are –6.10 for krautite, –6.88 for koritnigite and –6.83 for cobaltkoritnigite. We also estimated the log Ksp for magnesiokoritnigite as –2.0. Calculation of phase diagrams shows that all these phases originate under acidic conditions from solutions with high metal and arsenate concentration. They are restricted to local environments, to pockets that maintain such high concentrations over the time necessary for crystallisation of the krautite-group phases.

The new mineral magnesiokoritnigite (IMA 2013-049), ideally Mg(AsO3OH)·H2O, was found at the Torrecillas mine, Salar Grande, Iquique Province, Chile, where it occurs as a secondary alteration phase in association with anhydrite, chudobaite, halite, lavendulan, quartz and scorodite. Crystals of magnesiokoritnigite are colourless to pale-pink, thin to thick laths up to 2 mm long. Laths are elongated on [001], flattened on {010} and exhibit the forms {010}, {110}, {10}, {101}, {031} and {01}. The crystals also occur in dense deep-pink intergrowths. Crystals are transparent with a vitreous lustre. The mineral has a white streak, Mohs hardness of ∼3, brittle tenacity, conchoidal fracture and one perfect cleavage on {101}. The measured and calculated densities are 2.95(3) and 2.935 g cm– 3, respectively. Optically, magnesiokoritnigite is biaxial (+) with α = 1.579(1), β = 1.586(1) and γ = 1.620(1) (measured in white light). The measured 2V is 50(2)° and the calculated 2V is 50°. Dispersion is r < v, medium. The optical orientation is Y ≈ b; Z ^ c = 36° in obtuse β (note pseudomonoclinic symmetry). The mineral is non-pleochroic. The empirical formula, determined from electron-microprobe analyses, is (Mg0.94Cu0.03Mn0.02Ca0.01)Σ 1.00As0.96O5H3.19. Magnesiokoritnigite is triclinic, P, with a = 7.8702(7), b = 15.8081(6), c = 6.6389(14) Å, α = 90.814(6), β = 96.193(6), γ = 90.094(7)°, V = 821.06(19) Å3 and Z = 8. The eight strongest X-ray powder diffraction lines are [dobs Å (I)(hkl)]: 7.96(100)(020), 4.80(54)(101), 3.791(85)(10,210,1,31), 3.242(56)(02,1,012), 3.157(92)(21,30,230), 3.021(61)(11,141,21,221), 2.798(41)(02,032) and 1.908(43)(multiple). The structure, refined to R1 = 5.74% for 2360 Fo > 4σF reflections, shows magnesiokoritnigite to be isostructural with koritnigite and cobaltkoritnigite.