ИСТИНА

Phosphosiderite from Atacama (Chile) was studied by X-ray powder diffraction, Mössbauer spectroscopy and chemical analysis. The arsenic content reaches 9 wt% As2O5. The averaged formula obtained is [Fe0.96Al0.04)[(PO4)0.94(AsO4)0.06]2.04H2O. Isomorphic substitution As → P in the tetrahedral positions was established. The parameters of the monoclinic cell are a = 5.3258(6) Å, b = 9.8003(11) Å, c = 8.7116(9) Å, β = 90.64(1) º. The Mössbauer spectrum of the sample at room temperature is shown in Fig.1 It can be well fit as one resolved symmetric Lorentz doublet with RTISα-Fe = 0.430.01 mms−1, RTQS = 0.260.01 mms−1, FWHM = 0.300.01 mms−1. This model corresponds well to the crystal structure of phosphosiderite, which contains unique crystallographic position of iron. No traces of divalent iron or magnetic phases were found even with a thick sample. The obtained parameters are typical for high-spin trivalent iron in an octahedral oxygen environment. The substitution of Fe3+ by Al3+ should not affect the isomer shift because iron and aluminum octahedra are far from each other, and the change of the cell parameters is small as discussed above. The As-analog of phosphosiderite is not found in nature. The As-analogue of strengite is scorodite, FeAsO42H2O. Fe-O interatomic distances in phosphosiderite, strengite, and scorodite are similar, average Fe-O distances being about 200 pm. Moreover, electronegativities of phosphorus and arsenic are close. These facts allow us to suppose that the substitution of phosphorus by arsenic in phosphosiderite structure should affect the isomer shift. Despite the strong inhomogeneity of arsenic distribution in the crystals, we do not see any significant broadening of the absorption lines, which agrees with the supposition.