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Iron-containing potassium feldspar crystals are prepared using the hydrothermal synthesis in an alkaline medium at temperatures ranging from 500 to 526°C. The crystal structure of the synthetic potassium feldspar is refined [Ital Structures diffractometer, MoKalpha radiation, 1327 unique reflections with F > 4sigma(F), anisotropic approximation, R(F) = 0.044]. It is established that, under the given preparation conditions, the synthesis leads to the formation of the monoclinic modification with the following unit-cell parameters: a 8.655(7) Å, b = 13.101(9) Å, c = 7.250(g) Å, beta = 116.02(2)°, space group C2/m, and Z = 4. The cation distribution over crystallographically inequivalent tetrahedral positions T(1) and T(2) is determined and justified using X-ray diffraction data. According to this distribution, the iron-containing potassium feldspar is assigned to the low ferrialuminosilicate sanidine. The proposed structural formula K A=0.99 (Si1.2Fe0.5Al0.3) T(1)=2 (Si1.81Al0.19) T(2)=2 O8 agrees well with the data of the electron microprobe analysis. It is revealed that iron occupies the T(1) position and manifests itself as a majority rather than minority impurity element with respect to aluminum

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