Cacoxenite—a Complex Phosphate with a Modular Structure

Abstract

The crystal chemical peculiarities of cacoxenite from the Těškov quarry (Bohemia, Czech Republic) are studied using single crystal X-ray analysis and thermal X-ray analysis, electron-microprobe analysis, and infrared (IR) and Raman spectroscopy. The crystal structure of cacoxenite (hexagonal system, sp. gr. P6₃/m, a = 27.5677(4) Å, c = 10.5364(3) Å, V = 6934.64(16) ų) is refined to R₁ = 0.052 by 4637 independent reflections. The crystal structure is based on two independent structural blocks, which form an open-framework (the minimum free pore diameter is 15.1 Å). The crystal structure of cacoxenite is stable up to 190°C. The empirical formula of the mineral (based on 17 P atoms) is (\({\text{Fe}}_{{19.98}}^{{3 + }}\)Al_4.97)_Σ24.95O₆(PO₄)₁₇(OH_8.52Cl_3.48)_Σ12(H₂O)₂₄⋅ nH₂O (n ∼ 50). The most intense bands of IR and Raman spectra in a range from 400 to 1200 cm^–1 correspond to stretching and bending vibration modes of the phosphate groups. The presence of water in the structure is confirmed by the presence of an intense band in the IR spectrum in the area of 1624 cm^–1 (O–H vibrations), as well as the absorption band with maximum of 3357 cm^–1, which corresponds to stretching vibrations of the Fe(Al)–OH bond and H₂O molecules. The calculation of structural complexity of cacoxenite for the framework without H-corrections and with the addition of framework-related H-positions shows a very complex structure of the mineral (2312.464 bit/cell).