Crystal structure refinement and chemical formula of prosopite, CaAl₂F₄[(OH)_4-xF_x] x = 0.0-1.0

Prosopite is an alumino-fluoride of calcium mineral. The chemical compositions of prosopite samples obtained from Zacatecas (Mexico), Ivigtut (Greenland), and Katugin deposit (Eastern Siberia, Russia) were determined using scanning electron microscopy and energy-dispersive X-ray spectroscopy. The fluorine content (in apfu) was between 4 and 5, and those less than 4 were not observed. The empirical formula derived as a mean of chemical compositions of multiple grains from Ivigtut is (Ca_0.96Sr_0.04)Al_2.00F₄[(OH)_3.72F_0.28]. The crystal structure of prosopite [monoclinic; a = 6.7103(3) Å, b = 11.1619(5) Å, c = 7.3741(3) Å, β = 94.919(2)°; space group C2/c; Z = 4] was analyzed using single-crystal X-ray diffraction and was refined to the R value of 0.0185 (wR2 = 0.0554) using 791 unique reflections with |F_o| > 4σ(|F_o|). The positions of hydrogen atoms were determined at the position where residual electron density peaks appeared using the difference Fourier method. F^- and O^2- ions are distributed at each F and O site in order. The chemical structural formula, Ca_0.964(2)Sr_0.036Al₂F₄(OH)₄, obtained from the refinement of Ivgtut sample is approximately consistent with the result of chemical analysis. During the chemical analysis (Zacatecas, Ivigtut, and Katugin deposit samples) and refinement (Ivigtut sample), we assume that (OH)^- dissolution into F sites does not occur (but substitution of F^- in (OH) sites slightly occurs) and propose that the chemical structural formula of prosopite is expressed as CaAl₂F₄[(OH)_4-xF_x] (x = 0.0-1.0). The crystal structure of prosopite consists of two types of AlF₂(OH)₄ octahedra and one kind of CaF₆(OH)₂ dodecahedra. The size difference between F sites and (OH) sites can be observed in Ca dodecahedra and Al2 octahedra; however, evident differences in Al-F and Al-O distances are not observed in Al1 octahedral sites. Two hydrogen bonds (O1-H1...O2 and O2-H2...F2) are confirmed using bond valence sum calculations.