The paper is devoted to the investigation of the crystal chemistry of the halotrichite group XAl2(SO4)4 · 22H2O along the joints of the × = Fe-Mg-Mn-Zn compositional tetrahedron by means of X-ray powder diffraction and the Rietveld method on synthetic samples. Complete solid solution along the joints has been observed. The volume vs. < rx > plot (mean ionic radius of the X cation) indicate fairly large departures from the expected linearity. The reason/s of the departure from linearity is, at present, not clearly understood and could be possibly related to the combined effect of the different degree of filling of the d shell of the transition elements, the corresponding attraction/repulsion with the five facing lone pairs of the water molecules pertaining to the XO(H2O)5 octahedron, and to the different degree of ionicity of the X-O bond.

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