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Crystal chemistry of the halotrichite group XAl₂(SO₄)₄ · 22H₂O

Dipartimento di Scienze della Terra, Università di Roma "La Sapienza", P.le A. Moro 5, I-00185 Roma, Italy

^* E-mail: paolo.ballirano{at}uniromal.it

The paper is devoted to the investigation of the crystal chemistry of the halotrichite group XAl₂(SO₄)₄ · 22H₂O along the joints of the x = 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. < r_x > 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.

Key-words: sulfates, halotrichite-group, crystal chemistry, X-ray powder diffraction, Rietveld method.