American Mineralogist, Volume 90, pages 679–686, 2005 0003-004X/05/0004–679$05.00/DOI: 10.2138/am.2005.1739 679 INTRODUCTION The iron sulfate FeOHSO 4 was crystallized for the first time by Maus (1827) but the chemical formula he proposed for this compound was shown to be incorrect. Posnjak and Merwin (1922) determined some of the crystallographic and optical features and studied the stability over a limited temperature range of 50 to 200 °C. Synthetic crystals suitable for single- crystal X-ray analysis were obtained and structurally investigated by Johansson (1962). According to this author, the compound forms orthorhombic crystals with lattice parameters a J = 7.331(5), b J = 6.419(5), and c J = 7.142(5) Å [a J , b J , and c J are the lattice parameters measured by Johansson (1962) for the orthorhombic form], space group Pnma, and Z = 4. The structure contains two crystallographically independent iron atoms, each octahedrally coordinated by four oxygen atoms and two hydroxyl groups. The OH – groups are shared between two adjacent Fe-octahedra which are connected to each other to form chains, of {…Fe(OH)O 2 …} composition, running parallel to a. The four octahedral O atoms are shared with sulfate tetrahedra. They provide connectivity between the Fe-chains to make up a three-dimensional network which is based on [Fe 3+ O 4 (OH) 2 ] 7– octahedra and [SO 4 ] 2– tet- rahedra. Recently, FeOHSO 4 has received a lot of attention because its thermal decomposition and hydrolysis products are important for industrial application such as pigments, catalysis, and magnetic materials. Several studies (Mahapatra et al. 1990; Pelovski et al. 1996 and references therein) have been carried out on the quoted compound with different techniques and methods (derivatographic, thermogravimetric, powder X-ray phase analysis, and Mössbauer spectroscopy) with the aim of acquiring a deep knowledge of its stability, synthesis in various gaseous environments, and finally to investigate its behavior in the dehydration processes. The sample used for this study has been identified as a high- temperature phase derived from a metahohmannite compound during a synchrotron real-time powder diffraction experiment. In fact at about 100 °C hohmannite Fe 2 (H 2 O) 4 [O(SO 4 ) 2 ]·4H 2 O transforms to metahohmannite Fe 2 3+ (H 2 O) 4 [O(SO 4 ) 2 ] releasing * E-mail: f.scordari@geomin.uniba.it The order-disorder character of FeOHSO 4 obtained from the thermal decomposition of metahohmannite, Fe 3+ 2 (H 2 O) 4 [O(SO 4 ) 2 ] GENNARO V ENTRUTI, 1 FERNANDO SCORDARI, 1 EMANUELA SCHINGARO, 1 ALESSANDRO F. GUALTIERI, 2 AND CARLO MENEGHINI 3 1 Dipartimento Geomineralogico, Università di Bari, I-70125 Bari, Italy 2 Dipartimento di Scienze della Terra, Università di Modena e Reggio Emilia, I-41100 Modena, Italy 3 Dipartimento di Fisica “E.Amaldi”, Università di RomaTre, I-00146 Roma, Italy ABSTRACT The iron sulfate FeOHSO 4 studied was obtained as a dehydration product of metahohmannite Fe 2 (H 2 O) 4 [O(SO 4 ) 2 ] during a synchrotron real-time powder diffraction experiment. As quoted in the literature, FeOHSO 4 has iron atoms octahedrally coordinated with two hydroxyl groups and four sul- fate O atoms, while each hydroxyl group is bonded to two iron atoms. This compound is commonly described in the orthorhombic system with space group Pnma, lattice parameters a J = 7.33, b J = 6.42, and c J = 7.14 Å (a J , b J , and c J are the Johansson lattice parameters), and Z = 4. However a preliminary Rietveld refinement of the pattern at about 220 °C using the structural model from the literature yielded a poor fit of the observed data and a final R p value of about 23%. A careful analysis of the calculated powder diffraction pattern showed unexpected peaks, not observed in the experimental trace, for h = 2n + 1, while sharp reflections for h = 2n seemed to point to different lattice constants and space group. The recognition of the order-disorder character of the FeOHSO 4 compound was the key to successfully interpreting the unexpected features of the experimental powder pattern and the misfit with respect to the calculated pattern. In fact, FeOHSO 4 belongs to a family of OD structures formed by equivalent layers of symmetry Pbmm. Only two MDO (Maximum Degree of Order) polytypes are possible. MDO1 results from a regular alternation of stacking operators 2 1/2 and 2 –1/2 , and yields an orthorhombic structure with space group Pnma and lattice parameters a J = 7.33, b J = 6.42, and c J = 7.14 Å. MDO2 results from the 2 1/2 |2 1/2 |2 1/2 ... sequence of symmetry operators and yields a monoclinic structure with space group P2 1 /c, a M = 7.33, b M = 7.14, c M = 7.39 Å, and β = 119.7°. The analysis of one-dimensional stacking disorder was performed by fitting the observed XRPD pattern with a calculated intensity curve generated by DIFFaX. The disorder model was investigated by taking into account a probability matrix for the occurrence of OD layer sequences. The best fit (R p = 0.009) to the observed powder pattern was obtained with a 61:39 ratio of monoclinic and orthorhombic polytypes for a fully disordered OD layers sequence.