Laura Chelazzi

The structure–property relationship of four crystal forms of rifaximin

The crystal structures of four crystal forms of rifaximin, named α, β, δ and e, have been elucidated by a combination of single crystal and powder diffraction experiments with synchrotron and X-ray sources. The crystal structures of the δ and e forms have been determined from powder diffraction data alone. Hot stage microscopy has also been of paramount importance in the understanding of the β → α conversion process.

From 3D channelled frameworks to 2D layered structures in molecular salts of L-serine and DL-serine with oxalic acid

L-Serine and DL-serine have been treated with oxalic acid under different experimental conditions, e.g. crystallization from solution and slurry, kneading and dry mixing, yielding the molecular salts [L-serH]2[ox]·2H2O forms I and II, [L-serH][Hox] and [DL-serH]2[ox]·2H2O, all fully characterized by powder or single crystal X-ray diffractions; the relationship between the different crystal forms, relative thermal stability, dependence on relative humidity and possible interconversions have all been explored.

From ancient pigments to modern optoelectronic applications of arsenic sulfides: bonazziite, the natural analogue of β-As4S4 from Khaidarkan deposit, Kyrgyzstan

Abstract Bonazziite is a new mineral from Khaidarkan deposit, Kyrgyzstan and represents the natural analogue of the β-form of the well known As4S4 compound. It occurs as rare crystals up to 100 mm across associated with realgar, sulfur, wakabayashilite, alacra’nite, non-stoichiometric As4S4+x sulfides and stibnite in a calcite matrix. In thick section, bonazziite is opaque with a resinous lustre and a dark-orange streak. It is brittle; the Vickers hardness (VHN15) is 70 kg/mm2 (range: 60-76) (Mohs hardness of ~2½). In planepolarized incident light, bonazziite is strongly bireflectant and pleochroic from orange to light red. The mineral shows orange to red internal reflections. Between crossed polars, the mineral is strongly anisotropic with greyish to light-blue rotation tints. Reflectance percentages in air for Rmin and Rmax are 19.9, 22.2 (471.1 nm), 19.1, 21.3 (548.3 nm), 18.8, 19.7 (586.6 nm) and 17.8, 18.9 (652.3 nm), respectively. Bonazziite is monoclinic, space group C2/c, with a = 9.956(1), b = 9.308(1), c = 8.869(1) Å, b = 102.55(2)° and V = 802.3(2) Å3, Z = 4. The crystal structure [R1 = 0.0263 for 735 reflections with Fo > 4σ(Fo)] is based on the As4S4 cage-like molecule, in which each As atom links one As and two S atoms. The As4S4 molecule is identical to that found in the structure of realgar. The six strongest powder diffraction lines [d in Å (I/I0) (hkl)] are: 5.74 (100) (1̄11); 4.10 (60) (021); 3.92 (50) (1̄12); 3.12 (60) (022, 310); 2.95 (50) (221, 202); 2.86 (80) (2̄22, 1,31). A mean of six electron microprobe analyses gave the formula As3.95S4.05, on the basis of eight atoms. The new mineral has been approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification (IMA No. 2013-141) and named for Paola Bonazzi, in recognition of her seminal contributions to the study of arsenic sulfides and their alteration induced by exposure to light.