Heliophyllite: a discredited mineral species identical to ecdemite

Abstract. The type material for heliophyllite, preserved in the Swedish Museum of Natural History in Stockholm, was re-investigated through a combined EPMA (electron probe X-ray microanalysis), Raman, and X-ray powder diffraction (XRPD) and single-crystal study. EPMA chemical data, together with Raman and single-crystal structural studies, point to heliophyllite being identical to ecdemite. XRPD synchrotron data highlight the presence of a minor quantity of finely admixed finnemanite in the analyzed material, explaining the presence of some additional diffraction peaks, not indexable with the ecdemite unit cell, reported in the literature. The discreditation of heliophyllite has been approved by the IMA Commission on New Minerals and Mineral Names (proposal 19-H, 2019)

Neutron diffraction in gemology: Single-crystal diffraction study of brazilianite, NaAl3(PO4)(2)(OH)(4)

The chemical composition and the crystal structure of a gem-quality brazilianite from the Telírio pegmatite, near Linópolis, Minas Gerais, Brazil, [NaAl3(PO4)2(OH)4, a = 11.2448(5) Å, b = 10.1539(6) Å, c = 7.1031(3) Å, β = 97.351(4)°, V = 804.36(7) Å3, space group P21/n, Z = 4], have been reinvestigated by means of electron microprobe analysis in wavelength-dispersive mode, single-crystal X-ray and neutron diffraction. The chemical analysis shows that brazilianite from Telírio Claim approaches almost ideal composition. The neutron anisotropic structural refinement was performed with final agreement index R1 = 0.0290 for 211 refined parameters and 2844 unique reflections with Fo > 4σ(Fo), the X-ray refinement led to R1 = 0.0325 for 169 refined parameters and 2430 unique reflections with Fo > 4σ(Fo). The building-block units of the brazilianite structure consist of chains of edge-sharing AlO4(OH)2 and AlO3(OH)3 octahedra. Chains are connected, via corner-sharing, by P-tetrahedra to form a three-dimensional framework, with Na atoms located in distorted cavities running along [100]. Five independent H sites were located, here labeled as H(1), H(2a), H(2b), H(3), and H(4). The configuration of the OH groups, along with the complex hydrogen-bonding scheme, are now well defined. The O-H distances corrected for "riding motion" range between ~0.992 and ~1.010 Å, the O···O distances between ~2.67 and ~2.93 Å, and the O-H···O angles between ~151° and ~174°. The H(2a) and H(2b) are only ~1.37 Å apart and mutually exclusive (both with site occupancy factor of 50%). The differences between the crystal structure of brazilianite and wardite [ideally NaAl3(PO4)2(OH)4·2H2O] are discussed. This work fulfills the need for accurate crystal-chemical data for this gem mineral

Iron(II) modified natural zeolites for hexavalent chromium removal from contaminated water

Abstract Three different types of Fe(II)-modified natural zeolites were tested as supports in continuous-flow columns for the treatment of Cr(VI) contaminated water. The natural zeolites chosen as support were commercially available Zeosand (80% clinoptilolite), ATZ (79% phillipsite/chabazite), and ZS-55RW (90% Chabazite). All the examined modified zeolites turned out active for hexavalent chromium abatement, lowering its concentration below the European regulation level, even at relatively high flow rates (40 mL/h, linear velocity 15 cm/h). Zeosand, having a broader pH range of stability, was found to be the best one in terms of both Fe(II) uptake (0.54 wt%) and Cr removal (90 mg Cr/Kg zeolite)

Machine learning meets volcano plots: Computational discovery of cross-coupling catalysts

The application of modern machine learning to challenges in atomistic simulation is gaining attraction. We present new machine learning models that can predict the energy of the oxidative addition process between a transition metal complex and a substrate for C-C cross-coupling reactions. In turn, this quantity can be used as a descriptor to estimate the activity of homogeneous catalysts using molecular volcano plots. The versatility of this approach is illustrated for vast libraries of organometallic catalysts based on Pt, Pd, Ni, Cu, Ag, and Au combined with 91 ligands. Out-of-sample machine learning predictions were made on a total of 18 062 compounds leading to 557 catalyst candidates falling into the ideal thermodynamic window. This number was further refined by searching for candidates with an estimated price lower than 10 US$ per mmol. The 37 catalyst finalists are dominated by palladium phosphine ligand combinations but also include the earth abundant transition metal (Cu) with less common ligands. Our results indicate that modern statistical learning techniques can be applied to the computational discovery of readily available and promising catalyst candidates

THE CRYSTAL STRUCTURE OF NATURAL LIPSCOMBITE

editorial reviewe

Gabrielsonite revisited: crystal-structure determination and redefinition of chemical formula

A reinvestigation of gabrielsonite from the holotype specimen from Långban, central Sweden, using single-crystal synchrotron diffraction, electron-microprobe techniques and Fourier-transform infrared (FTIR) spectroscopy, Raman and Mo¨ssbauer spectroscopies show that the mineral is an anhydrous Fe3+-bearing arsenite and not a hydrous Fe2+-bearing arsenate, as originally proposed. The revised ideal chemical formula of gabrielsonite is PbFe3+(As3+O3)O. The mineral is related to the descloizite supergroup, but it differs through the valencies of the non-Pb cations Fe (M3+ vs. M2+) and As (3 + vs. 4 +) and through lower coordination of Pb (4 vs. 7–8) and As (3 vs. 4). The redefinition of gabrielsonite (proposal 17-G) has been approved by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association

On the Crystal-Chemistry of Rosasite and Parádsasvárite

We report the results of mineralogical and structural studies of para´dsasva´rite from Rudaba´nya, Hungary, and rosasite from Hayden, Arizona (USA). A preliminary investigation of the two minerals, which belong to the rosasite–malachite group, was conducted using Raman spectroscopy, X-ray diffraction, and EPMA. Para´dsasva´rite has the chemical formula (Zn1.91Cu0.06Mg0.02)R1.99(CO3)(OH)2, ideally Zn2(CO3)(OH)2, and rosasite has the formula (Cu1.14Zn0.84Mg0.02)R2(CO)3(OH)2, matching the general formula CuZn(CO3)(OH)2. The first single-crystal full structural study of rosasite from Hayden, based on synchrotron X-ray data, is reported here and fully confirms the results obtained by Perchiazzi (2006) on the basis of X-ray powder data. The presence of structural disorder in Cu-Zn distribution in these minerals is indicated by the streaking of reflections along c*. Rosasite is monoclinic, P21/a, a 12.2436(29) A, ˚ b 9.3555(19) A, ˚ c 3.1535(6) A, ˚ b¼98.69(3)8, and its crystal structure was refined to R1¼12.4%, wR2¼35.5%. The presence of Cu2þ causes a Jahn-Teller distortion of coordination polyhedra, a major feature of the rosasite crystal structure, particularly evident in the Me1 coordination polyhedron, which is fully occupied by Cu. A Rietveld study of para´dsasva´rite, based on synchrotron radiation data, shows it is isostructural with rosasite, monoclinic P21/a, a 12.253(4) A, ˚ b 9.348(3) A, ˚ c 3.167(1) A, ˚ b ¼ 97.700(4)8, and its crystal structure was refined to R1 ¼ 1.45%, wR2 ¼ 2.45%. The Me1 and Me2 coordination polyhedra in para´dsasva´rite, the former hosting Zn with a minor presence of Cu, the latter fully occupied by Zn, are as expected both more regular than the corresponding polyhedra in rosasite. Crystal-chemical considerations, based on literature chemical data for rosasite and Zn-rich malachite, indicate that a minimum Cu content of 0.4–0.5 apfu is necessary to stabilize the malachite-type structure

The Na-rich phosphate minerals from Malpensata granitic pegmatite, Piona, Lecco province, Italy

peer reviewe