Sulphur and Molybdenum Incorporation at the Calcite-Water Interface: Insights from Ab Initio Molecular Dynamics

<p>Sulphur and molybdenum trace impurities in speleothems (stalagmites and stalactites) can provide long and continuous records of volcanic activity, which are important for past climatic and environmental reconstructions. However, the chemistry governing the incorporation of the trace-element bearing species into the calcium carbonate phases forming speleothems is not well understood. Our previous work has shown that substitution as tetrahedral oxyanions [<i>X</i>O₄]^2- (<i>X</i>=S, Mo) replacing [CO₃]^2- in CaCO₃ bulk phases (except perhaps for vaterite) is thermodynamically unfavourable with respect to the formation of competing phases, due to the larger size and different shape of the [<i>X</i>O₄]^2-tetrahedral anions in comparison with the flat [CO₃]^2- anions, which implied that most of the incorporation would happen at the surface rather than the bulk of the mineral. Here we present an ab initio molecular dynamics study exploring the incorporation of these impurities at the mineral-water interface. We show that the oxyanions substitution at the aqueous calcite (10.4) surface is clearly favoured over bulk incorporation, due to the lower structural strain on the calcium carbonate solid. Incorporation at surface step sites is even more favourable for both oxyanions, thanks to the additional interface space afforded by the surface line defect to accommodate the tetrahedral anion. Differences between sulphate and molybdate substitution can be mostly explained by the size of the anions. The molybdate oxyanion is more difficult to incorporate in the calcite bulk than the smaller sulphate oxyanion. However, when molybdate is substituted at the surface, the elastic cost is avoided because the oxyanion protrudes out of the surface and gains stability via the interaction with water at the interface, which in balance results in more favourable surface substitution for molybdate than for sulphate. The detailed molecular-level insights provided by our calculations will be useful to understand the chemical basis of S- and Mo-based speleothem records.</p>

Sulphur and Molybdenum Incorporation at the Calcite-Water Interface: Insights from Ab Initio Molecular Dynamics

<p>Sulphur and molybdenum trace impurities in speleothems (stalagmites and stalactites) can provide long and continuous records of volcanic activity, which are important for past climatic and environmental reconstructions. However, the chemistry governing the incorporation of the trace-element bearing species into the calcium carbonate phases forming speleothems is not well understood. Our previous work has shown that substitution as tetrahedral oxyanions [<i>X</i>O₄]^2- (<i>X</i>=S, Mo) replacing [CO₃]^2- in CaCO₃ bulk phases (except perhaps for vaterite) is thermodynamically unfavourable with respect to the formation of competing phases, due to the larger size and different shape of the [<i>X</i>O₄]^2-tetrahedral anions in comparison with the flat [CO₃]^2- anions, which implied that most of the incorporation would happen at the surface rather than the bulk of the mineral. Here we present an ab initio molecular dynamics study exploring the incorporation of these impurities at the mineral-water interface. We show that the oxyanions substitution at the aqueous calcite (10.4) surface is clearly favoured over bulk incorporation, due to the lower structural strain on the calcium carbonate solid. Incorporation at surface step sites is even more favourable for both oxyanions, thanks to the additional interface space afforded by the surface line defect to accommodate the tetrahedral anion. Differences between sulphate and molybdate substitution can be mostly explained by the size of the anions. The molybdate oxyanion is more difficult to incorporate in the calcite bulk than the smaller sulphate oxyanion. However, when molybdate is substituted at the surface, the elastic cost is avoided because the oxyanion protrudes out of the surface and gains stability via the interaction with water at the interface, which in balance results in more favourable surface substitution for molybdate than for sulphate. The detailed molecular-level insights provided by our calculations will be useful to understand the chemical basis of S- and Mo-based speleothem records.</p>

Monofluorophosphates—New Examples and a Survey of the PO3F2− Anion

During a systematic study of monofluorophosphates, i.e., compounds comprising the tetrahedral anion PO3F2−, twelve, for the most part new, compounds were obtained from aqueous solutions. Crystal structure refinements based on single crystal X-ray diffraction data revealed the previously unknown crystal structures of CdPO3F(H2O)2, Cr2(PO3F)3(H2O)18.8, Pb2(PO3F)Cl2(H2O), (NH4)2M(PO3F)2(H2O)2 (M = Mg, Mn, Co), NH4Cr(PO3F)2(H2O)6, NH4Cu2(H3O2)(PO3F)2, (NH4)2Zn(PO3F)2(H2O)0.2, and (NH4)2Zn3(PO3F)4(H2O), as well as redeterminations of ZnPO3F(H2O)2.5 and (NH4)2Ni(PO3F)2(H2O)6. From the previously unknown crystal structures, CdPO3F(H2O)2 (space group P-1), Cr2(PO3F)3(H2O)18.8 (P-1), Pb2(PO3F)Cl2(H2O) (Pnma), NH4Cr(PO3F)2(H2O)6 (R-3m), (NH4)2Zn(PO3F)2(H2O)0.2 (C2/c), and (NH4)2Zn3(PO3F)4(H2O) (I-43d) each crystallizes in an unique crystal structure, whereas compounds (NH4)2M(PO3F)2(H2O)2 (M = Mg, Co) crystallize in the (NH4)2Cu(PO3F)2(H2O)2 type of structure (C2/m) and (NH4)2Mn(PO3F)2(H2O)2 in a subgroup thereof (P21/n, with a klassengleiche relationship of index 2), and NH4Cu2(H3O2)(PO3F)2 (C2/m) crystallizes isotypically with natrochalcite-type KCu2(H3O2)(SO4)2. A survey on the PO3F2− anion, including database entries of all inorganic compounds comprising this group, revealed mean bond lengths of P–O = 1.506(13) Å, P–F = 1.578(20) Å, and angles of O–P–O = 113.7(1.7)° and O–P–F = 104.8(1.7)°, using a dataset of 88 independent PO3F2− anions or entities. For those crystal structures of monofluorophosphates where hydrogen bonding is present, in the vast majority of cases, hydrogen bonds of the type D–H···F–P (D = O, N) are not developed.

An unexpected rhenium(IV)–rhenium(VII) salt: [Co(NH3)6]3[ReVIIO4][ReIVF6]4·6H2O

The title hydrated salt, tris[hexaamminecobalt(III)] tetraoxidorhenate(VII) tetrakis[hexafluoridorhenate(IV)] hexahydrate, arose unexpectedly due to possible contamination of the K2ReF6 starting material with KReO4. It consists of octahedral [Co(NH3)6]3+ cation (Co1 site symmetry 1), tetrahedral [ReVIIO4]− anions (Re site symmetry 1) and octahedral [ReIVF6]2− anions (Re site symmetries 1and \overline{3}). The [ReF6]2− octahedral anions (mean Re—F = 1.834 Å), [Co(NH3)6]3+ octahedral cations (mean Co—N = 1.962 Å), and the [ReO4]− tetrahedral anion (mean Re—O = 1.719 Å) are slightly distorted. A network of N—H...F hydrogen bonds consolidates the structure. The crystal studied was refined as a two-component twin.