Isomorphic Substitution in Vanadinite [Pb5(VO4)3C1] - Raman Spectroscopic Study

The Raman and infrared spectroscopy of three vanadinite [Pb5(VO4)Cl] specimens from three localities has been studied. Single crystal Raman spectra were obtained and the spectra were found to be both orientation and temperature dependent. Measurement of the Raman spectra at liquid nitrogen temperature enabled better band separation and increased intensities of weak bands through band narrowing enabling, the assignment of the bands in vanadinites to be made. Two types of isomorphous substitution are identified (a) substitution in the bulk of the crystal where electron beam microanalysis identifies the presence of calcium and copper. (b) surface substitution where infrared spectroscopy shows the isomorphous substitution of vanadate by phosphate and of chloride by hydroxyl groups

Structure of hydrated calcium carbonates: A first-principles study

The structures of both ikaite (CaCO3·6H2O) and monohydrocalcite (CaCO3·H2O) were computed at the PBE0 level of theory, using all electron Gaussian type basis sets. Correction for the long-range dispersion contribution was included for the oxygen–oxygen interactions by using an additive pairwise term with the atomic coefficients fitted against the calcite vs aragonite enthalpy difference. The potential chirality of monohydrocalcite is discussed, as well as the helical motifs created by the three-fold rototranslational axes parallel to the [001] direction. These elements represent a significant link between monohydrocalcite and vaterite, both appearing as intermediate species during CaCO3 crystallization from amorphous calcium carbonate. The hydrogen bond pattern, never fully discussed for monohydrocalcite, is here described and compared to the available experimental data. Both phases are characterized by the presence of hydrogen bonds of moderate to high strength. Water molecules in monohydrocalcite interact quite strongly with 2 View the MathML source units through such hydrogen bonds, whereas their interaction with each other is minor. On the contrary, water molecules in ikaite create a complex network of hydrogen bonds, where each water molecule is strongly hydrogen bonded to one View the MathML source anion and to one or two other water molecules

Raman and mid-IR spectroscopic study of the magnesium carbonate minerals - Brugnatellite and coalingite

Two hydrated hydroxy magnesium carbonate minerals brugnatellite and coalingite with a hydrotalcite-like structure have been studied by Raman spectroscopy. Intense bands are observed at 1094 cm-1 for brugnatellite and at 1093 cm-1 for coalingite attributed CO32- ν1 symmetric stretching mode. Additional low intensity bands are observed at 1064 cm-1. The existence of two symmetric stretching modes is accounted for in terms of different anion structural arrangements. Very low intensity bands at 1377 and 1451 cm-1 is observed for brugnatellite and the Raman spectrum of coalingite displays two bands at 1420 and 1465 cm-1 attributed to the (CO3)2- ν3 antisymmetric stretching modes. Very low intensity bands at 792 cm-1 for brugnatellite and 797 cm-1 for coalingite are assigned to the CO32- out-of-plane bend (ν2). X-ray diffraction studies by other researchers have shown these minerals are disordered. This is reflected in the difficulty of obtaining Raman spectra of reasonable quality and explains why the Raman spectra of these minerals have not been previously or sufficiently described. A comparison is made with the Raman spectra of other hydrated magnesium carbonate minerals

Stable prenucleation mineral clusters are liquid-like ionic polymers

Calcium carbonate is an abundant substance that can be created in several mineral forms by the reaction of dissolved carbon dioxide in water with calcium ions. Through biomineralization, organisms can harness and control this process to form various functional materials that can act as anything from shells through to lenses. The early stages of calcium carbonate formation have recently attracted attention as stable prenucleation clusters have been observed, contrary to classical models. Here we show, using computer simulations combined with the analysis of experimental data, that these mineral clusters are made of an ionic polymer, composed of alternating calcium and carbonate ions, with a dynamic topology consisting of chains, branches and rings. The existence of a disordered, flexible and strongly hydrated precursor provides a basis for explaining the formation of other liquid-like amorphous states of calcium carbonate, in addition to the non-classical behaviour during growth of amorphous calcium carbonate

Structural features of C-S-H(I) and its carbonation in air - A Raman spectroscopic study. Part II: Carbonated phases

The effects of carbonation of mechanochemically prepared C-S-H samples under ambient conditions for upto 6 months have been investigated by Raman spectroscopy and X-ray diffraction. The type and extent of carbonation are strongly dependent on the initial CaO/SiO2 (C/S) ratio of the samples. Amorphous calcium carbonate hydrate is formed within minutes upon exposure to air. It crystallizes, over time, to give primarily vaterite at C/S >= 0.67 and aragonite at C/S <= 0.50. Calcite was not observed as a primary carbonation product within the time frame investigated. Decalcification upon storage also initiates silicate polymerization. The dimeric silicate units seen in the calcium-rich phases polymerize rapidly to yield Q(2) silicate moieties. After 6 months, broad bands are seen in most spectra, ascribed to poorly ordered silica. C-S-H phases with C/S ratios of 0.75 and 0.67 are the most resistant to carbonation, and even after 6 months of storage, Q(2) silicate units still dominate their structures. The ability of Raman spectroscopy to probe the short-range order of poorly crystalline materials is ideal for investigations of C-S-H structure. Additionally, the technique's sensitivity toward the various calcium carbonate polymorphs illuminates the sequence of carbonation and decalcification processes during aging of C-S-H. Of particular importance is the identification of amorphous calcium carbonate as the first carbonation product. Additionally, the formation of aragonite as a carbonation product is related to the presence of SiO2 gel in the aged samples