Search

The role of subglacial abrasion in sustaining subglacial microbiomes is an area of growing interest. It is likely that subglacial abrasion produces a range of bioutilisable compounds, some released from fluid inclusions in the confines of the mineral matrix and some produced via the surface free radicals that crushing produces in a variety of different minerals. As yet, the geochemical reactions which are initiated by abrasion remain poorly understood. This is largely because of the multiple potential sources and sinks of free radicals that occur in even the most simple of multi-mineralic system during crushing and subsequent wetting of the mixture. We illustrate this assertion by conducting a series of laboratory experiments where common, "simple" silicate minerals, such as quartz and feldspar, are abraded and incubated with ultra-pure water. Gaseous reaction products included CH4, H2 and CO2. The concentrations of major ions (including Na+, K+, Mg2+, and Ca2+), dissolved silicate, dissolved iron and pH of the solution during incubation are reported. These results were used to calculate HCO3- in solution and the saturation index with respect to various minerals using PHREEQCi. We found that abrasion of silicates produces bio-available gases and has the potential to increase the concentration of various weathering products in solution. Further, we found that even trace amounts of carbonates, an impurity in many natural rocks and minerals, may significantly affect the reaction products, making it challenging to concretize the reactions taking place. These results highlight the importance of conducting abrasion experiments utilising natural samples to better understand how apparently negligeable changes in mineralogy might affect the reaction products of abrasion and crushing. [ABSTRACT FROM AUTHOR]

The epitaxial overgrowth of brushite (CaHP[O.sub.4] x 2[H.sub.2]O) by the interaction of phosphate-bearing, slightly acidic, aqueous solutions with gypsum (CaS[O.sub.4] x 2[H.sub.2]O) was investigated in situ using atomic force microscopy (AFM). Brushite growth nuclei were not observed to form on the {010} gypsum cleavage surface, but instead formed in areas of high dissolution, laterally attached to gypsum [101] step edges. During the brushite overgrowth the structural relationships between brushite (Aa) and gypsum (A2/a) result in several phenomena, including the development of induced twofold twining, habit polarity, and topographic effects due to coalescence of like-oriented crystals. The observed brushite growth is markedly anisotropic, with the growth rate along the main periodic bond chains (PBCs) in the brushite structure increasing in the order [101] > [101] > [010], leading to tabular forms elongated on [101]. Such a growth habit may result from the stabilization of the polar [101] direction of brushite due to changes in hydration of calcium ions induced by the presence of sulfate in solution, which is consistent with the stabilization of the gypsum [101] steps during dissolution in the presence of HP[O.sup.2-.sub.4] ions. The coupling between growth and dissolution was found to result in growth rate fluctuations controlled by the changes in the solution composition. Keywords: Brushite, gypsum, in situ AFM, epitaxy DOI: 10.2138/am.2010.3557

Field emission scanning electron microscopy (FESEM), two-dimensional X-ray diffraction (2DXRD), and transmission electron microscopy coupled with selected area electron diffraction (TEMSAED) analyses of the reactant/product textural relationship show that the thermal decomposition of Iceland spar single crystals according to the reaction CaC[O.sub.3(s)] [right arrow], Ca[O.sub.(s)] + C[O.sub.2(g)] is pseudomorphic and topotactic. This reaction begins with the formation of a mesoporous structure made up of up to four sets of oriented rod-shaped CaO nanocrystals on each rhombohedral cleavage face of the calcite pseudomorph. The four sets formed on [(10[bar.1]4).sub.calcite] display the following topotactic relationships: (1) [(1[bar.2]10).sub.calcite][parallel][(110).sub.CaO]; (2) [([bar.1]104).sub.calcite[parpendicular to]][(110).sub.Cao]; (3) [([bar.1]018).sub.calcite]][parallel][(110).sub.CaO]; and (4) [(0[bar.1]14).sub.calcite[perpendicular to]] [(110).sub.CaO]; with [[841].sub.calcite][parallel][[1[bar.1]O].sub.CaO] in all four cases. At this stage, the reaction mechanism is independent of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (i.e., air or high vacuum). Strain accumulation leads to the collapse of the mesoporous structure, resulting in the oriented aggregation of metastable CaO nanocrystals (~5 nm in thickness) that form crystal bundles up to ~1 [micro]m in cross-section. Finally, sintering progresses up to the maximum T reached (1150 [degrees]C). Oriented aggregation and sintering (plus associated shrinking) reduce surface area and porosity (from 79.2 to 0.6 [m.sup.2]/g and from 53 to 47%, respectively) by loss of mesopores and growth of micrometer-sized pores. An isoconversional kinetic analysis of non-isothermal thermogravimetric data of the decomposition of calcite in air yields an overall effective activation energy [E.sub.[alpha]] = 176 [+ or -] 9 k J/ mol (for [alpha] > 0.2), a value which approaches the equilibrium enthalpy for calcite thermal decomposition (177.8 kJ/mol). The overall good kinetic fit with the F I model (chemical reaction, first order) is in agreement with a homogeneous transformation. These analytical and kinetic results enable us to propose a novel model for the thermal decomposition of calcite that explains how decarbonation occurs at the atomic scale via a topotactic mechanism, which is independent of the experimental conditions. This new mechanistic model may help reinterpret previous results on the calcite/CaO transformation, having important geological and technological implications. Keywords: Calcite, lime, thermal decomposition, CaO nanocrystals, TEM-SAED, oriented aggregation, kinetics, topotactic

The thermogravimetry (TG), differential scanning calorimetry (DSC), X-ray thermodiffraction (XRTD) and environmental scanning electron microscopy (ESEM) are used to study the thermal dehydration of epsomite crystals grown in the presence and absence of organic additives. The combined use of XRTD, ESEM and kinetic thermal analyses is a novel approach to study thermally stimulated reactions, which helps in understanding the complex physical chemistry of organic-inorganic interactions.

We report a novel solvothermal route for the production of bassanite (CaSO4·0.5H2O) nanoparticles using amorphous Ca-ethoxide as a precursor. Bassanite nanorods, 120–200 nm in length, with the highest specific surface area reported so far (54 m2 g−1) and enhanced reactivity, are obtained at 78 °C and 1 atm. Such nanoparticles may find application in several fields, including biomaterials, drug delivery, and cultural heritage conservation. [ABSTRACT FROM AUTHOR]

Hydrated lime (Ca(OH)2) is a vernacular art and building material produced following slaking of CaO in water. If excess water is used, a slurry, called lime putty, forms, which has been the preferred craftsman selection for formulating lime mortars since Roman times. A variety of natural additives were traditionally added to the lime putty to improve its quality. The mucilaginous juice extracted from nopal cladodes has been and still is used as additive incorporated in the slaking water for formulation of lime mortars and plasters, both in ancient Mesoamerica and in the USA Southwest. Little is known on the ultimate effects of this additive on the crystallization and microstructure of hydrated lime. Here, we show that significant changes in habit and size of portlandite crystals occur following slaking in the presence of nopal juice as well as compositionally similar citrus pectin. Both additives contain polysaccharides made up of galacturonic acid and neutral sugar residues. The carboxyl (and hydroxyl) functional groups present in these residues and in their alkaline degradation byproducts, which are deprotonated at the high pH (12.4) produced during lime slaking, strongly interact with newly formed Ca(OH)2 crystals acting in two ways: (a) as nucleation inhibitors, promoting the formation of nanosized crystals, and (b) as habit modifiers, favoring the development of planar habit following their adsorption onto positively charged (0001)Ca(OH)2 faces. Adsorption of polysaccharides on Ca(OH)2 crystals prevents the development of large particles, resulting in a very reactive, nanosized portlandite slurry. It also promotes steric stabilization, which limits aggregation, thus enhancing the colloidal nature of the lime putty. Overall, these effects are very favorable for the preparation of highly plastic lime mortars with enhanced properties. [ABSTRACT FROM AUTHOR]

Despite being studied for more than one century, no consensus exists regarding the ultimate mechanism(s) of the thermal decomposition of dolomite [(CaMg(CO3)2]. To shed light on such a reaction, dolomite single crystals were calcined in air between 500 and 1000 °C, and in situ, in a TEM (high vacuum), following irradiation with the electron beam. In situ TEM shows that the decomposition involves the initial formation of a face centered cubic mixed oxide (Ca0.5Mg0.5O) with reactant/product orientation relationships [001]dolomite// <111>oxide, <4̅41>dolomite// <100>oxide, {112̅0}dolomite//{110} oxide, {112̅8}dolomite// {110}oxide and {101̅4}dolomite^{100} oxide ~12°. This phase undergoes de-mixing into oriented crystals of Mg-poor CaO and Ca-poor MgO solid solutions upon long-term e-beam exposure. Ex situ TEM, XRD, 2D-XRD, and FESEM analyses show the formation of porous pseudomorphs made up of oxide nanocrystals with similar parent/product orientation relationships, but with limited Ca/Mg substitution (up to ~9-11%) due to de-mixing (spinodal decomposition) of the metastable (Ca,Mg)O precursor. High ion diffusivity at T > 500 °C (ex situ experiments) favors the formation of pure CaO and MgO crystals during coarsening via oriented aggregation and sintering. These results show that the thermal decomposition of dolomite is topotactic and independent of pCO2. Formation of Mg-calcite nanocrystals (up to ~8 mol% Mg) during the so-called "half decomposition" is observed at 650-750 °C. This transient phase formed topotactically following the reaction of CaO nanocystals (solid solution with ~9 mol% Mg) with CO2 present in the air and/or released upon further dolomite decomposition. With increasing T, Mg- calcite transformed into calcite, which underwent decomposition following the known topotactic relationship: {101̅4}calcite//{110}CaO and <4̅41>calcite//<110>CaO. These observations solve the long standing controversy on the mechanism of the "two-stage" decomposition of dolomite, which assumed the direct formation of calcite during the so-called "half decomposition." [ABSTRACT FROM AUTHOR]

In this work, the potential of bio-inspired strategies for the synthesis of calcium sulfate (CaSO 4 ·nH 2 O) materials for heritage conservation is explored. For this, a nonclassical multi-step crystallization mechanism to understand the effect of calcein- a fluorescent chelating agent with a high affinity for divalent cations- on the nucleation and growth of calcium sulfate phases is proposed. Moving from the nano- to the macro-scale, this strategy sets the basis for the design and production of fluorescent nano-bassanite (NB-C; CaSO 4 ·0.5H 2 O), with application as a fully compatible consolidant for the conservation of historic plasterwork. Once applied to gypsum (CaSO 4 ·2H 2 O) plaster specimens, cementation upon hydration of nano-bassanite results in a significant increase in mechanical strength, while intracrystalline occlusion of calcein in newly-formed gypsum cement improves its weathering resistance. Furthermore, under UV irradiation, the luminescence produced by calcein molecules occluded in gypsum crystals formed upon nano-bassanite hydration allows the easy identification of the newly deposited consolidant within the treated gypsum plaster without altering the substrate's appearance., (© 2024 The Author(s). Small published by Wiley‐VCH GmbH.)

We report a novel solvothermal route for the production of bassanite (CaSO 4 ·0.5H 2 O) nanoparticles using amorphous Ca-ethoxide as a precursor. Bassanite nanorods, 120-200 nm in length, with the highest specific surface area reported so far (54 m 2 g -1 ) and enhanced reactivity, are obtained at 78 °C and 1 atm. Such nanoparticles may find application in several fields, including biomaterials, drug delivery, and cultural heritage conservation.

Hydrated lime (Ca(OH) 2 ) is a vernacular art and building material produced following slaking of CaO in water. If excess water is used, a slurry, called lime putty, forms, which has been the preferred craftsman selection for formulating lime mortars since Roman times. A variety of natural additives were traditionally added to the lime putty to improve its quality. The mucilaginous juice extracted from nopal cladodes has been and still is used as additive incorporated in the slaking water for formulation of lime mortars and plasters, both in ancient Mesoamerica and in the USA Southwest. Little is known on the ultimate effects of this additive on the crystallization and microstructure of hydrated lime. Here, we show that significant changes in habit and size of portlandite crystals occur following slaking in the presence of nopal juice as well as compositionally similar citrus pectin. Both additives contain polysaccharides made up of galacturonic acid and neutral sugar residues. The carboxyl (and hydroxyl) functional groups present in these residues and in their alkaline degradation byproducts, which are deprotonated at the high pH (12.4) produced during lime slaking, strongly interact with newly formed Ca(OH) 2 crystals acting in two ways: (a) as nucleation inhibitors, promoting the formation of nanosized crystals, and (b) as habit modifiers, favoring the development of planar habit following their adsorption onto positively charged (0001) Ca(OH) 2 faces. Adsorption of polysaccharides on Ca(OH) 2 crystals prevents the development of large particles, resulting in a very reactive, nanosized portlandite slurry. It also promotes steric stabilization, which limits aggregation, thus enhancing the colloidal nature of the lime putty. Overall, these effects are very favorable for the preparation of highly plastic lime mortars with enhanced properties.