Nanoscale phenomena during the growth of solid solutions on calcite {101¯4} surfaces

This work deals with the growth behaviour of calcite {101¯4} surfaces in contact with multicomponent aqueous solutions containing divalent cations (Ba²⁺, Sr²⁺, Mn²⁺, Cd²⁺, or Mg²⁺). The result is the formation of solid solutions, with calcite or aragonite as one of the end-members. In situ atomic force microscopy has revealed a wide variety of surface phenomena occurring during the formation of these solid solutions. Among them are: (1) the thickening of growth steps and the subsequent dissolution of surfaces followed by the nucleation of secondary three-dimensional nuclei on calcite surfaces, (2) the transition between growth mechanisms, (3) the formation of an epitaxial layer that armours the substrate from further dissolution and (4) the inhibitory effect of the newly formed surface on the subsequent growth (template effect). The two last phenomena can considerably limit coprecipitation as an effective mechanism for divalent metal uptake. All the phenomena described are a consequence of the interplay between thermodynamics, supersaturation of the aqueous solution with respect to the possible solid solutions and the crystallographic control of the surfaces on the cation incorporation, and indicates that there are many differences between the crystal growth of solid solutions and phases with fixed composition.     

Crystal chemistry and reaction relations of piemontites and thulites from highly oxidized low grade metamorphic rocks at Vitali,Andros Island,Greece

Piemontite- and thulite-bearing assemblages from highly oxidized metapelitic and metacalcareous schists associated with braunite quartzites at Vitali, Andros island, Greece, were chemically investigated. The Mn-rich metasediments are intercalated in a series of metapelitic quartzose schists, marbles, and basic metavolcanites which were affected by a regional metamorphism of the highP/T type (T=400–500° C,P>9 kb) and a later Barrovian-type greenschist metamorphism (T=400–500° C,P～-5–6 kb). Texturally and chemically two generations of piemontite (I and II) can be distinguished which may show complex compositional zoning. Piemontite I coexisted at highP/T conditions with braunite, manganian phengite (alurgite), Mn³⁺-Mn²⁺-bearing Na-pyroxene (violan), carbonate, quartz, hollandite, and hematite. Zoned grains generally exhibit a decreasing Mn³⁺ and an increasing Fe³⁺ and Al content towards the rim. Chemical compositions of piemontite I range from 2.0 to 32.1 mole % Mn³⁺, 0 to 25.6 mole % Fe³⁺, and 60.2 to 81.2 mole % Al. Up to 12.5 mole % Ca on the A(2) site can be substituted by Sr. Piemontites formed in contact or close to braunite (±hematite) attained maximum (Mn³⁺+Fe³⁺)Al_?1 substitution corrresponding to about 33 mole % Mn³⁺+Fe³⁺ in lowiron compositions and up to about 39 mole % Mn³⁺+ Fe³⁺ at intermediate Fe³⁺/(Fe³⁺+Mn³⁺) ratios. Piemontite II which discontinuously overgrows piemontite I or occurs as separate grains may have been formed by greenschist facies decomposition of manganian Na-pyroxenes according to the reaction: (1) $$\begin{gathered} {\text{Mn}}^{{\text{3 + }}} - Mn^{2 + } - bearing omphacite/chloromelanite \hfill \\ + CO_2 + H_2 O + HCl \pm hermatite \hfill \\ = piemontite + tremolite + albite + chlorite \hfill \\ + calcite + quartz + NaCl \pm O_2 . \hfill \\ \end{gathered} $$ Thulites crystallized in coexistence with Al-rich piemontite II. All thulites analysed are low-Fe³⁺ manganian orthozoisites with Mn^tot～-Mn³⁺ substituting for Al on the M(3) site. Their compositions range from 2.9 to 7.2 mole % Mn³⁺, 0 to 1.2 mole % Fe³⁺, and 91.8 to 96.7 mole % Al. Piemontites II in thulite-bearing assemblages range from 5.8 to 15.9 mole % Mn³⁺, 0 to 3.7 mole % Fe³⁺, and 83.7 to 93.6 mole % Al. By contrast, piemontites II in thulite-free assemblages are similarly enriched in Mn³⁺ + Fe³⁺ — and partially in Sr²⁺ — as core compositions of piemontite I (21.1 to 29.6 mole % Mn³⁺, 2.0 to 16.5 mole % Fe³⁺, 60.6 to 68.4 mole % Al, 0 to 29.4 mole % Sr in the A(2) site). The analytical data presented in this paper document for the first time a continuous low-Fe³⁺ piemontite solid solution series from 5.8 to 32.1 mole % Mn³⁺. Aluminous piemontite II is enriched by about 3 mole % Mn³⁺+Fe³⁺ relative to coexisting thulite in Fe³⁺-poor samples and by about 6 mole % Mn³⁺+Fe³⁺ in more Fe³⁺-rich samples. Mineral pairs from different samples form a continuous compositional loop. Compositional shift of mineral pairs is attributed to the effect of a variable fluid composition at constantP _fluid andT on the continuous reaction: (2) $$\begin{gathered} piemontite + CO_2 \hfill \\ = thulite + calcite + quartz \hfill \\ + Mn^{2 + } Ca_{ - 1} [calcite] + H{_2} O + O{_2} \hfill \\ \end{gathered} $$ Further evidence for a variable \(x_{H_2 O} \) and/or \(f_{O_2 } \) possibly resulting from fluid infiltration and local buffering during the greenschist metamorphism is derived from the local decomposition of piemontite, braunite, and rutile to form spessartine, calcite, titanite, and hematite by the reactions: (3) $$\begin{gathered} piemontite + braunite + CO_2 \hfill \\ = sperssartine + calcite + quartz \pm hermatite \hfill \\ + H{_2} O + O{_2} \hfill \\ \end{gathered} $$ and more rarely: (4) $$\begin{gathered} piemontite + quartz + rutile + braunite \hfill \\ = spessartine + titanite + hematite + H{_2} O + O{_2} . \hfill \\ \end{gathered} $$      

The coprecipitation of Sr2+ with calcite at 25°C and 1 atm

The incorporation of Sr²⁺ into calcite at earth surface aqueous conditions is affected by the absolute concentration of Sr²⁺, the presence of Ba²⁺ and NaCl in the solution, and the rate of precipitation. At solution ratios (molar) of Sr²⁺ to Ca²⁺ in the low 10⁻³ range, which yield calcites with several hundred ppm Sr²⁺, K^sr_calcite typically assumes a value between 0.10 and 0.20. Above these concentrations the value of k^Sr_calcite drops to approximately 0.06. Furthermore, if minor amounts of Ba²⁺ or large amounts of Na⁺ (0.48 M) are added to a dilute Sr²⁺ solution, a value around 0.06 for k^Sr_calciteis found. This “strontium concentration effect” and the associated “competitive cation effect” suggest that small amounts of Sr²⁺ may be incorporated into a limited number of nonlattice sites in calcite. Incorporation of Sr²⁺ into these sites, presumably defects, noticeably affects k^Sr_calcite only at low Sr²⁺ concentrations and in the absence of competition from other large cations.An increase in k^Sr_calcite with rate of precipitation, qualitatively similar to that found in other studies, was observed only when precipitation times were decreased from days to hours.For many geologic settings a partition coefficient for Sr²⁺ into calcite of 0.06 appears appropriate, but there are situations—very low Sr²⁺ concentrations, the presence of Mg²⁺, and fast precipitation rates—in which a larger value might better approximate natural partitioning.     

Origin of anhedral Sr-rich calcite in deep-water mixed sediment, north-east Australia

At burial depths of 800-1000 m, within the epicontinental Queensland Trough of north-east Australia (ODP Site 823), microcrystalline inter- and intraskeletal mosaics of anhedral (loaf-shaped, rounded) calcite have Sr²⁺ values ranging from below microprobe detection limits (<150 ppm) to 8100 ppm. Host rocks are well lithified, fine-grained mixed sediment to clayey wackestone and packstone of Middle and Late Miocene age. Petrography demonstrates that calcite precipitation has spanned shallow to deep burial, overlapping formation of framboidal pyrite in the upper 50 m; shallow-burial dolomitization (<300 m); and dedolomitization during sediment consolidation and incipient chemical compaction at greater (>400–500 m) depths. Petrographic observations illustrate that the calcite microfabric formed through coalescing crystal growth resulting from one or a combination of displacive growth in clay, porphyroid neomorphism of aragonite/vaterite, and clay replacement by calcite. Sr²⁺ mean concentrations in calcite between depths of 800 and 1000 m are similar to an expected equilibrium pore-water concentration, using a D^sr of 0.06, and may indicate active calcite precipitation. However, Sr²⁺ variation (2000–5000 ppm) within and among crystals, and concentrations that range well above predicted equilibrium values for a given depth, illustrate either variable Sr²⁺ retention during recrystallization of shelf-derived aragonite (and authigenic local vaterite) or relative uptake of Sr²⁺ during calcite precipitation with burial. Within the context of calcite formation during burial to 1 km, diagenetic attributes that affect the latter process include increased concentrations of pore-water Sr²⁺ with depth associated with aragonite recrystallization/dissolution; upward migration of Sr-rich pore water; and increased D^Sr related to local variation in precipitation/recrystallization rates, differential crystal sector growth rates and/or microvariation in aragonite distribution.     

Reliability of experimental partition coefficients in carbonate systems: Evidence for inhomogeneous distribution of impurity cations

Electron spin resonance (ESR) spectroscopy, thermal analysis and X-ray diffraction (XRD) analysis are used to study the mode of incorporation of a wide range of impurity ions (up to 1% Ni²⁺, Sr²⁺, Cd²⁺, Mg²⁺, Cu²⁺, Fe²⁺, Zn²⁺, Pb²⁺, Mn²⁺ and Ba²⁺) in calcites synthesised in aqueous solutions. It is shown that when certain impurity ions are incorporated in these calcites they become concentrated, together with other trace elements such as Mn²⁺, into hitherto unsuspected trace phases. The important conclusion is drawn that unless the scavenging of trace elements by these phases can be avoided or corrected for, any calcite—liquid partition coefficients measured may have limited significance. ESR spectroscopy has revealed the intense strain common in calcites grown by different methods, whilst thermal analysis suggested that such strain may significantly alter the thermodynamic properties of these calcites. ESR spectroscopic analysis of aragonites synthesised by methods previously reported in the literature has revealed the common occurrence of trace calcite at concentrations below the XRD detection limit and proves the tendency of this calcite to scavenge trace Mn. Using ESR spectroscopy to monitor trace calcite levels and structural imperfections in synthetic aragonites, the urea hydrolysis technique is modified to provide a method for the synthesis of high-purity strain-free aragonite available for experimental studies.     

Nature of Co-bearing ferromanganese crusts of the Magellan Seamounts (Pacific Ocean): Communication 2. Ion exchange properties of ore minerals

The results of experimental studies of ion exchange properties of Co-bearing ferromanganese crusts in the Magellan Seamounts (Pacific Ocean) are discussed. Maximum reactivity in reactions with the participation of manganese minerals (Fe-vernadite, vernadite) is typical of Na⁺, K⁺, and Ca²⁺ cations, whereas minimum activity is recorded for cations Pb²⁺ and Co²⁺. The exchange complex of ore minerals in crusts is composed of Na⁺, K⁺, Ca²⁺, Mg²⁺, and Mn²⁺ cations. The exchange capacity of manganese minerals increases from the alkali metal cations to rare and heavy metal cations. Peculiarities of the affiliation of Co²⁺, Mn²⁺, and Mg²⁺ cations in manganese minerals of crusts are discussed. In manganese minerals, Co occurs as Co²⁺ and Co³⁺ cations. Metal cations in manganese minerals occur in different chemical forms: sorbed (Na⁺, K⁺, Ca²⁺, Mn²⁺, Co²⁺, Cu²⁺, Zn²⁺, Cd²⁺, and Pb²⁺); sorbed and chemically bound (Mg²⁺, Ni²⁺, Y³⁺, La³⁺, and Mo⁶⁺); and only chemically bound (Co³⁺). It is shown that the age of crust, its preservation time in the air-dry state, and type of host substrate do not affect the ion exchange indicators of manganese minerals. It has been established that alkali metal cations are characterized by completely reversible equivalent sorption, whereas heavy metal cations are sorbed by a complex mechanism: equivalent ion exchange for all metal cations; superequivalent, partly reversible sorption for Ba²⁺, Pb²⁺, Co²⁺, and Cu²⁺ cations, relative to exchange cations of manganese minerals. The obtained results refine the role of ion exchange processes during the hydrogenic formation of Co-bearing ferromanganese crusts.     

Metastable phenomena on calcite {101̄4} surfaces growing from Sr2+–Ca2+–CO32− aqueous solutions

In situ atomic force microscopy (AFM) experiments, scanning electron microscopy (SEM) imaging and composition analysis, and X-ray diffraction have provided information about the growth, dissolution and transformation processes promoted by Sr²⁺–Ca²⁺–CO₃²⁻ aqueous solutions in contact with calcite {101̄4} surfaces. Experiments have shown a wide variety of surface phenomena, such as the influence of the Sr-bearing newly-formed surface on the subsequent growth (template effect), the growth and subsequent dissolution of surfaces and the nucleation of secondary three-dimensional nuclei on calcite surfaces. These phenomena reveal the metastability of the crystallisation system and are a consequence of the interplay between thermodynamics (the relative stability of the two calcite and aragonite structure solid solutions that can be formed), supersaturation of the aqueous solution with respect to the two possible solid solutions, and the crystallographic control of the surfaces on cation incorporation.     

Sr,Cd, Mn and Co distribution coefficients in calcite as a function of calcite precipitation rate

Distribution coefficients, as a function of precipitation rate, were determined for the metals Sr²⁺, Co²⁺, Mn²⁺ and Cd²⁺in calcite. A pH-stat was used to maintain a constant degree of-saturation, and hence precipitation rate, during each coprecipitation run. The precipitation rate was proportional to the degree of supersaturation and the mass of seed crystal introduced. Distribution coefficients (λ) as a function of rate were determined using radioactive isotopes for solutions with saturations $\text{Ω = 1}$ to $\text{Ω = 5.5}$. Strontium distribution coefficients increased with increasing precipitation rate, while Co, Mn and Cd distribution coefficients decreased with increasing precipitation rate. The following rate expressions (at 25°C) were derived: logλ_Sr = 0.249 log R ?1.57logλ_Mn = ?0.266 log R + 1.35logλ_Co = ?0.173 log R + 0.68logλ_Cd = ?0.194 log R + 1.46 where R is the observed precipitation rate in nmoles CaCO₃ per mg seed crystal per min.In separate experiments the uptake of radioactive isotopes was monitored during the recrystallization of calcite seed crystals. Rates of recrystallization were from 100 to 10, 000 times slower than the pH-stat experiments, but yielded distribution coefficients consistent with the above rate expressions.Using gross estimates of biogenic crystal growth rates, aragonite to calcite transformation rates, and the above Sr rate expression, biogenic calcite and diagenetic calcite Sr contents are estimated. These experiments indicate that in addition to solution composition, precipitation rate is a significant factor influencing the trace metal content of naturally occurring calcite.     

Electron paramagnetic resonance and polarized optical absorption spectra of Ni2+ in synthetic forsterite

The investigated Ni doped forsterite was grown with the floating zone technique. The EPR spectra were taken at room temperature using both 9.5 and 35 GHz. All specimens show EPR signals resulting from Mn²⁺ at M2 and Fe³⁺ at M1, M2, and Si positions. Ni²⁺ EPR signals are observed at 35 GHz but not at 9.5 GHz. The Ni²⁺ spectra are described by the spin Hamiltonian

Selected geochemical factors influencing diagenesis of eocene carbonate rocks,peninsular Florida,U.S.A.

The geochemical significance of three selected ions (Mg²⁺, Na⁺, and Sr²⁺) supports a model of dolomitization by brackish groundwater. This groundwater zone contains sufficient quantities of Mg²⁺ to facilitate dolomitization ($\text{Mg}\text{Ca}\text{ratios 1}$). Rising and falling of sea level and fluctuations of the phreatic zone related to climatic variations account for the thickness of the dolomite layers and the chemical distributions within these layers. Sodium concentrations in the calcite are 70–185 ppm, indicating formation in brackish water. Dolomite has sodium concentrations between 50–1400 ppm, suggesting formation in waters of similar salinity.Strontium in calcite ranges from 320–600 ppm, suggesting diagenesis in slightly saline waters in an open system. Dolomite contains 241 ppm Sr²⁺ on the average and calcite has 418 ppm Sr²⁺. The Sr²⁺ concentrations of the dolomite are characteristic of diagenesis in water less saline than sea water. Average strontium concentrations in the dolomite occur in two distinct groups, 260 ppm for dolomite with 39–43 mole-% MgCo₃ and 195 ppm for the dolomite with 44–50 mole-% MgCO₃. The difference in the Sr²⁺ concentrations of the two dolomite groups indicates the higher mole-% MgCO₃ dolomite recrystallized in a less saline environment than the lower mole-% MgCO₃ dolomite. These different environments are attributed to a relatively more saline coastal environment and a less saline inland environment.The more nearly stoichiometric dolomite (44–50 mole-% MgCO₃) has less scatter when mole-% MgCO₃ is plotted against Sr²⁺ and Na⁺. This suggests a greater approach to equilibrium with the dolomitizing fluid than the lower mole-% MgCO₃ (39–43) dolomite. The more saline environment has higher Mg/Ca ratios and promotes more calcium-rich dolomite during diagenesis because of the inhibition from competing foreign ions and because it is thermodynamically a more favorable environment which causes more rapid crystallization. The less saline waters allow recrystallization to proceed more slowly, producing better ordering in the dolomites, textural preservation and development of subhedral to euhedral rhombic crystals.     

Modulations in incommensurate (Ca1–xSrx)2MgSi2O7 single crystals

Single crystals of (Ca_1–xSr_x)₂MgSi₂O₇ slightly doped with 1000 ppm Mn²⁺ and with x ranging from 0.04 to 0.32 were grown from the melt in a mirror furnace applying the Czochalski technique. Transmission electron microscopy (TEM) revealed incommensurately modulated structures at room-temperature for all compositions in accordance with earlier studies by electron paramagnetic resonance (EPR). Electron diffraction patterns clearly show satellite reflections typical for two-dimensional modulation, and their successive destabilization with increasing Sr content. The modulation is of tartan-like appearance. Beyond a Sr/(Sr+Ca) ratio of about 0.32 the synthesis of stable solid solution åkermanite type crystals was proved not to be feasible, indicating the existence of a miscibility gap in the Sr åkermanite system. As presumed from the diffuse scattering around the satellite reflections, and suggested more conclusively by crystallographic processing of high resolution EM images the Sr ions incorporated into the incommensurate crystal phase are distributed in an ordered fashion and are partly adapted to the displacive modulation of the pure åkermanite. This means, occupational modulation even makes a contribution to the overall modulation characteristics in (Ca_1–xSr_x)₂ MgSi₂O₇.     

Calcite dissolution: Effects of trace cations naturally present in Iceland spar calcites

In situ dissolution experiments on a set of pure, optical quality Iceland spar calcite samples from four different localities showed etch pit step retreat rates to be inversely proportional to total inherent trace cation composition. Atomic absorption spectroscopy (AAS) revealed Fe²⁺, Mg²⁺, Mn²⁺ and Sr²⁺ in amounts varying from a few to hundreds of ppm. We used a very simple experimental set-up, with an Atomic Force Microscope (AFM) fluid cell and a droplet of MilliQ water. As the calcite dissolved and approached equilibrium with the solution, trace cations were released, which were then present for interaction with the dissolving surface. We monitored continuous free-drift dissolution, in situ, on fresh cleavage surfaces for up to 40 min. Dissolution produced one-layer-deep, rhombic etch pits that continually expanded as we collected images. The rhombohedral symmetry of calcite defines two obtuse and two acute edges on the cleavage surface of etch pits and these, as expected from previous work, had different dissolution rates. Despite identical experimental conditions for all samples, we observed lower step retreat rates for both obtuse and acute edges on calcite characterised by relatively high trace cation composition. Increased cation concentration, particularly Mn, was also correlated with rounding of obtuse-obtuse corners, resulting in obtuse step retreat rates similar to those for acute sides. Physcial limitations of the AFM technique were taken into account when measuring step rate retreat and results were collected only from single-layer etch pits, which represent crystalline calcite with minimal defects. Dissolution rates presented here are thus lower than previous reports for studies of deep etch pits and where the physical limitations of imaging may not have been considered. In addition to molecular-level proof that divalent cations inherent at ppm levels in the calcite affect the dissolution process, these results show that pure, optical quality Iceland spar calcite should not be considered pure in the chemical sense. The results imply that dissolution rates determined for ideal systems with pure, synthetic or natural, materials may be considered as the boundary condition for dissolution in real systems in nature, where cations are always present both in the solution and in the initial solid.     

Sorption of trace metals on calcite: Applicability of the surface precipitation model

Published Sorption isotherm data of Cd²⁺, Mn²⁺, Zn²⁺, and Co²⁺ on calcite are adequately described by the surface precipitation model which was originally developed by FArley et al. (1985) for the sorption of cations on metal oxides. In addition to monolayer adsorption, the model accounts for the formation of a surface phase with a composition that is described by a solid solution having as end members the sorbent calcium carbonate mineral and a pure carbonate precipitate of the sorbing trace metal. The model thus specifies a continuum between adsorption and precipitation. This feature is supported in the literature by observations on the reaction kinetics and the amount of surface coverage during trace metal sorption on calcite. The apparent adsorption constants of these trace metals, as derived from the model, can be ranked according to the degree to which their ionic radii match the ionic radius of Ca²⁺.     

Determination of the physico-chemical characteristics of hydrothermal fluids from the post-metamorphic Omai gold deposit, Guiana Shield, using analysis of ionic species by crush-leach technique and capillary electrophoresis

Summary Capillary electrophoresis (CE) or capillary ion analysis (CIA^TM) represents a relatively new, simple and fast analytical technique suited in particular to the analysis of aqueous fluids and leach solutions. The mechanisms responsible for separation in CE are based on the ion charge/size ratios. The application of capillary electrophoresis to a number of key vein-forming minerals is described for the first time for a Precambrian orogenic gold deposit. The Omai deposit is situated in the north-central part of the Barama-Mazaruni Supergroup, Guiana Shield, a Paleoproterozoic granitoid-greenstone belt metamorphosed during the Trans-Amazonian tectonothermal event at ca. 2.1Ga.Fluid inclusions extracted by crushing and leaching from 45 vein-forming mineral separates have been analyzed for ionic composition. The anionic species detected in the Omai fluids are , Cl^–, , , oxalate, , F^–, formate, , , acetate and propionate. Cationic species include , K⁺, Na⁺, Ca²⁺, Sr²⁺, Mg²⁺, Mn²⁺, Ba²⁺, Li⁺, Zn²⁺, and Cu²⁺. Calcium, potassium and sodium cations have been used for temperature calculation. The presence of ionization products of organic acids is described for the first time in an orogenic hydrothermal system. However, their origin is still unclear and more data are needed to assess their role in the hydrothermal systems. Gold was transported as sulfide complexes, although the thiosulfate involvement in the remobilization and transport of gold in the upper part of the hydrothermal system at Omai is plausible.Present addresses: Cambior Management Services, Rosebel Gold Mines NV, Paramaribo, SurinameUNESCO Center for Macromolecules, Division of Polymer Science, Department of Chemistry, University of Stellenbosch, Matieland, South Africa     

The uptake of cobalt into ferromanganese nodules,soils, and synthetic manganese (IV) oxides

Strong enrichments of cobalt occur in marine manganese nodules, soils, wads, and natural and synthetic minerals such as hollandite, cryptomelane, psilomelane, lithiophorite, birnessite, and δ-MnO₂. Previously, it was suggested that Co³⁺ ions in these minerals replace either Mn³⁺ or substitute for Fe³⁺ in incipient goethite epitaxially intergrown with δ-MnO₂. Neither of these interpretations is now considered to be satisfactory on account of the large discrepancy of ionic radius between octahedrally coordinated low-spin Co³⁺ and high-spin Mn³⁺ or Fe³⁺ in oxide structures. The close agreement between the ionic radii of Co³⁺ and Mn⁴⁺ suggests that some cobalt substitutes for Mn⁴⁺ ions in edge-shared [MnO₆] octahedra in many manganese(IV) oxide mineral structures. It is proposed that hydrated cations, including Co²⁺ ions, are initially adsorbed on to the surfaces of certain Mn(IV) oxides in the vicinity of essential vacancies found in the chains or sheets of edge-shared [MnO₆] octahedra. Subsequently, fixation of cobalt takes place as a result of oxidation of adsorbed Co²⁺ ions by Mn⁴⁺ and replacement of the displaced manganese by low-spin Co³⁺ ions in the [MnO₆] octahedra or vacancies.     

Optical absorption and mössbauer spectra of purple and green yoderite,a kyanite-related mineral

Mössbauer and polarized optical absorption spectra of the kyanite-related mineral yoderite were recorded. Mössbauer spectra of the purple (PY) and green yoderite (GY) from Mautia Hill, Tanzania, show that the bulk of the iron is Fe³⁺ in both varieties, with Fe²⁺/(Fe²⁺+Fe³⁺) ratios near 0.05. Combining this result with new microprobe data for PY and with literature data for GY gives the crystallochemical formulae: $$\begin{gathered} ({\text{Mg}}_{{\text{1}}{\text{.95}}} {\text{Fe}}_{{\text{0}}{\text{.02}}}^{{\text{2 + }}} {\text{Mn}}_{{\text{0}}{\text{.01}}}^{{\text{2 + }}} {\text{Fe}}_{{\text{0}}{\text{.34}}}^{{\text{3 + }}} {\text{Mn}}_{{\text{0}}{\text{.07}}}^{{\text{3 + }}} {\text{Ti}}_{{\text{0}}{\text{.01}}} {\text{Al}}_{{\text{3}}{\text{.57}}} )_{5.97}^{[5,6]} \hfill \\ {\text{Al}}_{{\text{2}}{\text{.00}}}^{{\text{[5]}}} [({\text{Si}}_{{\text{3}}{\text{.98}}} {\text{P}}_{{\text{0}}{\text{.03}}} ){\text{O}}_{{\text{18}}{\text{.02}}} ({\text{OH)}}_{{\text{1}}{\text{.98}}} ] \hfill \\ \end{gathered}$$ and PY and $$\begin{gathered} ({\text{Mg}}_{{\text{1}}{\text{.98}}} {\text{Fe}}_{{\text{0}}{\text{.02}}}^{{\text{2 + }}} {\text{Mn}}_{{\text{< 0}}{\text{.001}}}^{{\text{2 + }}} {\text{Fe}}_{{\text{0}}{\text{.45}}}^{{\text{3 + }}} {\text{Ti}}_{{\text{0}}{\text{.01}}} {\text{Al}}_{{\text{3}}{\text{.56}}} )_{6.02}^{[5,6]} \hfill \\ {\text{Al}}_{{\text{2}}{\text{.00}}}^{{\text{[5]}}} [({\text{Si}}_{{\text{3}}{\text{.91}}} {\text{O}}_{{\text{17}}{\text{.73}}} {\text{(OH)}}_{{\text{2}}{\text{.27}}} ] \hfill \\ \end{gathered}$$ for GY. The Mössbauer spectra at room temperature contain one main doublet with isomer shifts and quadrupole splittings of 0.36 (PY), 0.38 (GY) and 1.00 (PY), 0.92 (GY) mm s^?1, respectively. These values correspond to Fe³⁺ in six or five-fold coordination. The doublet components have anomalously large half widths indicating either accomodation of Fe³⁺ in more than one position (e.g., octahedraA1 and five coordinatedA2) or the yet unresolved superstructure. Besides strong absorption in the ultraviolet (UV) starting from about 25,000 cm^?1, the polarized optical absorption spectra are dominated by strong bands around 16,500 and 21,000 cm^?1 (PY) and a medium strong band at around 13,800 cm^?1 (GY). Position and polarization of these bands, in combination with the UV absorption, explain the colour and pleochroism of the two varieties. The bands in question are assigned to homonuclear metal-to-metal charge transfer transitions: Mn²⁺(A1) Mn³⁺(A1′) ? Mn³⁺(A1) Mn²⁺(A1′) and Mn²⁺(A1) Mn³⁺(A2 ? Mn³⁺(A1) Mn²⁺(A2) in PY and Fe²⁺(A1) Fe³⁺(A1′) ? Fe³⁺(A1) Fe²⁺(A1′) in GY. The evidence for homonuclear Mn²⁺ Mn³⁺ charge transfer (CTF) is not quite clear and needs further study. Heteronuclear FeTi CTF does not contribute to the spectra. In PY, additional weak bands were resolved at energies around 17,700, 18,700, 21,000, and 21,900 cm^?1 and assigned to Mn³⁺ in two positions. Weak bands around 10,000 cm^?1 in both varieties are assigned to Fe²⁺ spin-alloweddd-transitions. Very weak and sharp bands, around 15,400, 16,400, 21,300, 22,100, 23,800, and 25,000 cm^?1 are identified in GY and assigned to Fe³⁺ spin-forbiddendd-transitions.     

Magnetic behavior of trioctahedral micas with different octahedral Fe ordering

This contribution is finalized at the discussion of the magnetic structure of two samples, belonging to phlogopite–annite [sample TK, chemical composition ^IV(Si_2.76Al_1.24) ^VI(Al_0.64Mg_0.72 $ {\text{Fe}}_{1.45}^{2 + } $ Mn_0.03Ti_0.15) (K_0.96Na_0.05) O_10.67 (OH)_1.31 Cl_0.02] and polylithionite–siderophyllite joints [sample PPB, chemical composition ^IV(Si_3.14Al_0.86)^VI(Al_0.75Mg_0.01 $ {\text{Fe}}_{1.03}^{2 + } $ $ {\text{Fe}}_{1.03}^{3 + } $ Mn_0.01Ti_0.01Li_1.09) (K_0.99Na_0.01) O_10.00 (OH)_0.65F_1.35]. Samples differ for Fe ordering in octahedral sites, Fe²⁺/(Fe²⁺?+?Fe³⁺) ratio, octahedral composition, defining a different environment around Fe cations, and layer symmetry. Spin-glass behavior was detected for both samples, as evidenced by the dependency of the temperature giving the peak in the susceptibility curve from the frequency of the applied alternating current magnetic field. The crystal chemical features are associated to the different temperature at which the maximum in magnetic susceptibility is observed: 6?K in TK, where Fe is disordered in all octahedral sites, and 8?K in PPB sample, showing a smaller and more regular coordination polyhedron for Fe, which is ordered in the trans-site and in one of the two cis-sites.     

Broad distribution of crystal-field environments for Nd<Superscript>3+</Superscript> in calcite

Calcite crystals were grown from solution with single-crystal dimensions up to 3 mm and doped up to 0.1 at% with Nd³⁺ ions. Phase purity was verified by powder X-ray diffraction. The concentration of Nd³⁺ was measured by energy-dispersive spectrometry and Rutherford backscattering spectrometry. Micro X-ray fluorescence mapping of the calcite grains indicates uniform Nd distribution in as-grown crystal grains. X-ray absorption fine structure suggests that Nd³⁺ is substituted for Ca²⁺ with local lattice distortion. Temperature-dependent near-infrared spectroscopy of Nd³⁺ impurities in calcite reveals large inhomogeneous linewidths and smooth line profiles that are characteristic of glassy hosts, though the samples are well crystallized.     

Crystal morphology and surface structures of orthorhombic MgSiO3 perovskite

Orthorhombic MgSiO₃ perovskite is thought to be the most abundant mineral in the mantle of the Earth. Its bulk properties have been widely studied, but many geophysical and rheological processes are also likely to depend upon its surface and grain boundary properties. As a first step towards modelling these geophysical properties, we present here an investigation of the structures and energetics of the surfaces of MgSiO₃-perovskite, employing both shell-model atomistic effective-potential simulations, and density-functional-theory (DFT) calculations. Our shell-model calculations predict the {001} surfaces to be the energetically most stable surfaces: the calculated value of the surface energy being 2.2 J/m² for the MgO-terminated surface, which is favoured over the SiO₂-terminated surface (2.7 J/m²). Also for the polar surfaces {111}, {101} and {011} the MgO-terminated surfaces are energetically more stable than the Si-terminated surfaces. In addition we report the predicted morphology of the MgSiO₃ perovskite structure, which is dominated by the energetically most stable {001} and {110} surfaces, and which appears to agree well with the shape of grown single crystals.