Compressibility of nanocrystalline forsterite

We established an equation of state for nanocrystalline forsterite using multi-anvil press and diamond anvil cell. Comparative high-pressure and high-temperature experiments have been performed up to 9.6 GPa and 1,300°C. We found that nanocrystalline forsterite is more compressible than macro-powder forsterite. The bulk modulus of nanocrystalline forsterite is equal to 123.3 (±3.4) GPa whereas the bulk modulus of macro-powder forsterite is equal to 129.6 (±3.2) GPa. This difference is attributed to a weakening of the elastic properties of grain boundary and triple junction and their significant contribution in nanocrystalline sample compare to the bulk counterpart. The bulk modulus at zero pressure of forsterite grain boundary was determined to be 83.5 GPa.     

OH defects in forsterite

Polarized FTIR spectra of near endmember forsterite single crystals from Pamir, Tadzikistan show the existence of sharp strongly pleochroic absorption bands in the region of the OH stretching fundamental. Bands centered at 3674/3624, 3647/3598 and 3640/ 3592 cm^-1 are attributed to OH dipoles oriented parallel to [100]. An OH band doublet at 3570/3535 cm^-1 shows both, a strong absorption parallel to [100] and a strong component parallel to [001]. On the basis of the pleochroic scheme and under the assumption of vacancies on Si- and M-sites it is proposed that O1 is partially replaced by OH defects pointing to the vacant Si-site. O3 is donator oxygen of OH dipoles lying near the O3-O1 tetrahedral edge or roughly pointing to a vacant M2-site. Also O2 can act as donator oxygen of an OH group oriented along the O2-O3 edge of a vacant M1 octahedron. The splitting of the bands is explained by the presence of Fe²⁺ in cation sites surrounding the OH defects.     

Origin of chondritic forsterite grains

Iron-poor and refractory lithophile element (RLE) rich forsterite grains occur in all major types of unequilibrated chondrites. In our laser ablation inductively coupled mass spectrometry (LA-ICPMS) minor and trace element study we show that refractory forsterites (RF) from carbonaceous (CC), unequilibrated ordinary (UOC) and a Rumuruti chondrite (RC) have similar chemical compositions with high RLE concentrations and low concentrations of Mn, Fe, Co and Ni. Fractionation of RLEs and rare earth elements (REEs) is in agreement with formation by crystallization from a RLE rich silicate melt. Low concentrations and the fractionation of moderately siderophile elements (Fe, Co, Ni) in RFs suggests formation at low oxygen fugacity, possibly in equilibrium with primitive Fe,Ni metal condensates in a gas of solar composition. Anomalously high Ti in the parental melt can be explained by Ti³⁺/Ti⁴⁺ ∼1.5, supporting formation of RF in highly reducing conditions. Low Mn concentrations indicate formation at high temperatures (>∼1160 K). The model of formation of RFs and the accompanying physico-chemical conditions during their formation as well as their relation to non refractory olivine are discussed.     

High-temperature infrared properties of forsterite

Polarized emittance measurements were acquired for synthetic forsterite, the pure magnesium end member of the olivines group, on the whole infrared spectral range and up to the melting point by using CO₂ laser heating. The experimental data, fitted with a semi-quantum dielectric function model, allowed the retrieval of the temperature dependence of the absorption coefficient of forsterite both in the opaque and semi-transparent regions. The analysis of the phonon parameters indicates that the lattice dynamics evolve drastically with increasing temperature. The normal modes involving motions of the magnesium cations located in site 1 are the more impacted, and some of them vanish around 1,200 K. The results confirm that the enhancement of the lattice anharmonicity and the increasing mobility of the magnesium cations are closely linked and are at the origin of the anomalies observed in the evolution of the thermophysical properties. This complete set of spectroscopic data may be a step toward a more precise evaluation of the impact of thermal radiation heat transfer inside systems involving forsterite and quantification of their heat budget.     

Chemical etching of dislocations in forsterite

Chemical etching of pure melt-grown forsterite crystals is reported here for the first time. Cleaved forsterite crystals of known purity, and polished sections of the same crystals have been successfully etched revealing dislocations, subgrain boundaries, inclusions and growth imperfections.     

Determination of point of zero charge of PILCS with single and mixed oxide pillars prepared from Tunisian-smectite

Different pillared interlayer clays (PILCs) with single oxide pillars of Cr and Al and mixed oxide pillars of these metals have been prepared from a Tunisian purified smectite (Hp): Several Al/metal, OH/metal and metal/clay ratios were used in order to investigate the effect on the chemical and physical properties, specifically the point of zero charge of the synthesized pillared clays. These chromium pillared clays compounds at n meq (Nitrate) and at n meq (Chlorite) are noted pillared clay and pillared clays \(Hp(20,40,60)_{NO_3 } \) and Hp(20,40,60)_Cl. The structure of the pillared materials are studied by XRD and cationic exchange capacity, the textural property are investigated by the specific surface area S_BET. The acid-base chemistry “surface acidity” of these products was analysed by using acid-base potentiometric in order to determine the PZC of each sample. The resulting materials exhibited basal spacings in the range of 13.96–21.13 Å, with high surface areas (10.58–198 m² g^?1). Pillared clays prepared from Tunisian purified smectite showed an increase of basal spacings and surface areas. A relatively strong interaction between metal and aluminium in the pillars was observed. The samples are studied by acid-base potentiometric. This experimental method is used to determine the point of zero charge (PZC). The PZC values of the edge sites are 8.2 for Tunisian purified smectite and 6.8; 5.9; 5.19; 6.84; 5.86; 5.73; 6.78; 7.56 for the differents samples respectively: \(Hp(20,40,60meq)_{NO_3 } \) Hp(20,40,60meq)_Cl; Hp-Cr/Al; Hp-Al.     

Oxalate-promoted forsterite dissolution at low pH

We ran a series of 124 semi-batch reactor experiments to measure the dissolution rate of forsterite in solutions of nitric and oxalic acid solutions over a pH range of 0-7 and total oxalate concentrations between 0 and 0.35 m at 25 °C. We found that the empirical rate law for the dissolution of forsterite in these solutions is

     

Water diffusion in synthetic iron-free forsterite

The kinetics of hydrogenation of dry synthetic forsterite single crystals was determined by performing experiments under hydrothermal conditions. The experiments were performed at 1.5 GPa, 1000 °C for 3 h in a piston-cylinder apparatus, or at 0.2 GPa, 900–1110 °C, for 3–20 h in TZM cold-seal vessels. The oxygen fugacity was buffered using Fe–FeO or Ni–NiO powders. Polarized Fourier transform infrared spectroscopy was utilized to quantify the hydroxyl distributions in the samples after the experiments. Hydrogenation rates were measured parallel to the three crystallographic axes from profiles of water content as a function of position in the samples. The chemical diffusion coefficients are marginally slower than in natural iron-bearing olivine for the same diffusion process, but the anisotropy of diffusion is the same, with the [001] axis the fastest direction of diffusion and [100] the slowest. Fits of the diffusion data to an Arrhenius law yield similar activation energies for each of the crystallographic axes; a global fit to all the diffusion data gave 211 ± 18 kJ mol^–1, in reasonable agreement with the previous results for natural olivine. Thus hydrogenation most likely occurs by coupled diffusion of protons and octahedrally coordinated metal vacancies. The diffusion rates are fast enough to modify water contents within xenoliths ascending from the mantle, but probably too slow to permit a total equilibration of forsterite or olivine crystals.     

Microdenticles on naturally weathered hornblende

Microdenticles (with lengths in the micron-submicron range rather than tens of microns) are developed on the lateral surfaces of larger “classic” denticles on naturally weathered hornblende from weathered amphibolite of the Carroll Knob Complex in western North Carolina. Microdenticles share the shape and orientation of the larger more typical denticles, producing arrays of microdenticles that give the larger host denticle the appearance of a surface covered with imbricate pointed or rounded scales. The arrays of imbricate microdenticles are formed by low-temperature aqueous alteration during weathering of the Carroll Knob Complex hornblende; they are later-stage corrosion forms on already-corroded surfaces of hornblende that show larger-scale evidence of typical weathering. In the Carroll Knob occurrence, hornblende microdenticles are associated with dilute weathering solutions, suggesting possible control by extreme undersaturation of solutions with respect to hornblende.     

Electron paramagnetic resonance study of iridium in forsterite

Electron paramagnetic resonance of Ir²⁺ in forsterite is studied at Q-band frequency and room temperature. There are four equivalent spectra superimposed along the three crystallographic axes. The individual spectrum consists of four hyperfine lines of approximately equal intensity separated from each other by 42 G; one axis of the g tensor is near the c axis. Ir²⁺ is certainly substituted for Mg²⁺. Because of the fourfold degeneracy of the EPR spectrum, it may be suggested that iridium occurs at M₁. Taking into account that one eigenvector of the g tensor is nearly parallel to c, it seems also possible that the substitution takes place at the M₂ position. In this case, the lattice relaxation of the crystal structure around Ir²⁺ at M₂ must break the point symmetry m at M₂.     

Diffusion control of quartz and forsterite dissolution rates

An established engineering model is used to identify conditions where diffusion controls the dissolution of quartz and forsterite in packed beds. The model shows that diffusion control is favored at low advection flux, large grain size, high temperature, and high pH (if the reaction consumes H⁺). Quartz dissolution is chemical reaction controlled for most geochemically reasonable combinations of temperature, grain size, and flow rate. On the other hand, forsterite dissolution rates can be diffusion controlled for typical advection fluxes, grain sizes, temperatures, and pH’s. The apparent activation energy for diffusion-controlled reactions in a packed bed is much higher than the <∼20 kJ/mol value that is often used to identify diffusion controlled reactions. The models are quite general and can be adapted to deal with other mineral dissolution and precipitation reactions.     

Partitioning of manganese between forsterite and silicate liquid

Partition coefficients for Mn between forsterite and liquid in the system MgO-CaO-Na₂O-Al₂O₃-SiO₂ (+ about 0.2% Mn) were measured by electron microprobe for a variety of melt compositions over the temperature range 1250–1450°C at one atm pressure. The forsterite-liquid partition coefficient of Mn (mole ratio, MnO in Fo/MnO in liquid, designated D_mn^fo?Liq) depends on liquid composition as well as temperature: at 1350°C, D_Mn^Fo?Liqranges from 0.60 (basic melt, SiO₂ = 47wt%) to 1.24 (acidic melt, SiO₂ = 65wt%). At lower temperatures, the partition coefficient is more strongly dependent on melt composition.The effects of melt composition and temperature on D_Mn^fo?Liq can be separately evaluated by use of the Si:O atomic ratio of the melts. A plot of D_mn^Fo?Liq measured at various temperatures vs melt Si:O for numerous liquid compositions reveals discrete, constant-temperature curves that are not well defined by plotting D_Mn^Fo?Liq against other melt composition parameters such as melt basicity or MgO content. For constant Si:O in the melt, In D_Mn^Fo?Liq vs reciprocal absolute temperature is linear; however, the slope of the plot becomes more positive for higher values of Si:O, indicating a higher energy state for Mn²⁺ ions in acidic melts than in basic melts.Comparison of Mn partitioning data for the iron-free system used in this study with data of other workers on iron-bearing compositions suggests that the effect of iron on Mn partitioning between olivine and melt is small over the range of basalt liquidus temperatures.     

Equation of state and melting point studies of forsterite

We report in this paper results of lattice dynamical calculations of the thermodynamic properties, namely, the equation of state and the melting point of forsterite. The root mean square displacements of atoms and thermal parameters are also evaluated. It is observed from our results, that, a modified Lindemann criterion for melting may be proposed for an ionic ‘molecular’ solid like forsterite. Agreement among various theoretical and experimental results is satisfactory indicating that lattice dynamical studies based on microscopic quasiharmonic formulations help in understanding the macroscopic properties of forsterite, over a wide range of temperature and pressure.     

On the kinetics of textural equilibration in forsterite marbles

In the aureole of the Beinn an Dubhaich granite, Skye, the minimum observed median forsterite-calcite-calcite dihedral angle varies from 110° at the olivinein isograd to about 165° (the equilibrium value) at the granite-limestone contact. Laboratory experiments were performed to investigate the kinetics of this textural change. It was found that the rate of change of the forsterite-calcite-calcite dihedral angle followed approximately first-order kinetics with an activation energy of 48±4 kJ mol^-1 for fluid-present conditions, and 90 ±4 kJ mol^-1 for fluid-absent conditions. Scanning ion imaging demonstrated that, at least in the early stages of textural change, solution-reprecipitation of the calcite was the rate determining step in the fluid-present runs. The latter result and the value of the activation energy are both consistent with the activation energy found by previous authors for (albeit zeroth order) silicate-aqueous solution solution-reprecipitation reactions. The value of activation energy for the dry data does not correspond to those for either grain boundary or volume diffusion of oxygen or magnesium in forsterite. The mechanism for textural equilibration in the fluid-absent runs is uncertain. Application of the experimentally-derived rate equation to the Beinn an Dubhaich marbles gave activation energies higher than those obtained experimentally. It is concluded from consideration of grain growth effects that activation energies derived from the Beinn an Dubhaich marbles probably reflect textural equilibration under predominantly fluid-absent conditions.     

Simulation of forsterite protonation by the interatomic potential method

The interatomic potential method in an ionic approximation was used to model the protonation of forsterite crystals. The formation of isolated OH^? groups in iron-free and iron-bearing crystals and neutral clusters of protonated cation vacancies was considered. It was shown that the presence of trivalent impurities may significantly facilitate protonation processes owing to their reduction to a divalent state or formation of clusters with cation vacancies. In most cases, charge balancing of hydrogen-bearing defects by magnesium vacancies is energetically favorable over that involving silicon vacancies.     

Hypoplastic modelling of moisture-sensitive weathered rockfill materials

For broken rock materials under stress the process of weathering and consequently the degradation of the solid hardness may be accelerated under water. Thus, the resistance of particles against abrasion and breakage can be strongly influenced by a change of the moisture content of the grains. The focus of this paper is on modelling the essential mechanical properties of moisture-sensitive weathered coarse-grained rockfill materials using a hypoplastic constitutive model. The model takes into account the current void ratio, the effective stress, the strain rate and a moisture-dependent degradation of the solid hardness. Creep and stress relaxation during the process of degradation of the solid hardness are also included. It is shown that the results obtained from numerical simulations are in good agreement with experiments carried out with weathered granite.     

High-temperature deformation of forsterite single crystals doped with vanadium

Creep experiments have been performed on samples from a single crystal of vanadium-doped forsterite under controlled \(p_{{\text{O}}_2 } \) conditions to investigate the effects of the addition of substitutional defects in the tetrahedral lattice sites. The addition of vanadium causes marked changes in the flow behavior of the forsterite, with a net increase in the creep rate at high \(p_{{\text{O}}_2 } \) and a new \(p_{{\text{O}}_2 } \) -dependent flow regime at low \(p_{{\text{O}}_2 } \) conditions. These observations can be interpreted as resulting from changes in the majority defect species that maintain the charge neutrality within the crystal. A climb-controlled dislocation creep model for the high-temperature deformation of vanadium-doped forsterite is proposed in which either (i) movement of uncharged jogs is rate-limited by the diffusion of silicon via a vacancy mechanism or (ii) movement of positively charged jogs is rate-limited by diffusion of oxygen via a vacancy mechanism.     

风化壳淋积型稀土矿评述

传统观点认为,风化壳淋积型稀土矿床（即离子吸附型稀土矿床）中,稀土主要以吸附态赋存于风化壳黏土矿物表面,其他赋存形式占比较少。但近年来的同步辐射研究显示,稀土元素也同时以内层络合物形式存在,而内层络合物的存在可能会抑制稀土元素的离子交换。如何将风化壳淋积型稀土矿中各种形态的稀土元素有效地溶出,对于提高稀土资源利用率十分重要。本文选取南岭地区风化壳淋积型稀土矿石样品,选择混合酸（五酸）消解、盐酸消解、硝酸消解、硫酸铵浸提的前处理方式,采用电感耦合等离子体质谱（ICP-MS）测定,探讨样品中稀土元素的溶出情况。结果表明,采用混合酸（五酸）能够溶出样品中的全相稀土;盐酸或硝酸能够溶出以离子状态吸附于黏土矿物或铁锰氧化物中的稀土元素,以及以碳酸盐、磷酸盐等形式存在的稀土元素;硫酸铵浸提则只能将离子相的稀土元素置换出来。采用硝酸、盐酸溶出的稀土量,占混合酸（五酸）消解溶出稀土量（全相稀土）的71.7%～97.5%,硫酸铵浸提溶出的稀土量（离子相稀土）最低,仅是全相稀土量的9.1%～75.5%。Sc³⁺的离子半径明显小于其他稀土元素,其提取率也明显低于其他稀土元素;铈（Ce）不同于其他稀土元素的分异-富集特性,使其在硫酸铵浸提中提取率与其他稀土元素不一致。

     

Deeply weathered rock at victorian damsites

Two major features of weathering at Victorian damsites are joint controlled weathering and sub-basaltic weathering. These features have received attention in the literature from geologists generally engaged on engineering projects. The author has been fortunate to have been associated with geologic investigations at more than fifteen damsites where the features have been exposed.

Weathering at Australian sites is typical as it tends to occur where water is most often present. Barriers within or above the subject rock direct percolating waters to some locations and protect others.

The significant feature of joint controlled weathering is that the relevant barriers, the limonite-coated joints, are themselves the product of earlier weathering and provide the avenue for water penetration into the rock mass bound by the joints.

Sub-basaltic weathering appears to be due to the effect of the basalt cap in concentrating percolation through vertical joints.