Nanotopography of synthetic and natural dolomite crystals

Examination with scanning electron microscopy (SEM) and scanning force microscopy (SFM) revealed etch pits, layers and islands on dolomite crystal faces synthesized from calcite in Ca‐Mg‐Cl solutions at 200 °C and a wide variety of natural dolomites. Layers are broad, flat structures bounded by steps less than 100 nm high and greater than 1 μm wide. Islands are rounded topographic highs <20 nm high and <200 nm wide. The nanotopography of synthetic dolomite changed from islands throughout most of the reaction to layers at 100% dolomite. Island nanotopography formed on both Ca‐rich and near‐stoichiometric dolomite. Analyses of reaction products from dolomite synthesis indicates that there are no SFM‐detectable products formed in <10 h. SEM‐detectable products formed in 15 h. X‐ray diffraction (XRD)‐detectable products formed in ≈18 h, and the reaction went to completion in ≈40 h. Based on SFM analyses, the induction period for dolomitization in these experiments accounts for ≈20% of the total reaction time necessary to dolomitize CaCO₃ completely under the experimental conditions used here. Island nano‐ topography is inferred to occur at higher degrees of supersaturation than layer nanotopography for three reasons. First, island nanotopography on synthetic calcite and gypsum forms at higher supersaturations than layer nanotopography. Secondly, island nanotopography formed in solutions with higher degrees of supersaturation with respect to dolomite. Thirdly, the greater surface roughness of a crystal face composed of islands compared with layers indicates that island surfaces have higher surface energy than layer surfaces. Therefore, the stability of island surfaces requires a higher degree of supersaturation. Because islands and layers form under a wide range of conditions, their presence provides broadly applicable criteria for evaluating relative degrees of supersaturation under which ancient dolomite formed. Comparison of synthetic dolomites with natural dolomites demonstrates (1) similar nanotopography on natural and synthetic dolomites and (2) both natural planar and non‐planar dolomite may have island nanotopography.     

冻结过程中单孔冰结晶变形机制研究

孔隙水在冻结过程中产生的冰结晶压力导致了多孔材料的冻胀及破坏。本文通过理论分析分别给出了不同形状晶体的结晶压力计算模型,并分析了经典结晶压力计算公式的使用条件。建立了降温过程中孔隙冰晶生长模型,实现冰晶生长过程中的孔隙变形计算,分析了晶核密度、孔径大小、荷载和冰晶体积对孔隙冻胀变形的影响机制。结果表明：起始孔隙直径和长宽比的增大对结晶变形抑制作用的机理在于减少了冰晶体积中膨胀结晶的比例。荷载对孔隙变形的抑制机制在于,荷载的增大迫使冰晶更多地横向生长（长宽比增大）,导致膨胀结晶所占比例减小。孔隙中的晶体生长有完全填充模式和部分填充模式,在部分填充模式下,晶核密度、荷载和孔径的增大都会导致晶体在孔隙中的填充率增大,从而对孔隙结晶变形产生影响。本模型揭示了单个孔隙中冻胀变形机制,为解决多孔介质的冻胀变形与破坏问题提供了新的思路。     

The crystal chemistry of birefringent natural uvarovites. Part III. Application of the superposition model of crystal fields with a characterization of synthetic cubic uvarovite

Synthetic, flux-grown uvarovite, Ca₃Cr₂ [SiO₄]₃, was investigated by optical methods, electron microprobe analysis, UV-VIS-IR microspectrometry, and luminescence spectroscopy. The crystal structure was refined using single-crystal X-ray CCD diffraction data. Synthetic uvarovite is optically isotropic and crystallizes in the “usual” cubic garnet space group Ia3¯d [a=11.9973 Å, Z=8; 21524 reflections, R1=2.31% for 454 unique data and 18 variables; Cr–O=1.9942(6), Si–O=1.6447(6), Ca–O_a=2.3504(6), Ca–O_b= 2.4971(6) Å]. The structure of Ca₃Cr₂[SiO₄]₃ complies with crystal-chemical expectations for ugrandite group garnets in general as well as with predictions drawn from “cubically averaged” data of non-cubic uvarovite–grossular solid solutions (Wildner and Andrut 2001). The electronic absorption spectra of Cr³⁺ in trigonally distorted octahedra of synthetic uvarovite were analyzed in terms of the superposition model (SM) of crystal fields. The resulting SM and interelectronic repulsion parameters are =9532 cm^?1, =4650 cm^?1, power law exponent t ₄=6.7, Racah B₃₅=703 cm^?1 at 290 K (reference distance R ₀=1.995 Å; fixed power law exponent t ₂=3 and spin-orbit parameter ζ=135 cm^?1). The interelectronic repulsion parameters Racah B ₅₅=714 cm^?1 and C=3165 cm^?1 were extracted from spin-forbidden transitions. This set of SM parameters was subsequently applied to previously well-characterized natural uvarovite–grossular solid solutions (Andrut and Wildner 2001a; Wildner and Andrut 2001) using their extrapolated Cr–O bond lengths to calculate the energies of the spin-allowed bands. These results are in very good agreement with the experimentally determined band positions and indicate the applicability of the superposition model to natural 3d ^N prevailing systems in geosciences. Single-crystal IR absorption spectra of synthetic uvarovite in the region of the OH-stretching vibration exhibit one isotropic absorption band at 3508 cm^?1 at ambient conditions, which shifts to 3510 cm^?1 at 77 K. This band is caused by structurally incorporated hydroxyl groups via the (O₄H₄)-hydrogarnet substitution. The water content, calculated using an integral extinction coefficient ?=60417 cm^?2 l mol^?1, is c _H2O=33 ppm.     

Growth and morphology of high-germanium single crystals

Experimental data on the growth and study of high-germanium single crystals with better piezoelectric characteristics and with a working capacity at higher temperatures and pressures in comparison with pure quartz are reported.     

Electrical conduction and polarization of calcite single crystals

The electrical conductivity and polarization properties of calcite single crystals with three orientations, namely, a (00.1) plane perpendicular to the crystallographic c axis (10.0) plane parallel to the crystallographic c axis, and a (10.4) cleavage plane, were studied by both complex impedance and thermally stimulated depolarization current (TSDC) measurements. Conductivities for (00.1)-, (10.0)-, and (10.4)-oriented single calcite crystals at 400–600?°C were 1.16?×?10^?7?–?1.05?×?10^?5, 7.40?×?10^?8?–?4.27?×?10^?6, and 4.27?×?10^?7?–?2.86?×?10^?5 Ω^?1 m^?1, respectively, and the activation energies for conduction were 112, 103, and 101?kJ?mol^?1, respectively. The TSDC spectra verified the electrical polarizability of calcite crystals. The activation energy for depolarization, estimated from TSDC spectra, of the (00.1)-, (10.0)-, and (10.4)-oriented calcite substrates were 112, 119, and 114?kJ?mol^?1, respectively. Considering the correlation between the processes of conduction and electric polarization, we proposed the mechanisms of conduction and polarization in calcite on the assumption of oxide ion transport.     

Glide systems of hematite single crystals in deformation experiments

The critical resolved shear stresses (CRSSs) of hematite crystals were determined in compression tests for r-twinning, c-twinning and {a}<m>-slip in the temperature range 25 °C to 400 °C, at 400 MPa confining pressure, and a strain rate of 10^− 5 s^− 1 by Hennig-Michaeli, Ch., Siemes, H., 1982. Experimental deformation of hematile crstals betwen 25 °C and 400 °C at 400 MPa confining pressure. In: Schreyer, W. (Ed.) High Pressure Research in Geoscience, Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, p. 133–150. In the present contribution newly performed experiments on hematite single crystals at temperatures up to 800 °C at strain rates of 10^− 5 s^− 1 and 300 MPa confining pressure extends the knowledge about the CRSS of twin and slip modes. Optical observations, neutron diffraction goniometry, SEM forescatter electron images and electron backscatter diffraction are applied in order to identify the glide modes. Both twinning systems and {a}<m>-slip were confirmed by these methods. Besides the known glide systems the existence of the (c)<a>-slip system could be stated. Mechanical data establish that the CRSS of r-twinning decreases from 140 MPa at 25 °C to  5 MPa at 800 °C and for {a}<m>-slip from > 560 MPa at 25 °C to  40 MPa at 700 °C. At room temperature the CRSS for c-twinning is around 90 MPa and at 600 °C  60 MPa. The data indicate that the CRSSs above 200 °C seem to be between the values for r-twinning and {a}<m>-slip. For (c)<a>-slip only the CRSS at 600 °C could be evaluated to  60 MPa. Exact values are difficult to determine because other glide systems are always simultaneously activated.     

Neutron irradiation and post-irradiation annealing of rutile (TiO2−x ): effect on hydrogen incorporation and optical absorption

Neutron irradiation and post-irradiation annealing under oxidising and reducing conditions have been used to investigate H incorporation in, and the optical properties of, reduced (TiO_2−x ) rutile. Optical absorption in rutile is mainly due to a Ti³⁺ Ti⁴⁺ intervalence charge transfer effect. The main mechanism for H incorporation in rutile involves interstitial H not coupled to other defects, which has important implications for the rate of H diffusion, and possibly also on the electrical properties of rutile. Additional minor OH absorption bands in IR spectra indicate that a small amount of interstitial H is coupled to defects such as Ti³⁺ on the main octahedral site, and indicates that more than one H incorporation mechanism may operate. Concentration of oxygen vacancies has a controlling influence on the H affinity of rutile.     

The anisotropy of magnetic susceptibility in biotite, muscovite and chlorite single crystals

The anisotropy of magnetic susceptibility (AMS) of single crystals of biotite, muscovite and chlorite has been measured in order to provide accurate values of the magnetic anisotropy properties for these common rock-forming minerals. The low-field AMS and the high-field paramagnetic susceptibility are defined. For the high-field values, it is necessary to combine the paramagnetic deviatoric tensor obtained from the high-field torque magnetometer with the paramagnetic bulk susceptibility measured from magnetization curves of the crystals. This leads to the full paramagnetic susceptibility ellipsoid due to the anisotropic distribution of iron cations in the silicate lattice. The ellipsoid of paramagnetic susceptibility, which was obtained for the three phyllosilicates, is highly oblate in shape and the minimum susceptibility direction is subparallel to the crystallographic c-axes. The anisotropy of the susceptibility within the basal plane of the biotite has been evaluated and found to be isotropic within the accuracy of the instrumental measurements. The degree of anisotropy of biotite and chlorite is compatible with previously reported values while for muscovite the smaller than previously published values. The shape of the chlorite AMS ellipsoid for all the samples is near-perfect oblate in contrast with a wide distribution of oblate and prolate values reported in earlier studies. Reliable values are important for deriving models of the magnetic anisotropy where it reflects mineral fabrics and deformation of rocks.     

Synthesis of barite,celestite and barium-strontium sulfate solid solution crystals

Crystals of synthetic barite (BaSO₄), celestite (SrSO₄), and their solid solutions were recrystallized from brine by thermal cycling. Electron probe microanalyses show that the solid solutions are homogeneous within detection limits. The molar composition of solid solution crystals approximates the ideal value based on the ratio of equilibrium constants for barium and strontium sulfates.     

Thermoluminescence, electron paramagnetic resonance and optical absorption in natural and synthetic rhodonite crystals

Thermoluminescence, electron paramagnetic resonance and optical absorption properties of rhodonite, a natural silicate mineral, have been investigated and compared to those of synthetic crystal, pure and doped. The TL peaks grow linearly for radiation dose up to 4 kGy, and then saturate. In all the synthetic samples, 140 and 340°C TL peaks are observed; the difference occurs in their relative intensities, but only 340°C peak grows strongly for high doses. Al₂O₃ and Al₂O₃ + CaO-doped synthetic samples presented several decades intenser TL compared to that of synthetic samples doped with other impurities. A heating rate of 4°C/s has been used in all the TL readings. The EPR spectrum of natural rhodonite mineral has only one huge signal around g = 2.0 with width extending from 1,000 to 6,000 G. This is due to Mn dipolar interaction, a fact proved by numerical calculation based on Van Vleck dipolar broadening expression. The optical absorption spectrum is rich in absorption bands in near-UV, visible and near-IR intervals. Several bands in the region from 540 to 340 nm are interpreted as being due to Mn³⁺ in distorted octahedral environment. A broad and intense band around 1,040 nm is due to Fe²⁺. It decays under heating up to 900°C. At this temperature it is reduced by 80% of its original intensity. The pink, natural rhodonite, heated in air starts becoming black at approximately 600°C.     

天然金珍珠和染色金珍珠的致色因素和鉴定分析方法研究进展

金属离子、卟啉和类胡萝卜素是天然金珍珠的主要致色成分,有机染料是染色金珍珠的致色成分。鉴定天然金珍珠和染色金珍珠的主要手段有常规观察检测、紫外可见吸收光谱法、拉曼光谱法、荧光光谱等。本文结合天然金珍珠的致色成分和染色金珍珠的染色方法,对两类金珍珠的鉴定技术作了评述,认为常规观察检测是十分必要的,染色珍珠颜色分布不均匀,在孔眼内或表面破损处可看到染料的沉淀浓集;紫外可见吸收光谱法是目前较为常用的鉴定方法,天然金珍珠主要吸收峰位于350~360 nm,而染色金珍珠则出现明显的410~450 nm谱峰;应用拉曼光谱法鉴定可获得天然金珍珠在275 cm-1明显的特征峰,而染色金珍珠出现强的荧光背景峰;应用荧光光谱法鉴定金黄色天然珍珠在376 nm处光激发时谱峰是以472 nm为中心,而染色珍珠在372 nm处受光激发时谱峰位于436 nm。这些鉴定方法仍然在实验验证阶段,进一步优化和开发新的测试手段则是今后重要的发展方向。     

Synthesis, characterization and thermochemistry of a Pb-jarosite

The enthalpy of formation from the elements of a well-characterized synthetic Pb-jarosite sample corresponding to the chemical formula (H₃O)_0.74Pb_0.13Fe_2.92(SO₄)₂(OH)_5.76(H₂O)_0.24 was measured by high temperature oxide melt solution calorimetry. This value ( = −3695.9 ± 9.7 kJ/mol) is the first direct measurement of the heat of formation for a lead-containing jarosite. Comparison to the thermochemical properties of hydronium jarosite and plumbojarosite end-members strongly suggests the existence of a negative enthalpy of mixing possibly related to the nonrandom distribution of Pb²⁺ ions within the jarosite structure. Based on these considerations, the following thermodynamic data are proposed as the recommended values for the enthalpy of formation from the elements of the ideal stoichiometric plumbojarosite Pb_0.5Fe₃(SO₄)₂(OH)₆:  = −3118.1 ± 4.6 kJ/mol,  = −3603.6 ± 4.6 kJ/mol and S° = 376.6 ± 4.5 J/(mol K). These data should prove helpful for the calculation of phase diagrams of the Pb-Fe-SO₄-H₂O system and for estimating the solubility product of pure plumbojarosite. For illustration, the evolution of the estimated solubility product of ideal plumbojarosite as a function of temperature in the range 5-45 °C was computed (Log(K_sp) ranging from −24.3 to −26.2). An Eh-pH diagram is also presented.     

利用吸收和发光光谱技术分析高温高压天然富氢钻石的鉴定特征

对不同类型褐色钻石进行高温高压处理和结构特性研究是钻石研究中的难点和重点之一。前人对富氢钻石的研究主要集中于其特殊的生长结构以及其形成环境的探讨,而对富氢钻石经高温高压处理后的变化特征鲜有涉及。本文对经高温高压处理前后的富氢钻石的红外光谱、紫外可见吸收光谱以及光致发光光谱等谱学特征进行了对比,研究其鉴定特征。结果表明:高温高压处理前后的富氢钻石的光谱特征具有明显差异,特别是红外光谱,经处理后的钻石中与氮氢有关的吸收峰如3310 cm~(-1)、3232 cm~(-1)、3189 cm~(-1)等明显减弱甚至消失,并出现与孤氮有关的新的2688 cm~(-1)吸收峰;紫外可见光吸收光谱中,经处理的褐色钻石中的无选择性吸收(钻石呈褐色的原因)变为孤氮的典型吸收,即550 nm至短波的吸收以及N_3中心(415 nm)的吸收均明显增强,因此钻石也由原来的褐色变为黄色。钻石经处理前后的光致发光光谱中,与氮原子有关的缺陷类型、峰的强度以及缺陷组合也有变化。本文获得的光谱变化特征,为准确鉴定高温高压处理的黄色富氢钻石提供了依据,也为解释与氢和氮相关的晶格缺陷在高温条件下的变化机理提供了理论基础。     

Crystallisation of sodium sulfate: supersaturation and metastable phases

Crystallisation of sodium sulfate solutions by evaporation under controlled climatic conditions has revealed the existence of crystalline hydrated sodium sulfate salts not previously reported. The sodium sulfate phase crystallising and the concentration of the solution at the point of crystallisation depends on the climatic conditions (temperature and evaporation rate). During the rehydration of the anhydrous sodium sulfate phase, thenardite, another previously unreported phase was formed prior to the nucleation of the stable phase, mirabilite Na₂SO₄ · 10H₂O. The addition of organic inhibitors changes both the crystallisation and the rehydration behavior in this system.     

On spherical nanoindentations, kinking nonlinear elasticity of mica single crystals and their geological implications

In this paper, we show, using cyclic spherical nanoindentation experiments, that the deformation mechanisms in mica, including basal plane ruptures and delaminations, can be explained by invoking the presence of mobile dislocation walls, and incipient and regular kink bands. Our results clearly show that the energy dissipated or that was stored during the deformation of muscovite depends critically on its previous deformation history and/or the pre-existing defect concentration. Once nucleated, the dislocation-based incipient kink bands are believed to be responsible for the nonlinear elastic deformation and hysteretic loops obtained during cyclic loading. Moreover, a model is presented to estimate the number and distribution of dislocations and the energy consumed in their motion under the indenter. From the model, we also estimate the critical resolved shear stress for the motion of basal plane dislocations under the indenter. The implications of this work can be extended beyond mica to understand the nonlinear hysteretic deformation in other geological formations dominated by layered minerals.     

Petrology of fine-grained rock fragments and petrologic implications of single crystals from the Luna 20 soil

Only fine-grained rocks are present in the Luna 20 samples, and coarser grained rocks are represented by fragments of single crystals. A petrologic study has been made of 47 fine-grained crystalline rocks, microbreccias, and glassy aggregates. In addition, a total of 33 single crystals of pyroxene, plagioclase, olivine and spinel, in the size range 125 to 500 μ, have been examined using electron microprobe and single crystal X-ray diffraction techniques.The most abundant fine-grained crystalline rocks in the samples we have examined are recrystallized anorthositic norite and anorthositic troctolite. Gabbroic rocks, anorthosite, and KREEP basalt are present but not common. Most of the single crystals of pyroxene and plagioclase could have been derived from coarser grained noritic, troctolitic and anorthositic rocks. However, three of the 14 pyroxene crystals, and 2 of the 5 olivine crystals have $\text{Fe}\text{(Fe + Mg)}$ contents greater than 0.45 and are believed to have been derived from mare basalts or related rocks. Two relatively sodic crystals of plagioclase were found. One is a crystal zoned at least over the range An₈₅ to An₆₃, and the second is a homogeneous crystal of albite (~An₃).     

Dislocation generation, slip systems, and dynamic recrystallization in experimentally deformed plagioclase single crystals

Three samples of gem quality plagioclase crystals of An60 were experimentally deformed at 900 °C, 1 GPa confining pressure and strain rates of 7.5–8.7×10⁻⁷ s⁻¹. The starting material is effectively dislocation-free so that all observed defects were introduced during the experiments. Two samples were shortened normal to one of the principal slip planes (010), corresponding to a “hard” orientation, and one sample was deformed with a Schmid factor of 0.45 for the principal slip system [001](010), corresponding to a “soft” orientation. Several slip systems were activated in the “soft” sample: dislocations of the [001](010) and 110(001) system are about equally abundant, whereas 110{111} and [101] in ( 31) to ( 42) are less common. In the “soft” sample plastic deformation is pervasive and deformation bands are abundant. In the “hard” samples the plastic deformation is concentrated in rims along the sample boundaries. Deformation bands and shear fractures are common. Twinning occurs in close association with fracturing, and the processes are clearly interrelated. Glissile dislocations of all observed slip systems are associated with fractures and deformation bands indicating that deformation bands and fractures are important sites of dislocation generation. Grain boundaries of tiny, defect-free grains in healed fracture zones have migrated subsequent to fracturing. These grains represent former fragments of the fracture process and may act as nuclei for new grains during dynamic recrystallization. Nucleation via small fragments can explain a non-host-controlled orientation of recrystallized grains in plagioclase and possibly in other silicate materials which have been plastically deformed near the semi-brittle to plastic transition.     

Synthesis and optical absorption spectra of Cr2+-containing orthosilicates

By reacting chromium metal with appropriate oxides in a neutral atmosphere at high temperature it is possible to prepare Mg₂SiO₄ with the olivine structure containing Cr⁺⁺ and also Cr₂SiO₄. X-ray powder diffraction data for chromous orthosilicate may be indexed on an orthorhombic cell with a=5.690, b=11.262, and c=9.584 Å. The compound is not isotructural with olivine. Optical absorption spectra of Cr²⁺ in both hosts have been obtained and also indicate large differences between the two structures.     

Magnetic properties of the clinopyroxenes aegirine and hedenbergite: a magnetic susceptibility study on single crystals

Two natural clinopyroxene single crystals were investigated, an aegirine-augite (AEG) and a magnesian hedenbergite (HED). Both samples were carefully characterized by electron microprobe, X-ray diffraction, and Mössbauer spectroscopy. Magnetic susceptibility measurements of powdered samples reveal low temperature antiferromagnetic coupling and Curie-Weiss behaviour with T _N =7.5(5)?K, Θ _P =?19(1)?K for AEG, and T _N =31(1)?K, Θ _P =+21(1)?K for HED, respectively. Low temperature Mössbauer spectra exhibit relaxation phenomena. Magnetic susceptibility measurements of the single crystals show the direction of the magnetic moments to be lying within the a/c plane for both samples: 50(±2)° from a and 57(±2)° from c in AEG, and 45(±2)° from a and 60(±2)° from c in HED, respectively. The antiferromagnetic interchain interaction competes with the ferromagnetic intrachain interaction in both pyroxenes. In the magnesian hedenbergite a field induced magnetic transition is found. Its dependence on temperature, magnetic field and crystallographic direction is investigated and described.