Physics and Chemistry of Minerals最新文献

Understanding the dynamics of the lithosphere relies heavily on the scale-dependent rheology of minerals. While quartz, feldspar, and phyllosilicates are the key phases to govern the rheology of the crust and tectonic margins, olivine and other mafic phases control the same in the upper mantle. Phase transition, solid-state substitution, polymorphism, etc. also affect mineral phase rheology. High pressure–temperature deformation tests with natural, synthetic and analog materials have improved our interpretation of the geodynamic state of the lithosphere. However, deforming and studying a single crystal is not easy, because of the scarcity of specimens and laborious sample preparations. Experimental micro- to nanoindentation at room and/or elevated temperatures has proven to be a convenient method over mesoscale compressive testing. Micro- to nanoindentation technique enables higher precision, faster data acquisition and ultra-high resolution (nanoscale) load and displacement. Hardness, elastic moduli, yield stress, fracture toughness, fracture surface energy and rate-dependent creep of mono- or polycrystalline minerals are evaluated using this technique. Here, we present a comprehensive assessment of micro- to nano-mechanics of minerals. We first cover the fundamental theories of instrumented indentation, experimental procedures, pre- and post-indentation interpretations using various existing models followed by a detailed discussion on the application of nanoindentation in understanding the rheology and deformation mechanisms of various minerals commonly occur in the crust and upper mantle. We also address some of the major limitations of indentation tests (e.g., indentation size effect). Finally, we suggest potential future research areas in mineral rheology using instrumented indentation.

Tsavorite is the trade name for the green vanadium–chromium variety of grossular occurring in the Precambrian terrains in the areas of Merelani Hills (Tanzania) and Tsavo Park (Kenya) which are by far the most important source of gem grade specimens of tsavorite used for high jewellery. The tsavorite crystals from Merelani Hills exhibit a pink-red and yellow fluorescence when irradiated by common portable UV lamp, an unusual phenomenon among members of the garnet group. The electron density map calculated from the diffraction data and plotted against a grossular standard shows that an excess of negative charge is clearly pinpointed in the crystallographic site occupied by Al³⁺. The bulk elemental analysis shows that the most represented end-member, besides grossular, is the vanadium-bearing goldmanite garnet (3.82–4.08 mol %). The fluorometry with an excitation beam at 408 nm indicates a complex emission pattern with the most intense emissions at 701 and 716 nm and subordinately at 592 nm. The colour perception is dominated by the emission yellow band at 592 nm while the contribution of the red band modulates the colour ranging from bright orange to pink-red. The attribution of the emission at 592 nm is related to Mn²⁺ while the emissions at 701 and 716 nm could be related to the chromium content and/or to a possible fraction of vanadium as V²⁺. Because of the characteristic colour perceived under UV light, the use of a common led lamp can be useful as a diagnostic tool to easily identify tsavorite.

Modeling of eight mechanisms for the incorporation of Ti⁴⁺ and Cr³⁺ impurity components into phlogopite was carried out by a semi-empirical method using the GULP (General Utility Lattice Program) software. The calculation of thermodynamic mixing properties in the range of 1–7 GPa and 373–1573 K and the analysis of the structure geometry for the simulated solid solutions provided the following energy-preferred schemes of isomorphic substitution: ^VI(Mg²⁺) + 2^IV(Si⁴⁺) = ^VI(Ti⁴⁺) + 2^IV(Al³⁺) and ^VI(Mg²⁺) + 2^IV(Al³⁺) = ^VI(□) + 2^IV(Ti⁴⁺), ^VI(Mg²⁺) + ^IV(Si⁴⁺) = ^VI(Cr³⁺) + ^IV(Al³⁺), and 3^VI(Mg²⁺) = ^VI(Al³⁺) + ^VI(Cr³⁺) + ^VI(□). It is shown the scheme 2^VI(Mg²⁺) = ^VI(Ti⁴⁺) + ^VI(□) illustrating entrance of Ti with the formation of a vacancy is realized in the case of microconcentrations of Ti only. Accumulation of high-Ti contents associates with the formation of a vacancy in the octahedral site. This provides incorporation of Ti via the schemes ^VI(Mg²⁺) + 2^IV(Al³⁺) = ^VI(□) + 2^IV(Ti⁴⁺) and (Mg, Fe²⁺) + 2OH⁻ = Ti⁴⁺  + 2O²⁻ only. It is shown that incorporation of high-Cr concentrations (> 5.5 wt % Cr₂O₃) is accompanied by an increase in the number of vacancies in the octahedral site with an increase in the proportion of the dioctahedral component K(Al, Cr, □)₂AlSi₃O₁₀(OH)₂.

Brillouin light scattering spectroscopy was used along with detailed composition information obtained from electron probe microanalysis to study the influence of octahedral site chemistry on the elastic properties of natural biotite crystals. Elastic wave velocities for a range of directions in the ac and bc crystallographic planes were obtained for each crystal by application of the well-known Brillouin equation with refractive indices and phonon frequencies obtained from the Becke line test and spectral peak positions, respectively. In general, these velocities increase with decreasing iron content, approaching those of muscovite at low iron concentrations. Twelve of thirteen elastic constants for the full monoclinic symmetry were obtained for each crystal by fitting analytic expressions for the velocities as functions of propagation direction and elastic constants to corresponding experimental data, while the remaining constant was estimated under the approximation of hexagonal symmetry. Elastic constants (C_{11}), (C_{22}), and (C_{66}) are comparable to those of muscovite and show little change with iron concentration due to the strong bonding within layers. In contrast, nearly all of the remaining constants show a pronounced dependence on iron content, a probable consequence of the weak interlayer bonding. Similar behaviour is displayed by the elastic stability, which exhibits a dramatic decrease with increasing iron content, and by the elastic anisotropy within the basal cleavage plane, which decreases as the amount of iron in the crystal is reduced. This systematic dependence on iron content of all measured elastic properties indicates that the elasticity of biotite is a function of octahedral site chemistry and provides a means to estimate the elastic constants and relative elastic stability of most natural biotite compositions if the iron or, equivalently, magnesium, concentration is known. Moreover, the good agreement between the elastic constants of Fe-poor (Mg-rich) biotite and those of phlogopite obtained from first-principles calculation based on density functional theory indicates that the latter approach may be of use in predicting the elastic properties of biotites.

Three samples of fluorapophyllite-(K) with different cation composition were studied by the X-ray diffraction and IR-spectroscopy methods. The samples (K_0.72(NH₄)_0.20)_Σ0.92Na_0.08 Ca₄[Al_0.04Si_7.96O₂₀]F·8H₂O from Nidym and K_0.87Na_0.06 Ca₄[Si₈O₂₀]F·8H₂O from Akhaltsykhe localities are characterized by the deficiency of large cations K⁺ and (NH₄)⁺ in the A site; the chemical composition of Talnakh sample (K_0.94(NH₄)_0.06)_Σ1.00 Ca₄[Si₈O₂₀]F·8H₂O is close to the stoichiometric one. The large cations deficiency in the structure of Nidym and Akhaltsykhe samples is compensated by the presence of medium-sized Na⁺ cations, located 1.12–1.31 Å away from the A position. The planar coordination of Na is formed by four H₂O molecules. The occurrence of ammonium ions is found for the Nidym and Talnakh samples and their concentrations are estimated from the IR spectra and structure refinement. Polarized infrared measurements performed for fluorapophyllite-(K) samples oriented along the c-axis reveal pleochroic behavior of the OH absorption bands. The observed pleochroism is explained in terms of absorption from two OH vectors of the H₂O molecules in the crystal structure.

Carbonate minerals are major contributors to carbon sequestration in geological deposits; however, their nature and behavior remain unclear. Amorphous magnesium carbonate (AMC) is formed as a precursor to crystalline magnesium carbonates and as a product of thermal decomposition of nesquehonite (NSQ). In this study, the AMCs formed during the crystallization and decomposition of NSQ were investigated using X-ray diffraction (XRD) and atomic pair distribution function (PDF) methods. An AMC with a hydromagnesite-like structure (AMC-I) was formed immediately after mixing MgCl₂ and Na₂CO₃ solutions. After 5 min of stirring, no change was observed in the XRD pattern; however, the PDF pattern changed. This suggests that the medium-range ordered structure of AMC-I transformed into an intermediate structure (AMC-II) between AMC-I and NSQ. After 10 min of stirring, the AMC-II crystallized into NSQ. In the case of Rb₂CO₃, the AMC-II structure was formed immediately after the mixing of solutions and was stable for three days. AMC-II in the Rb₂CO₃ solution appeared to be in equilibrium with energetic local minima, indicating the existence of polyamorphism in AMC. When Cs₂CO₃ solution was used, the first precipitate had an AMC-I structure. By stirring for 5 min, the AMC-I was transformed to AMC-II, and after 10 min of stirring, a few quantities crystallized into NSQ. After three days, NSQ dissolved and transformed back into AMC-I. Thus, it is inferred that the crystallization of NSQ is significantly influenced by alkali cations in aqueous solutions. The AMC formed during the thermal decomposition also possesses the AMC-I structure.

Scapolite Na_2.25Ca_1.43K_0.30Fe_0.02[Al_4.23Si_7.77O₂₄]Cl_0.72(SO₄)_0.1(CO₃)_0.18 has been investigated by single-crystal X-ray diffraction, Infrared and Raman spectroscopy combined with diamond anvil cells (DAC) up to 19.6 GPa at room temperature, to understand its phase stability and the behaviors of CO₃²⁻. The experimental results show that a phase transition from the tetragonal phase (P4₂/n) to the triclinic phase occurs at 10.7 GPa, which is attributed to the pressure-induced rotation of tetrahedra around the bridging O atoms. The pressure–volume data were fitted to the second-order Birch–Murnaghan equation of state, yielding V₀ = 1102.6(8) ų and K₀ = 70.3(9) GPa for the tetragonal phase and V₀ = 1188.2(16) ų and K₀ = 33.0(3) GPa for the triclinic phase. In the triclinic phase, the larger compressibility is due to the increased degree of freedom in the crystal structure, and the anisotropy of the axial compressibility may be related to the ionic radius of the anionic group. From the Infrared absorption and Raman spectroscopy data, we speculate that CO₃²⁻ is extruded by the four-membered rings at the (001) plane during the whole pressure range. The high-pressure behavior of CO₃²⁻ in scapolite provides a possibility that carbon exists in the Earth’s interior as CO₃²⁻ coupled to silicates.

Diamonds from placers in the northern provinces of the Siberian Platform attract attention not only due to the unsolved problem of their sources but also due to the variety of properties they exhibit. In this work, electron paramagnetic resonance (EPR), infrared (IR), and photoluminescence (PL) methods were used to study cuboid diamonds from Istok (25 diamonds of Ib-IaA type) and dodecahedron diamonds from Mayat (21 diamonds of IaAB type) placers. The main impurity defects in the crystals studied are nitrogen, nitrogen-titanium, and oxygen-containing centers. The total amount of nitrogen in the form of C (P1 in EPR nomenclature), A, and B centers ranges from 34 to 903 ppm. For 19 out of 25 crystals from the Istok placer, the N3 titanium-related center is observed in the EPR spectra, but only one optical system with zero-phonon line (ZPL) at 635.7 nm is detected in the PL spectra in most cases. It can be assumed that the absence of an accompanying system with ZPL at 440.3 nm is due to its quenching by other centers in these crystals. For 12 out of 21 crystals from the Mayat placer, photoluminescence of oxygen-containing centers in the region of 600–800 nm in the form of a series of equidistant bands with an interval of ~ 31 meV was revealed. A comparison of the EPR and PL data has shown that this red broad band is not correlated with titanium-nitrogen paramagnetic centers OK1 and N3. Thus, the results obtained for diamond crystals from the Istok and Mayat placers show no connection between the N3 and OK1 paramagnetic centers and band in the 600–800 nm region, which questions the presence of oxygen in the structure of the N3 and OK1 centers. The nickel-nitrogen centers were found in crystals from the Mayat placer testifying about the peridotitic paragenesis of these diamonds.

The luminescence of the uranyl cation UO₂²⁺ depends on the local crystalline environment and is sensitive to structural influences. Steady-state photoluminescence emission spectra of the related uranyl silicates uranophane-α, uranophane-β, sklodowskite and haiweeite from various locations are presented and discussed in the light of structure–property relation. The four mineral species were chosen for their close relationships: uranophane-α and uranophane-β are polymorphs and share the underlaying topology with sklodowskite. Haiweeite, with different topology, shares the composing elements Ca, U, Si, O with uranophane, while in sklodowskite Mg replaces Ca. All species show some variability in their spectra, parameterized as a variation of the centroid wavelength. Those variations are linked to defects and structural disorder, relevant in studies of uranyl speciation and migration. We present empiric spectra of the four mineral species with the least influence of structural disorder. As an unexpected feature, a prominent—partly dominating—double peak structure occurs in the case of uranophane-α only, while it is absent in the spectra of the other species. Considering a model of luminescent transitions in the uranyl ion in more detail, this observation is discussed in the light of the polymorphism of uranophane. We show evidence that variable amounts of uranophane-β phase embedded in uranophane-α are possibly at the origin of this spectral signature. Growth of those uranophane-β clusters might be induced by defects in the uranophane-α lattice and further promoted by the polymorphism of uranophane.

To calculate the thermodynamic properties of recently discovered high-pressure mixed valence iron oxides in the system Fe–Mg–O, information on the equation of state of precursor inverse spinel phases along the magnetite–magnesioferrite join is needed. The existing equation of state data, particularly for magnesioferrite, are in poor agreement and no data exist for intermediate compositions. In this study, the compressibility of nearly pure magnesioferrite as well as of an intermediate ({{mathrm{Mg}}_{0.5}}^{vphantom{2+}}mathrm{Fe}_{0.5}^{2+}{mathrm{Fe}}_{2}^{3+}{mathrm{O}}_{4}^{vphantom{2+}}) sample have been investigated for the first time up to approximately 19 and 13 GPa, respectively, using single-crystal X-ray diffraction in a diamond anvil cell. Samples were produced in high-pressure synthesis experiments to promote a high level of cation ordering, with the obtained inversion parameters larger than 0.83. The room pressure unit cell volumes, V₀, and bulk moduli, K_T0, could be adequately constrained using a second-order Birch–Murnaghan equation of state, which yields V₀ = 588.97 (8) ų and K_T0 = 178.4 (5) GPa for magnesioferrite and V₀ = 590.21 (5) ų and K_T0 = 188.0 (6) GPa for the intermediate composition. As magnetite has K_T0 = 180 (1) GPa (Gatta et al. in Phys Chem Min 34:627–635, 2007. https://doi.org/10.1007/s00269-007-0177-3), this means the variation in K_T0 across the magnetite–magnesioferrite solid solution is significantly non-linear, in contrast to several other Fe–Mg spinels. The larger incompressibility of the intermediate composition compared to the two end-members may be a peculiarity of the magnetite–magnesioferrite solid solution caused by an interruption of Fe²⁺–Fe³⁺ electron hopping by Mg cations substituting in the octahedral site.