Physics and Chemistry of Minerals最新文献

Synthetic Sc-bearing clinozoisite on the Ca₂Al₃Si₃O₁₂(OH)-Ca₂Al₂Sc³⁺Si₃O₁₂(OH) join was studied by single-crystal X-ray diffraction to understand better the distribution of Sc³⁺ among the octahedral sites, M1-M3, and its effect on the structure of epidote-group minerals. Oxide starting materials of Ca₂Al₂(Al_1-p)Sc_pSi₃O_12.5 composition with p = 0.5 and 1.0 were employed, and clinozoisite was successfully synthesized at P_H2O = 1.2–1.5 GPa and T = 700–800 °C. The Sc content in clinozoisite varies and attains 0.61 atoms per formula unit (apfu) from p = 1.0 starting material. Two Sc-bearing clinozoisite crystals from the product of p = 0.5 starting material (Run 20) were used for X-ray crystal structural analysis. The unit-cell parameters are a = 8.8815(4), b = 5.6095(2), c = 10.1466(5) Å, β = 115.318(6)º, and V = 457.0(1) ų for 20B, and a = 8.885(1), b = 5.6119(4), c = 10.153(1) Å, β = 115.27(2)º, and V = 457.9(4) ų for 20D. The resulting Sc³⁺ occupancies among the octahedral sites are ^M1Al_1.0^M2Al_1.0^M3(Al_0.684(7)Sc³⁺_0.316) for the former and ^M1Al_1.0^M2Al_1.0^M3(Al_0.629(6)Sc³⁺_0.371) for the latter, i.e., Sc³⁺ exclusively occupies M3. The mean ionic distance of < M3–O > increases with increasing Sc content at M3, but it tends to be slightly shorter than the expected value using the regression line based on the structural data of synthetic Ca₂(Al, Me³⁺)₃Si₃O₁₂(OH) clinozoisite. It is due to the reduced distortion of M3O₆ octahedra caused by the short M3–O1 and M3–O8 distances. Although the angular variance ends up at a similar value to the Al-Fe³⁺ epidote, the variation of ∠Oi–M3-Oi angles is different. The Sc-bearing clinozoisite has greater ∠O1–M3–O1’, but smaller ∠O2–M3–O2’ and ∠O2–M3–O4 relative to Al-Fe³⁺ series ones. Due to different local chemical surroundings, multiple peaks are present in the OH stretching region of Raman spectra. Three OH-stretching peaks, centered at 3342, 3382, and 3468 cm⁻¹ are assigned to the local configuration O10–H···O4–(^M1Al^M1Al^M3Sc³⁺) and O10–H···O4–(^M1Al^M1Al^M3Al), and O10–H···O2, respectively.

To support the role of proximal and remote sensing in geological rare earth element (REE) resource exploration, we studied the reflectance spectroscopy of synthetic single- and mixed-REE phosphate phases. Synthesis yielded monazite for the elements La to Gd, and xenotime for Dy to Lu and Y. Visible-to-shortwave infrared (350–2500 nm) reflectance spectra of synthetic single-REE monazites and xenotimes can be used to identify the ions responsible for the absorption features in natural monazites and xenotimes. Nd³⁺, Pr³⁺ and Sm³⁺ produce the main absorption features in monazites. In natural xenotime, Dy³⁺, Er³⁺, Ho³⁺ and Tb³⁺ ions cause the prevalent absorptions. The majority of the REE-related absorption features are due to photons exciting electrons within the 4f subshell of the trivalent lanthanide ions to elevated energy levels resulting from spin-orbit coupling. There are small (< 20 nm) shifts in the wavelengths of these absorptions depending on the nature of the ligands. The energy levels are further split by crystal field effects, manifested in the reflectance spectra as closely spaced (∼ 5–20 nm) multiplets within the larger absorption features. Superimposed on the electronic absorptions are vibrational absorptions in the H₂O molecule or within [OH]⁻, [CO₃]²⁻ and [PO₄]³⁻ functional groups, but so far only the carbonate-related spectral features seem usable as a diagnostic tool in REE-bearing minerals. Altogether, our study creates a strengthened knowledge base for detection of REE using reflectance spectroscopy and provides a starting point for the identification of REE and their host minerals in mineral resources by means of hyperspectral methods.

Access to synchrotron X-ray facilities has become an important aspect for many disciplines in experimental Earth science. This is especially important for studies that rely on probing samples in situ under natural conditions different from the ones found at the surface of the Earth. The non-ambient condition Earth science program at the Advanced Light Source (ALS), Lawrence Berkeley National Laboratory, offers a variety of tools utilizing the infra-red and hard X-ray spectrum that allow Earth scientists to probe Earth and environmental materials at variable conditions of pressure, stress, temperature, atmospheric composition, and humidity. These facilities are important tools for the user community in that they offer not only considerable capacity (non-ambient condition diffraction) but also complementary (IR spectroscopy, microtomography), and in some cases unique (Laue microdiffraction) instruments. The availability of the ALS’ in situ probes to the Earth science community grows especially critical during the ongoing dark time of the Advanced Photon Source in Chicago, which massively reduces available in situ synchrotron user time in North America.

Nefedovite, Na₅Ca₄(PO₄)₄F, has been investigated by in situ high-temperature powder (30–690 °C) and single crystal (27–827 °C) X-ray diffraction and Raman spectroscopy. Nefedovite is tetragonal, space group I-4, a = 11.6560(2), c = 5.4062(2) Å, V = 734.50(2) ų (R₁ = 0.0149). Nefedovite is a 1D antiperovskite, since its crystal structure contains chains of corner-sharing anion-centered [FCa₄Na₂]⁹⁺ octahedra. The chains are parallel to the c direction. Nefedovite is stable up to 727 °C and undergoes a displacive phase transition in the temperature range 277–327 °C. With increasing temperature, the PO₄ tetrahedra in the crystal structure of nefedovite gradually rotate around the imaginary fourfold inversion axes aligning the O2^…O3 edge parallel to [110], which ultimately leads to the appearance of the mirror plane perpendicular to the c direction and the change of space group from I-4 (82) to I4/m (87). The crystal structure of nefedovite expands strongly anisotropically with the direction of the maximum thermal expansion oriented perpendicular to the chains of anion-centered octahedra. The information-based structural complexity analysis demonstrates that both low- and high-temperature modifications of nefedovite are structurally simple with the I_G,total value less than 100 bits per unit cell. The structural complexity decreases along the phase transition, which is typical for displacive phase transitions.

Aside from its economic value, davidite and its synthetic analogs may have potential applications in materials science. The unique properties of the crichtonite group minerals, including davidite-(La), make them attractive candidates for high-level waste (HLW) immobilization. We studied the thermal evolution of the metamict davidite-(La) from the Radium Hill, Australia. The investigation of the temperature-induced crystallization process was conducted, and the thermal expansion coefficients (TEC) for the recrystallized davidite (RD) were determined for the first time. Our results demonstrate that RD has relatively low TEC indicating its thermophysical stability. The following TECs of davidite- (La) for the temperature range 25–1200 °C were obtained: (overline{mathrm{alpha }}) _a = (overline{mathrm{alpha }}) _b = 9.96 (3) × 10^–6 ºC⁻¹; (overline{mathrm{alpha }}) _c = 10.79 (4) × 10^–6 ºC⁻¹. The character of the thermal expansion is in agreement with the structure characterized by layers stacked along the c axis. The volume TEC α_V = 24.81 (47)—36.80 (48) × 10^–6 ºC⁻¹. Davidite-(La) exhibits an almost isotropic thermal expansion and shows one of the most superior thermal performances in comparison to the other mineral-like phases utilized for the immobilization of HLW.

Iron hydroxide FeO₂H_x (x ≤ 1) and ferrous iron chloride FeCl₂ can adopt the HP-PdF₂-type (space group: (P{a_{overline 3 }}), Z = 4) structure in the lowermost mantle, potentially contributing to the geochemical cycles of hydrogen and chlorine within Earth’s deep interior, respectively. Here we investigate the high-pressure behavior of HP-PdF₂-type FeCl₂ by X-ray diffraction (XRD) and Raman measurements in laser-heated diamond anvil cells. Our results show that HP-PdF₂-type FeCl₂ can be formed at 60‒67 GPa and 1650‒1850 K. Upon cold decompression, the diffraction peaks at pressures above 10 GPa can be indexed to the HP-PdF₂-type structure. Intriguingly, the calculated cell volumes reveal a remarkable decrease of ΔV / V = ∼ 14% between 36 and 40 GPa, which is possibly caused by a pressure-induced spin transition of Fe²⁺ (HS: high-spin → LS: low-spin). We also observe distinct changes in Raman spectra at 33‒35 GPa, practically coinciding with the onset pressures of isostructural phase transition in XRD results. Our observations combined with previous studies conducted at megabar pressures suggest that HP-PdF₂-type FeCl₂, with a wide pressure stability range, if present in subducting slabs, could facilitate the transport of chlorine from the middle lower mantle to the outer core.

To investigate the quantitative relationship between the crystal structure and composition of Mn-bearing calcite, the solid solutions of Ca_1–xMn_xCO₃ (x = 0.1, 0.3, 0.5, 0.7, 0.9) with continuous MnCO₃ mol% content were synthesized at 1 GPa and 700 °C using high-purity CaCO₃ and MnCO₃ powders as starting materials. The run products were analysized by electron probe, powder X-ray diffraction and Raman spectroscopy. The CaO wt% and MnO wt% of the resulting products are consistent with the expected compositions. The powder X-ray diffraction results show that the products are single phase without any impurities. All diffraction peaks of samples with varying MnCO₃ mol% contents can be indexed by the calcite-type structure carbonates ACO₃ (R-3c space group; A is a divalent cation), confirming the previous results that there is the completely continuous solid solution between CaCO₃ and MnCO₃ end members. The unit-cell parameters and volumes of the solid solutions decrease as the MnCO₃ mol% content increases, presenting a linear relationship of Ca–Mn ideal miscibility, which is perfectly consistent with the rigid body model of A-site substitution in ACO₃. Besides, as MnCO₃ mol% content increases, the bond distance of A–O decreases linearly, while the bond distance of C–O changes like a parabola. Therefore, the addition of Mn makes the bond distance of A–O shorten, resulting in the decrease of unit-cell parameters and volumes for Ca_1–xMn_xCO₃. Furthermore, two exterior vibrations (T and L) of the crystal lattice and two internal vibrations (ν₄ and ν₁) within the CO₃²⁻ unit are assigned in the Raman spectra of these solid solutions. The characteristic vibration modes T, L, and ν₄ as a whole increase with the increasing of MnCO₃ mol% content, whereas the characteristic vibration mode ν₁ as a whole decreases with the increase of MnCO₃ mol% content. These variations in Raman vibration modes are related to the radius of substituted ions.

The equilibrium partitioning of Na and K between alkali feldspar and NaCl–KCl salt melt was determined at 800 (^circ)C, 850 (^circ)C, 900 (^circ)C, 950 (^circ)C and 1000 (^circ)C and close to ambient pressure. Four different natural gem-quality alkali feldspars with low degree of Al–Si ordering covering the range from orthoclase to high sanidine and with slightly different minor element concentrations were used as starting materials. The partitioning curves obtained for the four feldspars are indistinguishable indicating that Na–K partitioning independent of the differences of Al–Si ordering state and minor element concentrations existing amongst these feldspars. A sub-regular two parameter Margules type solution model was fitted to the partitioning data, and the excess Gibbs energy describing the thermodynamic non-ideality of the alkali feldspar solid-solution and the respective Margules parameters (W_{text {g}text {K}}) and (W_{text {g}text {Na}}) including their temperature dependence expressed as (W_g=W_h-TW_s) were determined:

The corresponding solvus has a critical temperature slightly above 650 (^circ)C and is well comparable with earlier direct experimental determinations of the low-sanidine-albite solvus curve. Comparison of the vibrational excess entropy determined from low-temperature heat capacity measurements with the total excess entropy derived from the temperature dependence of the excess Gibbs energy yields a negative configurational contribution to the excess entropy pointing towards short-range Na–K ordering on the alkali site.

The metamict fergusonite-(Y) with the formula (Y_0.70Ln_0.20Ca_0.13U_0.02Th_0.02)_∑1.07(Nb_0.72Ta_0.17W_0.06Ti_0.04)_∑1(O_3.97(OH)_0.11F_0.08Cl_0.03) · 2.12H₂O from the Blyumovskaya Pit, Ilmeny Mountains (Russia) was studied by the means of high-temperature X-ray diffraction, thermal analysis, Raman spectroscopy and microprobe analysis. Thermal expansion was studied for both tetragonal (α-fergusonite) and monoclinic (β-fergusonite) polymorphs. The expansion of β-fergusonite is anisotropic and strongly negative along the α₃₃. In contrast, α-fergusonite exhibits a relatively isotropic thermal expansion upon heating. The volume CTE (α_V) for β-fergusonite varies in the range 22.87(94)–75.4(2.5) × 10^–6 ºC⁻¹, whereas α-fergusonite has α_V = 32.33(57)–31.66(49) × 10^-6 ºC⁻¹ in the temperature range 850–1200 °C. After heating to 1100 °C, the mineral develops a porous texture, and the radioactivity is reduced by 37%, which can be attributed to the partial volatilization of some radionuclides. In situ experiments revealed the complete sequence of the thermal evolution of the metamict fergusonite-(Y) for the first time.

High-temperature X-ray diffraction measurements of calcium ferrite (CF)-type MgAl₂O₄ were performed in a temperature range of 300–673 K at atmospheric pressure. From temperature dependence of the unit cell volume, thermal expansivity (α) was determined to be α(T) = (2.46 ± 0.13) × 10^–5 + (1.2 ± 0.3) × 10^–8 T in 1/K. Thermoelastic parameters of isothermal bulk modulus at zero pressure (K_T0), its pressure derivative (K_T′) and temperature derivative [(∂K_T0/∂T)_P] of MgAl₂O₄ CF were optimized by iteration calculation combining the least squares fitting of a third-order Birch–Murnaghan equation of state to previous P–V–T data with α calculation using the Grüneisen relation equation, α = γ_thC_V/(K_T0V) where γ_th and C_V are thermal Grüneisen parameter and isochoric heat capacity, respectively. γ_th was constrained by the α measured in this study. When pressure data were rescaled by Au equations of state which are different from that adopted in the previous study and temperature data were corrected using pressure dependence of electromotive force of a W–Re thermocouple, K_T0, K_T′ and (∂K_T0/∂T)_P were assessed to be 216(4) GPa, 3.9(3) and − 0.027(3) GPa/K, respectively. It was suggested that the optimized α was about 17% lower than that determined by the previous study at 2000 K.