Volume 109, Issue 7

Physical properties of mantle minerals are essential for comprehensive geodynamic modeling. High-pressure experiments allow measurements of physical properties but fundamental insights into their evolution with pressure are often experimentally inaccessible. Here we report the first in situ experimental determination of the optical refractive index, its wavelength-dispersion, and optical absorption coefficient of ferropericlase up to ~140 GPa at room temperature. All these properties change gradually in dominantly high-spin (below ~50 GPa) and low-spin (above ~80 GPa) ferropericlase. However, in the mixed-spin state (i.e., significant presence of both high- and low-spin iron), the index dispersion and the absorption coefficient decrease by a factor of three and ~30%, respectively. These anomalies suggest that charge transport by small polaron is reduced in mixed-spin ferropericlase, providing fundamental insights into the factor-of-three lower electrical conductivity of ferropericlase at ~50–70 GPa.

Understanding the stability of magnesite in the presence of a hydrous fluid in the Earth’s upper mantle is crucial for modeling the carbon budget and cycle in the deep Earth. This study elucidates the behavior of magnesite in the presence of hydrous fluids. We examined the brucite-magnesite [Mg(OH) 2 -MgCO 3 ] system between 1 and 12 GPa by using synchrotron in situ energy-dispersive X-ray diffraction experiments combined with textural observations from quenched experiments employing the falling sphere method. By subjecting magnesite to varying pressure-temperature conditions with controlled fluid proportion, we determined the stability limits of magnesite in the presence of a fluid and periclase. The observed liquidus provides insights into the fate of magnesite-bearing rocks in subduction zones. Our findings show that magnesite remains stable under typical subduction zone gradients even when infiltrated by hydrous fluids released from dehydration reactions during subduction. We conclude that magnesite can be subducted down to and beyond sub-arc depths. Consequently, our results have important implications for the carbon budget of the Earth’s mantle and its role in regulating atmospheric CO 2 levels over geological timescales.

Knowing the influence of isomorphic substitutions on the acid-base properties of smectite edge surfaces is an important aspect of the detailed understanding of clay minerals’ interfacial properties with implications in the modeling of adsorption processes. We investigated the intrinsic acidity constants of Fe(II)/Fe(III) and Al(III) substituted edge surface sites of montmorillonite with a cis -vacant structure, which includes four crystallographic orientations perpendicular to [010], [0 1 0], [110], and [ 1 1 0], using the first-principles molecular dynamics (FPMD) based vertical energy gap method. Fe(II) and Fe(III) substitutions resulted, respectively, in a significant increase and decrease in p K a values of amphoteric groups directly associated with Fe octahedra. In addition, Fe(II) substitution increased the p K a values of the neighboring silanol sites, while Fe(III) substitution had a weak influence on these sites. The Al-substituted tetrahedra had amphoteric sites with higher p K a values than the non-substituted Si tetrahedra, and they increased significantly the p K a values of the sites bridging the tetrahedral and octahedral sheets on surfaces perpendicular to [010] and [110]. The acid-base properties of substituted and non-substituted surface sites of cis -vacant montmorillonite were used to build a state-of-the-art surface complexation model, which successfully reproduced the best available experimental acid-base titration data. This model was further used to predict acid-base properties of dioctahedral smectites (montmorillonite, beidellite, and nontronite) according to their cis - or trans -vacant structures and their layer chemistry. According to these predictions, these smectites exhibit very similar overall pH buffering properties despite significant differences in structure and chemistry. A detailed analysis of the acid-base properties as a function of crystallographic directions demonstrated, however, that these differences should have a large influence on the adsorption of ionic species.

The crystal-chemical variations of spinels grown as a function of cooling rate (Δ T /Δ t ) were analyzed via X-ray electron-microprobe (EPMA) maps. Maps were collected serially by using a fixed distance. Spinels solidified from a tholeiitic MOR basaltic liquid (B 100 ) cooled at cooling rates (Δ T /Δ t ) of 1, 7, 60, and 180 °C/h, between 1300 and 800 °C and at ambient P and f O 2 . As Δ T /Δ t increases, the amount of spinel is invariably <5 area% and its size decreases. Compared to the previous data set collected by common single and selected EPMA analytical points (112 analyses), the kinetic effects induced by Δ T /Δ t are here quantitatively captured by a large number of analyses (2052). The TiO 2 , Al 2 O 3 , MgO, and FeO tot show large compositional variations at low cooling rates (from 1 to 60 °C/h), and only the average TiO 2 concentration shows a well-defined trend as a function of Δ T /Δ t . However, calculated average cation amounts (apfu) unveil quantitative kinetic effects. When Δ T /Δ t increases (from 1 to 180 °C/h), only Ti 4+ shows a linear decreasing trend, whereas the other major Al 3+ , Fe 3+ , Fe 2+ , and Mg 2+ cations alone are scattered. Conversely, the sums of trivalent (Al 3+ +Fe 3+ ) and divalent (Mg 2+ +Fe 2+ ) cations quantitatively capture the effect of the Δ T /Δ t . These new outcomes could be the base of novel geospeedometers with significant implications in volcanology, geophysics, and material sciences in regard to silicate melt rheology on Earth. They should be extended to high-pressure, hydrated, and low oxygen fugacity conditions. Furthermore, the analytical approach used here to capture kinetic effects on spinel growth and compositions can be also applied to other crystalline phases grown from silicate liquids.

Magmatic oxygen fugacity ( f O 2 ) is a fundamental property to understanding the long-term evolution of the Earth’s atmosphere and the formation of magmatic-hydrothermal mineral deposits. Classically, the magmatic f O 2 is estimated using mineral chemistry, such as Fe-Ti oxides, zircon, and hornblende. These methods, however, are only valid within certain limits and/or require a significant amount of a priori knowledge. In this contribution, a new oxybarometer, constructed by data-driven machine learning algorithms using trace elements in zircon and their corresponding independent f O 2 constraints, is provided. Seven different algorithms are initially trained and then validated on a data set that was never utilized in the training processes. Results suggest that the oxybarometer constructed by the extremely randomized trees model has the best performance, with the largest R 2 value (0.91 ± 0.01), smallest RMSE (0.45 ± 0.03), and low propagated analytical error (~0.10 log units). Feature importance analysis demonstrates that U, Ti, Th, Ce, and Eu in zircon are the key trace elements that preserve f O 2 information. This newly developed oxybarometer has been applied in diverse systems, including arc magmas and mid-ocean ridge basalts, fertile and barren porphyry systems, and global S-type detrital zircon, which provide f O 2 constraints that are consistent with other independent methods, suggesting that it has wide applicability. To improve accessibility, the oxybarometer was developed into a software application aimed at enabling more consistent and reliable f O 2 determinations in magmatic systems, promoting further research.

Evaporation of silicate materials from Earth or its precursors may be important in shaping their primordial compositions represented by undifferentiated meteorites, e.g., enstatite chondrites; however, the conditions under which evaporation occurs and the extent of evaporation-induced elemental and isotope fractionation remain uncertain. Here, we experimentally determine the volatility and isotope fractionation of Si, Mg, Ca, Nb, and Ta during enstatite melt evaporation at 2423–2623 K using a high-temperature conical nozzle levitator. Homogenous glasses are recovered after experiments; then we use EPMA and LA-ICP-MS to measure the elemental compositions, MC-ICP-MS to measure the Si and Mg isotopes, and TIMS to measure the Ca isotopes. Our results show that the evaporation rates of Si are larger than Mg, and the mean vapor/melt isotope fractionation factors (α = R vapor /R melt ; R = isotope ratio) are 0.99585 ± 0.00002 for 29 Si/ 28 Si and 0.98942 ± 0.00130 for 25 Mg/ 24 Mg. However, neither evaporative loss of Ca, Nb, and Ta nor Ca isotope fractionation was observed within analytical uncertainty. In conjunction with previous studies, we find that in an evaporation experiment the saturation degree (partial vapor pressure/equilibrium vapor pressure) of Si ( S Si ) is larger than S Mg when Si is more volatile than Mg, and vice versa. If the Mg/Ca and Si/Ca ratios and isotopes in the bulk silicate Earth are attributed to the evaporation of enstatite chondrite-like precursors, evaporation temperatures >5000 K and S Si < S Mg are required.

Quartz is widely distributed in various magmatic-hydrothermal systems and shows variable textures and trace element contents in multiple generations, enabling quartz to serve as a robust tracer for monitoring hydrothermal fluid evolution. This study demonstrates that integrated high-resolution SEM-CL textures and trace element data of quartz can be used to constrain physicochemical fluid conditions and trace the genesis of quartz in porphyry ore-forming systems. The Bilihe deposit is a gold-only porphyry deposit located in the Central Asian orogenic belt, NE China. Four quartz generations were distinguished following a temporal sequence from early-stage dendritic quartz, unidirectional solidification textured quartz (UST quartz), gray banded vein quartz (BQ), to late-stage white calcite vein quartz (CQ), with the Au precipitation being mostly related to dendritic quartz, UST quartz, and BQ. The well-preserved dendritic quartz with sector-zoned CL intensities and euhedral oscillatory growth zones crystallized rapidly during the late magmatic stage. The relatively low Al contents of dendritic quartz were interpreted to be related to contemporaneous feldspar or mica crystallization, while the high-Ti contents indicate high-crystallization temperatures (~750 °C). The comb-layered UST quartz displays heterogeneous, patchy luminescence with weak zoning, hosts coeval melt and fluid inclusions, and retains the chemical characteristics of magmatic dendritic quartz. High-Ti and low-Al contents of UST quartz suggest a formation at relatively high temperatures (~700 °C) and high-pH conditions. Three sub-types can be defined for hydrothermal BQ (BQ1, BQ2, and BQ3) based on contrasting CL features and trace element contents. The Al contents increase from BQ1 to BQ2 followed by a drop in BQ3, corresponding to an initial decrease and subsequent increase in fluid acidity. Temperature estimates of BQ decrease from BQ1 (635 °C) to BQ3 (575 °C), which may, however, be disturbed by high growth rates and/or high-TiO 2 activities. The CQ typically displays a CL-bright core and CL-dark rim with oscillating CL intensities and is characterized by the lowest Ti and highest Al, Li, and Sb contents compared to the other quartz types, which suggests a deposition from more acidic and lower temperature fluids (~250 °C). Trace element patterns indicate that a coupled Si 4+ ↔ (Al 3+ ) + (K + ) element exchange vector is applicable to dendritic quartz, UST quartz, and BQ. By contrast, charge-compensated cation substitution of Si 4+ ↔ (Al 3+ , Sb 3+ ) + (Li + , Rb + ) is favored for CQ. The comparison with compiled trace element data of quartz from other porphyry Au, Cu, and Mo deposits worldwide suggests that Ti, Al, Li, K, and Ge concentrations, as well as Al/Ti and Ge/Ti ratios, have the potential to discriminate the metal fertility of porphyry mineralization.

Zhengminghuaite, ideally Cu 6 Fe 3 As 4 S 12 , is a new Cu-Fe arsenosulfosalt found in the Zimudang Carlin-type gold deposit in southwestern Guizhou, China. It occurs as irregular, commonly fractured grains of several to a few tens of micrometers in the brecciated gold ores and is paragenetically associated with the late ore-stage mineral assemblage, including realgar, orpiment, pyrite, chalcopyrite, arsenopyrite, aktashite, christite, quartz, and calcite. Zhengminghuaite is opaque with a metallic luster and a conchoidal or uneven fracture. The Vickers microhardness (VHN 10 ) is 219 kg/mm 2 (range 192–247 kg/mm 2 ), and the calculated density is 4.77(5) g/cm 3 . In reflected light, zhengminghuaite is whitish gray with weak bireflectance (whitish gray to pinkish tinted gray), very weak anisotropy, and no internal reflection. Electron microprobe analyses give the empirical formula (Cu 5.92 Hg 0.08 ) Σ6.00 (Fe 1.59 Hg 1.07 Zn 0.37 ) Σ3.03 (As 3.94 Sb 0.02 ) Σ3.96 S 11.93 on the basis of total cations = 13, with the simplified formula Cu 6 (Fe,Hg,Zn) 3 (As,Sb) 4 S 12 . Zhengminghuaite is trigonal, with space group R 3. Unit-cell parameters determined from the single-crystal X-ray diffraction data are as follows: a = 13.5373(17) Å, c = 9.2354(13) Å, and V = 1465.7(4) Å 3 ( Z = 3). The eight strongest lines in the X-ray diffraction pattern are [ d (Å) ( I , %) ( hkl )]: 3.0785 (67) (003), 3.0670 (100) ( 1 3 1 ), 2.6586 (89) ( 1 3 2), 1.8825 (97) (134), 1.8773 (82) (520), 1.6060 (89) (13 5 ), 1.6028 (81) (523), and 1.6012 (83) (26 1 ). The crystal structure of zhengminghuaite belongs to the nowackiite group and can be described as formed by (0001) layers composed of corner-sharing FeS 4 and CuS 4 tetrahedra that delimit two triangular cavities. Zhengminghuaite is the Fe-dominant analog at the divalent cations site of nowackiite (Cu 6 Zn 3 As 4 S 12 ) and aktashite (Cu 6 Hg 3 As 4 S 12 ). Paragenetic relationships indicate that zhengminghuaite and associated Hg- and Tl-sulfosalts precipitated in response to the increase in sulfidation state and decrease in temperature of the late-ore stage hydrothermal fluid.

A-type granites generally have much lower water, higher temperature, and incompatible element concentrations than I-type granitoids. Yet it remains unclear why I-A-type granitic complexes occur in convergent plate margins. Here we conduct geochemical analyses on apatite and mafic minerals from the late Cretaceous I-A-type granitic complex in Fuzhou area, SE China, aiming to decipher differentiation, fluid metasomatism, and degassing that primarily control the compositional diversity of felsic magmas. Apatites in both rock types are F-rich and show large H 2 O and δD variations, i.e., 341–3892 ppm H 2 O and –325 to +336‰ δD in I-type granitoids; 67–1366 ppm H 2 O and –251 to +1439‰ δD in A-type granites. H 2 O in apatite is negatively correlated with La/Sm and Sr/Y in the I-type granitoids, whereas it is positively correlated with Ce and total rare earth element (REE) concentrations in the A-type granites. Once H 2 O increases up to hundreds of ppm, both rock types show a rapid decrease of H 2 O/Ce, an increase of F/Cl, and extensive H isotope fractionation. Arfvedsonite occurs as a late crystallizing mineral in the A-type granite and has much higher contents of Na 2 O, K 2 O, F, and high field strength elements (HFSE) than hornblende in the I-type granitoids, indicating the addition of F-HFSE-rich alkaline fluids during its magmatic evolution. The consumption of arfvedsonite and formation of aegirine further indicate the role of fluid metasomatism and H 2 degassing via a reaction of 3Na 3 Fe 5 Si 8 O 22 (OH) 2 + 2H 2 O = 9NaFeSi 2 O 6 + 2Fe 3 O 4 + 6SiO 2 +5H 2 . The combined geochemical data demonstrate that the systematic differences in mineral assemblage, whole-rock composition, magma temperature, H 2 O content, and δD of apatite between the I- and A-type granites are likely attributed to varying degrees of differentiation, fluid metasomatism and magmatic degassing. The I-type granitoids experienced hornblende, biotite, plagioclase, K-feldspar, and apatite fractionation and close-system degassing. The A-type granite was likely formed from the I-type monzogranitic magma that was metasomatized by the mantle-derived F-HFSE-rich alkaline fluids to produce the peralkaline magma, which further experienced K-feldspar + plagioclase + biotite + apatite fractionation and open-system degassing. Further numerical estimation indicates that the primary magma of Fuzhou granitic complex contained ~3.0 wt% H 2 O, and the lower water content of A-type granite was likely attributed to strong degassing during its emplacement. Our results indicate that some peralkaline A-type granites can be generated from relatively water-poor I-type granitic magmas by fluid metasomatism and degassing.

Cuprozheshengite, Pb 4 CuZn 2 (AsO 4 ) 2 (PO 4 ) 2 (OH) 2 , is a new mineral species from Yunnan, China. It occurs as sub-millimeter greenish-blue hemispherical aggregates of microscopic blade-like crystals on hemimorphite and is closely associated with veszelyite and galena. Cuprozheshengite is brittle with irregular fracture and has a Mohs hardness of 2½–3 and perfect cleavages on {011}. The calculated density is 5.91 g/cm 3 . The empirical chemical formula of the holotype is (Pb 3.97 Na 0.04 Ca 0.01 ) Σ4.02 Cu 1.06 Zn 2.09 (AsO 4 ) 2 [(P 0.84 As 0.12 Si 0.01 ) Σ0.97 O 4 ] 2 (OH) 2 based on 18 O atoms per formula unit. Cuprozheshengite is triclinic, space group P 1 , with unit-cell parameters a = 4.7977(8), b = 8.5789(8), c = 10.3855(9) Å, α = 97.270(8)°, β = 101.902(12)°, γ = 91.495(11)°, V = 414.30(9) Å 3 , and Z = 1. Cuprozheshengite is a member of dongchuanite group, whose general formula is A 4 VI B IV B 2 ( X 1O 4 ) 2 ( X 2O 4 ) 2 (OH) 2 , where A is an interlayer cation with Pb being dominant; B are transition metals with two crystallographic positions, IV B has tetrahedral coordination and is fully occupied by Zn, while VI B has octahedral coordination and is dominated by Zn or Cu; X 1 and X 2 are cations with tetrahedral coordination, occupied by As and P. Like other dongchuanite group minerals, the structural framework of cuprozheshengite is composed of two heteropolyhedral columns along [100]. Type 1 columns comprise corner-linked [ IV B O 4 ] and [ X 2O 4 ] tetrahedra. Each tetrahedron is connected with three other tetrahedra in the columns. Type 2 columns have alternating [ VI B O 4 (OH) 2 ] octahedra with pairs of corner-connected [ X 1O 4 ] tetrahedra. These two columns are connected by corner-sharing between [ IV B O 4 ] and [ X 1O 4 ] tetrahedra to form layers parallel to (011). Pb atoms occupy two independent sites between the layers. Cuprozheshengite is named as the copper analog of zheshengite. Single-crystal X-ray diffraction reveals that As and P order over the X 1 and X 2 sites, with As tending to occupy X 1. Density functional theory (DFT) calculations confirm the occupancy propensity of As benefiting structural stability. The structural and stability studies of cuprozheshengite may have implications for local environmental governance. As a stable mineral in the water and elemental cycles after weathering, cuprozheshengite still has the potential to continually crystallize, fixing As into a stable crystalline waste form.

The crystal chemistry of inderborite, a B-rich mineral (B 2 O 3 ~41 wt%) with ideal formula CaMg[B 3 O 3 (OH) 5 ] 2 ·6H 2 O or CaMg[B 3 O 3 (OH) 5 ] 2 (H 2 O) 4 (H 2 O) 4 ·2H 2 O from the Inder Deposit, Kazakhstan, was re-investigated by a multi-methodological approach (single-crystal X-ray and neutron diffraction, electron probe micro-analysis in wavelength-dispersive mode, laser ablation multi-collector inductively mass spectrometry). The experimental findings show that the real chemical formula of inderborite from the Inder Deposit is virtually identical to the ideal one: the fraction of potential isomorphic substituents is insignificant. Boron is, therefore, the only industrially relevant element occurring in this mineral. The in situ B isotope composition of the Inder inderborite shows enrichment in the heavy 11 B isotope, giving a weighted mean δ 11 B NIST951 of +35.15 ± 0.49 ‰ (2σ, N = 6). Such a positive δ 11 B value falls within the range of values in which the source of boron is ascribable to marine reservoirs rather than to terrestrial ones. X-ray (at 293 K) and neutron (at 20 K) structure refinements confirm that the principal building block unit of the structure is a [B 3 O 3 (OH) 5 ] 2– ring, consisting of two BO 2 (OH) 2 tetrahedra (B-ion in sp 3 electronic configuration) and one planar-triangular BO 2 OH group (B-ion in sp 2 electronic configuration). In the [B 3 O 3 (OH) 5 ] 2– ring, all the oxygen atoms that are not shared between two boron atoms are protonated. The building units share corners with the CaO 2 (OH) 4 (OH 2 ) 2 polyhedra and Mg(OH) 4 (OH 2 ) 2 octahedra, forming hetero-polyhedral sheets parallel to (100). Subsequent hetero-polyhedral sheets are mutually connected only by H-bonding interactions, even mediated by the zeolitic (“interstitial”) H 2 O molecules. Ten out of 11 independent oxygen sites in the structure of inderborite are involved in H-bonds as donors or acceptors, and this reflects the pervasive effect of the H-bonding network. The role played by the complex H-bond network is expected to be substantial on the stability of the crystalline edifice, having effects within the single hetero-polyhedral sheet, between subsequent sheets, and in the bonding with the interstitial zeolitic H 2 O molecules. Finally, the potential utilizations of inderborite, as a B-bearing mineral, are discussed.

The new mineral bobfinchite (IMA2020-082), Na[(UO 2 ) 8 O 3 (OH) 11 ]·10H 2 O, was found in the Burro mine, Slick Rock district, San Miguel County, Colorado, U.S.A., where it occurs as an oxidation product of uraninite on asphaltite matrix in intimate association with gypsum, natrozippeite, metaschoepite, and uranopilite. Bobfinchite crystals are transparent to translucent, yellow, lozenge-shaped disks up to 0.3 mm wide. Crystals are flattened on [100] and exhibit the forms {100}, {011}, {021}, {0 2 1}, and {0 1 1}. Bobfinchite has a pale-yellow streak and emits very dim yellow fluorescence under 365 nm ultraviolet illumination. The crystals are brittle with very good {100} cleavage and irregular, stepped fracture. The Mohs hardness is ca. 2 based on scratch tests. The calculated density is 5.044 g/cm 3 based on the empirical formula and 5.036 g/cm 3 for the ideal formula. Bobfinchite is optically biaxial (–), with α = 1.690(5), β = 1.7205(5), and γ = 1.730(5) (white light). The measured 2 V , estimated from the interference figure, is 55(5)° and the calculated value is 59.1°. Dispersion is moderate, r > ν ; orientation: X = a , Y = b , Z = c ; pleochroism: X nearly colorless, Y yellow, Z yellow; X < Y ≈ Z . Electron microprobe analysis provided the empirical formula (Na 0.99 Pb 0.02 )[(UO 2 ) 7.99 O 3 (OH) 11 ]·10H 2 O. The five strongest X-ray powder diffraction lines are [ d obs in Å( I )( hkl )]: 7.34(100)(200), 3.59(50)(024), 3.23(60)(224), 3.18(36)(240), and 2.01(23)(624,551,208,640,346). Bobfinchite is orthorhombic, Pbcn , a = 14.6249(9), b = 14.0389(10), c = 16.6923(10) Å, V = 3427.2(4) Å 3 , and Z = 4. The structure of bobfinchite ( R 1 = 0.0330 for 3770 I > 4σ I ) is built from uranyl oxide-hydroxide sheets that adopt the fourmarierite topology, with interlayer Na + and H 2 O groups. Both the sheet and interlayer topology mimic those observed in natural and synthetic Na-metaschoepites studied previously, and as seen in other uranyl oxide hydrate minerals, charge balance is achieved at specific sites in the sheet through the substitution O 2– ↔ (OH) – .

[Ag 6 ] 4+ clusters are extremely rare in nature (only found in Ag-rich tetrahedrite group minerals). Due to their remarkable structures and some promising applications, a few synthesis phases that contain octahedral [Ag 6 ] 4+ clusters have been reported. However, the kinds of natural conditions that promote the formation of subvalent hexasilver clusters in tetrahedrite group minerals are still unclear. Kenorozhdestvenskayaite-(Fe), ideally Ag 6 (Ag 4 Fe 2 )Sb 4 S 12 □ is a new tetrahedrite group mineral containing a natural [Ag 6 ] 4+ cluster, found in the Yindongpo gold deposit, Weishancheng ore field, Henan Province, China. This new species occurs at the edges of galena crystals as anhedral grains of 2 to 20 μm in size and is associated with pyrargyrite, pyrrhotite, and siderite. Kenorozhdestvenskayaite-(Fe) is black in color with metallic luster. It is brittle with conchoidal fracture and has a calculated density of 5.329 g/cm 3 . The empirical formula calculated on the basis of cation = 16 apfu is M (2) Ag 6 M (1) (Ag 2.41 Cu 1.20 Fe 1.84 Zn 0.71 ) Σ6.16 X (3) (Sb 3.82 As 0.01 ) Σ3.83 S(1) S 11.60 S(2) □. It is cubic, with space group I 4 3 m , a = 10.7119(6) Å, V = 1229.1(2) Å 3 , and Z = 2. Since kenorozhdestvenskayaite-(Fe) is a new tetrahedrite group mineral containing a natural [Ag 6 ] 4+ cluster, its structure is comparable to the synthetic ternary phosphide (Ag 6 M 4 P 12 ) M6′ $\begin{array}{} \displaystyle {\rm M_6'} \end{array}$. The presence of the unusual mineral assemblages, i.e., pyrrhotite and pyrargyrite, as well as the other keno-end-member tetrahedrites, indicates a low- f S 2 state for the mineralization stage, probably a result of the fluid boiling process in an open system that likely contributed to the formation of S-deficient tetrahedrites.

Compressibility and pressure-induced structural evolution of kokchetavite, the hexagonal polymorph of KAlSi3O8, has been studied up to 11.8 GPa using synchrotron single-crystal X-ray diffraction. Two phase transitions were observed at pressures of ~0.3 and 10.4 GPa. Kokchetavite-I (as-synthesized, P 6/ mcc ) transforms into kokchetavite-II with the P 6 c 2 space group. Kokchetavite-II → kokchetavite-III phase transition at ~10.4 GPa is accompanied by a change of symmetry to probably orthorhombic. After pressure release, kokchetavite reverts to the initial single-crystal state with P 6/ mcc space group. A second-order Birch-Murnaghan equation of state was calculated for phase kokchetavite-II with coefficients V 0 = 1486(3) Å 3 , K 0 = 59(2) GPa.

Hard Mode IR powder absorption spectroscopy has been used to characterize local strain relaxation associated with Al/Si ordering in a suite of synthetic anorthite samples with structural states that vary from a high degree of Al/Si order through a metastable incommensurate structure at intermediate states of order to long-range order with I 1 symmetry. The dominant feature accompanying the changing structural states is line broadening, which has been quantified by autocorrelation analysis and is attributed to local heterogeneous strain variations on a length scale of at least 1–5 unit cells. The autocorrelation results are consistent with contributions to the line broadening as being due to order parameters for both the C 1 → I 1 and I 1 → P 1 transitions, which couple biquadratically, λ Qod2Qdispl2. $\begin{array}{} \displaystyle \lambda Q_{\text{od}}^{2}Q_{\text{displ}}^{2}. \end{array}$ Close correlation with enthalpy variations from previously published calorimetric data indicates that the driving force for ordering can be understood in terms of elimination of strain fields arising from accommodating more or less rigid AlO 4 and SiO 4 tetrahedra in the feldspar framework. The metastable incommensurate structure of anorthite is closely analogous to the stable incommensurate structure that develops at intermediate compositions in the plagioclase solid solution, confirming that the same strain relaxation mechanism dominates the properties and behavior of all structural states across the solid solution. Elimination of strain heterogeneity by ordering on the basis of I 1 symmetry determines the form of non-ideal mixing shown by the solid solution at high temperatures, and changes in elastic properties may contribute to a break in the slope of partitioning of trace elements between crystals and melt.

Amorphous calcium carbonate (ACC) is the least stable polymorph of calcium carbonates. It has been identified to play an important role in nature (e.g., biomineralization and speleothem formation), where it acts as a precursor for the transformation to more stable polymorphs such as calcite. Furthermore, the use of ACC in technical applications requires a robust understanding of the material’s properties. We present the first study that reveals the existence of a glass transition for synthetic and anhydrous ACC. The glass transition occurs at 339 °C. Such measurements are impossible with conventional differential scanning calorimetry (DSC) due to the high tendency of ACC to crystallize. Fast scanning DSC with heating rates of 500 °C/s and higher, however, can be used to separate the endothermic glass transition signature from the exothermic crystallization event since crystallization is shifted to higher temperatures. This allows the detection and quantification of the glass transition for ACC. These observations indicate that ACC is a structural glass and are especially significant because the synthesis of ACC, precipitation from a solution followed by lyophilization, contrasts with the more conventional and well-known route of glass formation–the rapid cooling of a melt. Moreover, we prove that a structural glass can be produced from a simple single-component carbonate system.