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Antigorite is the high‐temperature member of the serpentine group minerals and is broadly considered a primary carrier of water in the subducting oceanic lithosphere. It has a wavy crystal structure along its a‐axis and several polysomes with different m‐values (m = 13–24) have been identified in nature. The m‐value is defined as the number of tetrahedra in one wavelength and is controlled by the misfit between the octahedral and tetrahedral layers. The degree of misfit primarily depends on the volumes of the MgO6 octehedra and SiO4 tetrahedra within the layers, which vary as a function of pressure and temperature. However, it is not well understood which m‐values of antigorite are stable at different pressure and temperature conditions. To investigate the pressure dependence of the stability of different m‐values in antigorite, we performed first‐principles calculations for several polysomes (m = 14–19) at high pressure from 0 to 14 GPa and compared their enthalpies at static 0 K. We found that although the energy differences between polysomes are small, polysomes with larger m‐values are more stable at ambient pressure, while polysomes with smaller m‐values are more stable at elevated pressures. This suggests that the structure of antigorite in the oceanic lithosphere subducting into the deep Earth may gradually evolve into a different polysome structure than the antigorite samples observed at ambient or near‐surface pressure conditions. These changes in the m‐values are accompanied by a minor dehydration reaction. By modulating the available amount of free water in the system, antigorite polysomatism may influence the distribution of intermediate‐depth seismicity, such as the observance of double seismic zones. Plain Language Summary: Antigorite is a hydrous mineral known for being an important carrier of water into the Earth's interior and a seismicity trigger at intermediate‐depths down to ∼200 km. However, its crystal structure is complex (e.g., Uehara, 1998), and thus it is not well understood which structural variant (polysome) of antigorite is stable or dominant at high pressures and temperatures during subduction. In this study, several crystal structures of antigorite were calculated by first‐principles calculations, and their enthalpies (i.e., Gibbs free energy of a system at static 0 K) were compared to determine the stable structure as a function of pressure. The results indicate that the structure and chemical composition of antigorite likely changes under high pressure, resulting in the gradual release of water during the subduction process. As antigorite is thought to be the main trigger of seismicity in the subducting lithospheric mantle, its metasomatism may contribute to explain the reported distributions of intermediate‐depth earthquakes. Key Points: Antigorite crystal structures with various values of m (14–19) were determined by first‐principles calculation under pressureThe relative enthalpy shows that antigorite with smaller m‐values are stabilized with increased pressureAntigorite in suducting slabs may gradually dehydrate under high pressure as a result of changes to stable m‐values [ABSTRACT FROM AUTHOR]

In this paper, we evaluated the potentialities of Raman spectroscopy and electron backscattered diffraction (EBSD) in the microscopic characterization of Ca-REE fluorcarbonates (CRFC) belonging to the bastnäsite-synchysite series to provide a "road map" for further investigations with transmission electron microscopy (TEM). EBSD was efective in establishing the sample orientation, setting up the oriented cuts, and ascertaining the effective syntactic relationship among all the detected CRFC phases; however, it failed to distinguish between diferent polysomes. On samples with diferent orientations that were preventively ascertained by EBSD and characterized by scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS), micro-Raman spectroscopy allows for distinguishing between polysomes based on the diferences in intensity and position of the symmetric stretching vibration (v1) of the carbonate group C O 3 2 − in the region around 1080–1099 cm–1. However, as evidenced by TEM-EDS, what appears as a homogeneous polysome in backscattered electrons (BSE) images may be a disordered intergrowth of compositional faults with a bulk composition being matched with that of a real polysome only by accident. Therefore, we conclude that the Raman signal is sensitive to diferent Ca/(Ca+REE) ratios but not to any ordered distribution of Ca-poor and Ca-rich lamellae within the analyzed volume, making the unambiguous identification of a polysome tricky. Finally, several ordered polysomes were detected at the TEM scale, including a B2S and a long-range polytype with a 32 nm repeat distance along c. The possible implications of the detected microstructure for ore mineral formation are discussed. [ABSTRACT FROM AUTHOR]

Perrierite-(Ce) crystals from ejecta of the Laacher See volcano, Eifel, in Germany were studied by X-ray single-crystal diffraction and electron microprobe analysis. The composition and crystal structure of this sample is discussed in relation to the known properties of the chevkinite group minerals and related synthetic compounds. Taking into account the modular character of the chevkinite minerals, based on a rutile and a silicate module, the preferred formation of either the perrierite or the chevkinite structure type is correlated with the formal charge of the rutile and silicate modules. The rutile module is expected to carry a negative charge, compensated by a positive charge of the silicate module. On average, the charge modulus is observed to be larger for the chevkinite-type module stacking. It can drop to zero in perrierite-type structures rich in Sr or Ca. In such cases, it is generally not expected to rise above two. The perrierite-(Ce) described in this study crystallizes in space group C2/m. The anisotropic oxygen and rare-earth element displacements observed in this crystal indicate a local domain structure of P 2 1 / a symmetry, when compared to the P 2 1 / a symmetry of the synthetic perrierite La 4 Mg 2 Ti 3 O 8 (Si 2 O 7) 2 . [ABSTRACT FROM AUTHOR]

The unit-cell parameter a of antigorite (usually expressed as the polysome m value) has been determined as a function of temperature (T) and pressure (P) in the range of 600-650 °C, 25-45 kbar in weeklong piston-cylinder experiments. A well-characterized natural antigorite (with m = 16 and less abundant m = 15) was used as a starting material that coexisted with olivine, chlorite, Ti-humite, and aqueous fluid at run conditions. Transmission electron microscope (TEM) measurements on selected focused ion beam (FIB) wafers showed that antigorite m values after the experiments varied between 14 and 22. More than 40 punctual analyses for each run condition were acquired to determine the range and the primary m value. The most frequent antigorite m-value decreased systematically from 17-19 at 600 °C to 15-16 at 650 °C. The spacing of the m-isolines is getting narrower as the antigorite breakdown reaction is approached. The topology of the m-isolines is similar to that previously characterized for the simple MgO-SiO2-H2O (MSH) system. However, the isolines are shifted to about 50-100 °C higher temperatures due to the incorporation of Al into antigorite. Powder samples and FIB wafers of natural antigorite from the Tianshan UHP belt (China) with peak metamorphic conditions of ~35 kbar, ~520 °C were also investigated with TEM. Low Al-antigorite formed at peak metamorphic conditions displays a peak m value of 20-21, whereas high-Al antigorite formed during isothermal decompression displays a lower m value of 19. Combination of our results with the published data of m values from metamorphic antigorite that experienced various conditions allowed construction of a P-T-m diagram that can be used in future studies to better constrain formation conditions of serpentinites. The decrease of m values and the increase of Al in antigorite with increasing temperature result in small, continuous dehydration whereby the H2O content of antigorite changes from 12.4 to 12.1 wt%. Therefore, it is expected that a pore fluid is present during the prograde deformation of serpentinites. TEM observations showed that antigorite adjusted its Al content by segregation of chlorite at the nanoscale. Together with the observation that multiple m values are always present in a single sample, this result indicates that full equilibration of antigorite at the micrometer-scale is rare, with important implications for the interpretation of geochemical signatures obtained by in situ techniques. [ABSTRACT FROM AUTHOR]

A part of the system CaO‐SiO2–Al2O3–Fe2O3–MgO which is of relevance to iron‐ore sintering has been studied in detail. For a bulk composition corresponding to 10.45 wt% CaO, 5.49 wt% MgO, 69.15 wt% Fe2O3, 13.37 wt% Al2O3 and 1.55 wt% SiO2 synthesis runs have been performed in air in the range between 1100 and 1300°C. Products have been characterized using reflected‐light microscopy, electron microprobe analysis and diffraction techniques. At 1250°C, an almost phase‐pure material with composition Ca2.99Mg2.67Fe3+14.58Fe2+0.77Al4.56Si0.43O36 has been obtained. The compound corresponds to the first Si‐containing representative of the M14+6nO20+8n polysomatic series of so‐called SFCA phases (Silico‐Ferrites of Calcium and Aluminum) with n = 2 and is denoted as SFCA‐III. Single‐crystal diffraction investigations using synchrotron radiation at the X06DA beamline of the Swiss Light Source revealed that the chemically homogenous sample contained both a triclinic and monoclinic polytype. Basic crystallographic data are as follows: triclinic form: a = 10.3279 (2) Å, b = 10.4340 (2) Å, c = 14.3794 (2) Å, α = 93.4888 (12)°, β = 107.3209 (14)° and γ = 109.6626 (14)°, V = 1370.49 (5) Å3, Z = 2, space group ; monoclinic form: a = 10.3277 (2) Å, b = 27.0134 (4) Å, c = 10.4344 (2) Å, β = 109.668 (2)°, V = 2741.22 (9) Å3, Z = 4, space group P21/n. Structure determination of both modifications was successful using diffraction data from the same allotwinned crystal. A description of the observed polytypism within the framework of OD‐theory is presented. Triclinic and monoclinic SFCA‐III actually correspond to the two possible maximum degree of order structures based on OD‐layers containing three spinel (S) and one pyroxene (P) modules (⟨S3P⟩). The existence of SFCA‐III in industrial iron‐ore sinters has yet to be confirmed. Polytypism is likely to occur in other SFCA‐members (SFCA, SFCA‐I) relevant to sintering as well, but has so far been neglected in the characterization of industrial samples. Our results shed light on this phenomenon and may therefore be also helpful for better interpretation of the powder diffraction patterns that are used for phase analysis of iron‐ore sinters. [ABSTRACT FROM AUTHOR]

The crystal structure of a new polytype of magnesiotaaffeite-2N′2S, ideally Mg3BeAl8O16 (trimagnesium beryllium octaaluminium hexadecaoxide), is described in space-group symmetry P-3m1. It has been identified in a fragment of a mineral sample from Burma (Myanmar). The new polytype is composed of two Mg2Al4O8 (S)- and two BeMgAl4O8 (N′)-modules in a stacking sequence N′SSN′′ which differs from the N′SN′S-stacking sequence of the known magnesiotaaffeite-2N′2S polytype. The crystal structure can be derived from a close-packed arrangement of O atoms and is discussed with regard to its polytypism and its Cr3+ chromophore content.

The crystal structure of a new polytype of magnesiotaaffeite-2N'2S, ideally Mg3BeAl8O16 (trimagnesium beryllium octaaluminium hexadecaoxide), is described in space-group symmetry P3m1. It has been identified in a fragment of a mineral sample from Burma (Myanmar). The new polytype is composed of two Mg2Al4O8 (S)- and two BeMgAl4O8 (N')-modules in a stacking sequence N'SSN" which differs from the N'SN'S-stacking sequence of the known magnesiotaaffeite-2N'2S polytype. The crystal structure can be derived from a close-packed arrangement of O atoms and is discussed with regard to its polytypism and its Cr3+ chromophore content. [ABSTRACT FROM AUTHOR]

Högbomite-group minerals are complex Fe-Mg-Zn-Al-Ti oxides related to the spinel group. Their polysomatic structure is composed of spinel (S) and nolanite (N) modules. The new polysome magnesiohögbomite-2N4S (IMA 2010-084) was found in the Sør Rondane Mountains, East Antarctica. It occurs in Mg-Al-rich, Si-poor skarns, characterized by a corundum-spinel-phlogopite-clinochlore assemblage. The new magnesiohögbomite polysome formed during the retrograde metamorphic stage. Magnesiohögbomite-2N4S appears macroscopically orange red, the streak is light orange colored. Euhedral crystals are hexagonal plates or prisms with cleavage planes on {001}. The mineral is optically uniaxial (-) and pleochroic with O = reddish brown and E = pale brown. The mean refractive index calculated from reflectance data in air at 589 nm is 1.85(3). The calculated density is 3.702(2) g/cm³. The Mohs hardness is 6.5-7, and VHN300 = 1020-1051, mean 1032 kg/mm². The crystal structure of the new polysome magnesiohogbomite-2jV45 has been solved and refined (R1 = 2.74%) from single-crystal XRD data. The crystal chemical formula is T10M24O46(OH) 2 where T and M represent tetrahedral and octahedral sites. The mineral is hexagonal, space group P63mc (no. 186), a = 5.71050(10), c = 27.6760(4) Å, Z = 1, V = 781.60(2) ų. The strongest lines in the powder XRD pattern [d(Å), I(%), hkl] are: 2.8561(4), 37, 110; 2.6120(3), 39,109; 2.42818(16), 100,116; 2.4160(4), 39,1070; 2.01181(13), 50,208; 1.54892(16), 35,2170; 1.42785(6), 57,220. Strongest peaks in Raman spectra are at 302,419, 479, 498, 709, 780, and 872 cm-1, with a broad OH-characteristic absorption around 3400 cm-1. The mean chemical composition (wt%) is SiO2 0.05, TiO2 7.08, SnO2 0.15, Al2O3 66.03, Cr2O3 0.02,Fe2O3 0.50, FeO 4.87, MnO 0.06, MgO 18.71, CaO 0.01, ZnO 0.96, NiO 0.01, CoO 0.02, F 0.06, Cl 0.01, H2O 1.00, sum 99.51. The simplified formula is (Mg8.2Fe1.2Zn0.2)2+ (Al22.7Fe0.1)3+ TiDue to image rights restrictions, multiple line equation(s) cannot be graphically displayed. O46(OH) 2 and ideal formula is Mg10Al22Ti2O46(OH)2. This mineral is a solid solution between the two ideal end-members, (Mg,Fe,Zn) Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed.(Al,Fe) Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed.TiDue to image rights restrictions, multiple line equation(s) cannot be graphically displayed.O46(OH)2 and (Mg,Fe,Zn) Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed.(Al,Fe) Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed.O46(OH)2. [ABSTRACT FROM AUTHOR]

Discrete, well-ordered, single-phase crystals (the Hanic phase) form when powders the bulk composition of Ca5Al6MgSiO17 are allowed to react in the absence of a melt. If, on the other hand, either the powders or crystals of Ca5Al6MgSiO17 are molten and then cooled, complex intergrowths form. These intergrowths consist of modules having the composition and structure of Ca3Al4MgO10 and gehlenite (Ca2Al2SiO7), forming a polysomatic series in which the modules occur in varied proportions. High-resolution transmission electron microscope (HRTEM) images reveal the details of these intergrowths. Mass balance is achieved by the formation of this polysomatic series together with Mg-substituted mayenite (Ca12−xMgxAl14O33). The tetrahedral sheets in gehlenite have close structural similarities to those of Ca5Al6O14. Based on our HRTEM images, we hypothesize chemical substitutions between gehlenite and Ca5Al6O14, resulting in gehlenite-like modules that are depleted in Si and correspondingly enriched in Al and Ca. [ABSTRACT FROM AUTHOR]

A new occurrence of schreyerite, V2Ti3O9, has recently been found in metamorphic rocks of the Sludyanka complex at the southern shore of Lake Baikal, Russia. In contrast to previously known schreyerite lamellae, which are intergrown with rutile, crystals from the Sludyanka complex occur as isolated single crystals associated with titanite, allowing single-crystal X-ray diffraction experiments. The chemical composition was determined with an electron microprobe giving the composition (V1.785Cr0.157Fe0.036)(Ti2.536V0.468)O9. A peculiarity of this schreyerite sample is the partial substitution of V4+ for Ti4+. The crystal structure was determined by single-crystal X-ray diffraction and was refined in the monoclinic space group C2/c [a = 17.102(2), b = 5.0253(5), c = 7.0579(8) Å, β = 106.636(10)º] to R1 = 2.84%. The structure is in agreement with the qualitative model of Grey et al. (1973) determined on the basis of electron diffraction and X-ray powder data for synthetic (Fe,Cr)2Ti3O9. Reinvestigation of schreyerite from the type locality indicates that this sample has cell dimensions and symmetry corresponding to the Sludyanka sample. The structure of schreyerite may be considered as a 1:1 polysome composed of slabs of berdesinskiite, V2TiO5, and Ti2O4 (a high-pressure phase of TiO2 with α-PbO2 structure). [ABSTRACT FROM AUTHOR]