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The thermal behaviour of a natural allanite-(Ce) has been investigated up to 1073 K (at room pressure) by means of in situ synchrotron powder X-ray diffraction and single-crystal neutron diffraction. Allanite preserves its crystallinity up to 1073 K. However, up to 700 K, the thermal behaviour along the three principal crystallographic axes, of the monoclinic β angle and of the unit-cell volume follow monotonically increasing trends, which are almost linear. At T  > 700–800 K, a drastic change takes place: an inversion of the trend is observed along the a and b axes (more pronounced along b ) and for the monoclinic β angle; in contrast, an anomalous increase of the expansion is observed along the c axis, which controls the positive trend experienced by the unit-cell volume at T  > 700–800 K. Data collected back to room T , after the H T experiments, show unit-cell parameters significantly different with respect to those previously measured at 293 K: allanite responds with an ideal elastic behaviour up to 700 K, and at T  > 700–800 K its behaviour deviates from the elasticity field. The thermo-elastic behaviour up to 700 K was modelled with a modified Holland–Powell EoS; for the unit-cell volume, we obtained the following parameters: V T0  = 467.33(6) Å 3 and α T0 ( V ) = 2.8(3) × 10 –5  K −1 . The thermal anisotropy, derived on the basis of the axial expansion along the three main crystallographic directions, is the following: α T0 ( a ):α T0 ( b ):α T0 ( c ) = 1.08:1:1.36. The T -induced mechanisms, at the atomic scale, are described on the basis of the neutron structure refinements at different temperatures. Evidence of dehydroxylation effect at T  ≥ 848 K are reported. A comparison between the thermal behaviour of allanite, epidote and clinozoisite is carried out.

The crystal chemistry and crystal structure of the rare earth element phosphates, monazite-(Ce), Ce(PO 4 ), and xenotime-(Y), Y(PO 4 ), as well as the arsenates, gasparite-(Ce), Ce(AsO 4 ), and chernovite-(Y), Y(AsO 4 ), from the hydrothermal quartz-bearing fissures, related to pegmatites overprinted by amphibolite facies, cropping out at Mt. Cervandone, Western Alps, Piedmont, Italy, have been investigated by means of electron microprobe analysis in wavelength dispersion mode and single-crystal X-ray diffraction. The chemical data reveal the occurrence of a full solid solution among the isostructural chernovite-(Y) and xenotime-(Y) with tetragonal symmetry, whereas a wide miscibility gap is observed for the isostructural gasparite-(Ce) and monazite-(Ce) of Mt. Cervandone, with monoclinic symmetry. A significant chemical heterogeneity has been observed for several investigated samples, especially related to the Th content, which is locally enriched in ThSiO 4 grains. The analysis of the refined structural models demonstrates the significant control played by the composition of the tetrahedrally-coordinated (As,P)-bearing sites on the bulk unit-cell volume, and on the size and shape of the (REE)-coordination polyhedra.

We investigated, by scanning and transmission electron microscopy (SEM, TEM), wavelength- and energy-dispersive spectroscopy (WDS, EDS), and electron diffraction tomography (EDT), several (Y-REE-U-Th)-(Nb-Ta-Ti) oxides from the Garnet Codera dike pegmatite (Central Italian Alps). These oxides have compositions in the samarskite-(Y) field and yield an amorphous response from the single-crystal X-ray diffractometer. Backscattered electron images reveal that the samples are zoned with major substitutions involving (U+Th) with respect to (Y+REE). At the TEM scale, the samples show a continuous range of variability both in terms of composition and in radiation damage, and the amount of radiation damage is directly correlated with the U-content. Areas with high U-content and highly damaged show crystalline, randomly oriented nanoparticles that are interpreted as decomposition products of the metamictization process. On the other hand, areas with lower U-content and radiation dose contained within 0.7×1016 α-event/mg, although severely damaged, still preserve single-crystal appearance. Such areas, noticeably consisting of relicts of the original samarskite structure, were deeply investigated by electron diffraction techniques. Surprisingly, the retrieved crystal structure of untreated samarskite is consistent with aeschynite and not with ixiolite (or columbite), as believed so far after X-ray diffraction experiments on annealed samples. In particular, the resolved structure is a niobioaeschynite-(Y), with Pnma space group, cell parameters a = 10.804(1), b = 7.680(1), c = 5.103(1) A, and composition (Y0.53Fe0.22Ca0.10U0.09Mn0.07)Σ=1(Nb1.07Ti0.47Fe0.34 Ta0.07W0.06)Σ=2O6 If this finding can be confirmed and extended to the other members of the group [namely samarskite-(Yb), calciosamarskite, and ishikawaite], then the samarskite mineral group should be considered no longer as an independent mineral group but as part of the aeschynite group of minerals.It is finally suggested that the rare crystalline sub-micrometric ixiolite domains, occasionally spotted in the sample by TEM, or the nanoparticles detected in highly metamict areas interpreted as decomposition product of the metamictization process, which may have in fact the ixiolite structure, act as seeds during annealing, leading to the detection of ixiolite peaks in the X-ray powder diffractograms.

The crystal structure and crystal chemistry of meyerhofferite, ideally CaB 3 O 3 (OH) 5 ·H 2 O, was investigated by a multi-methodological approach based on titrimetric determination of boron, gravimetric determination of calcium, determination of fluorine by ion selective electrode, determination of water content by heating, other minor elements by inductively coupled plasma atomic emission spectroscopy, along with single-crystal synchrotron X-ray and neutron diffraction. The concentration of more than 50 chemical elements was measured. The combination of these techniques proves that the composition of meyerhofferite approaches the ideal one (i.e., (Ca 1.012 Mg 0.003 ) (B 2.984 Si 0.001 )O 3 (OH) 5 ·1.018H 2 O), with only a modest fraction of Mg (with MgO ≈ 0.03 wt%) replacing Ca, and with Si the only potential substituent of tetrahedral B (with SiO 2 ≈ 0.02 wt%). The content of REE and other minor elements is, overall, not significant, including that of fluorine as a potential OH − substituent (i.e., < 0.01 wt%). These findings have some relevant geochemical and technical implications, here discussed. The X-ray and neutron structure model obtained in this study prove that the building units of the structure of meyerhofferite consist of: two BO 2 (OH) 2 tetrahedra and one BO 2 (OH) triangle, linked by corner-sharing to form [B 3 O 3 (OH) 5 ] 2− rings, and distorted Ca-polyhedra (with CN = 8, CaO 3 (OH) 4 (OH 2 )), linked by edge-sharing to form infinite chains along [001]. The B 3 O 3 (OH) 5 rings are connected to the Ca-polyhedra chains by corner- and edge-sharing, on two sides of the chains. These heteropolyhedral chains, made by Ca-polyhedra and B 3 O 3 (OH) 5 rings, are mutually connected through hydrogen bonding only, giving rise to the tri-dimensional edifice of meyerhofferite. The neutron structure refinement showed no evidence of static or dynamic disorder pertaining to the H sites; their libration regime was found to be significantly anisotropic. At least seven of the nine oxygen sites of the structure are involved in H-bonding, as donors or as acceptors . The role played by the H-bonding scheme on the physical properties of meyerhofferite is discussed.

Kernite, ideally Na (OH) ∙3H O, is a major constituent of borate deposits and one of the most important mineral commodities of B. The chemical composition and crystal structure of kernite from the Kramer Deposit (Kern County, California) were investigated by a suite of analytical techniques (i.e., titrimetric determination of B content, gravimetric method for Na, ion selective electrode for F, high- mass loss for H O content, inductively coupled plasma atomic emission spectroscopy for REE and other minor elements, elemental analysis for C, N, and H contents) and single-crystal X‑ray (at 293 K) and neutron (at 20 K) diffraction. The concentrations of more than 50 elements were measured. The general experimental formula of the kernite sample used in this study is Na (OH) ∙3.01H O. The fraction of other elements is, overall, insignificant: excluding B, kernite from the Kramer Deposit does not act as geochemical trap of other technologically relevant elements (e.g., Li, Be, or REE). The X‑ray and neutron structure model obtained in this study confirms that the structure of kernite is built up by: two (crystallographically independent) triangular BO OH groups and two tetrahedral BO groups, which share corner-bridging O atoms to form threefold rings, giving chains running along [010], and NaO (OH)(OH ) and NaO (OH)(OH polyhedra. Positional disorder of two H sites of H O molecules was observed by the neutron structure refinement and corroborated by the maximum-entropy method calculation, which consistently provided a model based on a static disorder, rather than a dynamic one. The H-bonding network in the structure of kernite is complex, pervasive, and plays a primary role on its structural stability: the majority of the oxygen sites are involved in H-bonding, as or as . The potential utilizations of kernite, as a source of B (B ~50 wt%), are discussed, on the basis of the experimental findings of this study.

The new mineral marchettiite (IMA2017-066) is the natural equivalent of ammonium hydrogen urate. It has a simple molecular formula C 5 H 7 N 5 O 3 and can be alternatively written as (NH 4 )C 5 H 3 N 4 O 3 . Marchettiite was found in a cleft at Mount Cervandone, Devero Valley, Piedmont, Italy, where it occurs as aggregates of opaque pale pink to white, platy prismatic crystals. This mineral has a white streak, dull and opaque lustre, it is not fluorescent and has a hardness of 2–2.5 (Mohs’ scale). The tenacity is brittle and crystals have a good cleavage parallel to 001. The calculated density is 1.69 g/cm 3 . Marchettiite is biaxial (–) with 2V of 47.24°; the optical properties of marchettiite were determined by periodic-DFT methods providing the following values: α = 1.372, β = 1.681 and γ = 1.768. No twinning was observed. Electron microprobe analyses gave the following chemical formula: C 4.99 H 6.97 N 4.91 O 3.00 . Although the small crystal size did not allow refinement of structural data by single-crystal diffraction, we were able to refine the structure by powder micro X-ray diffraction. Marchettiite has space group P$\\bar{1}$and the following unit-cell parameters: a = 3.6533(2) Å, b = 10.2046(7) Å, c = 10.5837(7) Å, α = 113.809(5)°, β = 91.313(8)°, γ = 92.44(1)° and V = 360.312 Å 3 . The strongest lines in the powder diffraction pattern [ d in Å ( I )( hkl )] are: 9.784(50)(001); 8.663(80)(01$\\bar{1}$); 5.659(100)(011); 3.443(100)(10$\\bar{1}$); 3.241(70)(003) and 3.158(100)(1$\\bar{1}\\bar{1})$. Marchettiite is named after Gianfranco Marchetti, the mineral collector who found this mineral.

The high-pressure behavior of the natural arsenate gasparite-(Ce) [Ce 0.43 La 0.24 Nd 0.15 Ca 0.11 Pr 0.04 Sm 0.02 Gd 0.01 (As 0.99 Si 0.03 O 4 )] from the Mt. Cervandone mineral deposit (Piedmont Lepontine Alps, Italy), has been studied by in situ single-crystal synchrotron X-ray diffraction up to 22.01 GPa. Two distinct high-pressure ramps have been performed, using a 16:3:1 methanol:ethanol:water solution and helium as P -transmitting fluids, respectively. No phase transition occurs within the pressure range investigated, whereas a change in the compressional behavior has been observed at ~ 15 GPa. A second-order Birch-Murnaghan EoS was fitted to the P-V data, leading to a refined bulk modulus of 109.4(3) GPa. The structural analysis has been carried out on the basis of the refined structure models, allowing the description of the deformation mechanisms accommodating the bulk compression in gasparite-(Ce) at the atomic scale, which is mainly controlled by the compression of the Rare Earth Elements coordination polyhedra, while the AsO 4 tetrahedra behave as a quasi-rigid units. A micro-Raman spectroscopy analysis, performed at ambient conditions, suggests the presence of hydroxyl groups into the structure of the investigated gasparite-(Ce).

The crystal structure and crystal chemistry of kurnakovite from Kramer Deposit (Kern County, California), ideally MgB3O3(OH)5·5H2O, were investigated by single-crystal neutron diffraction (data collected at 293 and 20 K) and by a series of analytical techniques aimed to determine its chemical composition. The concentration of more than 50 elements was measured. The empirical formula of the sample used in this study is Mg0.99(Si0.01B3.00)Σ3.01O3.00(OH)5·4.98H2O. The fraction of rare earth elements (REE) and other minor elements are, overall, insignificant. Even the content of fluorine, as a potential OH-group substituent, is insignificant (i.e., ~0.008 wt%). The neutron structure model obtained in this study, based on intensity data collected at 293 and 20 K, shows that the structure of kurnakovite contains: [BO2(OH)]-groups in planar-triangular coordination (with the B-ions in sp2 electronic configuration), [BO2(OH)2]-groups in tetrahedral coordination (with the B-ions in sp3 electronic configuration), and Mg(OH)2(H2O)4-octahedra, connected into (neutral) Mg(H2O)4B3O3(OH)5 units forming infinite chains running along [001]. Chains are mutually connected to give the tri-dimensional structure only via hydrogen bonding, and extra-chains \"zeolitic\" H2O molecules are also involved as \"bridging molecules.\" All the oxygen sites in the structure of kurnakovite are involved in hydrogen bonding, as donors or as acceptors. The principal implications of these results are: (1) kurnakovite does not act as a geochemical trap of industrially relevant elements (e.g., Li, Be, or REE), (2) the almost ideal composition makes kurnakovite a potentially good B-rich aggregate in concretes (for example, used for the production of radiation-shielding materials for the elevated ability of 10B to absorb thermal neutrons), which avoids the risk to release undesirable elements, for example sodium, that could promote deleterious reactions for the durability of cements.

The new mineral marchettiite (IMA2017-066) is the natural equivalent of ammonium hydrogen urate. It has a simple molecular formula C5H7N5O3 and can be alternatively written as (NH4)C5H3N4O3. Marchettiite was found in a cleft at Mount Cervandone, Devero Valley, Piedmont, Italy, where it occurs as aggregates of opaque pale pink to white, platy prismatic crystals. This mineral has a white streak, dull and opaque lustre, it is not fluorescent and has a hardness of 2-2.5 (Mohs' scale). The tenacity is brittle and crystals have a good cleavage parallel to {001}. The calculated density is 1.69 g/cm3. Marchettiite is biaxial (-) with 2V of 47.24°; the optical properties of marchettiite were determined by periodic-DFT methods providing the following values: α = 1.372, β = 1.681 and γ = 1.768. No twinning was observed. Electron microprobe analyses gave the following chemical formula: C4.99H6.97N4.91O3.00. Although the small crystal size did not allow refinement of structural data by single-crystal diffraction, we were able to refine the structure by powder micro X-ray diffraction. Marchettiite has space group P1 and the following unit-cell parameters: a = 3.6533(2) Å, b = 10.2046(7) Å, c = 10.5837(7) Å, α = 113.809(5)°, β = 91.313(8)°, γ = 92.44(1)° and V = 360.312 Å3. The strongest lines in the powder diffraction pattern [d in Å (I)(hkl)] are: 9.784(50)(001); 8.663(80)(011); 5.659(100)(011); 3.443(100)(101); 3.241(70)(003) and 3.158(100)(111). Marchettiite is named after Gianfranco Marchetti, the mineral collector who found this mineral.

CaCeAl 2 (Fe 3+ 2/3 □ 1/3 )[Si 2 O 7 ][SiO 4 ]O(OH), allanite-(Ce) and rare earth element (REE)-bearing epidote occur as globular aggregates and platy prismatic crystals in miarolitic cavities in a niobium, yttrium, fluorine (NYF) granitic pegmatite at Baveno, Verbania, Southern Alps, Italy. These samples were investigated by means of an electron probe micro-analyser (EPMA) and single-crystal X-ray diffraction. Our EPMA results show that the globular aggregates have the highest REE content in the core portion and decreases to REE-bearing epidote towards the rim whereas the prismatic crystals are characterized by marked oscillatory zoning that have the highest REE contents at the rim of the crystal. The unit-cell parameters of “allanites” have an intermediate unit-cell between CaCeAl 2 (Fe 3+ 2/3 □ 1/3 )[Si 2 O 7 ][SiO 4 ]O(OH), allanite-(Ce) and REE-free epidote, because reflect the strong chemical heterogeneity of the samples which form complete solid solutions. Hydrothermal fluids control the activity and precipitation of incompatible elements like high field strength elements (HFSE), Sc and REE by hydrous F-rich fluids below the critical temperature which allow to deposit accessory minerals in the cavities with decreasing temperature. The source of REE and Y are the sheet and REE-silicates like siderophyllite-annite, and gadolinite-(Y) which underwent partial to complete decomposition by the activity of aggressive F-rich hydrothermal fluids.