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The crystal structure and chemical composition of boussingaultite from Pécs-Vasas, Mecsek Mountains, South Hungary, were investigated by single-crystal neutron diffraction (at 20 K) along with a series of chemical analytical techniques [i.e., gravimetric determination of sulfates, EDTA titrimetric determination of magnesium, ion selective electrode for F and Cl, indirect gravimetric determination of ammonium as (NH4,Rb,Cs,K) tetraphenylborate, inductively coupled plasma atomic emission spectroscopy for REE and other minor elements, elemental analysis for C, N, and H content, high-T mass loss for H2O content]. The concentrations of more than 50 elements were measured. The experimental formula of the boussingaultite is: [(NH4)1.77K0.22)Σ1.99[(Mg0.95Mn0.06)Σ1.01(H2O)5.7](SO4)1.99. Neutron data analysis confirms that the structure of boussingaultite is built up by isolated Mg(H2O)6- octahedra, along with isolated NH4- and SO4-tetrahedra connected by a complex H-bonds network. Mg2+ is completely solvated by H2O molecules in a typical octahedral bonding configuration. All the seven independent oxygen sites in the structure are involved in H-bonds, as donors or as acceptors. The geometry of all the H2O molecules, bonded to Mg, is in line with that usually observed in crystalline compounds. The H2O molecules show moderate-strong H-bonds, with H···Oacceptor and Odonor···Oacceptor ranging between 1.72–1.87 and 2.70–2.84 Å, respectively, along with Odonor-H···Oacceptor angles between 168–178°. The four independent N-H···O bonds show H···Oacceptor and Ndonor···Oacceptor distances ranging between 1.81–2.00 and 2.84–2.98 Å, respectively, with N-H···O angles between 158–176°. All the H-bonds of the H2O molecules and of the NH4-group involve the oxygen sites of the SO4-group as acceptors: the SO4-group is, therefore, the "bridging unit" between the NH4 and the Mg(H2O)6 units, via H-bonds. Our structure refinement proved, unambiguously, that the partial K4+ vs. NH+ replacement generates a local disorder. K lies at the N site, and its bonding configuration can be described by a distorted polyhedron with CN = 8. However, the K+ vs. NH+ replacement implies 4 a change in the configuration of the SO4- tetrahedron, through a sort of rotation of the polyhedron. This is the first evidence of the presence of a partial picromerite component in the boussingaultite structure, which gives rise to a local disorder likely due to the significantly different bonding configurations of the two cations. Our refinement also revealed that Mn2+ replaces Mg2+ at the Mg site. No evidence of distortion of the octahedron is observed in response to such a replacement, but the fraction of Mn2+ is modest. An analysis of previous Raman and IR results is provided, and is compared with the experimental results of this study. [ABSTRACT FROM AUTHOR]

The crystal structure and crystal chemistry of meyerhofferite, ideally CaB3O3(OH)5·H2O, 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., (Ca1.012Mg0.003) (B2.984Si0.001)O3(OH)5·1.018H2O), 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 SiO2 ≈ 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 BO2(OH)2 tetrahedra and one BO2(OH) triangle, linked by corner-sharing to form [B3O3(OH)5]2− rings, and distorted Ca-polyhedra (with CN = 8, CaO3(OH)4(OH2)), linked by edge-sharing to form infinite chains along [001]. The B3O3(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 B3O3(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. [ABSTRACT FROM AUTHOR]

Kernite, ideally Na2B4O6(OH)2∙3H2O, 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-T mass loss for H2O 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 Na1.99B3.99O6(OH)2∙3.01H2O. 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 BO2OH groups and two tetrahedral BO4 groups, which share corner-bridging O atoms to form threefold rings, giving chains running along [010], and NaO4(OH)(OH2) and NaO2(OH)(OH2)3 polyhedra. Positional disorder of two H sites of H2O 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 donors or as acceptors. The potential utilizations of kernite, as a source of B (B2O3 ~50 wt%), are discussed, on the basis of the experimental findings of this study. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The crystal structure and crystal chemistry of wardite, ideally NaAl3(PO4)2(OH)4·2H2O, was investigated by single-crystal neutron diffraction (data collected at 20 K) and electron microprobe analysis in wavelength-dispersive mode. The empirical formula of the sample used in this study is: (Na0.91Ca0.01)Σ = 0.92(Al2.97Fe3+0.05Ti0.01)Σ = 3.03(P2.10O8)(OH)4·1.74H2O. The neutron diffraction data confirm that the crystal structure of wardite can be described with a tetragonal symmetry (space group P41212, a = b = 7.0577(5) and c = 19.0559(5) Å at 20 K) and consists of sheets made of edge-sharing Na-polyhedra and Al-octahedra along with vertex-sharing Al-octahedra, parallel to (001), connected by P-tetrahedra and H bonds to form a (001) layer-type structure, which well explains the pronounced {001} cleavage of the wardite crystals. The present data show that four crystallographically independent H sites occur in the structure of wardite, two belonging to a H2O molecule (i.e., H1–O6–H2) and two forming hydroxyl groups (i.e., O5–H3 and O7–H4). The location of the hydrogen atoms allows us to define the extensive network of H bonds: the H atoms belonging to the H2O molecule form strong H bonds, whereas both the H atoms belonging to the two independent hydroxyl groups form weak interactions with bifurcated bonding schemes. As shown by the root-mean-square components of the displacement ellipsoids, oxygen and hydrogen atoms have slightly larger anisotropic displacement parameters compared to the other sites (populated by P, Al and Na). The maximum ratio of the max and min root-mean-square components of the displacement ellipsoids is observed for the protons of the hydroxyl groups, which experience bifurcated H-bonding schemes. A comparative analysis of the crystal structure of wardite and fluorowardite is also provided. [ABSTRACT FROM AUTHOR]

The crystal chemistry of a ferroaxinite from Colebrook Hill, Rosebery district, Tasmania, Australia, was investigated by electron microprobe analysis in wavelength-dispersive mode, inductively coupled plasma-atomic emission spectroscopy (ICP-AES), Fe Mössbauer spectroscopy and single-crystal neutron diffraction at 293 K. The chemical formula obtained on the basis of the ICP-AES data is the following: $$ ^{X1,X2} {\text{Ca}}_{4.03} \,^{Y} \left( {{\text{Mn}}_{0.42} {\text{Mg}}_{0.23} {\text{Fe}}^{2 + }_{1.39} } \right)_{\varSigma 2.04} \,^{Z1,Z2} \left( {{\text{Fe}}^{3 + }_{0.15} {\text{Al}}_{3.55} {\text{Ti}}_{0.12} } \right)_{\varSigma 3.82} \,^{T1,T2,T3,T4} \left( {{\text{Ti}}_{0.03} {\text{Si}}_{7.97} } \right)_{\varSigma 8} \,^{T5} {\text{B}}_{1.96} {\text{O}}_{30} \left( {\text{OH}} \right)_{2.18} $$ . The Fe Mössbauer spectrum shows unambiguously the occurrence of Fe and Fe in octahedral coordination only, with Fe/Fe = 9:1. The neutron structure refinement provides a structure model in general agreement with the previous experimental findings: the tetrahedral T1, T2, T3 and T4 sites are fully occupied by Si, whereas the T5 site is fully occupied by B, with no evidence of Si at the T5, or Al or Fe at the T1- T5 sites. The structural and chemical data of this study suggest that the amount of B in ferroaxinite is that expected from the ideal stoichiometry: 2 a.p.f.u. (for 32 O). The atomic distribution among the X1, X2, Y, Z1 and Z2 sites obtained by neutron structure refinement is in good agreement with that based on the ICP-AES data. For the first time, an unambiguous localization of the H site is obtained, which forms a hydroxyl group with the oxygen atom at the O16 site as donor. The H-bonding scheme in axinite structure is now fully described: the O16- H distance (corrected for riding motion effect) is 0.991(1) Å and an asymmetric bifurcated bonding configuration occurs, with O5 and O13 as acceptors [i.e. with O16··· O5 = 3.096(1) Å, H··· O5 = 2.450(1) Å and O16- H··· O5 = 123.9(1)°; O16··· O13 = 2.777(1) Å, H··· O13 = 1.914(1) Å and O16- H··· O13 = 146.9(1)°]. [ABSTRACT FROM AUTHOR]

The crystal structure and chemical composition of boussingaultite from Pécs-Vasas, Mecsek Mountains, South Hungary, were investigated by single-crystal neutron diffraction (at 20 K) along with a series of chemical analytical techniques [i.e., gravimetric determination of sulfates, EDTA titrimetric determination of magnesium, ion selective electrode for F and Cl, indirect gravimetric determination of ammonium as (NH4,Rb,Cs,K) tetraphenylborate, inductively coupled plasma atomic emission spectroscopy for REE and other minor elements, elemental analysis for C, N, and H content, high-T mass loss for H2O content]. The concentrations of more than 50 elements were measured. The experimental formula of the boussingaultite is: [(NH4)1.77K0.22)Σ1.99[(Mg0.95Mn0.06)Σ1.01(H2O)5.7](SO4)1.99. Neutron data analysis confirms that the structure of boussingaultite is built up by isolated Mg(H2O)6- octahedra, along with isolated NH4- and SO4-tetrahedra connected by a complex H-bonds network. Mg2+ is completely solvated by H2O molecules in a typical octahedral bonding configuration. All the seven independent oxygen sites in the structure are involved in H-bonds, as donors or as acceptors. The geometry of all the H2O molecules, bonded to Mg, is in line with that usually observed in crystalline compounds. The H2O molecules show moderate-strong H-bonds, with H···Oacceptor and Odonor···Oacceptor ranging between 1.72–1.87 and 2.70–2.84 Å, respectively, along with Odonor-H···Oacceptor angles between 168–178°. The four independent N-H···O bonds show H···Oacceptor and Ndonor···Oacceptor distances ranging between 1.81–2.00 and 2.84–2.98 Å, respectively, with N-H···O angles between 158–176°. All the H-bonds of the H2O molecules and of the NH4-group involve the oxygen sites of the SO4-group as acceptors: the SO4-group is, therefore, the "bridging unit" between the NH4 and the Mg(H2O)6 units, via H-bonds. Our structure refinement proved, unambiguously, that the partial K4+ vs. NH+ replacement generates a local disorder. K lies at the N site, and its bonding configuration can be described by a distorted polyhedron with CN = 8. However, the K+ vs. NH+ replacement implies 4 a change in the configuration of the SO4- tetrahedron, through a sort of rotation of the polyhedron. This is the first evidence of the presence of a partial picromerite component in the boussingaultite structure, which gives rise to a local disorder likely due to the significantly different bonding configurations of the two cations. Our refinement also revealed that Mn2+ replaces Mg2+ at the Mg site. No evidence of distortion of the octahedron is observed in response to such a replacement, but the fraction of Mn2+ is modest. An analysis of previous Raman and IR results is provided, and is compared with the experimental results of this study. [ABSTRACT FROM AUTHOR]