On the crystal-chemistry of meyerhofferite, CaB3O3(OH)5·H2O

Abstract

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.

A neutron diffraction study of the hydrous borate inderborite, CaMg[B3O3(OH)5]2(H2O)4·2H2O

“…The experimental findings about the complex and pervasive nature of the H-bonding scheme in inderborite confirm the previous results for other hydrous borates (e.g., Lotti et al 2018Lotti et al , 2019Gatta et al 2019Gatta et al , 2020Gatta et al , 2022aGatta et al , 2002b. We expect that a thermal, compressional or chemical perturbation of the H-bonding scheme would easily lead to a phase transition, or even to a decomposition of this mineral.…”

“…), i.e., CaO 11.2(2) wt% and MgO 7.9(2) wt%. As commonly observed for hydrous borates (e.g., Gatta et al 2022b), even in this case a modest degeneration of the crystals was observed under the electron beam.…”

“…However, based on the structural model, the correct form of the chemical formula of inderborite should be given as: CaMg[B3O3(OH)5]2(H2O)4·2H2O. The chemical analysis here performed showed that there are not major substituents of Ca, Mg or B (Table 1), so that the real chemical composition of the inderborite from Inder is virtually identical to the ideal one, and the concentration of industrially critical elements Gatta et al 2022a) and also probertite (CaNa[B5O7(OH)4]•3H2O, Gatta et al 2022b) show no significant concentration of isomorphic substituents of the principal elements; as a result, their experimental formulae are basically identical to the ideal ones. The chemical purity of the hydrous borates was already reported as a common feature of this mineral population that does not depend on the nature of the deposit (e.g., Gatta et al 2019Gatta et al , 2020Gatta et al , 2022b), but appears to be rather governed by the crystal structure selectivity and, likely, it is also the results of iterated dissolution and recrystallization in lacustrine environments, which could promote purification.…”

“…colemanite (Lotti et al 2017(Lotti et al , 2018(Lotti et al , 2019, kurnakovite (Gatta et al 2019;Pagliaro et al 2021), kernite (Na2B4O6(OH)2•3H2O, Comboni et al 2020a;Gatta et al 2020), meyerhofferite (Ca2[B6O6(OH)10]•2H2O, Comboni et al 2020b, Gatta et al 2022a, inyoite (CaB3O3(OH)5•4H2O, Comboni 2022), probertite (CaNa[B5O7(OH)4]•3H2O, Gatta et al 2022b), and inderite (Comboni et al 2023) by a multi-methodological approach. Our experiments were aimed to search for potential Brich aggregates in construction materials (e.g., Portland or Sorel cements), due to the efficient radiationshielding capacity of 10 B (its high cross section for the 10 B(n,α) 7 Li reaction is ~3840 barns; Carter et al 1953, Sears 1986, Palmer and Swihart 1996, Rauch and Waschkowski 2002.…”