Zharchikhite, AlF(OH)2: a novel structure type related to α-PbO2.

The crystal structure of zharchikhite, AlF(OH)2, from the Zharchikhinskoe deposit (Buryatia, Russia) is solved here using single-crystal X-ray diffraction. The mineral is monoclinic, space group P21/c, a = 5.1788 (4), b = 7.8386 (4), c = 5.1624 (4) Å, ß = 116.276 (10)°, V = 187.91 (3) Å3 and Z = 4. Zharchikhite demonstrates a novel structure type roughly related to the α-PbO2 structure type and different from other compounds of the Al-F-OH system. The crystal structure of zharchikhite is based on the octahedral pseudoframework built from zigzag chains of edge-sharing AlF2(OH)4 octahedra; adjacent chains are linked via F vertices and the pseudoframework contains wide channels.

Crystal structure refinement, low- and high-temperature X-ray diffraction and Mössbauer spectroscopy study of the oxoborate ludwigite from the Iten'yurginskoe deposit.

This paper reports an investigation of the chemistry, crystal structure refinement and thermal behavior (80-1650 K) of ludwigite from the Iten'yurginskoe deposit (Eastern Chukotka, Russia). Its chemical composition was determined by electron microprobe analysis, giving an empirical formula (Mg1.70Fe2+0.29Mn0.01)Σ2.00(Fe3+0.90Al0.08Mg0.02)Σ1.00O2(BO3). A refinement of the crystal structure from single-crystal X-ray diffraction data (SCXRD) was provided for the first time for ludwigite from this deposit (R = 0.047). The structure can be described as a framework composed of [MO6]n- octahedra and isolated [BO3]3- triangles located in triangular interstices of the framework. Based on a comprehensive analysis of SCXRD and Mössbauer spectroscopy data, the M1 site is occupied by Mg, M2 and M3 by Mg and Fe2+, M4 by Fe3+, Mg and Al. There are also oxo-centered [O4M4]n+ and [O2M5]n+ polyhedra building up a framework with the [BO3]3- triangles located in its hexagonal interstices. No indications of magnetic ordering are found in the temperature range investigated. The Fe2+ → Fe3+ oxidation occurs above 600 K, and is accompanied by a decrease of the unit-cell parameters and subsequent incomplete solid-phase decomposition with the formation of hematite, warwickite and magnetite. The mineral melts at temperatures above 1582 K. The thermal expansion of ludwigite is slightly anisotropic, which is explained by a dense packing of the [MO6]n- octahedra as well as a virtually perpendicular orientation of the oxo-centered double chains to each other. At room temperature, maximum expansion is along the c axis (αc = 9.1 × 10-6 K-1) and minimum expansion is in the ab plane (αa = 8.6 × 10-6, αb = 7.6 × 10-6 K-1), which is due to the preferred orientation of the [BO3]3- triangles. A comparison of the thermal behavior of three oxoborates of the ludwigite group, namely azoproite (Mg,Fe2+)2(Fe3+,Ti,Mg,Al)O2(BO3), vonsenite (Fe2+,Mg)2(Fe3+,Mn2+,Sn,Al)O2(BO3) and ludwigite (Mg,Fe2+,Mn)2(Fe3+,Al,Mg)O2(BO3), is provided.

New data on the crystal chemistry of the natural two-layer aluminosilicates latiumite and tuscanite.

The crystal chemistry of the natural microporous two-layer aluminosilicates (2D zeolites) latiumite and tuscanite is re-investigated based on new data on the chemical composition, crystal structures, and infrared and Raman spectra. The CO32--depleted and P- and H-enriched samples from Sacrofano paleovolcano, Lazio, Italy, are studied. Both minerals are monoclinic; latiumite P21, a = 12.0206 (3), b = 5.09502 (10), c = 10.8527 (3) Å, ß = 107.010 (3)°, V = 635.60 (3) Å3 and tuscanite P21/a, a = 23.9846 (9), b = 5.09694 (15), c = 10.8504 (4) Å, ß = 107.032 (4)°, V = 1268.26 (8) Å3. The obtained crystal chemical formulae (Z = 2 for both minerals) are [(H3O)0.48(H2O)0.24K0.28](Ca2.48K0.21Na0.21Sr0.06Mg0.04)(Si2.86Al2.14O11)[(SO4)0.70(PO4)0.20](CO3)0.10 for latiumite and [(H3O)0.96(H2O)0.58K0.46](Ca4.94K0.44Na0.45Sr0.09Mg0.08)(Si5.80Al4.20O22)[(SO4)1.53(PO4)0.33](CO3)0.14 for tuscanite. These minerals are dimorphous. Both latiumite and tuscanite show distinct affinity for the PO43- anion. Hydrolytic alteration of these minerals results in partial leaching of potassium accompanied by protonation and hydration which is an important precondition for the existence of ion/proton conductivity of related materials.

Infrared spectroscopy as a tool for the analysis of framework topology and extra-framework components in microporous cancrinite- and sodalite-related aluminosilicates.

The identification of the framework type in multilayer cancrinite- and sodalite-group minerals and synthetic compounds is predominantly based on the unit-cell dimensions determined from powder or single-crystal X-ray diffraction data, by analogy with previously characterized samples with known crystal structures. However, topological type of the framework cannot be reliably determined in this way because of the different possible ABC staking sequences and different sets of cages with the same number of layers in the repeat unit. To solve this problem, additional criteria are required. The use of infrared (IR) spectroscopy makes it possible to distinguish topologically different types with the same unit-cell parameters. The most important diagnostic range in the IR spectrum (the "finger-print region", from 510 to 760 cm-1) corresponds to the O-T-O bending vibrations (T = Si, Al). The spectral bands at 705 ± 8, 528 ± 5, 547 ± 4, and 555 ± 3 cm-1 indicate the presence of the sodalite, Losod, liottite, and giuseppettite cages, respectively. The band at 528 ± 5 cm-1 shifts towards ∼518 cm-1 in the case when Losod cage hosts carbonate group. The IR spectrum in the "finger-print region" can be also used to identify a mineral species belonging to two-layer or three-layer minerals with different extra-framework compositions. The wavenumber of the antisymmetric stretching mode of the 12CO2 molecule, which is a common admixed extra-framework constituent in minerals belonging to the cancrinite and sodalite groups, depends on the kind of the host cage or channel: 2340-2343 cm-1 for the sodalite cage, 2338 cm-1 for the Losod cage, and 2351-2353 cm-1 for the liottite cage and wide channel in the cancrinite-type framework.

Investigation of thermal behavior of mixed-valent iron borates vonsenite and hulsite containing [OM4]n+ and [OM5]n+ oxocentred polyhedra by in situ high-temperature Mössbauer spectroscopy, X-ray diffraction and thermal analysis.

The investigation of elemental composition, crystal structure and thermal behavior of vonsenite and hulsite from the Titovskoe boron deposit in Russia is reported. The structures of the borates are described in terms of cation-centered and oxocentred polyhedra. There are different sequences of double chains and layers consisting of oxocentred [OM4]n+ tetrahedra and [OM5]n+ tetragonal pyramids forming a framework. Elemental composition was determined by energy-dispersive X-ray spectroscopy (EDX). Oxidation states and coordination sites of iron and tin in the oxoborates are determined using Mössbauer spectroscopy and compared with EDX and X-ray diffraction data (XRD). According to results obtained from high-temperature Mössbauer spectroscopy, the Fe2+ to Fe3+ oxidation in vonsenite and hulsite occurs at approximately 500 and 600 K, respectively. According to the high-temperature XRD data, this process is accompanied by an assumed deformation of crystal structures and subsequent solid-phase decomposition to hematite and warwickite. It is seen as a monotonic decrease of volume thermal expansion coefficients with an increase in temperature. A partial magnetic ordering in hulsite is observed for the first time with Tc ≃ 383 K. Near this temperature, an unusual change of thermal expansion coefficients is revealed. Vonsenite starts to melt at 1571 K and hulsite melts at 1504 K. Eigenvalues of thermal expansion tensor are calculated for the oxoborates as well as anisotropy of the expansion is described in comparison with their crystal structures.

Functionality in metal-organic framework minerals: proton conductivity, stability and potential for polymorphism.

Rare metal-organic framework (MOF) minerals stepanovite and zhemchuzhnikovite can exhibit properties comparable to known oxalate MOF proton conductors, including high proton conductivity over a range of relative humidities at 25 °C, and retention of the framework structure upon thermal dehydration. They also have high thermodynamic stability, with a pronounced stabilizing effect of substituting aluminium for iron, illustrating a simple design to access stable, highly proton-conductive MOFs without using complex organic ligands.

Crystal structure of the OH-dominant gadolinite-(Y) analogue (Y,Ca)2(Fe,□)Be2Si2O8(OH,O)2 from Heftetjern pegmatite, Norway.

A hydroxyl-dominant analogue of gadolinite-(Y) (OH-Gad) has been discovered in the Heftetjern granitic pegmatite, southern Norway, in association with late-stage rare-earth-element containing minerals. The empirical formula, based on ten O atoms per formula unit, is (Y1.285Ca0.55Ce0.07La0.04Nd0.01)Σ1.955(Fe2+0.57□0.43)Be2.02Si1.995O8.48(OH)1.52. The mineral is monoclinic, space group P21/c, a = 4.7514 (10), b = 7.5719 (16), c = 9.9414 (2) Å, ß = 90.015 (4)°, V = 357.663 (3) Å3 and Z = 2. The density calculated using the empirical formula is 3.903 g cm-3. The crystal structure was refined to R = 0.0217 for 776 reflections with I > 2σ(I). OH-Gad is isostructural with gadolinite-(Y) and it is characterized by the predominance of OH- over O2- at the anionic Ø-site. The refined crystal-chemical formula is: A(Y1.25Ca0.55Ce0.2)X(Fe2+0.57□0.43)ZBe2TSi2O8Ø[(OH)0.86O0.59(OH)*0.55] (Z = 2). The possible orientation and local environment of the hydroxyl group were suggested based on bond-valence sum calculations and geometrical analysis of the crystal structure. The infrared spectrum confirms disordering of H atoms. OH-Gad seems to be a potentially new mineral, the first simultaneously hydroxyl- and iron-dominant member of the gadolinite subgroup. It is an OH-analogue of gadolinite-(Y) and an Fe2+-analogue of hingganite-(Y).

Minerals with metal-organic framework structures.

Metal-organic frameworks (MOFs) are an increasingly important family of advanced materials based on open, nanometer-scale metal-organic architectures, whose design and synthesis are based on the directed assembly of carefully designed subunits. We now demonstrate an unexpected link between mineralogy and MOF chemistry by discovering that the rare organic minerals stepanovite and zhemchuzhnikovite exhibit structures found in well-established magnetic and proton-conducting metal oxalate MOFs. Structures of stepanovite and zhemchuzhnikovite, exhibiting almost nanometer-wide and guest-filled apertures and channels, respectively, change the perspective of MOFs as exclusively artificial materials and represent, so far, unique examples of open framework architectures in organic minerals.