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The present work is part of a systematic mineralogical and petrographic characterization of mica-bearing xenoliths from Somma-Vesuvius volcano (Roman Magmatic Province, southern Italy). Skarns, composite skarns-marbles and cumulates from Pompeii Plinian eruption (AD 79), and skarns and syenite from Avellino eruption (3945 ± 10 cal yr BP) were investigated to define the crystal chemistry of the Somma-Vesuvius trioctahedral micas and to draw inferences on petrogenetic processes to which they were subjected. Xenoliths were characterized by means of polarized optical microscopy, scanning electron microscopy (SEM-EDS), X-ray powder diffraction (XRPD), and bulk-rock geochemical analyses. Mica crystals were studied using electron microprobe analysis (EMPA) and single-crystal X-ray diffraction (SCXRD). Micas from skarns are variably associated with Mg ± Ca silicates (clinopyroxene, vesuvianite, humite, clinohumite, chondrodite, forsterite, and garnet), other sporadic silicates (anorthite, sodalite, titanite, and britholite), apatite, calcite, various types of oxides, as well as rare sulfides and halides. In composite skarn-marble rocks, the mineral assemblages show some differences compared to skarns, as a lack of clinopyroxene and the presence of dolomite. Cumulate samples consist of mica and clinopyroxene, whereas syenite is mainly composed of mica, K-feldspar, feldspathoids, and clinopyroxene. Together with mica, apatite occurs in all the lithotypes. Trace element arrays are scattered for skarn and composite skarn-marble samples. The REE patterns have a general enrichment in light (La, Ce, Pr, Nd) and medium (Sm, Eu, Gd, Tb, Dy) rare earth elements, in some cases with slight positive Gd anomaly. Cumulate samples generally have low amounts of Ba, Sr, Zr, and Th, while syenite exhibits low concentrations of trace elements, except for Rb, Cs, and Tl. Mica crystals occurring in the studied xenoliths are phlogopite with different Al and Mg contents at the octahedral site, a negligible tetraferriphlogopite component and variable dehydrogenation degree. All samples belong to the 1M polytype (C2/m and C2 space group) and have a wide range of unit-cell parameters, especially of the c axis [5.3055(1) ≤ a ≤ 5.3218(1) Å, 9.1893(1) ≤ b ≤ 9.2188(4) Å, 10.1803(2) ≤ c ≤ 10.2951(2) Å]. The shortest c-cell parameter pertains to de-hydrogenated phlogopite from Avellino skarn, whereas OH-rich phlogopite from Pompeii composite skarns-marbles has a c-cell parameter that approximates that of the end-member phlogopite. Overall, it is observed that the crystal chemistry of the micas studied here extends the known range of the other Vesuvian micas from the literature. The Ti-depletion and the wide degree of dehydrogenation of phlogopites from skarns and composite skarns-marbles suggest that the studied samples originated under variable pressure conditions. In addition, the presence of humite in the mineral assemblage seems to indicate the occurrence of devolatilization reactions. The scarce mica occurrence in cumulate and mainly in syenite, instead, may depend on pressure conditions in the magma storage system exceeding the mica stability. [ABSTRACT FROM AUTHOR]

Volcanic activity at Mt. Vulture lasted about 750 ka and produced Si[O.sub.2]-undersaturated volcanic rocks that can be classified as old (~700 ka), intermediate (~600-550 ka), and young (~130 ka). The intermediate deposits consist of pyroclastic falls and flows and lavas with compositions ranging from phonolite to foidite. A recent revision of the stratigraphic setting allowed these deposits to be classified into one synthem (the Barile Synthem) and further subdivided into four subsynthems (Toppo S. Paolo, Rionero, Vulture-S. Michele, and Ventaruolo). In the present investigation, trioctahedral micas from sample VUT 191 in the Vulture-S. Michele Subsynthem are considered. The host rock has modal diopside (20.2%), analcime (22.8%), plagioclase (27.8%), hauyne (5%), phlogopite (8.9%), and magnetite (6.3%). The micas were studied using chemical (EPMA, C-H-N, SIMS), structural (SCXRD), and spectroscopic (Mossbauer) methods. EPMA of 36 crystals from thin sections and 6 discrete crystals selected for the structural analysis showed remarkable compositional variability, as follows (in wt%): Si[O.sub.2] = 33.14-38.01, [Al.sub.2][O.sub.3] = 15.56-20.45, MgO = 13.[O.sup.2-]20.81, Fe[O.sub.tot] = 6.34-14.08, Ti[O.sub.2] = 2.34-6.02, [K.sub.2]O = 6.03-9.48, [Na.sub.2]O = 0.50-0.78, and BaO = 0.89-4.06; all crystals proved to be phlogopite. Elemental C-H-N analyses yielded [H.sub.2]O = 2.86 [+ or -] 0.36 wt%. The water content was also determined by SIMS on two single crystals, labeled VUT 191_2 and VUT 191_19, which yielded values of 3.81 [+ or -] 0.12 and 1.72 [+ or -] 0.08 wt% [H.sub.2]O, respectively. Mossbauer investigation showed that all the iron in VUT191 mica is octahedral with [Fe.sub.2+] 25.5% and [Fe.sup.3+] = 74.5%, confirming that Vulture micas are particularly [Fe.sup.3+]-rich, as also found from previous investigations. Structure refinements using anisotropic displacement parameters were performed in space group C2/m and converged at 1.89 < R [less than or equal to] 3.17, 2.09 [less than or equal to] [R.sub.w] [less than or equal to] 3.43%. All of the analyzed micas belong to the 1M polytype but exhibit remarkable variations in the c parameter from 10.1569(4) to 10.2458(4) [Angstrom]. The chemical and structural parameters indicate that the studied micas can be divided into two groups: the first encompassing strongly dehydrogenated micas affected mainly by Ti-oxy [[sup.IV][M.sup.2+] + 2(OH) [left and right arrow] WTi4+ + 2[0.sup.2-] + [H.sub.2]] and [M.sup.3+]-oxy [[sup.IV] [M.sup.2+] + (OH)-[left and right arrow] W[M.sup.3+] + [O.sup.2-] + 1/2 [H.sub.2], with [M.sup.3+] = [Fe.sup.3+], [Al.sup.3+] substitutions. The second group consist of samples in which vacancy-bearing mechanisms, 2 [sup.VI][M.sup.2+] [left and right arrow] [sup.VI][Ti.sup.4+]+ [sup.VI] [cube root]and 3[sup.IV][M.sup.2+] [left and right arrow] 2'a[M.sup.3+] +[sup.VI][cube root] occur. Keywords: Volcanic phlogopite, Mossbauer spectroscopy, structure refinement, CHN, SIMS, crystal chemistry, substitution mechanisms

Mt. Vulture trioctahedral micas-1M mainly consist of phlogopite-annite solid solutions with a minor component of brittle micas. However, both Li-free and Li- and F-rich compositions may coexist in the same volcano-stratigraphic level. We report the results of electron microprobe analysis (EMPA), secondary ion mass spectrometry (SIMS), and single-crystal X-ray diffraction (SCXRD) for three crystals (LC7-27Go, LC7-3Go, and LCT-1R) that are representative of both compositions. EMPA and SIMS showed that sample LC7-1R is richer in Ti[O.sub.2], [Li.sub.2]O, and F and poorer in [H.sub.2]O than the other samples. Structure refinements using anisotropic displacement parameters for the three samples, performed in space group C2/m, converged at 3.46 [less than equal to] R [less than or equal to] 4.34, 3.89 [less than or equal to] [R.sub.w] [less than or equal to] 4.39. When considering bond distances, the three samples can be described as homo-octahedral whereas mean atomic numbers suggest that only LC7-1R is meso-octahedral. Significant differences occur among the samples for some distortion parameters commonly used for micas. In particular, LCT-1R displays higher values of [BLD.sub.M2], [shift.sub.M2] and lower values of [DELTA][K.sub.04] and [t.sub.K-O4] than those of samples LC7-3Go and LC7-27Go. These differences are ascribed to F- and Ti-substitutions. Cation distributions were obtained by combining EMPA, SIMS, and SCXRD data after analysis of the effect of normalization schemes commonly used in mica formula recalculation. In LCT-3Go and LCT-27Go, Ti is incorporated according to the Ti-Tschermak mechanism, whereas more than one Ti-substitution mechanism occurs in LC7-1R. For the latter sample, octahedral vacancies are present leading to a complex substitution pattern and complex structural distortions related to the special octahedral compositions that were determined. Keywords: Analysis, chemical (mineral), EPMA and SIMS investigation, crystal structure, trioctahedral micas--1M, major and minor elements, Ti-F-bearing phlogopites, trace elements and REE, Li-bearing micas, XRD data, single-crystal structure refinement

The iron sulfate FeOHS[O.sub.4] studied was obtained as a dehydration product of metahohmannite [Fe.sub.2][([H.sub.2]O).sub.4][O[(S[O.sub.4]).sub.2]] during a synchrotron real-time powder diffraction experiment. As quoted in the literature, FeOHS[O.sub.4] has iron atoms octahedrally coordinated with two hydroxyl groups and four sulfate O atoms, while each hydroxyl group is bonded to two iron atoms. This compound is commonly described in the orthorhombic system with space group Pnma, lattice parameters [a.sub.j] = 7.33, [b.sub.j] = 6.42, and [c.sub.j] = 7.14 A ([a.sub.j], [b.sub.j], and [c.sub.j] are the Johansson lattice parameters), and Z = 4. However a preliminary Rietveld refinement of the pattern at about 220 [degrees]C using the structural model from the literature yielded a poor fit of the observed data and a final [R.sub.p] value of about 23%. A careful analysis of the calculated powder diffraction pattern showed unexpected peaks, not observed in the experimental trace, for h = 2n + 1, while sharp reflections for h = 2n seemed to point to different lattice constants and space group. The recognition of the order-disorder character of the FeOHSO4 compound was the key to successfully interpreting the unexpected features of the experimental powder pattern and the misfit with respect to the calculated pattern. In fact, FeOHS[O.sub.4] belongs to a family of OD structures formed by equivalent layers of symmetry Pbmm. Only two MDO (Maximum Degree of Order) polytypes are possible. MDO1 results from a regular alternation of stacking operators [2.sub.1/2] and [2.sub.-1/2], and yields an orthorhombic structure with space group Pnma and lattice parameters [a.sub.j] = 7.33, [b.sub.j] = 6.42, and [c.sub.j] = 7.14 A. MDO2 results from the [2.sub.1/2][12.sub.1/2] [12.sub.1/2] ... sequence of symmetry operators and yields a monoclinic structure with space group P[2.sub.1]/c,[a.sub.M] = 7.33, [b.sub.M] = 7.14, [c.sub.M] = 7.39 A, and [beta] = 119.7[degrees]. The analysis of one-dimensional stacking disorder was performed by fitting the observed XRPD pattern with a calculated intensity curve generated by DIFFaX. The disorder model was investigated by taking into account a probability matrix for the occurrence of OD layer sequences. The best fit ([R.sub.p] = 0.009) to the observed powder pattern was obtained with a 61:39 ratio of monoclinic and orthorhombic polytypes for a fully disordered OD layers sequence.

Research has been conducted on tetrasubstituted 2,3-dihydrofurans. The authors demonstrate that the stereoselective synthesis of these compounds has been carried out in n-butylpyridinium tetrafluoroborate as solvent.

Fluorine is an important proxy for magmatic differentiation processes in the shallow parts of volcanic plumbing systems. Fluorphlogopite is one of the more important fluorine carriers in magmatic rocks. In the present study, a full crystal chemical investigation of fluorophlogopite 1M from Piano delle Concazze, Mt. Etna volcano, Italy, is carried out. The fluorophlogopite occurs in a benmoreitic lava from prehistoric volcanic activity at Mt. Etna (post-caldera forming phase of the "Ellittico" eruptive center; ~15 ka BP). It is primarily associated with fluorapatite covered with amorphous SiO2 and crystallized during syn/post-eruption pneumatolytic stages. The mica sample studied here is among the most Fe- and Ti-rich fluorophlogopite found in nature. EPMA data yielded the following mean chemical formula for this mineral (K0.83Na0.13)(Fe0.442+Fe0.093+Mg2.18Al0.05Ti0.23Mn0.01)(Al0.92Si3.08)O10.64 (Cl0.01F1.35). Structure refinements on four fluorophlogopite crystals, performed in space group C2/m, converged at R = 0.03-0.04, with cell parameters in the ranges a = 5.323-5.324, b = 9.219-9.222, c = 10.116-10.119 Å, β = 100.1-100.3°. Major substitutions are OH- ↔ F-,M3+ -oxy (VIM2++OH- ↔ VIM3++O2-) and Ti-oxy substitution: VIM2++2(OH)2+ ↔ VITi4+ +2O2- . The fluorophlogopite from Piano delle Concazze exhibits the shortest c-parameter with respect to other fluorophlogopites found in nature. The short c parameter is essentially due to the absence of the hydroxyl group in favor of F- and especially of O2- and to the thus increased attractive interaction between the interlayer cation and the anion content (F-, O2-) located at the O4 site. A comparison with other natural fluorophlogopites (namely from Biancavilla, Etna and Presidente Olegario, Brazil) show intermediate crystal-chemical features for the Piano delle Concazze fluorophlogopite. Particularly at Etna, differences in the chemical composition of the crystallized fluorophlogopites could be related to the various extent of enrichments by transfer of a gas phase achieved in specific parts of the volcanic plumbing system. [ABSTRACT FROM AUTHOR]

A 3T mica polytype from Kasenyi (southwest Uganda), kamafugite, was studied by electron probe microanalysis (EPMA), single-crystal X-ray diffraction (SCXRD), micro-Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) to characterize its crystal chemistry and the relationships with phlogopites from the same rock but showing different stacking sequence and to get insights into factors affecting polytypism in Ugandan phlogopites. EPMA data gave: SiO2 = 38.7(2), Al2O3 = 13.08(9), MgO = 20.4(2), TiO2 = 4.8(1), MnO = 0.03(3), FeOtot = 5.51(9), Cr2O3 = 0.90(7), NiO = 0.11(5), SrO = 0.03(3), ZnO = 0.04(3), ZrO2 = 0.01(2), K2O = 9.64(5), Na2O = 0.29(1), BaO = 0.15(5), F = 0.13(5), and Cl = 0.01(1) wt%. The analyzed sample may be classified as a Ti-rich phlogopite. X-ray photoelectron spectroscopy provided Fe3+/Fe2+ and O2-/OH equal to ~0.75 and 7.14, respectively, which are in agreement with the results of previous Mössbauer investigation on the BU1 phlogopites from the same rock and with the structural formula of the studied crystal. Infrared spectra showed a shoulder at ~3660 cm-1 in the OH- stretching region (~3740-3600 cm-1), which is assigned to MgMgFe3+-OH--K-O2- local configurations. No evidences of vacancy substitutions were observed. Structure refinement based on single-crystal X-ray diffraction data was performed in space group P3112 using anisotropic displacement parameters and converged to R1 = 4.34 and wR2 = 3.33%. Unit-cell parameters are: a = b = 5.3235(3) and c = 30.188(2) Å. Geometrical and chemical considerations point to a disordered cation distribution over T1 and T2 tetrahedral sites, whereas partial cation ordering characterizes the octahedral sites with high-charge cations preferentially located as expected on M2 and M3. Tetrahedral bond length distortion and angular variance parameters describe more distorted polyhedra in 3T polytype than those found in coexisting 1M and 2M1 polytypes. Finally, the overall crystal-chemical features indicate the occurrence of the following substitution mechanisms in the studied sample: Ti-oxy [VIM2++2(OH) - ↔ VITi4++2(O2-)+H2↑] and Al, Fe3+, Cr-oxy [VIM2++(OH)- ↔ VIM3++O2-+1/2H2)↑]; Al, Fe3+-Tschermak [VIM2++IVSi4+ ↔ VI (Al3+, Fe3+)+IVAl3+]; XIIK++IVAl3+ ↔ IVSi4++XII□; tetraferriphlogopite [IVFe3+↔ IVAl]. [ABSTRACT FROM AUTHOR]

The 2M1-phlogopite from the potassic gabbronorite (Black Hill, Australia) has been studied by electron microprobe and single-crystal X-ray diffraction analyses. The crystal-chemical formula was (K0.95Na0.01)(Al0.15Mg1.27Fe1.162+Fe0.043+Ti0.384+)(Si2.85Al1.15)O10.76F0.11Cl0.03OH1.10. The structural analysis has shown that the crystal has the cell parameters a = 5.352(1), b = 9.268(1), c = 20.168(1) Å, and β = 95.10(1)° and exhibits symmetry lowering from the ideal space group C2/c to C1. An octahedral cation ordering pattern was revealed from the refined site-scattering powers. Specifically, using the scattering species Mg vs. Fe, it was found that the M1 site at z = 0 was occupied principally by Mg (~77%) and subordinately by Fe (~23%), whereas that at z = 0.5 was completely occupied by Fe; the M2 sites at z = 0 displayed ~88% Mg and ~12% Fe, whereas those at z = 0.5 were occupied by ~86% Fe and ~14% Mg. The analysis of geometrical features shows that the Ti uptake in the structure via the Ti-oxy mechanism induces structural distortions of different extents on the z = 0 and z = 0.5 layers, with stronger effects for the layer at z = 0. Minor chemical and structural differences, instead, affect the T sheets at z = 0 and z = 0.5. [ABSTRACT FROM AUTHOR]

The crystal structures of tobelite and Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed -rich muscovite from the sedimentary rocks of the Armorican sandstones (Brittany, France) have been solved for the first time by single-crystal X-ray diffraction. The structural study was integrated by electron probe microanalyses, and X-ray photoelectron and micro-Fourier transform infrared spectroscopy. The crystals belong to the 2M2 polytype with the following unit-cell parameters: a = 9.024(1), b = 5.2055(6), c = 20.825(3) Å, and β = 99.995(8)° for tobelite and a = 9.027(1), b = 5.1999(5), c = 20.616(3) Å, and β = 100.113(8)° for NH Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed -rich muscovite. Structure refinements in the space group C2/c converged at R1 = 8.01%, wR2 = 8.84% and R1 = 5.59%, wR2 = 5.63% for tobelite and NH Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed -rich muscovite, respectively. X-ray photoelectron spectroscopy revealed nitrogen environments associated either with inorganic (B.E. 401.31 eV) or organic (B.E. 398.67 eV) compounds. Infrared spectra showed, in the OH--stretching region (3700-3575 cm-1), two prominent bands, centered at ~3629 and ~3646 cm-1, and two shoulders at ~3664 and ~3615 cm-1 that were assigned to Al3+Al3+⇔-OH- arrangements having OH- groups affected by different local configurations. In addition, a series of overlapping bands from about 3500 to 2700 cm-1 characteristic of the NH Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed -stretching vibrations, a main band at ~1430 and a shoulder at ~1460 cm-1 that were associated to the Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed -bending vibration (v4) were also present. The ammonium concentration was semi-quantitatively estimated in both crystals from the absorbance of the OH--stretching and Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed -bending vibrations in the infrared spectra. An additional estimate was obtained for the Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed -rich muscovite by considering the normalized peak area between K2p3/2 and N1s in the X-ray photoelectron spectrum. The obtained values are in agreement with those derived from the interlayer spacing in the simulated X-ray powder diffraction spectra. The results of this integrated approach converged to Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed for Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed muscovite. [ABSTRACT FROM AUTHOR]

The crystal chemistry of 2M1 micas from Bunyaruguru kamafugite (southwest Uganda) was studied by electron probe microanalysis, single-crystal X-ray diffraction, Mössbauer and Fourier transform infrared spectroscopy. Chemical analyses showed that the studied crystals are Ti-rich, F-poor phlogo-pites with an annitic component, Fetot/(Fetot + Mg), ranging from 0.15 to 0.22. Unit-cell parameters from single-crystal X-ray data are in the range: 5.3252(1) ≤ a ≤ 5.3307(1), 9.2231(3) ≤ b ≤ 9.2315(3), 20.1550(6) ≤ c ≤ 20.1964(8) Å, and 94.994(2) ≤ β ≤ 95.131(2)°. Anisotropic structure refinements, in the space group C2/c, converged to 2.77 ≤ R1 ≤ 3.52% and 2.91 ≤ wR2 ≤ 4.02%. Mössbauer spectroscopy showed that the studied sample has: VIFe2+ = 60(1)%, VIFe3+ = 24(1)%, and IVFe3+ = 16(1)%. FTIR investigations pointed to the occurrence of Fe3+-oxy substitutions and ruled out the presence of vacancy mechanisms. The overall crystal-chemical features are consistent with the following substitutions: tetraferriphlogopite [IVFe3+ ↔ IVAl]; Ti-oxy [VIM2+ + 2 (OH)- ↔ VITi4+ + 2 (O2-) + H2↑] and Al, Fe3+, Cr-oxy [VIM2+ + (OH)- ↔ VIM3+ + O2- + ½ (H2)↑]; Al, Fe3+-Tschermak [VIM2+ + IVSi4+ , ↔ VIM3+ + IVAl]; kinoshitalite [XIIK + IVSi4+ ↔ XIIBa2+ + IVAl] and [XIIK+ + IVAl3+ ↔ IVSi4+ + XII□]. The estimation of the OH- content for Ugandan mica-2M1 was obtained, for the first time, from the linear regression equation c = 0.20(2) x OH- (gpfu) + 19.93(2) derived from literature data of 2M1-samples with known OH- content. The orientation of the O-H vector with respect to c* was found in the range from 2.0 to 6.9°. [ABSTRACT FROM AUTHOR]

The iron sulfate FeOHSO4 studied was obtained as a dehydration product of metahohmannite Fe2(H2O)4[O(SO4)2] during a synchrotron real-time powder diffraction experiment. As quoted in the literature, FeOHSO4 has iron atoms octahedrally coordinated with two hydroxyl groups and four sulfate O atoms, while each hydroxyl group is bonded to two iron atoms. This compound is commonly described in the orthorhombic system with space group Pnma, lattice parameters aJ = 7.33, bJ = 6.42, and cJ = 7.14 Å (aJ, bJ, and cJ are the Johansson lattice parameters), and Z = 4. However a preliminary Rietveld refinement of the pattern at about 220 °C using the structural model from the literature yielded a poor fit of the observed data and a final Rp value of about 23%. A careful analysis of the calculated powder diffraction pattern showed unexpected peaks, not observed in the experimental trace, for h = 2n + 1, while sharp reflections for h = 2n seemed to point to different lattice constants and space group. The recognition of the order-disorder character of the FeOHSO4 compound was the key to successfully interpreting the unexpected features of the experimental powder pattern and the misfit with respect to the calculated pattern. In fact, FeOHSO4 belongs to a family of OD structures formed by equivalent layers of symmetry Pbmm. Only two MDO (Maximum Degree of Order) polytypes are possible. MDO1 results from a regular alternation of stacking operators 2½ and 2-1/2, and yields an orthorhombic structure with space group Pnma and lattice parameters aJ = 7.33, bJ = 6.42, and cJ = 7.14 Å. MDO2 results from the 2½|2½|2½...sequence of symmetry operators and yields a monoclinic structure... [ABSTRACT FROM AUTHOR]

The iron sulfate FeOHSO4 studied was obtained as a dehydration product of metahohmannite Fe2(H2O)4[O(SO4)2] during a synchrotron real-time powder diffraction experiment. As quoted in the literature, FeOHSO4 has iron atoms octahedrally coordinated with two hydroxyl groups and four sulfate O atoms, while each hydroxyl group is bonded to two iron atoms. This compound is commonly described in the orthorhombic system with space group Pnma, lattice parameters aJ = 7.33, bJ = 6.42, and cJ = 7.14 Å (aJ, bJ, and cJ are the Johansson lattice parameters), and Z = 4. However a preliminary Rietveld refinement of the pattern at about 220 °C using the structural model from the literature yielded a poor fit of the observed data and a final Rp value of about 23%. A careful analysis of the calculated powder diffraction pattern showed unexpected peaks, not observed in the experimental trace, for h = 2n + 1, while sharp reflections for h = 2n seemed to point to different lattice constants and space group. The recognition of the order-disorder character of the FeOHSO4 compound was the key to successfully interpreting the unexpected features of the experimental powder pattern and the misfit with respect to the calculated pattern. In fact, FeOHSO4 belongs to a family of OD structures formed by equivalent layers of symmetry Pbmm. Only two MDO (Maximum Degree of Order) polytypes are possible. MDO1 results from a regular alternation of stacking operators 2½ and 2-1/2, and yields an orthorhombic structure with space group Pnma and lattice parameters aJ = 7.33, bJ = 6.42, and cJ = 7.14 Å. MDO2 results from the 2½|2½|2½...sequence of symmetry operators and yields a monoclinic structure... [ABSTRACT FROM AUTHOR]

The crystal structure of moganite from the Mogán formation on Gran Canaria has been re-investigated using high-resolution synchrotron X-ray diffraction (XRD) and X-ray/neutron pair distribution function (PDF) analyses. Our study for the first time reports the anisotropic atomic displacement parameters (ADPs) of a natural moganite. Rietveld analysis of synchrotron XRD data determined the crystal structure of moganite with the space group I2/a. The refined unit-cell parameters are a = 8.7363(8), b = 4.8688(5), c = 10.7203(9) Å, and β = 90.212(4)°. The ADPs of Si and O in moganite were obtained from X-ray and neutron PDF analyses. The shapes and orientations of the anisotropic ellipsoids determined from X-ray and neutron measurements are similar. The anisotropic ellipsoids for O extend along planes perpendicular to the Si-Si axis of corner-sharing SiO4 tetrahedra, suggesting precession-like movement. Neutron PDF result confirms the occurrence of OH over some of the tetrahedral sites. We postulate that moganite nanomineral is stable with respect to quartz in hypersaline water. The ADPs of moganite show a similar trend as those of quartz determined by single-crystal XRD. In short, the combined methods can provide high-quality structural parameters of moganite nanomineral, including its ADPs and extra OH position at the surface. This approach can be used as an alternative means for solving the structures of crystals that are not large enough for single-crystal XRD measurements, such as fine-grained and nanocrystalline minerals formed in various geological environments. [ABSTRACT FROM AUTHOR]