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Hydrous wadsleyite ([beta]-[Mg.sub.2]Si[O.sub.4]) with 2.8 wt% water content has been synthesized at 15 GPa and 1250[degrees]C in a multi-anvil press. The unit-cell parameters are: a = 5.6686(8), b = 11.569(1), c = 8.2449(9) [Angstrom], [beta] = 90.14(1)[degrees], and V = 540.7(1) [[Angstrom].sup.3], and the space group is I2/m. The structure was refined in space groups Imma and I2/m. The room-pressure structure differs from that of anhydrous wadsleyite principally in the increased cation distances around O1, the non-silicate oxygen. The compression of a single crystal of this wadsleyite was measured up to 61.3(7) GPa at room temperature in a diamond anvil cell with neon as pressure medium by X-ray diffraction at Sector 13 at the Advanced Photon Source, Argonne National Laboratory. The experimental pressure range was far beyond the wadsleyite-ringwoodite phase-transition pressure at 525 km depth (17.5 GPa), while a third-order Birch-Murnaghan equation of state (EoS) [[V.sub.0] = 542.7(8) [[Angstrom].sup.3], [K.sub.TO] = 137(5) GPa, K' = 4.6(3)] still fits the data well. In comparison, the second-order fit gives [V.sub.0] = 542.7(8) [[Angstrom].sup.3], [K.sub.T] = 147(2) GPa. The relation between isothermal bulk modulus of hydrous wadsleyite [K.sub.TO] and water content [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] is: KTO = 171 ( 1)-12(1 ) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (up to 2.8 wt% water). The axial-compressibility [[beta].sub.c] is larger than both [[beta].sub.a] and [[beta].sub.b], consistent with previous studies and analogous to the largest coefficient of thermal expansion along the c-axis. Keywords: Compressibility, hydrous wadsleyite, neon, orthorhombic DOI: 10.2138/am.2010.3533

We present a detailed [sup.29]Si NMR spectroscopic study of isotopically enriched samples of forsterite and of anhydrous and hydrous wadsleyite and ringwoodite ([alpha], [beta], and [gamma] phases of [Mg.sub.2]Si[O.sub.4]), which complement previous extensive studies of these minerals by XRD and vibrational spectroscopy, [sup.VI]Si is not detected in any of the phases at levels of about 0.1 to 0.5%. When coupled with recent theoretical calculations on ringwoodite, this suggests the possibility of re-ordering of high-temperature octahedral-tetrahedral disorder during cooling. Cross-polarization ([sup.29]Si{[sup.1]H } CPMAS) NMR supports the protonation of O1 oxygen atoms in hydrous wadsleyite without formation of significant amounts of Si-OH groups, in contrast, new NMR peaks appear in hydrous ringwoodite that cross-polarize very rapidly, indicating very short Si-H distances and the presence of Si-OH, as expected from models in which much of the [H.sup.+] substitutes into [Mg.sup.2+] vacancies. Static NMR spectra provide new constraints on chemical shift anisotropies in wadsleyite and are fully consistent with the cubic structure of ringwoodite. Spin-lattice relaxation in all phases is much better fitted by a stretched exponential function than with a more conventional '[T.sub.1]' exponential, as expected when relaxation is dominated by paramagnetic impurities. However, the effects of paramagnetic impurity on ion contents on relaxation, and on the formation of newly observed minor peaks that may result from 'pseudo-contact shifts,' appear to depend on mineral structure, and will require considerable future study to understand in detail. Keywords: NMR spectroscopy, forsterite, wadsleyite, ringwoodite, water in mantle

Single-crystal X-ray diffraction has been used to measure the thermal expansion coefficients of forsterite, wadsleyite, ringwoodite, and their hydrous forms at ambient pressure, from temperatures as low as 133 K to as high as 919 K. Second-order polynomial fitting to ln(V/[V.sub.0]) vs. T was applied to derive the expansion coefficients in the form of [alpha] = [a.sub.l] x T + [a.sub.0]. The single crystal of anhydrous wadsleyite persisted up to 859 K. Hydrous forsterite was observed to dehydrate at 919 K, whereas hydrous wadsleyite started to dehydrate at 655 K. The crystal of ringwoodite with 0.20 wt% broke down at 911 K, and the two ringwoodite samples with 0.74 and 2.4 wt% [H.sub.2]O, were observed to break down with an irreversible unit-cell expansion at 808 and 606 K, respectively. From room temperature to high temperatures in this study, the mean thermal volume expansion coefficients are 36.4(5) and 38.1 (9) x [10.sup.-6] [K.sup.-1], respectively, for anhydrous and hydrous forsterite; 28.5(5) and 35.8(8) x [10.sup.-6] [K .sup.-1], respectively, for the anhydrous and hydrous wadsleyite; 30.6(9), 35.2(15), and 34.9(7) x [10.sup.-6] [K.sup.-1] for the samples ofringwoodite with 0.2, 0.74, and 2.4 wt% [H.sub.2]O, respectively. Thus, forsterite, wadsleyite, and ringwoodite all have larger thermal expansion coefficients in their hydrous forms than in their anhydrous forms. Keywords: Thermal expansion, forsterite, wadsleyite, ringwoodite, single crystal

The crystal structure of hydrous pure magnesium forsterite ([Mg.sub.2]Si[O.sub.4]) containing 8900 ppmw [H.sub.2]O, synthesized at 12 GPa and 1250[degrees]C, has been refined. The major hydration mechanism appears to be M1 cation vacancy with protonation of the O1-O2 octahedral edge of M1. Raman spectra up to 41 GPa show strong coupling between the two [A.sub.g] modes (824.4and 856.2 [cm.sup.-1]). Mode Gruneisen parameters [[gamma].sub.i], related to Mg(2)[O.sub.6] translation mode decrease relative to anhydrous forsterite. This is attributed to the fact that the M2 site shows full occupancy compared to the M1 site, and the vacancy predominantly occurs at the M1 site. Pressure dependencies of four OH vibrations in the region 3548-3615 [cm.sup.-1] suggest that positional ordering of hydrogen ion (proton) takes place with increasing pressure. The OH mode at 3615 [cm.sup.-1] shows complex response as a function of pressure: the ([partial derivative]v/[partial derivative]P) slope changes from -0.43 below l0 GPa to 1.97 between 10 and 20 GPa, and does not show clear pressure dependence above 20 GPa. Single-crystal X-ray data were used to assign the OH band to the structural sites, O1 and O2. The proton is closer to O2 than O1. The distance between the proton and O2 is 0.96 [Angstrom]. In view of the empirical relation between OH-stretching frequencies and O ... O distances (Libowitzky 1999), the pressure dependence of the OH mode (3615 [cm.sup.-1]) is well correlated with the O1-O2 distance and the degree of hydrogen bonding. Keywords: Forsterite, X-ray diffraction, Raman spectroscopy, hydration mechanism, high pressure

We present here high-resolution [sup.29]Si MAS NMR data for synthetic samples of forsterite ([alpha]- [Mg.sub.2]Si[O.sub.4]), wadsleyite ([beta]), and ringwoodite ([gamma]). Enrichment to >99% [sup.29]Si provides greatly enhanced signal-to-noise ratios and thus great sensitivity to small features in the spectra. At a detection limit of 0.1 to 0.5%, no six-coordinated Si ([sup.vi]si) is observed in any of the polymorphs, although these results could be consistent with theoretical predications of 1 to 2% Mg-Si site disorder in ringwoodite if re-ordering occurs rapidly during cooling. Several small [sup.iv]Si peaks in ringwoodite samples may be related to residual defects from this process. In forsterite and wadsleyite, several very small 'extra' peaks are observed, many of which are at positions far outside the known range of chemical shifts for [sup.29]Si in silicates. These may be caused by 'pseudo-contact' shifts from dipolar interactions with unpaired electron spins on trace impurities of paramagnetic transition metal cations. Keywords: NMR spectroscopy, forsterite, ringwoodite, wadsleyite, high-pressure studies

A suite of pure magnesian wadsleyite ([beta]-[Mg.sub.2]Si[O.sub.4]) samples containing 0.005, 0.38, 1.18, and 1.66 wt% [H.sub.2]O was studied by single-crystal X-ray diffraction to determine the effects of hydration on cation ordering and crystal symmetry. Separate compressibility experiments were carried out to 9.6 GPa to determine the effects of hydration on isothermal equations of state. Crystal-structure refinements at ambient conditions show cation vacancies order onto the M3 site. The most hydrous sample (1.6 wt% [H.sub.2]O) displayed monoclinic symmetry with [beta] = 90.090(7)[degrees], whereas the samples with lower content were statistically orthorhombic. The density of wadsleyite decreases with increasing water content at STP according to the empirical relation, [rho] = 3.470(2) -0.046(2) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] g/[cm.sup.3] (with [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] in wt% [H.sub.2]O). Bulk moduli and pressure derivatives of wadsleyite are [K.sub.T0] = 173(5) GPa, [K.sub.0]' = 4.1(15) for 0.005 wt% [H.sub.2]O; [K.sub.T0] = 161(4) GPa, [K.sub.0]' = 5.4(11) for 0.38 wt% [H.sub.2]O; [K.sub.T0] = 158(4) GPa, [K.sub.0]' = 4.2(9) for 1.18 wt% [H.sub.2]O; and [K.sub.T0] = 154(4) GPa, [K.sub.0]' = 4.9(11) for 1.66 wt% [H.sub.2]O. Variation of the bulk modulus of wadsleyite with water content is non-linear, which may be attributable to softening of the structure by ordering of vacancies onto two non-equivalent M3 sites (M3a and M3b) and an accompanying dilution of orthorhombic symmetry. Keywords: Wadsleyite, bulk modulus, equation of state, nominally anhydrous minerals, mantle Transition Zone

Hydrogen incorporation in aluminous orthopyroxene may control the generation of melt and dominate the seismic properties at the base of the Earth's lithosphere. To clarify the substitution mechanism of H, we have synthesized, characterized, and refined the crystal structure of this potentially significant variant of orthopyroxene. The experimentally produced crystals ate small needles up to approximately 20 x 20 x 100 [micro]m in size. Electron microprobe chemical analysis indicates about 11.7 wt% [Al.sub.2][O.sub.3]. FTIR spectra indicate 7500 ppmw [H.sub.2]O with absorbance features qualitatively similar to natural mantle orthopyroxenes. TEM imagery indicates that the phase is pure orthopyroxene with low concentrations of defects and inclusions. Cell-parameter refinement from single-crystal X-ray diffraction gives a = 18.1876(7) [Angstrom]; b = 8.7352(7) [Angstrom]; c - 5.1789 (5) [Angstrom], V- 822.79(11) [[Angstrom].sup.3], which is 1.2% smaller than pure Mg anhydrous orthoenstatite. The crystal structure has been refined from single-crystal X-ray intensity data measured using a rotating anode X-ray generator, micro-focused X-ray beam, and CCD detector system. The refined structure indicates about 5% vacancy in M2 and significant Al occupancy in both M1 and T2, consistent with its composition, [([Mg.sub.095,0.05]).sup.M2], [([Mg.sub.0.79][Al.sub.0.21]).sup.M1], [([Al.sub.0.25][Si.sub.0.75]).sup.T2] [Si.stup.T1] [O.sub.6]. The existence of hydrous orthopyroxene in the mantle could absorb water released from olivine on decompression to delay the onset of melting in the spinel stability region in mantle peridotite compositions. Keywords: Orthopyroxene, crystal synthesis, high-pressure studies, X-ray data, crystal structure

Raman spectra in the range 80 to 4000 [cm.sup.-1] of wadsleyite II ([Fo.sub.90] with 2.0 wt% [H.sub.2]O and [Fo.sub.88] with 2.7 wt% [H.sub.2]O) have been measured in a diamond-anvil cell with solid rare-gas pressure-transmitting media to 51.4 GPa at room temperature. The ambient Raman spectrum of wadsleyite II is closely similar to wadsleyite modified with bands in frequency regions where the Si[O.sub.4] tetrahedral and OH stretching vibrations of hydrous ringwoodite occur. The most intense, characteristic wadsleyite II modes at 709 and 911 [cm.sup.-1] ([Si.sub.2][O.sub.7] and Si[O.sub.3] symmetric stretching vibrations, respectively) shift continuously to 51.4 GPa showing no evidence for a change in the crystal structure. A striking feature in the high-pressure Raman spectra of wadsleyite II is a significant growth in intensity in the mid-frequency range (300-650 [cm.sup.-1] at [10.sup.-4] GPa and 400-750 [cm.sup.-1] at 51.4 GPa) under compression accompanied by the appearance of new Raman modes near 40 GPa, perhaps a result of resonance electronic Raman scattering. In the OH stretching frequency range, the Raman spectrum of wadsleyite II exhibits at least six modes and their high-pressure behavior agrees with that of [Fo.sub.90] hydrous wadsleyite: OH stretching modes above 3530 [cm.sup.-1] remain approximately constant up to at least 21.8 GPa whereas OH modes at frequencies Keywords: Hydrous wadsleyite II, Raman spectroscopy, diamond-anvil cell, transition zone, high pressure

The single-crystal elastic properties of phase A have been investigated by Brillouin spectroscopy at ambient conditions. The Voigt-Reuss-Hill average for the adiabatic bulk and shear moduli are [K.sub.s] = 106(1) GPa and [mu] = 61(1) GPa, respectively. The present acoustic measurements resolve discrepancies between the bulk moduli obtained in earlier compression studies. The axial compressibility of the hexagonal (P[6.sub.3]) structure is highly anisotropic with the c-axis, which is perpendicular to the distorted close-packed layers, approximately 21% stiffer than the a-axis, in agreement with previous compression studies. The hydration of forsterite to phase A decreases the bulk and shear moduli by about 18 and 21%, respectively, while both compressional [V.sub.p] and shear [V.sub.s] wave velocities decrease by about 7%. These results suggest that water could be identified seismologically if phase A is present in abundance in cold subducted slabs. Keywords: Single-crystal elasticity, phase A, Brillouin spectroscopy, water in subduction zones

High pressure optical and near infrared spectra of a single crystal of ringwoodite with composition [([Mg.sub.0.90][Fe.sub.0.10]).sub.2]Si [O.sub.4] were measured to 21.5 GPa. The spectrum at ambient pressure shows a strong band at 12 265 [cm.sup.-1] with two shoulders at 8678 [cm.sup.-1] and 17 482 [cm.sup.-1]. The bands at 12 265 [cm.sup.-1] and at 8678 [cm.sup.-1] are due to spin-allowed crystal field transitions of octahedral [Fe.sup.2+], while the band at 17 482 [cm.sup.-1] is most likely due to [Fe.sup.2+] [right arrow] [Fe.sup.3+] charge transfer. The absorption edge due to ligand-to-metal charge transfer occurs close to 30 000 [cm.sup.-1]. With increasing pressure, both the crystal field and the charge transfer bands shift to higher frequencies. Whereas this is expected for the crystal field bands, this blue shift is surprising for an intervalence charge transfer band. Moreover, neither the crystal field nor the charge transfer bands broaden or intensify significantly with pressure. These results have major implications for radiative heat transfer in the Earth's mantle. It has commonly been assumed that radiative heat transfer is blocked in the mantle, because it was believed that the red shift and the increased intensity of charge transfer bands with pressure would effectively make mantle minerals opaque throughout the near infrared and visible range. Our results demonstrate that this effect does not occur for ringwoodite with a Mg/Fe ratio realistic for the Earth's mantle. Quite to the contrary, the mean free path of photons in ringwoodite actually increases with pressure, because the absorption bands move away from the maximum of the blackbody radiation.

The survival of coesite in ultrahigh-pressure (UHP) rocks is most commonly attributed to rapid exhumation, continuous cooling during uplift, and inclusion in strong phases that can sustain a high internal over-pressure during decompression. Exceptions to all of these criteria exist. Perhaps less attention has been paid to the role of fluid infiltration in the preservation of coesite. We used infrared spectroscopy to measure water contents of coesite and coesite pseudomorphs in a variety of UHP rocks. In all cases, OH concentrations in coesite are below the detection limit of ~100 ppm [H.sub.2]O. The silica phases surrounding coesite, however, show varying amounts of [H.sub.2]O. This is most spectacularly observed in pyrope quartzites from the Dora-Maira massif that contain at least three phases of silica replacing coesite, also distinguished by varying color of cathodoluminescence (CL): palisade-textured quartz (< 100 ppm [H.sub.2]O, red-violet CL); 'mosaic' quartz, which is actually chalcedony (up to 0.4 wt% [H.sub.2]O, yellow/brown CL); and a rare, highly hydrated silica phase interpreted to be opal (~7 wt% [H.sub.2]O, dark blue CL). Very similar signatures are observed in a grospydite xenolith from the Roberts Victor kimberlite. The quartz replacing coesite in other UHP samples studied contains on the order of 500 ppm [H.sub.2]O or less, and most measurements are under the detection limit of our technique. We infer that palisade quartz forms under dry or nearly dry conditions and at high temperatures during dilation of the host phase. The formation of hydrous silica phases such as chalcedony and opal, however, must take place at much lower temperatures, after cracking of the host phase, which allows external fluids to infiltrate. Delay of fluid infiltration to low temperatures, where kinetics are slow even in the presence of water, is the most critical factor in the preservation of coesite.

Seven separate samples of hydrous ringwoodite with compositions ranging from F[O.sub.100] to F[O.sub.89] and hydrogen contents from 0.2 to 1.1 wt% were synthesized in the 5000 ton multi-anvil press at the Bayerisches Geoinstitut. Synthesis conditions ranged from 18 to 22 GPa and 1400 to 1500 [degrees]C. The crystals were characterized by single-crystal X-ray diffraction, electron microprobe, IR and Mossbauer spectroscopy, and by analytical and high-resolution transmission electron microscopy. The crystals are optically isotropic, and the Fe-bearing samples are deep blue in color. Mossbauer spectroscopy and ELNE spectroscopy applied to the Fe-bearing samples indicates about 10% of the iron is in the ferric state. High-resolution TEM examination of one of the Fe-bearing samples indicates that the crystals are homogeneous and free of significant inclusions or exsolution features. Infrared spectra show a broad absorption band extending from about 2500 to 3600 [cm.sup.-1] with maxima ranging from 3105 for the pure magnesian samples to 3150 [cm.sup.-1] for the F[O.sub.89] samples. The crystal structures of the seven ringwoodite samples were refined by X-ray single-crystal diffraction. Refinement of cation site occupancies indicates full occupancy of the tetrahedral site for all samples, whereas the occupancy of the octahedral site appears to decrease systematically with H content. The principal hydration mechanism involves octahedral cation vacancies. The IR spectra are consistent with protonation of the short O-O approach on the tetrahedral edge, which would imply partial Mg-Si disorder.

Wadsleyite [[Beta]-[(Mg,Fe).sub.2]Si[O.sub.4]] is known to accept variable amounts of H and may be an important reservoir of H in the transition zone of the mantle. The crystal structure of [Fo.sub.94.6] hydrous wadsleyite ([Mg.sub.1.730][Fe.sub.0.098] [Al.sub.0.008][Si.sub.0.0991][H.sub.0.355][O.sub.4]) synthesized at 1400 [degrees] C and 17 GPa was refined in space group I2/m from 1784 measured intensities of which 830 were unique, and, of these, 650 were of intensity greater than 4[Sigma]. Unit cell parameters are a = 5.6715(7), b = 11.582(2), c = 8.258(1) [Angstrom], and [Beta] = 90.397(9) [degrees]. The final R(F) for all reflections was 0.026 ([R.sub.w] = 0.024); goodness of fit was 1.68. An H position was located on the nonsilicate O atom (O1). Partial occupancy of a normally vacant tetrahedral site adjacent to M3 was observed. This is postulated to be the result of Si moving from M3 to the adjacent tetrahedral void on decompression. Deviation from orthorhombic symmetry appears to result from ordering of H, (Mg, Fe), and possibly Si, within the two nonequivalent M3 sites.

We report a structure refinement ofa [P2.sub.1]/m ferromagnesian cummingtonite crystal with Fe/(Mg + Fe) = 0.37 at 140 K; this crystal was inverted from the C2/m structure, which is stable at room temperature. The phase-transition temperature is [approximately]240 K. The most significant structural changes associated with the C2/m-to-[P2.sub.1]/m phase transition are as follows: (1) one crystallographically independent O-rotated silicate chain becomes two, with the A chain S rotated and the B chain O rotated; (2) the M4 site collapses, with a marked decrease (0.020 [Angstrom]) in the average M4-O6 bond distance; and (3) the largest decrease in the isotropic displacement factor is for the M4 cation. From 295 to 140 K, total changes of 14 and 11 [degrees] occur in the O5-O6-O5 kinking angles of the A and B chains, respectively. The volume of the M1 octahedron decreases more than that of the other three M[O.sub.6] polyhedra, whereas the volume of the M3 octahedron is essentially unchanged. All four M[O.sub.6] polyhedra become more regular as a result of the C2/m-to-[P2.sub.1]/m transformation. These results provide new evidence that the effective size of the M4 cation controls the relative stability of the C2/m and [P2.sub.1]/m structures. The results also provide a new explanation for the unresolved broadening of the cummingtonite Raman spectra measured at low temperatures by Wang et al. (1988).

The structure of rutile-type compounds shows no substantial changes with the addition of H, as proved by the comparative analysis of the detailed X-ray structures of a synthetic anhydrous crystal and a hydrous rutile crystal. The structures of the two types of synthetic anhydrous and hydrous rutile crystals were based on the single-crystal x-ray data. The a/c ratios and the O position were also nearly the same for both the structures.

A hypothetical, ordered model of hydrous wadsleyite has the formula Mg7Si4O14(OH)2 and belongs to a space group Pmmb. Nonequivalent O1 sites have two H atoms. IR and Raman O-H stretching absorptions exhibit strong C-axis polarization. Hydration relieves the underbonding of O1 and overbonding of bridging O2. The model predicts greater distortion of silicate tetrahedra on partial hydration, while full hydration of O1 decreases the distortion. Trivalent octahedral cations Fe3+, Cr3+ and Al3+ exhibit higher solubility in wadsleyite than in olivine under hydrous conditions. The model prediction of maximum water content suggests that several oceans of H2O may be present in the transition zone of the mantle.

Three-dimensional, single-crystal X-ray diffraction data is used to define the crystal structures of a synthetic H-bearing aluminous stishovite containing nearly 1.6 weight% of Al2O3, and a pure-silica stishovite. Analysis of the results of the experiments verify that Al is interchangeable with Si in octahedral sites in conditions simulating the conditions of earth's mantle.

The crystal structures of natural F-bearing chondrodite [[Mg.sub.4.64][Fe.sub.0.28][Mn.sub.0.014][Ti.sub.0.023] [([Si.sub.1.01][O.sub.4]).sub.2]][F.sub.1.02][OH.sub.0.97]] from the Tilley Foster mine (Brewster, New York), F-bearing titanian clinohumite [[Mg.sub.8.805][Fe.sub.0.006][Ti.sub.0.214][([Si.sub.0.993][O.sub.4]).sub.4] [F.sub.0.484][OH.sub.0.516]] from Kukh-i-Lal (Tadjikistan) and F-free titanian hydroxylclinohumite [[Mg.sub.7.378][Fe.sub.1.12][Mn.sub.0.052][Ni.sub.0.014][Ti.sub.0.453] [([Si.sub.0.996][O.sub.4]).sub.4][OH.sub.1.0]] from Val Malenco (Italy) were refined in space group [P2.sub.1]/b (unique axis a) from single-crystal neutron diffraction data, collected on a four-circle diffractometer at the High Flux Isotope Reactor at Oak Ridge National Laboratory. Accurate H atom positions were determined at 295 K, 100 K, and 20 (10) K. Only one H position of approximately 50% occupancy was observed for each structure, which confirms a disordered H model. Time-of-flight single-crystal neutron data were also collected at 295 K and 20 K for the Val Malenco clinohumite as an additional check on space group symmetry. The crystal structure of the Kukh-i-Lal clinohumite was further investigated by X-ray single-crystal refinement at 295 K and by piezoelectric measurements. A few, very weak, symmetry-forbidden reflections were observed for each crystal at both ambient and lower temperatures. The same reflections were observed by all methods used. No temperature dependence is indicated, as no additional peaks appear at low temperature, and the intensity of the reflections are sample dependent. It appears that the real structure is made up of [P2.sub.1] and Pb domains so that violations are due to ordering of both H and Ti. No distinct piezoelectric effect was observed that would indicate the absence of a center of symmetry. This points to the simultaneous presence of various enantiomorphic domains, which cancels the piezoelectric effect of individual domains. The decrease in unit-cell volume with F substitution in clinohumites can be explained by the higher concentration of H-site vacancies and the coupled cationic and anionic substitution on the M3 and O/F site.

The crystal structure and hydrogen positions of serandite, Na[Mn.sub.2][[Si.sub.3][O.sub.8](OH)], have been refined from single-crystal X-ray and time-of-flight neutron diffraction data at ambient conditions. The proton occupies an asymmetric, double-well position between O3 and O4, confirming one of the shortest asymmetric hydrogen bonds known in minerals with d(O3 ... O4) = 2.464(1) [Angstrom] (X-ray) and 2.467(1) [Angstrom] (neutron). The proton position closest to O3 has about 84% occupancy and an O-H distance of 1.078(3) [Angstrom], and the position closest to O4 has an occupancy of 16% and an O-H distance of 1.07(1) [Angstrom]. The d(H ... O) of these hydrogen bonds is 1.413(3) [Angstrom] and 1.41(1) [Angstrom], respectively. Hydrogen bond angles are 164 [degrees] for H1 and 168 [degrees] for H2. The [Si.sup.IV]-OH bond length [1.628(1) [Angstrom]] is intermediate in length among the three other Si-O bonds in the dominantly (84%) hydrated Si1 tetrahedron. These new structure data for a very strong hydrogen bond may be useful for extending spectroscopy-structure correlation diagrams into the region of very low energy O-H stretching.