Pressure and temperature dependence of cation distribution in Mg-Mn olivine

Synthetic (Mg_0.51, Mn_0.49)₂SiO₄ olivine samples are heat-treated at three different pressures; 0, 8 and 12 GPa, all at the same temperature (～500° C). X-ray structure analyses on these single crystals are made in order to see the pressure effect on cation distribution. The intersite distribution coefficient of Mg and Mn in M1 and M2 sites, K _D = (Mn/Mg) _M1/(Mn/Mg) _M2, of these samples are 0.192 (0 GPa), 0.246 (8 GPa) and 0.281 (12 GPa), indicating cationic disordering with pressure. The small differences of cell dimensions between these samples are determined by powder X-ray diffraction. Cell dimensions b and c decrease, whereas a increases with pressure of equilibration. Cell volume decreases with pressure as a result of a large contraction of the b cell dimension. The effect of pressure on the free energy of the cation exchange reaction is evaluated by the observed relation between the cell volume and the site occupancy numbers. The magnitude of the pressure effect on cation distribution is only a fifth of that predicted from the observed change in volume combined with thermodynamic theory. This phenomenon is attributed to nonideality in this solid solution, and nonideal parameters are required to describe cation distribution determined in the present and previous experiments. We use a five-parameter equation to specify the cationic equilibrium on the basic of thermodynamic theory. It includes one energy parameter of ideal mixing, two parameters for nonideal effects, one volume parameter, and one thermal parameter originated from the lattice vibrational energy. The present data combined with some of the existing data are used to determine the five parameters, and the cation distribution in Mg-Mn olivine is described as a function of temperature, pressure, and composition. The basic framework of describing the cationic behavior in olivine-type mineral is worked out, although the result is preliminary: each of the determined parameters is not accurate enough to enable us to make a reliable prediction.     

Vibrational spectra of single-crystal topaz

A single-crystal of topaz was studied by Raman spectroscopy to assign the internal modes of the high-frequency range and to compare with infrared data. All active modes exhibit an important Davydov splitting (150 cm^?1) but we have found a small Bethe splitting (14.5 cm^?1) consistent with a very regular SiO₄ tetrahedron. Because of a high value of v ₁ (～920 cm^?1) the Raman active modes present a mixed v ₁/v ₃ character. Finally the substitution of OH for F splits an A _g internal mode and lead to some proper modes at 3650 cm^?1, 3639 cm^?1 and 1165 cm^?1.     

Infrared spectra of olivine polymorphs: α, β phase and spinel

Infrared (IR) absorption spectra are presented for olivine (α) and spinel (γ) phases of A₂SiO₄ (A=Fe, Ni, Co) and Mg₂GeO₄. IR spectra of β phase (“modified spinel”) Co₂SiO₄ and of α Mg₂SiO₄ are also included. These results provide reference spectra for the identification of olivine high-pressure polymorphs. Isostructural and isochemical correlations are used to support a general interpretation of the spectra and to predict the spectrum of γ Mg₂SiO₄. A γ Mg₂GeO₄ sample equilibrated at 1,000° C shows evidence of partial inversion, but one equilibrated at 730° C does not. This suggests that partial inversion could occur in silicate spinels at elevated temperatures and pressures, however no evidence of inversion is seen in the ir spectra of the silicates in this study.     

Vibrational interactions of tetrahedra in silicate glasses and crystals

The vibrational interactions of oxide tetrahedra and the correlation off vibrational bands with structural entities in variously polymerized idealized silicate structural units have been investigated with normal-coordinate calculations. Raman and infrared intensities as well as frequencies were calculated, yielding good qualitative agreement with observed glass and crystal spectra. Structures considered include those in the binary metal-oxide-silica systems, and analogous structures with Al or Ge replacing every other Si. The fitted force constants for stretching of bonds from Si to bridging oxygens (Obr) and non-bridging oxygens (Onb) show close correlation with bond lengths in crystals. In all-silicate units the high-frequency Raman modes due to symmetric T-Onb stretching show essentially no coupling between unlike types of tetrahedra (O-types), classified on the basis of numbers of Obr versus Onb. Calculated spectra of units with more than one O-type (e.g. double chain) have multiple high-frequency Raman modes. Interconnection of different O-types in glasses is very probable on the basis of unsplit low/mid-frequency Si-Obr stretching bands in the measured spectra of all-Si glasses. In Si-Al and Si-Ge structures oscillation of bridging oxygens may play an important role in the high-frequency Raman. There may be more high-frequency modes than there are different types of tetrahedra, and behavior in such modes is complex, not readily described by “one-mode/two-mode” terminology. High frequency antisymmetric (both infrared- and Raman-active) and low- to mid-frequency symmetric (Raman active) T-O stretching modes may sometimes be identified with individual T-O-T groups, although this depends on the degree of polymerization.     

Vibrational interactions of tetrahedra in silicate glasses and crystals

Normal coordinate calculations have been carried out on partially polymerized simple silicate crystals, including Li and Na di- and metasilicates, Li and Gd pyrosilicates, thortveitite and rankinite. In the antisymmetric Si-O stretching modes which are active at 800–1200 cm^?1 in infrared spectra, Si-Obr vibrations occur at higher frequencies than Si-Onb vibrations if the bonds have equivalent strengths. However, this relationship is usually reversed when bridging oxygens are overbonded and non-bridging oxygens are underbonded in terms of Pauling bond strengths, a situation which is generally more common in crystals. An observed bimodality of the high-frequency envelope in infrared spectra of glasses in the alkali oxide-silica systems may be somewhat fortuitous, with the high frequency component (ca. 1100 cm^?1) representing underbonded non-bridging oxygens and saturated bridging oxygens, and the lower-frequency component (ca. 1000 cm^?1) mainly oversaturated bridging oxygens. Significant differences between crystals and glasses in the number and location of the main high-frequency infrared peaks suggest that there are short-range bonding rearrangements in the glasses, and that crystallite models are not applicable. Mid-frequency (600–800 cm^?1) infrared modes in silicates more polymerized than the pyrosilicate (Si₂O₇) appear to be mostly antisymmetric modes in which Si rattles against bridging oxygens, rather than symmetric stretching modes.     

Vibrational interactions of tetrahedra in silicate glasses and crystals

Normal coordinate calculations, producing synthetic infrared and Raman spectra, were carried out on melilites, pyroxenes, silica polymorphs and feldspars. Atomic motions are complex in the high-frequency Raman modes of melilites and aluminous pyroxenes. The symmetric T-Onb stretching vibrations of Si and Al tetrahedra with different numbers of bridging oxygens are separate from each other, but may combine individually with oscillation of bridging oxygens between Si and Al tetrahedra. The latter type of vibration tends to dominate as Al/Si increases. The frequencies of these vibrational components and the degree of such intermixing depend on T-O force constants, which vary greatly depending on local bonding configurations; individual bands in the high-frequency Raman cannot in general be assigned to single structural entities or fixed combinations thereof. Calculations confirm that in some Al-Si glasses such as jadeite and spodumene, i.e. those in which all Al can be tetrahedral without non-bridging oxygens, Al-O-Al linkages or linkage of more than two tetrahedra by a single oxygen, aluminum is predominantly in tetrahedral coordination. Other Al-Si glasses which are richer in aluminum or which have non-bridging oxygens may contain Al tetrahedral triclusters, non-tetrahedral Al, or both. On the basis of distinctive 450–750 cm^?1 infrared bands, both silica and feldspar glasses resemble tridymite and related stuffed derivatives, not other crystalline silica polymorphs or feldspars. Either these glasses have a structure like that of tridymite on a local scale, or the disorder of the glasses causes drastic modification to the vibrations in question.     

The effect of melt composition on the wetting angle between silicate melts and olivine

The wetting angle between silicate melts containing Ca, Li, Na, or K and olivine single crystals have been measured as part of an investigation of the dependence of the solid-liquid interfacial energy on melt composition and olivine orientation. The wetting angle increases with increasing silica content of the melt on (100) surfaces, but decreases with increasing silica content on (010) and (001) surfaces. For a given silica content, the wetting angle on (100) decreases in going from Ca to Li to Na to K, while the wetting angle on (010) and (001) increases in going from Ca to K-bearing melts. Based on published values for liquid-vapor interfacial energies, the observed changes in wetting angle with changes in melt composition indicate that the solid-liquid interfacial energy increases with increasing silica content of the melt for the (100) surface. However, for (010) and (001) surfaces, the variation of the solid-liquid interfacial energy with silica content depends upon whether Ca or K is present in the melt. In addition, the solid-liquid interfacial energy depends upon the orientation of the olivine in the following manner: _sl ⁽⁰¹⁰⁾ _sl ⁽⁰⁰¹⁾ _sl ⁽¹⁰⁰⁾ .     

Vibrational spectra and the ferroelectric phase transition of colemanite

The infrared and far infrared (10–4000 cm^–1) polarized, single crystal reflectivity of the calcium borate mineral colemanite, CaB₃O₄(OH)₃·H₂O, have been measured at room temperature and at several temperatures below the ferroelectric phase transition at approximately 270 K. Over the entire frequency range studied, only minor spectral changes are seen at the ferroelectric phase transition.     

The effect of liquid composition on the partitioning of Ni between olivine and silicate melt

We report the results of experiments designed to separate the effects of temperature and pressure from liquid composition on the partitioning of Ni between olivine and liquid, \(D_{\text{Ni}}^{\text{ol/liq}}\). Experiments were performed from 1300 to 1600 °C and 1 atm to 3.0 GPa, using mid-ocean ridge basalt (MORB) glass surrounded by powdered olivine in graphite–Pt double capsules at high pressure and powdered MORB in crucibles fabricated from single crystals of San Carlos olivine at one atmosphere. In these experiments, pressure and temperature were varied in such a way that we produced a series of liquids, each with an approximately constant composition (~12, ~15, and ~21 wt% MgO). Previously, we used a similar approach to show that \(D_{\text{Ni}}^{\text{ol/liq}}\) for a liquid with ~18 wt% MgO is a strong function of temperature. Combining the new data presented here with our previous results allows us to separate the effects of temperature from composition. We fit our data based on a Ni–Mg exchange reaction, which yields \(\ln \left( {D_{\text{Ni}}^{\text{molar}} } \right) = \frac{{ -\Delta _{r(1)} H_{{T_{\text{ref}} ,P_{\text{ref}} }}^{ \circ } }}{RT} + \frac{{\Delta _{r(1)} S_{{T_{\text{ref}} ,P_{\text{ref}} }}^{ \circ } }}{R} - \ln \left( {\frac{{X_{\text{MgO}}^{\text{liq}} }}{{X_{{{\text{MgSi}}_{ 0. 5} {\text{O}}_{ 2} }}^{\text{ol}} }}} \right).\) Each subset of constant composition experiments displays roughly the same temperature dependence of \(D_{\text{Ni}}^{\text{ol/liq}}\) (i.e.,\(-\Delta _{r(1)} H_{{T_{\text{ref}} ,P_{\text{ref}} }}^{ \circ } /R\)) as previously reported for liquids with ~18 wt% MgO. Fitting new data presented here (15 experiments) in conjunction with our 13 previously published experiments (those with ~18 wt% MgO in the silicate liquid) to the above expression gives \(-\Delta _{r(1)} H_{{T_{\text{ref}} ,P_{\text{ref}} }}^{ \circ } /R\) = 3641 ± 396 (K) and \(\Delta _{r(1)} S_{{T_{\text{ref}} ,P_{\text{ref}} }}^{ \circ } /R\) = ? 1.597 ± 0.229. Adding data from the literature yields \(-\Delta _{r(1)} H_{{T_{\text{ref}} ,P_{\text{ref}} }}^{ \circ } /R\) = 4505 ± 196 (K) and \(\Delta _{r(1)} S_{{T_{\text{ref}} ,P_{\text{ref}} }}^{ \circ } /R\) = ? 2.075 ± 0.120, a set of coefficients that leads to a predictive equation for \(D_{\text{Ni}}^{\text{ol/liq}}\) applicable to a wide range of melt compositions. We use the results of our work to model the melting of peridotite beneath lithosphere of varying thickness and show that: (1) a positive correlation between NiO in magnesian olivine phenocrysts and lithospheric thickness is expected given a temperature-dependent \(D_{\text{Ni}}^{\text{ol/liq}} ,\) and (2) the magnitude of the slope for natural samples is consistent with our experimentally determined temperature dependence. Alternative processes to generate the positive correlation between NiO in magnesian olivines and lithospheric thickness, such as the melting of olivine-free pyroxenite, are possible, but they are not required to explain the observed correlation of NiO concentration in initially crystallizing olivine with lithospheric thickness.     

Vibrational spectra,lattice dynamics and chemical constitution of oxides

The relationships among lattice dynamics of oxides and their chemical constitution are discussed in terms of the concept of dynamic crystal chemistry. A new approach to the design of a dynamic model of an ionic-covalent crystal combining molecular force constants with explicit treatment of long-range electrostatic forces in a lattice is outlined and its applications exemplified. The conditions of introduction of scaled quantum chemical force constants of a suitable molecular system into the secular equation for lattice vibrations are determined rigorously. The importance of unified treatment of stability conditions and various dynamic properties of crystals including phonon spectra, macroscopic elastic and piezoelectric constants etc., is emphasized. Several results of ab initio quantum chemical SCF computations by the gradient method for molecular species containing silicon-oxygen bonds are presented and are used to explain some empirical regularities in silicate structures.     

The structures of albite and jadeite composition glasses quenched from high pressure

The structures of jadeite and albite composition glasses quenched from 10 and 15 kbar, respectively, have been studied using radial distribution analysis. It is found that aluminum does not undergo a coordination increase at these pressures, in agreement with recent Raman and NMR studies of these glass compositions. Radial distribution functions (RDF's) indicate that the structural response of albite and Jadeite composition melts to pressure up to 10–15 kbar has to do primarily with a distortion of the tetrahedral sites, a slight collapse of the tetrahedral rings forming the framework, and a small loss of short range order resulting in a more densely packed framework structure. The RDF's do not provide any clues concerning T-O-T angular changes at these pressures. It is suggested that the observed structural changes should be closely related to those that take place in melts of rhyolitic composition. Such changes in rhyolitic melts would result in a density increase and viscosity decrease with increasing pressure similar to that observed for melts of albite and jadeite composition.     

Experimental palagonitization of basaltic glasses of varied composition

The rate of palagonitization of three chemically different types of basaltic glasses has been determined experimentally as a function of temperature (20–90 ° C) and time (3.5–14 months) in both fresh and saline water. Between ca. 40 ° C and 70 ° C there is a marked increase in the rate of transformation of the glasses, especially those of alkali basalt composition. The alteration process also accelerates after ca. 10 months at temperature higher than 70 ° C. These phenomena are possibly related to stepwise losses of the major elements, and minimum activation temperatures for the oxide/ion—water metasomatism.     

The influence of bulk composition on the speciation of water in silicate glasses

Infrared spectroscopy was used to determine the concentrations of molecular water and hydroxyl groups in hydrous rhyolitic, orthoclasic, jadeitic, and Ca–Al-silicate glasses synthesized by quenching of melts from elevated presure and temperature. The rhyolitic glasses and some of the Ca–Al-silicate glasses were quenched from water-vapor-saturated melts and used to determine the solubility of water in melts of these compositions. For all compositions studied, hydroxyl groups are the dominant hydrous species at low total water contents, whereas molecular water dominates at elevated water contents. Although the trends in species concentrations in all these compositions are similar, the proportions of the two hydrous species are influenced by silicate chemistry: increasing silica content and K relative to Na both favor molecular water over hydroxyl. Results on rhyolitic glass demonstrate that molecular water is also favored by decreasing temperature at T<850°C. For rhyolitic glasses quenched from vapor-saturated melts, the mole fraction of molecular water is proportional to water fugacity for P(H₂O)1500 bars, demonstrating that the behavior of molecular water is approximately Henrian at total water contents up to at least several weight percent. Data on water solubility for albitic, orthoclasic, and Ca–Al-silicate melts to higher pressures can also be fit by assuming Henrian behavior for molecular water and can be used to set constraints on the partial molar volume of water in these melts. The demonstration of Henry's law for molecular water in these liquids provides a link between spectroscopic measurements of microscopic species concentrations and macroscopic thermodynamic properties.     

A note on the Raman spectra of water-bearing albite glasses

The Raman spectra of albite glasses with 4.5 and 6.6 weight percent water have been obtained, and are compared with that of a dry sample. The hydrous glasses show bands near 3600 cm^?1 due to O-H stretching, and a previously unreported weak band near 1600 cm^?1 due to bending of molecular H₂O. Other weak spectral features are discussed, and the effect of dissolved water on the aluminosilicate framework vibrations is considered.     

The oxygen-isotope composition of chondrules and isolated forsterite and olivine grains from the Tagish Lake carbonaceous chondrite

The oxygen-isotope compositions (obtained by laser fluorination) of hand-picked separates of isolated forsterite, isolated olivine and chondrules from the Tagish Lake carbonaceous chondrite describe a line (δ¹⁷O = 0.95 * δ¹⁸O − 3.24; R² = 0.99) similar to the trend known for chondrules from other carbonaceous chondrites. The isolated forsterite grains (Fo_99.6-99.8; δ¹⁸O = −7.2‰ to −5.5‰; δ¹⁷O = −9.6‰ to −8.2‰) are more ¹⁶O-rich than the isolated olivine grains (Fo_39.6-86.8; δ¹⁸O = 3.1‰ to 5.1‰; δ¹⁷O = −0.3‰ to 2.2‰), and have chemical and isotopic characteristics typical of refractory forsterite. Chondrules contain olivine (Fo_97.2-99.8) with oxygen-isotope compositions (δ¹⁸O = −5.2‰ to 5.9‰; δ¹⁷O = −8.1‰ to 1.2‰) that overlap those of isolated forsterite and isolated olivine. An inverse relationship exists between the Δ¹⁷O values and Fo contents of Tagish Lake isolated forsterite and chondrules; the chondrules likely underwent greater exchange with ¹⁶O-poor nebular gases than the forsterite. The oxygen-isotope compositions of the isolated olivine grains describe a trend with a steeper slope (1.1 ± 0.1, R² = 0.94) than the carbonaceous chondrite anhydrous mineral line (CCAM_slope = 0.95). The isolated olivine may have crystallized from an evolving melt that exchanged with ¹⁶O-poor gases of somewhat different composition than those which affected the chondrules and isolated forsterite. The primordial components of the Tagish Lake meteorite formed under conditions similar to other carbonaceous chondrite meteorite groups, especially CMs. Its alteration history has its closest affinities to CI carbonaceous chondrites.     

Structure vs. composition: A solid-state H and Si NMR study of quenched glasses along the Na2O-SiO2-H2O join

A suite of six hydrous (7 wt.% H₂O) sodium silicate glasses spanning sodium octasilicate to sodium disilicate in composition were analyzed using ²⁹Si single pulse (SP) magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, ¹H-²⁹Si cross polarization (CP) MAS NMR, and fast MAS ¹H-NMR. From the ²⁹Si SPMAS data it is observed that at low sodium compositions dissolved water significantly depolymerizes the silicate network. At higher sodium contents, however, dissolved H₂O does not affect a significant increase in depolymerization over that predicted based on the Na/Si ratio alone. The fast MAS ¹H-NMR data reveal considerable complexity in proton environments in each of the glasses studied. The fast MAS ¹H-NMR spectra of the highest sodium concentration glasses do not exhibit evidence of signficantly greater fractions of dissolved water as molecular H₂O than the lower sodium concentration glasses requiring that the decrease in polymerization at high sodium contents involves a change in sodium solution mechanism. Variable contact time ¹H-²⁹Si cross polarization (CP) MAS NMR data reveal an increase in the rotating frame spin lattice relaxation rate constant (T_1ρ*) for various Qⁿ species with increasing sodium content that correlates with a reduction in the average ¹H-²⁹Si coupling strength. At the highest sodium concentration, however, T_1ρ* drops significantly, consistent with a change in the Na₂O solution mechanism.     

Structure of hydrous aluminosilicate glasses along the diopside-anorthite join: A comprehensive one- and two-dimensional H and Al NMR study

We have taken a systematic approach utilizing advanced solid-state NMR techniques to gain new insights into the controversial issue concerning the dissolution mechanisms of water in aluminosilicate melts (glasses). A series of quenched anhydrous and hydrous (∼2 wt% H₂O) glass samples along the diopside (Di, CaMgSi₂O₆)—anorthite (An, CaAl₂Si₂O₈) join with varying An components (0, 20, 38, 60, 80, and 100 mol %) have been studied. A variety of NMR techniques, including one-dimensional (1D) ¹H and ²⁷Al MAS NMR, and ²⁷Al → ¹H cross-polarization (CP) MAS NMR, as well as two-dimensional (2D) ¹H double-quantum (DQ) MAS NMR, ²⁷Al triple-quantum (3Q) MAS NMR, and ²⁷Al → ¹H heteronuclear correlation NMR (HETCOR) and 3QMAS/HETCOR NMR, have been applied. These data revealed the presence of SiOH, free OH ((Ca,Mg)OH) and AlOH species in the hydrous glasses, with the last mostly interconnected with Si and residing in the more polymerized parts of the structure. Thus, there are no fundamental differences in water dissolution mechanisms for Al-free and Al-bearing silicate melts (glasses), both involving two competing processes: the formation of SiOH/AlOH that is accompanied by the depolymerization of the network structure, and the formation of free OH that has an opposite effect. The latter is more important for depolymerized compositions corresponding to mafic and ultramafic magmas.Aluminum is dominantly present in four coordination (Al^IV), but a small amount of five-coordinate Al (Al^V) is also observed in all the anhydrous and hydrous glasses. Furthermore, six-coordinate Al (Al^VI) is also present in most of the hydrous glasses. As Al of higher coordinations are favored by high pressure, Al^VIOH and Al^VOH may become major water species at higher pressures corresponding to those of the Earth’s mantle.     

Geochemistry and oxygen isotopic composition of olivine in kimberlites from the Arkhangelsk province: Contribution of mantle metasomatism

The paper presents data on the composition of olivine macrocrysts from two Devonian kimberlite pipes in the Arkhangelsk diamond province: the Grib pipe (whose kimberlite belongs to type I) and Pionerskaya pipe (whose kimberlite is of type II, i.e., orangeite). The dominant olivine macrocrysts in kimberlites from the two pipes significantly differ in geochemical and isotopic parameters. Olivine macrocrysts in kimberlite from the Grib pipe are dominated by magnesian (Mg# = 0.92–0.93), Ti-poor (Ti < 70 ppm) olivine possessing low Ti/Na (0.05–0.23), Zr/Nb (0.28–0.80), and Zn/Cu (3–20) ratios and low Li concentrations (1.2–2.0 ppm), and the oxygen isotopic composition of this olivine δ¹⁸O = 5.64‰ is higher than that of olivine in mantle peridotites (δ¹⁸O = 5.18 ± 0.28‰). Olivine macrocrysts in kimberlite from the Pionerskaya pipe are dominated by varieties with broadly varying Mg# = 0.90–0.93, high Ti concentrations (100–300 ppm), high ratios Ti/Na (0.90–2.39), Zr/Nb (0.31–1.96), and Zn/Cu (12–56), elevated Li concentrations (1.9–3.4 ppm), and oxygen isotopic composition δ¹⁸O = 5.34‰ corresponding to that of olivine in mantle peridotites. The geochemical and isotopic traits of low-Ti olivine macrocrysts from the Grib pipe are interpreted as evidence that the olivine interacted with carbonate-rich melts/fluids. This conclusion is consistent with the geochemical parameters of model melt in equilibrium with the low-Ti olivine that are similar to those of deep carbonatite melts. Our calculations indicate that the variations in the δ¹⁸O of the olivine relative the “mantle range” (toward both higher and lower values) can be fairly significant: from 4 to 7‰ depending on the composition of the carbonate fluid. These variations were formed at interaction with carbonate fluid, whose δ¹⁸O values do not extend outside the range typical of mantle carbonates. The geochemical parameters of high-Ti olivine macrocrysts from the Grib pipe suggest that their origin was controlled by the silicate (water–silicate) component. This olivine is characterized by a zoned Ti distribution, with the configuration of this distribution between the cores of the crystals and their outer zones showing that the zoning of the cores and outer zones is independent and was produced during two episodes of reaction interaction between the olivine and melt/fluid. The younger episode (when the outer zone was formed) likely involved interaction with kimberlite melt. The transformation of the composition of the cores during the older episode may have been of metasomatic nature, as follows from the fact that the composition varies from grain to grain. The metasomatic episode most likely occurred shortly before the kimberlite melt was emplaced and was related to the partial melting of pyroxenite source material.     

The natural occurrence of hydroxide in olivine

Polarized infrared (IR) spectra of olivine single crystals from 17 different localities show a tremendous variability in both mode and abundance of hydroxide (OH) incorporation. Kimberlitic olivines contain the most total OH at an estimated concentration level of 976 H/10⁶Si, whereas olivines from basalts contain the least at 3 H/10⁶Si. Olivines of metamorphic and hydrothermal origin have widely varying concentration levels intermediate between those of basalts and kimberlites. Over 30 distinct OH absorption bands have been identified. Most of these bands are not unique to individual localities but may be found in samples from several different localities. Pleochroism is consistent among localities, but relative band intensities vary. No evidence is found for molecular H₂ in olivine. Hydrous minerals have been identified in olivine by their characteristic OH absorption bands. Serpentine is commonly found and is clearly distinguishable from intrinsic OH. Talc is present in one sample. Prominent OH bands at 3572 and 3525 cm^?1 are attributed to humite group minerals. San Carlos, Arizona, olivines annealed in the presence of H₂O develop absorption bands which are found in natural samples, however the OH absorption spectra of these annealed olivines are not identical to those of any single natural crystal. Sharp-band OH abundances in annealed samples are an order of magnitude lower than the maximum measured in natural specimens. The mechanical properties determined from these annealed olivines may not be directly applicable to mantle olivine because both the OH sites and concentrations are different.