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Pyroxene crystals are common in meteorites but few compositions have been recognized in astronomical environments. We present quantitative room-temperature spectra of 17 Mg-- Fe-- and Ca--bearing ortho- and clinopyroxenes, and a Ca-pyroxenoid in order to discern trends indicative of crystal structure and a wide range of composition. Data are produced using a Diamond Anvil Cell: our band strengths are up to 6 times higher than those measured in KBr or polyethylene dispersions, which include variations in path length (from grain size) and surface reflections that are not addressed in data processing. Pyroxenes have varied spectra: only two bands, at 10.22~$\mu$m and 15.34~$\mu$m in enstatite (En$_{99}$), are common to all. Peak-wavelengths generally increase as Mg is replaced by Ca or Fe. However, two bands in MgFe-pyroxenes shift to shorter wavelengths as the Fe component increases from 0 to 60 per cent. A high-intensity band shifts from 11.6~$\mu$m to 11.2~$\mu$m and remains at 11.2~$\mu$m as Fe increases to 100~per~cent; it resembles an astronomical feature normally identified with olivine or forsterite. The distinctive pyroxene bands between 13~ and 16~$\mu$m show promise for their identification in MIRI spectra obtained with JWST. The many pyroxene bands between 40 and 80~$\mu$m could be diagnositic of silicate mineralogy if data were obtained with the proposed SPICA telescope. Our data indicate that comparison between room-temperature laboratory bands for enstatite and cold $\sim 10-K$ astronomical dust features at wavelengths $\gtrsim 28~\mu$m can result in the identification of (Mg,Fe)- pyroxenes that contain 7--15 % less Fe-- than their true values because some temperature shifts mimic some compositional shifts. Therefore some astronomical silicates may contain more Fe, and less Mg, than previously thought., Comment: 16 pages, 10 figures.accepted in MNRAS

The phonon component of thermal diffusivity (D) was measured up to temperatures (T) of ~1350 K using laser-flash analysis from leucite, analcime, pollucite, nepheline, sodalite, and petalite, many of which were single crystals. From electron microprobe analysis, nepheline is a solid solution, Na2(Na1.2K0.71)Al3.88Si4.10O16, whereas the other minerals have nearly endmember chemical compositions. From near-IR spectra, hydroxyl contents range from ~200 ppm to 1 wt%. At 298 K, D is low (0.51–0.75 mm2 s−1) for the zeolites and the solid-solution nepheline but moderate (1.9–2.2 mm2 s−1) for the endmember feldspathoids. For leucite, D decreases with increasing T until reaching the tetragonal to cubic phase transition whereupon D increases. A complex polynomial is required to describe D(T) for leucite, due to the displacive transition. For the other samples, as observed for most minerals, D decreases up to dehydration which terminated the runs and is described by FT −G + HT where G varies from 0.14 to 1.44 and H is negligible to 0.0006 K−1. Available heat capacity and volumetric data were used to calculate thermal conductivity as a function of T. For sodalite and petalite, k decreases with T, whereas for the remaining phases, k is roughly constant and low, ~1.5 Wm−1 K−1.

Thermal diffusivity ( D ) was obtained from cristobalite formed via melting slightly hydrous fused silica using laser flash analysis. Natural samples have lower D due to contamination ( e.g. , with feldspar) or to varying amounts of pore space. Data collected from cristobalite from 298 to 1300 K is fit by 14.11×106 T −2.+0.000493 T mm2/s. Values of D ( e.g. , 1.85 mm2/s at 298 K) and thermal conductivity are similar to those for rhyolite and silica glass, but higher than their melt values, consistent with cristobalite being highly disordered, but without the additional degree of freedom for a melt. Formation of snowflake obsidian is discussed in view of our synthesis experiments.

The phonon component of thermal diffusivity ( D ) was measured up to temperatures ( T ) of ~1000–1500 K from laser-flash analysis from four chemically distinct, gem quality single crystals and six cleavage flakes or mats of orthopyroxenes, plus synthetic protoenstatite. From electron microprobe analysis, Mg/(Mg + Fe) varies from 0.65 to 1.0 and Al, Mn, Cr, Ti, and Na occur as minor impurities. From visible spectroscopy, Fe 3+ content is small to negligible. From near-IR spectroscopy, hydroxyl contents range from ~ 0 to 600 ppm. Heating large crystals redistributes H + among various sites: thus, thermal history influences speciation. For Mg/(Mg + Fe) ~ 0.9, at 298 K, D is 1.8, 1.9, and 3.2 mm 2 s −1 along [010], [100], and [001]. For all samples, D decreases with increasing T. Roughly, anisotropy is independent of T , and unaffected by transition to the protoenstatite structure, which causes D to drop by ~10 %. Low-order polynomials describe D −1 ( T ) for orthopyroxenes. A second-order polynomial describes the dependence of D 298 on Mg content. Only 300 ppm OH suffices to lowers D by ~10 % whereas Fe 3+ makes D decrease more rapidly with T but coupled Al substitution has the opposite effect. Available heat capacity and volumetric data were used to calculate orthopyroxene and protoenstatite thermal conductivity.

Thermal diffusivity (D) at high temperature (T) was measured from 15 samples of commercial SiO2 glasses (types I, II, and III with varying hydroxyl contents) using laser-flash analysis (LFA) to isolate vibrational transport, in order to determine effects of impurities, annealing, and melting. As T increases, Dglass decreases, approaching a constant (~ 0.69 mm2s− 1) above ~ 700 K. From ~ 1000 K to the glass transition, the slope of D is small but variable. Increases of D with T of up to 6% correlate with either low water and/or low fictive temperature and are attributed to removal of strain and defects during annealing. Upon crossing the glass transition, D substantially decreases to 0.46 mm2s− 1 for the anhydrous melt. Hydration decreases Dglass, makes the glass transition occur over wider temperature intervals and at lower T, and promotes nucleation of cristobalite from supercooled melt. Due to the importance of thermal history, a spread in D of about 5% occurs for any given chemical type. Combining prior steady-state, cryogenic data with our average results on type I glass provides thermal conductivity (klat = ρCPD) for type I: klat increases from ~ 0 K, becoming nearly constant above 1500 K, and drops by ~ 30% at Tg. We find that D− 1(T) correlates with thermal expansivity times temperature from ~ 0 K to melting due to both properties arising from anharmonicity.

Thermal diffusivity (D) was measured using laser-flash analysis (LFA) from oriented single-crystal albite and glasses near LiAlSi3O8, NaAlSi3O8, CaAl2Si2O8, LiAlSi2O6 and CaMgSi2O6 compositions. Viscosity measurements of the supercooled liquids, over 2.6 × 108 to 8.9 × 1012 Pa s, confirm strongly non-Arrhenian behavior for CaAl2Si2O8, and CaMgSi2O6, and near-Arrhenian behavior for the others. As T increases, D glass decreases, approaching a constant near 1,000 K. Upon crossing the glass transition, D decreases rapidly. For feldspars, D for the melt is ~15% below D of the bulk crystal, whereas for pyroxenes, this difference is ~40%. Thermal conductivity (k lat = ρC P D) of crystals decreases with increasing T, but k lat of glasses increases with T because heat capacity (C P ) increases with T more strongly than density (ρ) and D decrease. For feldspars, k lat for the melt is ~10% below that of the bulk crystal or glass, whereas this decrease for pyroxene is ~50%. Therefore, melting substantially impedes heat transport, providing positive thermal feedback that could promote further melting.

To determine which factors affect thermal diffusivity ( D ) of mineral aggregates, we compared accurate measurements of D for five quartzites, two microcrystalline samples (chert and agate), chalcedony, and two partially crystalline opal specimens (of type opal-CT) with single-crystal quartz. Samples were characterized using infrared (IR) spectroscopy, electron microprobe analysis, optical microscopy, and X-ray powder diffractometry (XRD). Using laser flash analysis, we measured D of the quartzites and microcrystalline samples between room temperature and 1000 °C, and D of the opal specimens and chalcedony up to the temperatures where these samples failed (300 and 650 °C, respectively). Data between 20 and 500 °C can be fit by 1/ D = AT + B . Values of A and B for quartzites, microcrystalline samples, opal specimens, and chalcedony are distinct from each other and from those of directionally averaged quartz single-crystals. Lower D at room temperature correlates with inflated B values of polycrystalline samples and results from porosity (all samples), hydration (all samples), grain boundaries (microcrystalline samples and chalcedony), and disorder (opal specimens and chalcedony). Dehydration and pore space reduction alter the temperature derivatives of D ; dehydration causes the low A in chalcedony and opal. Above 600 °C, D changes negligibly with temperature and is lower than directionally averaged quartz, suggesting an increase in contact resistance as cracks open during heating. Thermal cracking is greatest in samples with large grain sizes and abundant, fluid-filled pores. The prevalence of cracking in polycrystalline samples suggests that high-temperature laboratory measurements generally underestimate heat transport properties in geologic environments, wherein confining pressures limit thermal expansion.

The phonon component of thermal diffusivity ( D ) was measured from 8 single-crystals and 2 polycrystalline clinopyroxene samples at temperatures ( T ) up to a maximum of ~1000 to 1600 K, using laser-flash analysis. Electron microprobe analysis shows that we have two samples of near end-member diopside, two augites, one end-member and one impure jadeite, aegerine, and three near end-member spodumenes. Hydroxyl contents determined from near-IR spectroscopy range from ~ 0 to 70 ppm as H 2 O. Two directions constrain D for clinopyroxenes, consistent with phonon symmetry. Anisotropy = ( D ⊥c − D ||c )/ D ||c is near 40 % for all samples and independent of T , where || and ⊥ indicate the direction heat flows relative to the c -axis. Thermal diffusivity decreases with increasing T and approaches a constant ( D sat ) near 1400 K. The temperature dependence of 1/ D is well described by low-order polynomial fits. At 298 K and for near end-member compositions, cation mass and bond lengths strongly affect D ⊥c whereas D ||c is essentially constant and large, 3.9 mm 2 s −1 . For solid solutions, D at 298 K decreases from end-member values as impurity content increases, but D at saturation is little affected. Excluding Li-rich clinopyroxenes, D ⊥c,sat ~ 0.5 mm 2 s −1 and D ||c,sat ~ 0.9 mm 2 s −1 . The values pertain to lithospheric and mantle compositions of clinopyroxene above 1400 K, and lead to bulk thermal conductivity at saturation being 2.4 W m −1 K −1 . Pressure deriviatives are predicted from models as ∂(ln k )/∥ P = 4.2 to 4.7 %GPa −1 .

Thermal diffusivity (D) was measured using laser-flash analysis from oriented single-crystal low-sani- dine (K0.92Na0.08Al0.99Fe 3+ Si2.95O8), and three glasses near KAlSi3O8. Viscosity measurements of the three super- cooled liquids, in the range 10 6.8 to 10 12.3 Pa s, confirm near- Arrhenian behavior, varying subtly with composition. For crystal and glass, D decreases with T, approaching a constant near 1,000 K: Dsat * 0.65 ± 0.3 mm 2 s -1 for bulk crystal and *0.53 ± 0.03 mm 2 s -1 for the glass. A rapid decrease near 1,400 K is consistent with crossing the glass transition. Melt behavior is approximated by D = 0.475 ± 0.01 mm 2 s -1 . Thermal conductivity (klat) of glass, calculated using previous heat capacity (CP) and new density data, increases with T because CP strongly increases with T. For melt, klat reaches a plateau near 1.45 W m -1 K -1 , and is always below klat of the crystal. Melting of potassium feldspars impedes heat transport, providing positive thermal feedback that may promote further melting in continental crust.

The phonon component of thermal diffusivity ( D ) from 12 single crystals in the spinel family was measured at temperatures ( T ) of up to ~2000 K, using laser-flash analysis. Synthetic disordered spinel, 4 gemstones near MgAl 2 O 4 , nearly ZnAl 2 O 4 , 4 “hercynites” [(Mg,Fe 2+ )(Al,Fe 3+ ) 2 O 4 ], and 2 magnetites (nearly Fe 3 O 4 ) were characterized using optical spectroscopy and electron microprobe analysis. The magnetic transition in Fe 3 O 4 is manifest as a lambda curve in 1/ D , but otherwise, D decreases with increasing T and approaches a constant ( D sat ) at high T . Part of the decrease in D as T increases results from disordering above ~700 K: these two effects were distinguished by making multiple heating runs. At 298 K, D decreases strongly as either cation substitution or Mg-Al disorder increases, whereas D sat is moderately perturbed by substitutions. For both ordered and (equilibrium) disordered spinels and hercynites, the temperature dependence of 1/ D is best described by low-order polynomial fits. For spinel, combining our data with previous cryogenic studies of thermal conductivity ( k ) constrains the T dependence of D and k from ~0 K to melting. The response of D to disorder, impurity content, and cation mass for the aluminates is used to constrain D ( T ) for γ-Mg 2 SiO 4 and ringwoodite. Pressure derivatives are provided by relationships such as ∂ln( k lat )/∂ P = ∂ln( K T )/∂ P . Our results show that the phonon contribution to heat transport in Earth’s transition zone is high, particularly for large proportions of ringwoodite.

Lattice thermal diffusivities of eleven orientated natural and synthetic quartz samples were measured using a laser-flash apparatus (LFA). For α-quartz, thermal diffusivity (D) decreases with temperature, so that directionally averaged values of D(T) between 20° and 500°C can be fit with D(T) = 1/(0.0017T + A), where A varies from 0.12 to 0.18 among the samples and D is in mm2/s. For β-quartz, D decreases very slightly or is constant with temperature. Values of D measured along [001] exceed those measured along [100] at all temperatures studied. A sharp decrease of D(T) marks the α-β phase transition, consistent with correlation of 1/D with C P in the damped harmonic oscillator model. Due to the rapidity of the laser-pulse and the dynamic nature of the measurements, the raw data show evidence of latent heat being released at the transition temperature over ∼50 ms. D(T) values within 10 K of the transition are affected both by latent heat release and by the phase change. For our suite of samples, D at all temperatures varies by ±7% from the overall average. This range is outside the 2% experimental uncertainty. To ascertain causes of D variations, we characterized out samples using infrared absorption spectroscopy, electron microprobe analyses and secondary ion mass spectrometry. Differences between the samples seem to result from the interplay of different impurity types: cations that replace Si in the silicate tetrahedra, cations squeezing into the lattice, and OH. Although limitations in quantifying amounts of trace impurities and poor understanding of where impurities reside in the lattice hamper interpretation, it seems that samples containing more substitutional impurities (e.g., Al3+) have higher diffusivities along [001], those with more interstitial defects (e.g., Li+) have higher diffusivities along [100], and that OH reduces D.

Thermal diffusivity ( D ) data from 12 oriented single crystals and seven polycrystalline samples of olivine group minerals were acquired with the laser-flash method at temperatures ( T ) of up to ~1500 °C. Samples included forsterite, Fe-Mg binary olivines, sinhalite, and chrysoberyl; specimens were characterized using infrared spectroscopy and electron microprobe analysis. Crystal orientation and chemistry both affect D . For our single crystals, D [100] > D [001] > D [010] at all temperatures. Thermal diffusivity decreases with increasing T and becomes virtually constant at high temperatures. At room temperature, D [001] of pure forsterite has the highest observed values, but substitution of a small amount of Co in forsterite (0.3 wt% CoO) lowers D by ~20%. Substitution of ~10% Fe for Mg in forsterite, as in typical mantle olivine, lowers D by ~50%. At room temperature, mantle olivine has D = 3.25, 1.66, and 2.59 mm2/s for the [100], [010], and [001] orientations, respectively. The values decrease to 0.93–0.87 mm2/s at 790–985 °C for [100], 0.54–51 mm2/s at 590–740 °C for [010] and 0.83–0.79 mm2/s at 740–890 °C for [001]. Two dunite samples have D of 0.55–0.56 mm2/s at 890–1080 °C, showing the effect of preferred orientation of grains dominated by [010]. Thermal diffusivity of polycrystalline samples is controlled by the large amounts of olivine present; minor phases offset the curves for D ( T ) from the value of the olivine mineral. Our laser-flash measurements isolate the phonon component of heat transfer from radiative transfer and show that the phonon contribution becomes nearly constant for the high temperatures expected in the mantle. The other microscopic component (diffusive radiative transfer) depends strongly on temperature and this temperature dependence likely exerts greater control on mantle convection.

Absorption coefficients associated with atomic motions of species expected in astronomical environments are determined from infrared measurements of various hydrosilicates, hydrated magnesium oxide, and the Al-bearing chain silicate, sapphirine. Band types measured include O‐H stretching modes near 3 μm, Si‐O stretching motions near 10 μm, Si‐O‐Si bends near 14 μm, O‐Si‐O bends near 20 μm, and translations of cations such as Mg and Ca near 50‐200 μm. We obtain data from films of varying thickness and use a ratioing method. First, bandstrengths of O‐H fundamentals were determined from spectra obtained from films of controlled thicknesses, generally 6 μm. The O‐H absorbance strength was then used to accurately determine thickness for a thinner film of each mineral (found to be

Recently, X-ray spectroscopy indicated that the low-spin (LS) electronic configuration of Fe 2+ and Fe 3+ ions substituting in silicate perovskite of various compositions, and also in magnesiowustite is stable at room temperature above pressures of ∼49 to 120 GPa [J. Badro, G. Fiquet, F. Guyot, J.-P. Rueff, V.V. Struzhkin, G. Vanko, G. Monaco, Iron partitioning in Earth's mantle: toward a deep lower mantle discontinuity, Science 300 (2003), 789–791.; J. Badro, J.-P. Rueff, G. Vanko, G. Monaco, G. Figuet, F. Guyot, Electronic transition in perovskite: possible non-convecting layers in the lower mantle, Science 305 (2004) 383–386.; J. Li, V.V. Struzhkin, H.K. Mao, J. Shu, R.J. Hemley, Y. Fei, B. Mysen, P. Dera, V. Prakapenka, G. Shen, Electronic spin state of iron in lower mantle perovskite, Proc. Natl. Acad. Sci. 101 (2004) 14027–14030.; J.M. Jackson, W. Sturhahn, G. Shen, J. Zhao, M.Y. Hu, D. Errandonea, J.D. Bass, Y. Fei, A synchrotron Mossbauer study of (Mg,Fe)SiO 3 perovskite up to 120 GPa, Am. Mineral. 90 (2005) 199–205.; J.F. Lin, V.V. Struzhkin, S.D. Jacobsen, M.Y. Hu, P. Chow, J. Kung, H. Liu, H.K. Mao, R.J. Hemley, Spin transition of iron in magnesiowustite in the Earth's lower mantle, Nature 436 (2005) 377–380.]. Simple thermodynamic relationships combined with crystal field theory provide a minimum Clausius–Clapeyron slope of ∼0.23 to 0.4 GPa/K for the high-spin (HS) to low-spin transition of Fe ions in perovskite, consistent with lower mantle temperatures stabilizing the disordered HS state. This computation of ∂ P / ∂ T utilizes experimentally determined parameters only ( P and T at the transition), and is supported by microstructural analysis of the change in volume and entropy considerations. Similarly, (∂ P / ∂ T ) min ∼0.18 to 0.31 GPa/K for magnesiowustite. High spin Fe 2+ should be stable in silicates throughout Earth's mantle for compositions of Fe / (Mg + Fe) ∼0.1, consistent with disorder prevailing at high temperature, although partial conversion of octahedral Fe 3+ to LS may occur. Similarly, partial conversion of octahedral Fe 2+ in magnesiowustite to the LS is possible for Fe-rich compositions, which transform at relatively low pressures.

The distinct patterns, relatively low intensities and peak positions of overtone-combination bands of silicates and oxides suggest that the 5-8 μm spectral region can provide clues for the dust composition when near optically thick conditions exist for the 10-μm silicate feature. We present 1000-2500 cm -1 room-temperature laboratory spectra obtained from powders of silicate, aluminate and nitride minerals and silicate glasses. The spectra exhibit overtone absorption bands with mass absorption coefficients ∼ 100 times weaker than the fundamentals. These data are compared with the 5-8 μm spectra of deeply embedded young stellar objects observed with the Short Wavelength Spectrometer on the Infrared Space Observatory. Fits of the laboratory data to the observations, after subtraction of the 6.0-μm H 2 O ice feature and the 6.0-μm feature identified with organic refractory material, indicate that crystalline melilite (a silicate) or metamict hibonite (a radiation-damaged crystalline aluminate) may be responsible for much of the 6.9-μm absorption feature in the observations, with melilite providing the best match. A weaker 6.2-μm absorption in the young stellar object spectra is well matched by the spectra of hydrous crystalline amphibole silicates (actinolite and tremolite). Relative abundances of Si-O in room-temperature amphiboles to low-temperature H 2 O ice are in the range 0.46-3.9 and in melilite are in the range 2.5-8.6. No astronomical feature was matched by the overtones of amorphous silicates because these bands are too broad and peak at the wrong wavelength. Hence, this analysis is consistent with the 10-μm features of these objects being due to a mixture of crystalline and amorphous silicates, rather than only amorphous silicates.

Heat capacity (CV) and entropy (S) as a function of temperature were calculated for phases in the CaO-Al2O3 system from vibrational spectra using a quasi-harmonic model. Calculated values of CV at 298 K for the calcium aluminates may have uncertainties as small as ±1%, based on comparison with published calorimetric data for CaO, Al2O3, and CaAl2O4. For hibonite (CaAl12O19), we predict CP as 519.3 J/mol-K and S as 391.7 J/mol-K, at 298 K. For grossite (CaAl4O7), calculated values of CP as 195.9 J/mol-K and of S as 172.0 J/mol-K are slightly smaller than the available calorimetric data at 298 K, consistent with calorimetric data having been obtained from samples containing ∼10 wt% hibonite impurities. Thermal conductivity at 298 K (k0) is predicted from peak widths of the vibrational modes using the damped harmonic oscillator model of a phonon gas. Calculations of k0 for CaO and Al2O3 differ from the measurements by 17% and 5%, respectively. The discrepancy for lime is larger due to uncertainties in its peak widths. This comparison suggests that our results for the calcium aluminates should be more accurate than conventional measurements of k0 which are commonly uncertain by ∼25%.

Diffusion of heat by radiation in the mantle has previously been associated with absorption of light by Fe 2+ . The near-IR absorptions of structurally incorporated OH − provide another mechanism to convert the energy of photons into lattice vibrations. The effective diffusive radiative thermal conductivity due to protonation ( k rdf,wet ) may be important within the mantle because low absorption strengths, as expected for trace to minor OH − contents, allow light to travel long distances in the medium. Available calibrations of band strength with OH − content are used to develop a formula for k rdf,wet as a function of temperature, grain size, and hydroxyl concentration that is generally applicable to mantle phases. These results are compared to new data on the phonon contribution for olivine and to calculations of Fe 2+ radiative transfer. As T increases, k rdf,wet increases. As OH − content increases, k rdf,wet initially increases, reaches a maximum near 100–1000 ppm hydroxyl content as H 2 O, and then decreases at higher concentrations. A similar dependence on grain size exists, with the maximum k rdf,wet near d ∼ 1 cm. Grain sizes and OH − contents of hydrous upper mantle samples foster radiative transport via protonation. For example, at 1500 K, sub-cm grain sizes with ∼10–100 ppm H 2 O, provide k rdf,wet that is ∼1/4 of k rdf arising from the combined effect of overtones and Fe 2+ transitions, and ∼1/8 of the lattice contribution to k of olivine. Given that OH − concentrations vary among mantle minerals, the above results suggest that mantle heat flow is heterogeneous and anisotropic. The dependence of both k rdf and viscosity on grain size and hydration suggests that the dynamical behavior in or near subducting slabs will be complex.

Infrared reflectance, thin-film absorbance, and polarized Raman spectra were acquired from ~100 to ~4000 cm−1 from polycrystalline garnets along the pyrope (Py = Mg3Al2Si3O12)-majorite [Mj = Mg3(MgSi)Si3O12 ] join. We measured Py100, Mj39, Mj45, Mj80, and natural single-crystal, nearly end-member pyrope. Consistency is obtained between measured absorption spectra and absorption coefficients calculated through Kramers-Kronig analysis of reflectivity ( R ) data, if R -values are scaled to account for losses through scattering. The widths of the lattice modes double from each end-member toward the middle of the binary, whereas frequencies are nearly independent of composition. Strong polarization of Raman peaks of all our samples except pyrope shows that the space group is reduced from cubic, even for the Al-rich majorites. The largest number of IR or Raman peaks is observed for Mj80. The number of peaks for Al-rich majorites lies between those seen for cubic pyrope and I 41/ a majorite. The behavior of symmetry-breaking modes supports the I 41/ acd space group, which was inferred from X-ray data for Mj contents near 30 to 45% (Nakatsuka et al. 1999a). Also, the internal mode types that are expected for isolated SiO4 tetrahedra in any garnet-like structure do not fully describe the patterns of peaks seen for some intermediate compositions along the Py-Mj join. The appearance of Sioct-O-Sitet bending modes in the available spectral data for Mj contents near 30–40 and 80–90%, but not near 45–50%, is attributed to two-mode behavior. Specifically, disruption of the structural chain -Sioct-O-Sitet-O-Sioct-O-Sitet-in Si-rich majorites by units such as ![Graphic][1] alters the electrostatic balance so that the internal modes are not entirely described by motions of isolated SiO4 tetrahedra. Short chains are formed at both high and low Al contents, promoting coupling of Si octahedral and tetrahedral vibrations. Near Mj50, each tetrahedron is linked (on average) to four octahedra containing two Al-, one Si-, and one Mg, and the Sioct-O-Sitet bending modes disappear. Order-disorder in majorite garnets is characterized not only by increased peak widths, but also by the presence of modes due to the bridging O atoms, whereas splittings and the appearance of new peaks indicate the space group for non-cubic garnets. [1]: /embed/inline-graphic-1.gif

A band of pure crystalline forsterite (100 per cent Mg2SiO4) occurs at 69.67 μm at room temperature (295 K); for olivines with ≳10 per cent Fe the corresponding feature is at ≳73 μm. The Mg-rich forsterite feature is observed in a variety of ISO LWS spectra, but the corresponding Fe-rich olivine feature is not. For the 10 astronomical sources in our sample, the forsterite band peaks in the 68.9–69.3 μm range and narrows with decreasing peak wavelength. This is consistent with the shortwards shifting of the peak observed when laboratory samples are cooled to 77 K (69.07 μm) and 3.5 K (68.84 μm). The shifted peak is produced by lattice contraction and the sharpening is due to a decrease in phonon density at lower temperatures. However, the astronomical bands are narrower than those of the laboratory samples. By comparing the laboratory and astronomical peak wavelengths, we deduce characteristic forsterite 69-μm band temperatures that are in the 27–84 K range for the eight post-main-sequence objects in our sample. These values are shown to be consistent with the local continuum temperatures derived using a β=1.5 dust emissivity index, similar to derived interstellar values of the opacity index. For the pre-main sequence-objects HD 100546 and MWC 922, the characteristic 69-μm forsterite band temperatures (127±18 and 139±10 K, respectively) are significantly higher than those of the post-main-sequence objects and are more than twice as high as their local continuum temperatures deduced using β=1.5. The assumption of large grains (β=0) can produce agreement between the derived 69-μm and continuum temperatures for one of these objects but not for the other – a spatial separation between the forsterite and continuum-emitting grains may therefore be implied for it. We conclude that observations of the peak wavelength and FWHM of the 69-μm forsterite band show great promise as a new diagnostic of characteristic grain temperatures.

Vibrational spectra are used to determine key physical properties of phases thought to be important in Earth’s transition zone. Single-crystal infrared (IR) reflectance spectra were measured for synthetic Mg, Fe, Ni, or Co-bearing silicates with the spinel structure. Peak parameters (frequency, damping coefficient, and oscillator strength) were determined for the fundamentals, and for overtones, up to 3rd order. On average, the frequencies and damping coefficients of the overtones are simple multiples of the corresponding parameters of the fundamental modes. Absorption spectra of thin films were measured at pressures ( P ) up to 370 kbar for γ-Mg2SiO4 and up to 200 kbar for γ-Fe2SiO4. Widths are nearly constant, but frequencies (νi) increase either linearly or quadratically with P . For weak peaks, the absorption widths have values close to their corresponding damping coefficients. For γ-Fe2SiO4, ambient IR data predict heat capacity ( C V = 126 ± 2.5 J/mol·K), shear modulus ( G = 875 ± 15 kbar), and sound velocities ( u P = 8.20 ± 0.05, u S = 4.25 ± 0.06 km/s) at 298 K; pressure data give ∂ G /∂ P = 0.06, 0.44, or 0.91 if for the bulk modulus, ∂ K /∂ P = 5, 4.5, or 4, respectively, and an average mode Gruneisen parameter of = 1.45 ± 0.4, which implies that thermal expansivity is (21 ± 1) × 10−6/K. For γ-Mg1.2Fe0.8SiO4, ambient IR data predict G = 1120 ± 50 kbar, u P = 9.12 ± 0.20, and u S = 5.18 ± 0.20 km/s at 298 K, assuming that the frequency of the acoustic mode is 225 to 240 cm−1. This calculation uses K S = 1995 kbar, which was obtained from recent compression data by assuming ∂ K /∂ P = 4. The above values provide a smooth quadratic dependence of K S and G on Fe/(Fe + Mg). The trends suggest that ∂ K /∂ P remains at 4 whereas ∂ G /∂ P drops from ~1 to ~0.5 as Fe content increases in ringwoodite. Acoustic fundamentals or overtones were used successfully here to provide u , G , and their P and T derivatives for silicate spinels. This method should work for other simple structures, and may be generally applicable.

Pre-solar grains of silicon carbide found in meteorites and interpreted as having had an origin around carbon stars from their isotopic composition, have all been found to be of the beta-SiC polytype. Yet to date fits to the 11.3 microns SiC emission band of carbon stars had been obtained only for alpha-SiC grains. We present thin film infrared (IR) absorption spectra measured in a diamond anvil cell for both the alpha- and beta- polymorphs of synthetic SiC and compare the results with previously published spectra taken using the KBr matrix method. We find that our thin film spectra have positions nearly identical to those obtained previously from finely ground samples in KBr. Hence, we show that this discrepancy has arisen from inappropriate `KBr corrections' having been made to laboratory spectra of SiC particles dispersed in KBr matrices. We re-fit a sample of carbon star mid-IR spectra, using laboratory data with no KBr correction applied, and show that beta-SiC grains fit the observations, while alpha-SiC grains do not. The discrepancy between meteoritic and astronomical identifications of the SiC-type is therefore removed. This work shows that the diamond anvil cell thin film method can be used to produce mineral spectra applicable to cosmic environments without further manipulation.

Forty garnets are anisotropic among 48 in a suite spanning the quaternary system Mg 3 Al 2 Si 3 O 12 - Fe 3 Al 2 Si 3 O 12 - Ca 3 Al 2 Si 3 O 12 -Mn 3 Al 2 Si 3 O 12 (Py-Al-Gr-Sp). The eight isotropic specimens are too thin (>0.4 mm) for detection of weak anisotropy. Birefringence (delta ) in the remaining 40 garnets is low (delta = 0.0001 to 0.0006) and undulatory in appearance, suggesting that most optical anomalies in quaternary garnets, including the pyrope-almandine-spessartine ternary, originate through residual strain. Multiple or alternate origins are not precluded for the few samples with nearly uniform retardation or unusual sector twinning. An inverse correlation exists between degree of birefringence and stresses encountered during tectonic deformation. That is, mantle garnets from kimberlites (e.g., Py 37 Al 36 Gr 27 ) have the highest delta values. As geologic setting is difficult to separate from composition, the inverse trend suggests that birefringence arises partially from internal factors. We propose that the mismatch in size between Ca (super 2+) and Mg (super 2+) exacerbates retention of residual strain. Short-range ordering is not entirely ruled out, but clustering has only been inferred for synthetics in the middle of the Py-Gr binary, a composition range unknown for natural samples, and domains are precluded in the natural samples by the recent X-ray studies of samples from similar localities.

Polarized single-crystal infrared (IR) reflectance spectra measured for synthetic fayalite (Fe2SiO4) possess all but one of the peaks mandated by symmetry plus a few weak bands attributable to overtones. Fayalite has a band at 86 cm−1, which is below the frequency range commonly examined, suggesting that vibrational spectra of ferrous minerals may need remeasuring. B2u polarizations were also measured for natural monticellite (nearly MgCaSiO4) and synthetic Mg2Si18O4. Thin film absorption data were collected for the above species, Co2SiO4, Mn2SiO4, Mg2SiO4, (Mg1.9Fe0.1)SiO4, Mg2GeO4 and Mn2GeO4. The relationships of IR peak positions (from these data and from previous single-crystal studies) with the inverse square root of the average cation mass were used to make band assignments, to estimate the positions for missing bands, and to determine which peaks are degenerate. The pressure dependencies of monticellite IR peaks were measured and resemble those of other olivines.

High-pressure infrared (IR) spectra of most lattice modes and of hydrous impurities of Fe-bearing wadsleyite were measured from O to 23 GPa. At ambient conditions, 29 of the 35 expected IR bands were revealed through a combination of reflection and absorption measurements. Absorption spectra of a magnesium end-member at ambient conditions were similar. The pressure dependence of eight OH vibrations were established to 12 GPa. The degree of hydrogen bonding is inferred from the values of the hydroxyl frequencies and their response to pressure: Five hydroxyls with low v have negative ∂v/∂P and thus moderate to strong hydrogen bonding, whereas three OH groups with high v have near-zero or positive values of ∂v/∂P and thus weak or no hydrogen bonding. This information, the response of the OH bands to Fe/Mg exchange, and X ray data were used to assign the most intense OH bands to the structural sites, O(1) and O(2). These assignments of hydroxyl vibrations are consistent with previous theoretical models. Our study of high-pressure OH vibrations in nominally anhydrous wadsleyite indicates that hydrogen impurities in wadsleyite are stable during compression. Our data are consistent with the previously observed minor structural change at around 9 GPa which is possibly due to a slight change in the compression mechanism.

Infrared (IR) reflectance and absorbance spectra obtained from polycrystalline and single-crystal perovskites of MgSiO3 and (Mg0.9Fe0.1)SiO3 compositions yielded virtually all the expected fundamental modes. Near-IR absorbance spectra indicate that the polycrystals were anhydrous, whereas the single crystal contained about 0.06 wt % H2O. A Kieffer-type model was used to calculate heat capacity, entropy, and Debye temperature for the orthorhombic perovskite phase. The calculated values at 298 K are Cp = 82 ± 2 J/mol K and S = 56 ± 2 J/mol K. Our calculated Cp agrees with calorimetric data within 5%. Above 500 K, the Debye temperature is nearly constant and equals 960±30 K. The bulk modulus is calculated from vibrational frequencies to be 265±13 GPa which lies within 2% of recent experiments. The Clapeyron slope for the phase transition between ilmenite and orthorhombic perovskite is calculated to be 1.110.6 MPa/K which is close to the experimental determination.

Polarized single-crystal infrared (IR) reflectance spectra measured for natural ilmenite (FeTiO 3 ) possessed the eight peaks mandated by symmetry plus weak bands attributable to overtones. IR and Raman peaks were correlated between these data and previous polycrystalline studies of 14 isostructural compounds by comparing intensity patterns. This enabled assignment of symmetries. Frequencies for unobserved bands were estimated from relationships of IR and Raman bands with cell volume and cation mass. Different trends exist for high- and low-frequency bands indicating that the former are essentially internal motions of octahedra containing X 4+ , while the latter are translations of the M 2+ . Heat capacities calculated from vibrational spectra lie 5 % below calorimetric measurements from 150 to 1100 K. Measured entropy is bracketed by the sum of lattice entropy calculated from a Kieffer-type model plus a constant magnetic contribution and the sum S 1at plus S mag plus constant d-electron exchange and Fe-Ti charge transfer contributions. These results are consistent with electronic processes being temperature dependent, as expected for distorted octahedra. Accurate prediction of S and Cp for ferrous compounds will require knowledge of visible as well as vibrational spectra. Bulk moduli for isostructural compounds calculated from a modified Brout model lie within 5 % of experiment. K T is predicted for the eight compounds

Wadsleyite (β-(Mg,Fe)2SiO4) is a major constituent of the Earth's transition zone and is known to accommodate OH. The portion of the transition zone between 400–550 km could be an important source or sink for hydroxyl in plumes and slabs intersecting this region. Micro-infrared spectroscopy has been carried out on the β-phase and coexisting metastable olivine synthesized in a multianvil apparatus at 14 GPa and 1550–1650 K under hydrous conditions. Single-crystal and polycrystal specimens of both phases were analyzed in the 1800–8500 cm−1 frequency region to determine the speciation, abundances, and partitioning behavior of the hydrous components in coexisting β-phase and olivine. β-phase spectra consistently show three distinct OH bands at 3329, 3580, and 3615 cm−1. OH concentrations range from 10000–65000 H/106 Si. A strong positive correlation of grain size and extent of transformation with OH concentration in the β-phase indicates that grain-growth and transformation rates are enhanced in a hydrous environment. Olivine spectra are variable, but consistently show a prominent broad-band absorbance representing molecular H2O, consistent with the infrared signature of the starting material. OH concentrations in olivine range from

All 17 infrared lattice modes for yttrium aluminum, yttrium gallium, and yttrium iron garnets Y3R2(RO4)3 are obtained from single‐crystal reflection and absorption spectroscopy, and the powder technique. Assignments for 12 modes are firmly establish based on chemical substitution among these garnets and within related series. Frequencies of the modes associated with translation of yttrium are nearly constant across the series. Frequencies of all other modes decrease quasilinearly as cell volume increases. Values the high and low frequency dielectric constants are in good agreement with independent measurements; also, the Lyddane–Sachs–Teller relationship was found to hold. Calculations of bulk moduli from vibrational spectra using a modification of Brout’s formula linearly correlate with experimental values for KT.

Thermal diffusivity ( D ) as a function of temperature ( T ) was determined from common and brittle micas both parallel and perpendicular to the basal plane using laser-flash analysis, which does not suffer from problems with contact losses and radiative gains. Samples were characterized using the electron microprobe and visible spectroscopy. For the two orientations, D differs by almost a factor of 10, such that for flow within the cleavage plates D is ∼2 mm 2 s –1 , similar to many silicate minerals. Thermal diffusivity decreases as F or Fe contents increase. Data are best described as D = F T –G + H T , where F, G, and H are fitting parameters. Thermal conductivity up to 1000 K is calculated from D , density, and heat capacity for muscovite, phlogopite, and biotite. The pressure dependence of k is inferred from recent data from muscovite for flow across the basal plane.

Far-infrared spectra of laboratory silicates are normally obtained at room temperature even though the grains responsible for astronomical silicate emission bands seen at wavelengths >20 micron are likely to be at temperatures below ~150 K. In order to investigate the effect of temperature on silicate spectra, we have obtained absorption spectra of powdered forsterite and olivine, along with two orthoenstatites and diopside clinopyroxene, at 3.5+-0.5 K and at room temperature (295+-2K). To determine the changes in the spectra the resolution must be increased from 1 to 0.25 cm^-1 at both temperatures since a reduction in temperature reduces the phonon density, thereby reducing the width of the infrared peaks. Several bands observed at 295 K split at 3.5 K. At 3.5 K the widths of isolated single bands in olivine, enstatites and diopside are ~ 90% of their 295 K-widths. However, in forsterite the 3.5-K-widths of the 31-, 49- and 69-micron bands are, respectively, 90%, 45% and 31% of their 295 K widths. Due to an increase in phonon energy as the lattice contracts, 3.5-K-singlet peaks occur at shorter wavelengths than do the corresponding 295-K peaks; the magnitude of the wavelength shift increases from \~ 0-0.2 micron at 25 micron to ~0.9 micron at 80 micron. Changes in the relative absorbances of spectral peaks are also observed. The temperature dependence of lambda_pk and bandwidth shows promise as a means to deduce characteristic temperatures of mineralogically distinct grain populations. In addition, the observed changes in band strength with temperature will affect estimates of grain masses and relative mineral abundances inferred using room-temperature laboratory data., 11 pages, 7 figures including figures 3a and 3b. includes latex and eps files. Accepted by MNRAS on 15th March 2001

Infrared absorption measurements were taken from 100 to 5000 cm−1 of a natural chondrodite and three dense hydrous magnesium silicates: phase A, phase B, and superhydrous phase B (shy-B). Raman spectra were also acquired from phase B and the chondrodite. Roughly half of the lattice modes are represented and our data are the first report of the low frequency modes. Comparison of our new spectra to symmetry analyses suggests that multiple sites for hydrogen exist for all the phases. The shy-B we examined crystallizes in P21nm with two OH sites. Models for the density of states are constructed based on band assignments for the lattice modes and for the OH stretching vibrations. Heat capacity CP and entropy S calculated using Kieffer's formulation should be accurate within 3% from 200 to 800 K. Model values for CP at 298 K are 299.6 J/mol-K for chondrodite, 421.5 J/mol-K for phase A, 529.4 J/mol-K for shy-B, and 618.9 J/mol-K for phase B. Model values for S2980 are 234.2 J/mol-K for chondrodite, 303.5 J/ mol-K for phase A, 377.9 J/mol-K for shy-B, and 473.3 J/mol-K for phase B. Debye temperatures are near 1000 K.

We present a laboratory study of silicate glasses of astrophysically relevant compositions including olivines, pyroxenes and melilites. With emphasis on the classic Si-O stretching feature near 10 microns, we compare infrared spectra of our new samples with laboratory spectra on ostensibly similar compositions, and also with synthetic silicate spectral data commonly used in dust modeling. Several different factors affect spectral features including sample chemistry (e.g., polymerization, Mg/Fe ratio, oxidation state and Al-content) and different sample preparation techniques lead to variations in porosity, density and water content. The convolution of chemical and physical effects makes it difficult to attribute changes in spectral parameters to any given variable. It is important that detailed chemical and structural characterization be provided along with laboratory spectra. In addition to composition and density, we measured the glass transition temperatures for the samples which place upper limits on the formation/processing temperatures of these solids in space. Popular synthetically-generated optical functions do not have spectral features that match any of our glass samples. However, the ~10 microns feature generated by the synthetic data rarely exactly matches the shape and peak position of astronomically observed silicate features. Our comparison with the synthetic spectra allows astronomers to determine likely candidates amongst our glass samples for matching astronomical observations., Comment: 46 pages, 14 figures, 4 tables

The phonon component of thermal diffusivity (D) for ten synthetic single-crystals (LiF, NaCl, NaI, NaI:Tl, KCl, KBr, CsI, CsI:Tl, AgCl, and AgBr) with the B1 and B2 structures was measured from ambient temperature (T) up to ∼1093 K using contact-free, laser-flash analysis, from which effects of ballistic radiative transfer were removed. We investigated optical flats from different manufacturers as well as pellets made from compressed powders of most of the above chemical compositions plus LiI, NaBr, KI, RbCl, RbBr, RbI, CsCl, CsBr, and AgI. Impurities were characterized using various spectroscopic methods. With increasing T,D decreases such that near melting the derivatives ∂D/∂T are low, −0.0006±0.0004 mm2 s−1 K−1. Our results are ∼16% lower than D298 previously obtained with contact methods, which are elevated by ballistic radiative transfer for these infrared (IR) windows, and are well described by either D−1 following a low order polynomial in T, or by D−1∝T+n, where n ranges from 1.0294 to 1.9429. In...

The phonon component of thermal diffusivity (D) for eleven compounds (synthetic SrTiO3, SrTiO3:Fe3+, BaTiO3, KTaO3, KNbO3, NdGaO3, YAlO3, YAlO3:Tm, LaAlO3, La0.29Sr0.66Al0.65Ta0.35O3, and natural Ca1.01Mn0.001Fe0.007Ti0.99O3) with various perovskite structures was measured from ambient temperature (T) up to ∼2000 K using contact-free, laser-flash analysis, from which effects of ballistic radiative transfer were removed. Structural transitions (e.g., orthorhombic to tetragonal) below 800 K were manifest as sharp steps in 1/D. Above 800 K, structural transitions occur over intervals of ∼150 K. Similarly broad peaks accompany changes from colorless to black, attributable to partial reduction in Ti, Nb, or Ta from contact with graphite coatings. Otherwise, D decreases with increasing T and, if substitutional disorder exists, approaches a constant (Dsat) near 1600 K. Our data are best described as D−1 following a low order polynomial in T. Ordered, cubic perovskites occupy a single trend for D(T)−1, defining t...

Single-crystal Raman and infrared reflectivity data including high pressure results to over 200 kbar on a natural, probably fully ordered MgAl2O4 spinel reveal that many of the reported frequencies from spectra of synthetic spinels are affected by disorder at the cation sites. The spectra are interpreted in terms of factor group analysis and show that the high energy modes are due to the octahedral internal modes, in contrast to the behavior of silicate spinels, but in agreement with previous data based on isotopic and chemical cation substitutions and with new Raman data on gahnite (∼ ZnAl2O4) and new IR reflectivity data on both gahnite and hercynite (∼Fe0.58Mg0.42Al2O4). Therefore, aluminate spinels are inappropriate as elastic or thermodynamic analogs for silicate spinels. Fluorescence sideband spectra yield complementary information on the vibrational modes and provide valuable information on the acoustic modes at high pressure. The transverse acoustic modes are nearly pressure independent, which is similar to the behavior of the shear modes previously measured by ultrasonic techniques. The pressure derivative of all acoustic modes become negative above 110 kbar, indicating a lattice instability, in agreement with previous predictions. This lattice instability lies at approximately the same pressure as the disproportionation of spinel to MgO and Al2O3 reported in high temperature, high pressure work.

Radiation-induced smoky color and associated electron paramagnetic resonance (EPR) signals develop only in potassium feldspar (KalSi_3O_8) free of structurally bound molecular water. Fluid inclusion water does not influence coloration. The integrated intensity of each of the four bands (11,600, 16,200, 19,100, and 27,200 cm^(−1)) in the optical absorption spectra are linearly correlated with the doubly-integrated intensity of a broad, asymmetric first derivative at g_(eff)=2.027 in EPR spectra. In microcline, the EPR pattern is resolved into an asymmetric six-line pattern at g_(eff)=2.024 and a single derivative at g_(eff)=2.009 which, based on analogy to alkali-silicate glass, are due respectively to [SiO_4/K^+]^(2+) and a hole shared between two nonbonding oxygens on Si. We propose that structural water inhibits formation of smoky centers in feldspar by releasing atomic hydrogen during irradiation which destroys centers while diffusing towards a stable site.

Far-infrared, mid-IR, and Raman powder spectra were measured on six phases (bromellite, chrysoberyl, phenakite, bertrandite, beryl, and euclase) in the system BeO-Al2O3-SiO2-H2O. A single-crystal absorption spectrum of IR fundamentals in beryl is also presented, which more closely resembles the powder absorption spectrum than it does absorption spectra calculated from single-crystal reflection data. Assignments of the SiO4 and BeO4 internal vibrations are made in accordance with each mineral's symmetry and composition and by comparison to structural analogs. Heat capacities C v calculated for these partial band assignments agree with C v derived from experimental C p for all six phases, provided that Kieffer's (1979c) model is slightly modified to correctly enumerate both Si-O and Be-O stretching modes in the high frequency region (>750 cm−1). Si-O stretching bands were found to out-number Be-O stretching modes in the high-energy region of the vibrational spectra with two exceptions: (1) For those phases containing oxygen ions not coordinated to silicon, vibrations occurring at v>1,080 cm−1 that are attributable to Be-O (H) stretching must be treated separately in the model in order to calculate C v accurately. (2) Minerals consisting entirely of interlocking Si and Be tetrahedra (i.e., phases without Al or OH) can be modeled by one optic continuum representing all optical modes. These results, along with the occurrence of very low energy lattice vibrations for Be-silicates within Al, suggests that although Be-O bonds are generally weaker than neighboring Si-O bonds, Be mimics the network-forming characteristic of Si to a limited extent.

Ferrous and ferric iron concentrations in feldspars with low total iron content (

Optical functions (n and k) of cosmic dust species like forsterite (Mg2SiO4) are required at all wavelengths to quantify the temperature and amount of dust. Astronomers combine optical functions of forsterite and olivine in different ways, which will affect radiative transfer models. We investigated what recent updates to the ultraviolet-visible-near-infrared (UV-VIS-NIR) laboratory spectra of forsterite and the choice of forsterite n, k dataset will have on radiative transfer models. We measured the UV-VIS-NIR transmission spectra of synthetic forsterite, MgO, SiO2, olivine (Fo90), and meteoritic olivine (pallasite). We derived optical functions for these and compared the UV-IR behavior of our k, absorption cross-section 〈Cabs〉, and total flux to that of “astronomical silicate” and olivine. Laboratory-derived k is substantially lower than “astronomical silicate” k at λ ~ 0.2–5 µ m. In the IR, different laboratory n and k produce equivocal 〈Cabs〉, whereas total flux is different for “astronomical silicate” versus laboratory n, k. From 0.35–5 µ m, the choice of “forsterite” k values has the most effect on modeled quantities. For environments with significant UV flux, astronomers should use recent UV-VIS-NIR laboratory n, k.

Some gem-quality labradorite phenocrysts in Miocene basaltic lava from Lake County, Oregon, have a pink schiller due to metallic copper; some have a transparent red or green color. The copper content of the crystals varies systematically with color: pale-yellow labradorite sections have 0–40 ppm CuO; greens have about 100 ppm CuO; reds have 150 to 200 ppm CuO; schiller-bearing laths have 80 to 300 ppm CuO. The variation of Cu content among different crystals is primary and reflects a variation in magma chemistry during plagioclase fractionation. Similarity of absorption spectra of the red zones to that of copper-ruby color in glass shows that the red arises from the intrinsic absorption of colloidal Cu^0 particles that are too small to scatter light (