Local relaxation around [6]Cr 3+ in synthetic pyrope–knorringite garnets, [8]Mg 3 [6](Al 1-X Cr X 3+) 2 [4]Si 3O 12, from electronic absorption spectra

Pyrope-knorringite garnets, Mg₃(Al_1-X Cr³⁺_X)₂Si₃O₁₂ with X=0.25, 0.50, and 1.00, were synthesized between 9 and 16 GPa and 1300 and 1600 °C, using multianvil high-pressure techniques. The garnets with X=0.25 and 0.50 are fine-grained, pink and violet in color. The end-member knorringites with X=1.00 are black when compact and gray when coarse-grained. The fine powder is greenish gray in natural light and pale pink under a tungsten lamp. Powder remission spectra in the wavenumber range 30 000–10 000 cm^–1 on finely powdered crystals were measured by two different methods: (I.) by the use of a small integrating sphere for small samples or (II.) microscope-spectrometric measurement using diffusely reflected radiation from a 45° illuminated microsample. Both methods yielded similar diffuse reflectance spectra. The following crystal-field parameters of ^[6]Cr³⁺ were determined for garnets with X=0.25, 0.50, 1.00: 10 Dq=17 856, 17 596, 17 286 cm^–1; and B=654, 677, 706 cm^–1; nephelauxetic ratio =(B_field/B_free)= 0.71, 0.74, 0.77. The -values indicate decreasing covalency of the Cr–O bond with increasing Cr content. The 10 Dq value for together with the mean Cr–O distance in end-member knorringite, 1.96 Å (Novak and Gibbs 1971), were used to calculate from the spectral data, local mean Cr–O distances (Langer 2001a) as a function of composition. The results indicate relatively strong local site relaxation with a value of =0.77.     

Measurement of the polarized absorption spectra of synthetic transition metal-bearing silicate microcrystals in the spectral range 44,000-4,000 cm−1

A new single beam microtechnique has been developed for measuring the polarized absorption spectra in the region 44,000-4,000 cm^?1. Spectra of a natural garnet (Spess₇₀Alm₃₀), measured by the microtechnique and by conventional macrotechniques, are consistent and thus prove the applicability of the microtechnique described. It is possible to obtain well resolved spectra down to about 13,000 cm^?1 with crystals as small as about 10 μm. Thus spectra of crystals obtained in routine high-pressure high-temperature silicate syntheses can be measured. The polarized spectra of Mn³⁺, Fe³⁺, Fe²⁺, and Cr³⁺ in the following synthetic silicate minerals are presented: piemontite (I), acmite (II), orthoferrosilite (III), and kyanite (IV) or uvarovite (V), respectively. O-Cr³⁺, O-Mn³⁺, and O-Fe²⁺ charge transfer band maxima in the UV region are identified at 38,700 cm^?1, in V; at 33,200, 35,300, and 39,000 cm^?1, in I; and at 32,800, 35,200, and 37,300 cm^?1, in III, respectively. Bands in the region ≦25,000 cm^?1 are assigned to spin-allowed and spin-forbidden dd transitions as predicted from crystal field theoretical considerations for the foregoing ions in the respective structures.     

High-pressure electronic absorption spectroscopy of natural and synthetic Cr<Superscript>3+</Superscript>-bearing clinopyroxenes

Comparison of polarized optical absorption spectra of natural Ca-rich diopsides and synthetic NaCrSi2O6 and LiCrSi2O6 clinopyroxenes, evidences as vivid similarities, as noticeable differences. The similarities reflect the fact that in all cases Cr3+ enters the small octahedral M1-site of the clinopyroxene structure. The differences are due to some iron content in the natural samples causing broad intense near infrared bands of electronic spin-allowed dd transitions of Fe2+(M1, M2) and intervalence Fe2+/Fe3+ charge-transfer transition, and by different symmetry and different local crystal fields strength of Cr3+ in the crystal structures. The positions of the spin-allowed bands of Cr3+, especially of the low energy one caused by the electronic 4 A 2g → 2 T 1g transition, are found to be in accordance with mean M1–O distances. The local relaxation parameter ε calculated for limCr 3+ → 0 from the spectra and interatomic á Cr - O ñ \left\langle {Cr - O} \right\rangle and á Mg - O ñ \left\langle {Mg - O} \right\rangle distances yields a very high value, 0.96, indicating that in the clinopyroxene structure the local lattice relaxation around the “guest” ion, Cr3+, deviates greatly from the “diffraction” value, ε = 0, than in any other known Cr3+-bearing systems studied so far. Under pressure the spin-allowed bands of Cr3+ shift to higher energies and decrease in intensity quite in accordance with the crystal field theoretical expectations, while the spin-forbidden absorption lines remain practically unshifted, but also undergo a strong weakening. There is no evident dependence of the Racah parameter B of Cr3+ reflecting the covalence of the oxygen-chromium bond under pressure: within the uncertainty of determination it may be regarded as practically constant. The values of CrO6 octahedral modulus, k\textpoly\textloc k_{\text{poly}}^{\text{loc}} , derived from high-pressure spectra of natural chromium diopside and synthetic NaCrSi2O6 kosmochlor are very close, ~203 and ~196 GPa, respectively, being, however, nearly twice higher than that of MgO6 octahedron in diopside, 105(4) GPa, obtained by Thompson and Downs (2008). Such a strong stiffening of the structural octahedron, i.e. twice higher value of k\textCr3 + \textloc k_{{{\text{Cr}}^{3 + } }}^{\text{loc}} comparing with that of k\textMg2 + \textloc k_{{{\text{Mg}}^{2 + } }}^{\text{loc}} , may be caused by simultaneous substitution of Ca2+ by larger Na+ in the neighboring M2 sites at so-called jadeite-coupled substitution Mg2+ + Ca2+ → Cr3+ + Na+. It is also remarkable that the values of CrO6 octahedral modulus of NaCrSi2O6 kosmochlor obtained here are nearly twice larger than that of 90(16) GPa, evaluated by high-pressure X-ray structural refinement by Origlieri et al. (2003). Taking into account that the overall compressibility of the clinopyroxene structure should mainly be due to the compressibility of M1- and M2-sites, our k\textCr3 + \textloc k_{{{\text{Cr}}^{3 + } }}^{\text{loc}} -value, ~196 GPa, looks much more consistent with the bulk modulus value, 134(1) GPa.     

Hydroxyl in olivines from mantle xenoliths in kimberlites of the Siberian platform

From a total of 335 olivine crystal grains, crystallographically orientated platelets and, where possible, parallelepipeds were prepared, chemically analysed by electron microprobe, examined under the polarisation microscope, and studied by polarised FTIR microscope-absorption-spectrometry in the _OH vibrational range, 3,000–3,800 cm^–1. The 335 crystal grains were extracted from 174 different specimens of Yakutian upper mantle material, including 97 xenoliths that represent all the rock types occurring in all the kimberlites of the Siberian shield. The other specimens were mega- and macrocrysts or inclusions in diamonds and garnets. Analysis of the polarised _OH-spectra allowed distinction between hydroxyl in non-intrinsic separate inclusions, NSI, and in isolated local or condensed extended defects, intrinsic to the olivines, ILD or CED, respectively. As the two latter types cannot be distinguished by vibrational spectroscopy, and as they are presumably interconnected by intracrystalline condensation reactions of the type n [ILD][CED]_n, we propose to symbolise them as [ILD/CED]. Of the total of 70 polarised _OH-bands that were found in the whole set, 17 are caused by NSI, 53 by [ILD/CED]. Total mean integrated _OH-band intensities, (̄_int)_total with ̄_int=(_||a+_||b+_||c)_int/3, were determined from the spectra. They yielded the contents of structurally unallocated water, using the recent calibration of the IR-method (Bell et al. 2003). The range is 0<wt. ppm (H₂O)_total<419. Olivines included in diamonds were found to be free of hydroxyl (detection limit of the single crystal IR-spectrometry, ca. 1 wt. ppm water). The total water contents of the different types of olivines increase in the sequence groundmass crystals < megacrysts < macrocrysts. NSI are: (1) Serpentine plus talc with _OH in the range 3,704–3,657 cm^–1, either polarised along a of the olivine matrix (Pbnm setting) or unpolarised. Approximately 232 olivines out of the 335 contain such NSI. Serpentine and talc occur mostly together, in rare cases one of them alone and if so, mostly talc. (2) Mg-edenite or Mg-pargasite occur rarely and with _OH at 3,709–3,711 cm^–1. NIS types (1) and (2) are presumably formed by metasomatic alterations of the host olivines, the orientated ones probably in the mantle, the unorientated ones during later stages. (3) The spectra of 23 olivine crystals, displayed specifically a _OH-band, polarised c>a>b, at 3,327–3,328 cm^–1, an energy typical of _OH in hydrous wadsleyite. We assume this phase to be present as NIS in the respective olivines, possibly as relic phase. (4) Weak bands between 3,175 and 3,260 cm^–1 polarised along c, are tentatively assigned to molecular water NSI with relatively strong hydrogen bonds to the matrix. We did not find larger clusters of molecular water, i.e. liquid-like water with its characteristic broad band centred at ca. 3,400 cm^–1. We did also not find any humite minerals as an NSI. Of the 53 _OH-bands intrinsic to olivine, the 29 most abundant and strong ones were subject to further analysis in terms of OH^–-bearing structural defects [ILD/CED]. Nearly all these bands are strongly polarised along a. Two bands at 3,672 and 3,535 cm^–1 are assigned to boron-related defects, [ILD/CED]_B. Five bands at 3,573, 3,563, 3,541, 3,524 and 3,512 cm^–1 are intensity-correlated and are assigned to Si-depleted titan-clinohumite-like defects, [ILD/CED]_(thl). The other, so far unidentified _OH of [ILD/CED] are suggested to originate from OH^– in different types of (Mg, Fe)-depleted defects recently predicted and discovered by TEM. These are called [ILD/CED]_(KWK). Eight mostly strong bands of them occur at energies higher than 3,573 cm^–1, [ILD/CED]_(KWK)-H, 13 strong to medium strong bands occur below 3,500 cm^–1, [ILD/CED]_(KWK)-L. Such intrinsic defects may occur alone, [ILD/CED]_(thl) and [ILD/CED]_(KWK)-H, or in different combinations with each other, [ILD/CED]_(KWK)-H+[ILD/CED]_(thl), [ILD/CED]_(KWK)-H+[ILD/CED]_(KWK)-L and [ILD/CED]_(KWK)-H+[ILD/CED]_(thl)+[ILD/CED]_(KWK)-L. Though there are indications that the occurrences of such types and combinations of the intrinsic OH^–-bearing defects in the olivines are related to the types and genetic peculiarities of their host rocks, straightforward and simple correlations do not exist. The reasons for this and also for the great number of varieties of intrinsic [ILD/CED] are discussed.Editorial responsibility: J. Hoefs

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Synthesis,lattice constants and OH-valence vibrations of an orthorhombic amphibole with excess OH in the system Li2O-MgO-SiO2-H2O

Orthorhombic amphiboles with excess OH, which can be schematically deduced from anthophyllite by the combined substitutions Mg²⁺ + O^2–Li⁺+OH^– and Mg²⁺2 Li⁺, were synthesized at 750–875° C/1 kbar in the system Li₂O-MgO-SiO₂-H₂O. Their phase relations are presented for 800° C/1 kbar . An amphibole with the analytical composition 2.70 wt% Li₂O, 31.1 wt% MgO, 63.0 wt% SiO₂, and 3.29 wt% H₂O has lattice constants a ₀ 18.588 (11), b ₀ 17.966 (10), c ₀ 5.262 (3) Å, V ₀ 1,757.2 (1.5) ų (referred to Space Group Pnma). The OH-valence vibrational spectrum of this amphibole showed v _OH bands at 3,667, 3,708, and 3,725 (shoulder) cm^–1, which are ascribed to OH in the configurations (MgMgMg)-OH, (MgMgMg)-OH-Li (Li in the A-site) of the pseudotrigonal (M1M1M3)-OH arrangement in the amphibole structure, and to Si-OH, respectively. No explanation can at present be offered for an additional shoulder at 3,695 cm^–1. The proposed structural formula is (Li_{0.27 0.73})(Li_1.11 Mg_0.89)· (Mg₅)(Si_8.01O_21.20(OH)_0.80)(OH)_2.00.     

Geologic and societal factors affecting the international oceanic transport of aggregate

Crushed stone and sand and gravel are the two main sources of natural aggregate, and together comprise approximately half the volume and tonnage of mined material in the United States. Natural aggregate is a bulky, heavy material without special or unique properties, and it is commonly used near its source of production to minimize haulage cost. However, remoteness is no longer an absolute disqualifier for the production of aggregate. Today interstate aggregate routinely is shipped hundreds of kilometers by rail and barge. In addition, during 1992, the United States imported 1,317,000 metric tons of aggregate from Canada and 1,531,000 metric tons from Mexico. A number of ports on the Atlantic Coast and Gulf Coast of the United States receive imports of crushed stone from foreign sources for transport to various parts of the eastern United States. These areas either lack adequate supplies of aggregate or are augmenting their supplies because they have difficulties meeting current demand. These difficulties may include poor stone quality, environmental permitting problems, or transportation. Certain societal and geologic conditions of New York City and Philadelphia along the Atlantic Coast, and Tampa and New Orleans along the Gulf Coast, are discussed to demonstrate the different combinations of issues that contribute to the economic viability of importing crushed stone.     

Application of stacking and inversion techniques to three-dimensional wide-angle reflection and refraction seismic data of the Eastern Alps

We present new methods for the interpretation of 3-D seismic wide-angle reflection and refraction data with application to data acquired during the experiments CELEBRATION, 2000 and ALP 2002 in the area of the Eastern Alps and their transition to the surrounding tectonic provinces (Bohemian Massif, Carpathians, Pannonian domain, Dinarides). Data was acquired on a net of arbitrarily oriented seismic lines by simultaneous recording on all lines of seismic waves from the shots, which allows 2-D and 3-D interpretations. Much (80%) of the data set consists of crossline traces. Low signal to noise (S/N) ratio in the area of the young orogens decreases the quality of travel time picks. In these seismically heterogeneous areas it is difficult to assign clearly defined arrivals to the seismic phases, in particular on crossline record sections.
In order to enhance the S/N ratio, signal detection and stacking techniques have been applied to enhance the Pg -, Pn - and PmP phases. Further, inversion methods have been developed for the interpretation of WAR/R-data, based on automated 1-D inversion ( Pg ) and the application of the delay time concept ( Pn ). The results include a 3-D velocity model of the crust based on Pg waves, time and depth maps of the Moho and a Pn -velocity map. The models based on stacked data are robust and provide a larger coverage, than models based on travel time picks from single-fold (unstacked) traces, but have relatively low resolution, especially near the surface. They were used as the basis for constructing models with improved resolution by the inversion of picks from single-fold data. The results correlate well with geological structures and show new prominent features in the Eastern Alps area and their surrounds. The velocity distribution in the crust has strong lateral variations and the Moho in the investigation area appears to be fragmented into three parts.     

Polarized electronic absorption spectra of colourless chalcocyanite,CuSO4, with a survey on crystal fields in Cu2+ minerals

Polarized electronic absorption spectra of colourless chalcocyanite, CuSO₄, have been measured using microscope-spectrometric techniques. The spectra are characterized by a structured and clearly polarized band system in the near-infrared spectral range with components centred at 11,720, 10,545, 9,100, and 7,320 cm^?1, which have been assigned to crystal field d–d transitions of Cu²⁺ cations in pseudo-tetragonally elongated CuO₆ polyhedra with point symmetry C _i ( \(\bar{1}\) ). The polarization behaviour is interpreted based on a D ₂(C ₂″) pseudo-symmetry. Crystal field calculations were performed for the actual triclinic point symmetry by applying the Superposition Model of crystal fields, as well as in terms of a ‘classic’ pseudo-tetragonal crystal field approach yielding the parameters Dq _(eq) = 910, Dt = 395, and Ds = 1,336 cm^?1, corresponding to a cubically averaged Dq _cub = 679 cm^?1. A comparative survey on crystal fields in Cu²⁺ minerals shows that the low overall crystal field strength in chalcocyanite, combined with a comparatively weak pseudo-tetragonal splitting of energy levels, is responsible for its unique colourless appearance among oxygen-based Cu²⁺ minerals. The weak crystal field in CuSO₄ can be related to the lower position of the SO₄ ^2? anion compared to, e.g. the H₂O molecule in the spectrochemical series of ligands.     

Compression moduli of Cr3+-centered octahedra in a variety of oxygen-based rock-forming minerals

High-pressure electronic absorption spectra at room temperature and at pressures 10^?4<P[GPa]<8 were measured in the spectral range 380<λ[nm] <780(26218>ν?[cm^?1]>12820) on analysed single crystal slabs, about 20?μm thick, of Cr³⁺-bearing spinel (I), kyanite (II), corundum (III), pyrope (IV) and uvarovite (V) using DAC-cell techniques in combination with single-beam microscopespectrometry. Ligand field theoretical evaluation of the spectra yielded following results: (i)?the octahedral crystal field parameter, 10Dq_Cr3+[6], linearly shifts on increasing pressure to higher energies with slopes, (δ10Dq_Cr3+[6]/δP), of 103.1 (I), 99.5 (II), 104.0 (III), 111.7 (IV) and 110.3?[cm^?1/GPa] (V) (reliability parameters r≥0.92), (ii)?The Racah-parameter B_Cr3+[6], reflecting the covalency of the Cr–O bonds, does not significantly change with pressure up to 8?GPa, in those cases where it could be evaluated from the spectra (III, IV, V). This result is contrary to the behaviour of B_Cr3+[6] with increasing temperature (Taran et?al. 1994) and shows that P and T are not inversely correlated parameters with respect to B_Cr3+[6], a decrease of which reflects an increase in covalency. (iii)?This result enabled to extract octahedral compression moduli, k_Cr3+[6], from the pressure slopes of 10Dq_Cr3+[6]: 312+48 (I), 297+70 (II), 298+44 (III), 275+35 (IV), 257+32?GPa (V). Quotients k_cr3+[6]/k_bulk,phase are nearly the same (ca. 1.6) for I, II, IV and V but significantly lower (ca. 1.1) for III. Deviations between spectroscopically determined k_Cr3+[6] and published k_Al[6], obtained by HP-XRD on ruby and pyrope, are interpreted by lattice strain induced by [Cr³⁺,?Al3+?1]^[6] substitution.     

Hyperonic equation of state

An equation of state for cold matter at neutron star densities, >10¹⁴ gm/cm³, is evaluated. The gas is considered to be a degenerate mixture of neutrons, protons, leptons, hyperons and massive baryons. We derive the equilibrium equations including the effects of nuclear interactions among all the hadrons.