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1 Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada
2 State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, P.R. China
Correspondence: * E-mail: mfleet{at}uwo.ca
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ABSTRACT |
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 Top Abstract IntroductionExperimental methodsXANES theoryResults and discussionAcknowledgmentsReferences cited |
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Key Words: XANES • calcite • aragonite • dolomite • carbonate hydroxylapatite • oldhamite
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INTRODUCTION |
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TopAbstractIntroductionExperimental methodsXANES theoryResults and discussionAcknowledgmentsReferences cited |
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This study presents the local electronic structure and stereochemistry of some Ca-bearing carbonate minerals (calcite, aragonite, dolomite, ferroan dolomite, and manganoan calcite), synthetic carbonate hydroxylapatite (CHAP), and CaS (synthetic oldhamite) using Ca L2,3-edge X-ray absorption near-edge structure (XANES) spectroscopy. The Ca L2,3-edge of calcite and basic calcium phosphate (hydroxylapatite) reagent have been studied by Naftel et al. (2001) using XANES and of calcite by Garvie et al. (1995) using electron-loss near-edge structure (ELNES) spectroscopy. More recently, Ca L2,3-edge XANES has been used to study the electronic structure of CaO (Ko et al. 2007) and the transformation of amorphous CaCO3 to calcite in sea urchin larval spicules (Politi et al. 2008). The present synthetic CHAP samples were from studies on the structural locations of the carbonate ion in hydroxylapatite (Fleet et al. 2004; Fleet 2009). Oldhamite is commonly associated with enstatite chondrites and enstatite achondrites (e.g., Floss and Crozaz 1990), and also occurs terrestrially as an alteration product of anhydrite. Synthetic CaS has been investigated by S K- and L2,3-edge XANES and shown to have an interesting electronic structure (Farrell et al. 2002; Kravtsova et al. 2004).
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EXPERIMENTAL METHODS |
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TopAbstractIntroductionExperimental methodsXANES theoryResults and discussionAcknowledgmentsReferences cited |
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. Carbonate hydroxylapatite (CHAP) was synthesized at 2–4 GPa and 1400 °C in a piston-cylinder apparatus (Fleet et al. 2004), and characterized by powder X-ray diffraction and Fourier transform infrared spectroscopy (FTIR). CHAP samples PC16, PC17, and PC18 were also investigated by X-ray structure analysis (Fleet et al. 2004; Fleet 2009), but PC15, PC24, and PC26 have not been reported previously. Calcium monosulfide (synthetic oldhamite) was prepared by dry reaction of high-purity calcium metal and sulfur in an evacuated sealed silica glass tube. The two reagents were weighed to excess of the stoichiometric proportion of sulfur, and reacted in the sealed glass tube at 600 °C overnight, and 800 °C for 1 h. This initial product was then ground and reloaded with excess sulfur into a 20 cm long sealed silica glass tube, and heated in a horizontal tube furnace at 700 °C overnight, and 800 °C for 1 h, maintaining a thermal gradient along the tube sufficient to allow any excess sulfur to condense at the cool end. Calcium L2,3-edge X-ray absorption spectra were collected in 2001 on the high-resolution spherical grating monochromator (SGM) of the Canadian Synchrotron Radiation Facility, located at the Synchrotron Radiation Center (SRC), University of Wisconsin-Madison. The SGM has since been relocated to the Canadian Light Source, University of Saskatchewan, Saskatoon. The SRC storage ring operates at either 800 MeV or 1 GeV with a ring current of 60–180 or 40–80 mA, respectively. The SGM has a 1200 lines/mm grating that provides high-resolution light for the photon energy range 270 to 700 eV. The incoming monochromatic light intensity (Io) was measured using a gold mesh detector downstream of the sample chamber. Spectra were collected simultaneously in both total electron yield (TEY), using specimen current, and fluorescence yield (FY), using a channel plate detector. The TEY recording mode probes the near surface (<70 Å depth) and is sensitive to near-surface impurities and alteration, whereas the FY mode is more representative of the bulk material. All samples were crushed to powders using an agate mortar and pestle, then spread on double-sided carbon tape and mounted onto a stainless steel sample disk before introduction to the experimental chamber. The untreated CaS sample was handled and transferred in a nitrogen-filled glove bag: three other samples were exposed to air for 5, 12, and 37 h. The Ca L2,3-edge spectra were collected from 345 to 360 eV at intervals of 0.1 eV, with a total scan time of about 6.1 min. Duplicate spectra for the CaCO3 reagent collected over an interval of two days showed excellent agreement in respect to relative position and intensity of spectral features (Figs. 1 and 2).
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XANES THEORY |
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TopAbstractIntroductionExperimental methodsXANES theoryResults and discussionAcknowledgmentsReferences cited |
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L = ±1, J = 0 or ±1. For the L2,3-edge, the core electrons are 2p, which have dipole-allowed transitions to s- and d-like final states. Since the Ca2+ cation has a 3d0 electron configuration and the 3d channel is much stronger than the 4s channel because of the large wavefunction overlap, the 2p absorption is determined by the transition probability of 2p63d0 
2p53d1 (Van der Laan and Kirkman 1992). Whereas simple one-particle density of states theories, involving separate initial and final states, have been successfully applied to the interpretation of K-edge spectra, they have failed completely to describe XANES of compounds of K, Ca, and the lighter 3d transition metals, especially in regard to the ratio of intensities, shape, and position of the L2 and L3 edges (Zaanen et al. 1985). This problem was resolved by adopting an atomic multiplet approach to the calculation of L2,3-edge XANES spectra (e.g., De Groot et al. 1990; Himpsel et al. 1991; Van der Laan and Kirkman 1992). The calculations have to take into account the 2p-3d and 3d-3d Coulomb and exchange interactions, the 2p and 3d spin-orbit interactions, and the crystal field acting on the 3d states. In particular, the 3d spin-orbit interaction has a significant influence on the branching ratio between the 2p1/2 and 2p3/2 peaks. Representative calculated spectra for the Ti4+ (3d0) cation are illustrated in De Groot et al. (1990) and Van der Laan and Kirkman (1992): both of these studies show that the relative intensities and energy positions of the spectral features vary markedly but systematically with the crystal-field splitting (10Dq).
De Groot et al. (1990) showed that the multiplet XANES spectrum of 3d0 cations in octahedral (Oh symmetry) coordination is comprised of seven peaks (or lines) of widely different intensities. Using the spectrum of calcite-structure CaCO3 as an example (Fig. 1
), Ca2+ is in octahedral coordination and the four principal peaks (a1, a2, b1, and b2) loosely correspond to the familiar 2p3/2-t2g, 2p3/2-eg, 2p1/2-t2g, 2p1/2-eg final states, respectively, with two of the seven states contributing to peak a1. The peak separations (a2-a1) and (b2-b1) are related non-linearly to the 3d crystal-field splitting, and (b1-a1) and (b2-a2) are related to the spin-orbit splitting of the 2p level. The relative intensities of the pairs a1,a2 and b1,b2 also change systematically with the crystal-field splitting; e.g., a1 and b1 are not present at 10Dq = 0.0 eV, but become dominant over a2 and b2, respectively, at higher values of 10Dq. Importantly, De Groot et al. (1990) showed that the small leading peaks (e.g., peaks 1 and 2 in Fig. 1), which were previously attributed to core hole effects, are part of the multiplet spectrum (see also Himpsel et al. 1991). For 3d0 cations in Oh symmetry, there are two small peaks leading a1 in spectra for octahedral coordination, and one leading each of a1 and a2 in spectra for tetrahedral and cubic coordinations. Crystal-field splittings (10Dq) are positive for octahedral coordination (t2g below eg) and negative for tetrahedral and cubic coordinations (eg below t2g). In summary, X-ray absorption at the L2,3 edge of 3d0 cations is dominated by exchange interactions, spin-orbit splitting, and crystal field effects. In consequence, the L2,3-edge XANES reflect electronic structure and crystal-chemical details within the first coordination sphere, but reveal little or no information of the intermediate or extended structure.
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RESULTS AND DISCUSSION |
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TopAbstractIntroductionExperimental methodsXANES theoryResults and discussionAcknowledgmentsReferences cited |
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to 4. In general, the total electron yield (TEY) recording method resulted in a greater signal strength and signal-to-noise ratio than the fluorescence yield (FY) method. Therefore, the TEY XANES spectra are used throughout this paper. Energy positions and relative intensity of spectral features were equivalent in all details for the two recording methods, suggesting that there were no significant crystal-chemical changes with depth. The spectral features for the Ca L2,3-edge are limited to a narrow range in energy (347–355 eV) and are not presently calibrated on an absolute energy scale. The present instrument calibration is similar to that of Naftel et al. (2001) and is shifted by about +1 eV relative to the ELNES spectra of Garvie et al. (1995). Measurements of the energy separation of the Ca L2 and L3 edges (b1-a1 and b2-a2 in Tables 1, 2, and 3) increase slightly from 3.3 to 3.4 eV in the sequence carbonates, CHAP and CaS. All of these values are generally consistent with the 2p spin-orbit splitting for Ca metal determined experimentally by X-ray photo-electron spectroscopy (XPS; 3.5 eV in Fuggle and Mårtensson 1980; 3.7 eV in Fink et al. 1985). The tabulated data emphasize that the positions of the various spectral peaks in the XANES spectra are essentially constant within each of the three groups of minerals and compounds presently investigated.
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and Table 1. Calcium, with an electronegativity of 1.0, is a hard-shell divalent cation in oxy structures, forming largely ionic bonds with oxygen. In the structure of calcite (space group R
m), Ca2+ has site symmetry and is in octahedral coordination with oxygen. The CaO6 octahedra share corners with carbonate ions, such that (0001) layers of Ca2+ cations alternate with (0001) layers of carbonate ions, stacked in the c-axis direction (e.g., Effenberger et al. 1981). The structure of dolomite (space group R) is similar to the calcite structure, except that now (0001) layers of Mg2+ cations alternate with the (0001) layers of Ca2+ cations (e.g., Caspi et al. 2005). The geometries of the CaO6 octahedra in calcite and dolomite are similar and close to ideal Oh symmetry (Fig. 5a).
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, and the relative peak intensities are essentially unchanged, with all studies pointing to a moderate crystal-field splitting for the Ca2+ cation in calcite, with a value of about +1.5 eV. The agreement between the relative intensities of a and b peaks and calculated spectra in the literature is better for Van der Laan and Kirkman (1992) than for De Groot et al. (1990), but both of these sets of calculations are for Ti4+ rather than Ca2+, and the peak width functions used in these studies were fairly arbitrary. A striking feature of the present study is the similarity of the Ca L2,3-edge XANES spectra for calcite, CaCO3, dolomite, ferroan dolomite, and manganoan calcite (Fig. 2; Table 1). These spectra are in fact identical within error of measurement. There are evidently no contributions to the Ca L2,3-edge XANES spectra from the different intermediate and extended structural chemistries of dolomite, ferroan dolomite, and manganoan calcite. This agreement between compounds of different chemistry but related crystal structures is a further confirmation of the local nature of X-ray absorption at the L2,3 edges of Ca2+ and other light 3d transition-metal cations.
In contrast, the Ca L2,3-edge XANES spectrum of the CaCO3 polymorph aragonite differs markedly from that of calcite and calcite-structure CaCO3 (Fig. 2; Table 1). The a1 and b1 peaks are weak, and a2 and b2 are shifted slightly to lower energy. Calcium is in an irregular ninefold coordination with site symmetry m (Fig. 5b) in the aragonite structure (space group Pmcn; e.g., Caspi et al. 2005). The CaO9 polyhedra share edges and corners with each other and with one edge of the carbonate groups. The Ca2+ cations and carbonate groups are again in alternate (001) layers, analogously to calcite, but now the carbonate groups are staggered within (001) layers and tilted about 2° out of the (001) plane. Six O2 atoms are in a chair configuration on one side of the CaO9 polyhedron (Fig. 5b) and capped by a transverse crest of three O1 atoms. Compared to the octahedron in calcite, the larger (ninefold-coordinated) polyhedron is expected to result in a weaker crystal field, but the geometry of the polyhedron does not reveal clear evidence for the sign of the crystal-field splitting. For this we have to turn to the weak peaks leading a2 and b2 in the XANES spectrum. First, the peaks labeled c and d do not appear to be part of the aragonite spectrum. Their positions (Table 1) and intensities (Fig. 2) are consistent with the presence of a minor amount of either calcite or dolomite, both of which are likely contaminant phases. When c and d are removed from the list of peaks, it is apparent that peaks 1, 2, a1, and b1 all shift toward the main peaks of the L3 and L2 edges, with 1 and 2 shifting together by the same amount, relative to calcite, and a1 and b1 shifting the most. The resulting a2-a1 and b2-b1 peak separations are both 0.8 eV. Using the model XANES spectra for the atomic multiplet theories of De Groot et al. (1990) and Van der Laan and Kirkman (1992), the peak positions and intensities confirm these peak assignments for aragonite and establish that 10Dq is positive. If 10Dq were negative, a1 would be located in between the leading peaks 1 and 2, and then the separations a2-a1 and b2-b1 would be no longer equal. In summary, the Ca L2,3-edge XANES spectrum shows that the crystal field at the Ca site of aragonite is weak and has positive 10Dq.
Carbonate hydroxylapatite
The Ca L2,3-edge XANES spectra of six synthetic carbonate hydroxylapatites (CHAP) are compared in Figure 3 and Table 2. The spectra are superficially similar to that of basic calcium phosphate in Naftel et al. (2001), but the present interpretation is more detailed. In the structure of hydroxylapatite (HAP; space group P63/m), isolated PO4 tetrahedra are linked by Ca2+ cations in two distinct structural positions, giving a formula of Ca14Ca26(PO4)6(OH)2. The hydroxyl groups are located in the large c-axis channel defined by triclusters of Ca2 at heights of z = 1/4,3/4. The Ca1 site has site symmetry 3 and the Ca2+ cation is in 6+3 coordination. The coordination polyhedron approximates to a tricapped trigonal prism (Fig. 5c): there are six nearest-neighbor O atoms and three distant equatorial ones. Ca2 has site symmetry m and Ca2+ is in 6+1 coordination. The polyhedron is irregular but has some similarity to a distorted octahedron (Fig. 5c); there are six nearest-neighbor O atoms of PO4 tetrahedra and one nearest-neighbor O atom of the channel hydroxyl group.
The complex crystal chemistry of the carbonate ion in apatites has been largely resolved in recent X-ray structure studies (Fleet et al. 2004; Fleet and Liu 2007, 2008). The carbonate ion is present in two structural locations; in the apatite channel (type A) and substituting for the phosphate group (type B). CHAP biomineralization is Na- and Mg-bearing, in minor amounts, and contains generally equal amounts of types A and B carbonate. The six CHAP samples presently investigated (Table 2) were synthesized at high temperature and pressure (Fleet et al. 2004). They are Na- and Mg-free but have cation vacancies at the two Ca sites to charge balance the substitution of phosphate by carbonate; i.e., Ca10–y
y[(PO4)6–2y(CO3)2y][(OH)2–2x(CO3)x]. The carbonate contents of PC16, PC17, and PC18 were calculated from site occupancies determined by X-ray structure analysis (Fleet et al. 2004) and of PC15, PC24, and PC26 by FTIR spectroscopy after Fleet (2009). PC18 and PC26 both contain a significant amount of carbonate (i.e., 1.08 and 0.87 pfu type A and 0.49 and 0.65 pfu type B, respectively), but the other CHAP samples contain only minor amounts of carbonate and are considered to have end-member HAP compositions for the purposes of this study.
The empty 3d orbitals of the Ca2+ cation on the Ca1 site must be split complexly because of the three equatorial O atoms but, clearly, the most stable orbital will be dz2, which projects normal to the (0001) face of the trigonal prism formed by O atoms of O1 and O2 (Fig. 5c). Therefore, the Ca1 site is expected to be associated with a weak crystal field, because of the high coordination of the Ca2+ cation, and a negative 10Dq. In the case of the Ca2 polyhedron, dz2 is destabilized by the near-axial O atoms O2 and OH (Fig. 5c), and the equatorial ring of five O atoms will tend to destabilize the dxy and dx2-y2 orbitals, leaving dxz and dyz as the most stable set. Therefore, the crystal-field splitting of Ca2 should have a positive 10Dq.
The Ca L2,3-edge XANES of the near-end-member HAP samples are clearly composite because there are four weak peaks in the L3-edge spectrum (Fig. 3). The Ca2 component is represented by the peaks 1, 2, a1, a2, b1, and b2. This spectrum is similar to that of calcite (Figs. 1 and 2) but with weaker development of a1 and b1, consistent with the smaller crystal-field splitting and positive 10Dq predicted from the coordination geometry. The features a1o and b1o are the only two resolved peaks of the Ca1 spectrum; the small leading peak of the L3 edge is hidden by the intensity of the Ca2 spectrum and a2o and b2o are overlapped with a2 and b2, respectively. Also, b1o is resolved only as a shoulder in the TEY-XANES of PC15, PC16, and PC17, but is present as a distinct peak in the corresponding fluorescence yield spectra. The weak shoulder at about 352 eV in the XANES of PC15 and PC17 is presently unexplained, but might be a surface contribution to these spectra (cf. Himpsel et al. 1991).
This interpretation of the Ca L2,3-edge XANES spectrum of HAP and CHAP results in uniform peak separations (Table 2) consistent with a weak crystal field of positive 10Dq for Ca2 and a very weak crystal field of negative 10Dq for Ca1. The only feature characteristic of the presence of significant amounts of carbonate in CHAP is the relatively weak intensity of the leading peak 2. The profile of this peak in the XANES spectra of PC18 and PC26 is clearly flat. Since all of our X-ray structure refinements of type A CHAP have indicated that the minor amounts of charge-balancing vacancies (and Na) are more-or-less evenly distributed between Ca1 and Ca2 (e.g., Fleet et al. 2004; Fleet and Liu 2004, 2007), we speculate that the loss in intensity of peak 2 is associated with the change in coordination of Ca2 on introduction of carbonate into the apatite channel. In PC18 and PC26, all channel hydroxyl ions are replaced by carbonate ions. The Ca2+ cation on the Ca2 site is then coordinated to carbonate O atoms at about z = 0.33,0.67 instead of hydroxyl O atoms at about z = 0.2,0.7. Noting that the substitution mechanism is [CO3 = 2OH], this is the only significant change to the coordination of Ca2+ in end-member type A CHAP.
Oldhamite (CaS)
Oldhamite is an ionic monosulfide with the halite structure (space group Fm3m), placing Ca2+ in ideal octahedral (Oh symmetry) coordination. Both synthetic CaS and the related compound MgS (which occurs naturally as niningerite) react with air to release H2S. At room temperature and pressure, synthetic CaS is diamagnetic and a classical insulator. The band gap is given variously as 4.434 or 2.143 eV (Stepanyuk et al. 1989) and lies between the S 3p bonding orbitals in the valence band and predominantly empty Ca 3d (and to a lesser extent Ca 4s 
*) antibonding orbitals in the conduction band (Shameem Banu et al. 1998). The partial densities of unoccupied states in the conduction band of CaS were calculated in Kravtsova et al. (2004). They confirmed that the bottom of the conduction band is formed by Ca d states and S d, s, and p states. Moreover, the low-lying Ca 3d band was a ready explanation for the characteristic strong doublet peaks at the S K- and L2,3-edges of CaS (Farrell et al. 2002), which were both reproduced by full multiple-scattering theoretical simulations in Kravtsova et al. (2004). A constant theme in XAS studies of the electronic structure of alkaline earth and 3d transition metal monosulfides is hybridization between empty metal 3d orbitals and S antibonding orbitals; namely, S 3p * for S K-edge XANES and S 3s * for S L2,3-edge XANES (Farrell et al. 2002; Kravtsova et al. 2004). The most relevant result of our earlier density of unoccupied states calculation for the present study is that the S 3d channel is closest to the Fermi level and separated from the Ca 3d channel by about 1 eV.
The Ca L2,3-edge XANES spectra of CaS and of the three air-exposed samples are compared in Figure 4 and peak positions and separations are summarized in Table 3. The most evident feature of these spectra is that the two principal edge peaks are doublets: the a2,a2o and b2,b2o pairs are of near-equal intensity in the untreated CaS spectrum, but a2o and b2o become progressively stronger with increasing amounts of exposure to air. The peaks 1, 2, a1, a2, b1, and b2 for the untreated CaS sample are consistent with Ca2+ in ideal octahedral (Oh symmetry) coordination with monosulfide, resulting in a crystal field of moderate strength and positive 10Dq. This Ca L2,3-edge XANES spectrum is thus equivalent to that of calcite-structure CaCO3, where Ca2+ is in octahedral coordination with O atoms, except that the peaks 1, 2, a1, and b1 are displaced to lower energy; the small leading peaks (1 and 2) by about 0.3 eV and a1 and b1 by about 0.5 eV (Tables 1 and 3). The second peaks of the prominent doublets in this spectrum (a2o and b2o; Fig. 4) are not readily explained. Indeed, a full interpretation of the Ca L2,3-edge XANES spectrum of CaS is not possible in the absence of further theoretical simulation and more rigorous experimentation. However, the edge-peak doublets could represent the separate S 3d (peaks a2 and b2) and Ca 3d (peaks a2o and b2o) channels identified in the density of unoccupied states calculation (Kravtsova et al. 2004), because they are separated by only about 0.5 eV.
Alternatively, the second peaks in the doublets (a2o and b2o) could represent oxide contamination in the synthetic oldhamite used in the present study. The CaS sample preparations were not investigated by X-ray powder diffraction after the synchrotron radiation study, but past experience with the method of synthesis and subsequent handling (Farrell et al. 2002) suggests that they are unlikely to contain significant amounts of bulk oxide. Also, we recently investigated CaS reagent (Alfa Aesar Co.; 99.95%) by X-ray powder diffraction, and found it to be essentially pure CaS. There was a very weak peak near the 2
position of the 200 reflection of CaO, with an intensity of about 0.2% of the 200 reflection of CaS and increasing only slightly to 0.3% on exposure to air for 48 h. Moreover, the background to powder diffraction scans was featureless, consistent with the absence of a significant component of amorphous oxide material. Because the Ca L2,3-edge XANES method only probes the outermost layers of samples, oxide or sulfate contamination could be present as a film or margin on grains representing only a small fraction of the bulk: e.g., a film with a thickness of 50 Å on a 5 µm spherical grain accounts for only about 0.6% of the total volume. However, a further complicating observation is that the XANES spectra collected by fluorescence yield (FY) are equivalent in all details to the corresponding TEY spectra illustrated in Figure 4. Thus, the contamination would have to be constant with depth beyond the lowest level probed by the fluorescence yield method; i.e., to a depth of several hundred angstroms. Also, because the proportions of the a2,b2 and a2o,b2o pairs of peaks in XANES spectra collected by TEY and FY are essentially the same, the grain margin would have to be a uniform mixture of sulfide and contaminant, rather than contaminant dominant at the outer margin grading into pure CaS with depth. Moreover, the contamination is unlikely to be halite-structure CaO. Although the a2o-a1 and b2o-b1 peak separations are consistent with the ELNES spectrum of CaO in Garvie et al. (1995), the a1 and b1 peaks do not appear to be composite and do not have sufficient intensity to result from overlapped CaS and CaO XANES spectra.
Peak intensities for the other three Ca L2,3-edge XANES spectra in Figure 4 change progressively with increasing exposure to air and new peaks appear at a1o and b1o, strengthening in proportion to the decrease in intensity of a2 and b2 and increase in intensity of a2o and b2o. Therefore, the weak peaks a1o and b1o and a component of the a2o and b2o peaks belong to the XANES spectrum of an air-alteration product. In this case, the principal peaks of the air oxidation spectra are readily obscured by overlap with a2o and b2o because the principal absorption edge peaks for all of the presently studied Ca-O environments are located in restricted energy ranges, namely 350.3–350.4 and 353.5–353.7 eV for L3 and L2 edges, respectively (Tables 1 and 2). Qualitatively, the air-oxidation XANES spectra show that CaS is progressively consumed by reaction with air, and the reaction is rate-limited after about 12 h. Also, the Ca2+ cation is in a very weak crystal field, consistent with a high-coordination number. Thus, a Ca sulfate phase is the likely product of the air oxidation of CaS. This conclusion is supported by the occurrence of the anhydrite + oldhamite assemblage at Ronneburg, Thuringen, Germany (e.g., Witzke, personal communication).
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ACKNOWLEDGMENTS |
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TopAbstractIntroductionExperimental methodsXANES theoryResults and discussionAcknowledgmentsReferences cited |
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Footnotes |
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MANUSCRIPT RECEIVED February 24, 2009; MANUSCRIPT ACCEPTED May 5, 2009
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