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Low-OH apatite of the compositional range [Ca.sub.4.99-5.06][(P[O.sub.4]).sub.2.98-3.00 [F.sub.0.51-0.48][Cl.sub.0.38-0.36]O[H.sub.0.14-0.12] was synthesized and characterized structurally by synchrotron-based single-crystal X-ray diffraction (XRD), and multiple nuclear magnetic resonance (NMR) spectroscopic techniques. The average structure is hexagonal with space group [P6.sub.3]/m. The presence of scattering in the single-crystal diffraction data set, which is incommensurate within the average hexagonal structure, suggests the presence of localized short-range monoclinic domains. Complex lineshapes in the [sup.31]P and [sup.19]F MAS NMR spectra are also consistent with the presence of an incommensurate phase. No evidence was detected for splitting of the Ca2 site into two distinct sites (as had been previously reported for hexagonal ternary apatites). Structure refinement and [sup.19]F {[sup.35]Cl} TRAPDOR NMR experiments verified intercolumnal neighboring of F and Cl atoms (inter-column distance of 2.62 [Angstrom]) within this low-OH- apatite suggesting that long-range neighboring ofF and Cl within the apatite anion channels is feasible. Keywords: Apatite, NMR spectroscopy, single-crystal XRD, Mars, Moon, PGE, synthesis

Phosphohedyphane, [Ca.sub.2][Pb.sub.3][(P[O.sub.4]).sub.3]Cl, space group P[6.sub.3]/m, a = 9.857(1), c = 7.130(2) [Angstrom], V = 599.9(2) [[Angstrom].sup.3], Z = 2, is a new mineral from the Capitana mine, Copiapo, Atacama Province, Chile and has been identified from numerous other deposits world-wide. At the Capitana mine, it occurs as transparent, colorless, tapering hexagonal prisms, as individuals up to about 0.5 mm in length and 0.1 mm in diameter, which are commonly doubly terminated. Crystals often occur in subparallel intergrowths and irregular clusters. Phosphohedyphane forms as a secondary mineral in the oxidized zone of the Capitana mine, a Cu-Pb-Ag deposit, where it closely associated with quartz, duftite, and bayldonite. Crystals exhibit core-to-rim chemical zonation and electron analyses of cores/rims yielded CaO 9.24/7.76, PbO 67.60/69.35, [P.sub.2][O.sub.5] 18.40/17.00, [As.sub.2][O.sub.5] 2.73/3.68, Cl 3.32/3.22, -O = Cl-0.75/-0.73, total 100.54/100.28 wt%. The name phosphohedyphane is for the relationship of the mineral to hedyphane. The mineral has an apatite structure with ordering of Ca and Pb in the two non-equivalent large cation sites, as in hedyphane. The structure refinement indicates that the Ca2(6h) site is completely occupied by Pb and the Cal(4f) site is occupied by 92% Ca and 8% Pb. The tetrahedral site refines to 91% P and 9% As. The refinement indicates the 0,0,0 position to be fully occupied by Cl. The ordering of Ca and Pb in phosphohedyphane has important implications with respect to the chlorapatite-pyromorphite solid solution series. An analysis of compositions of natural members of the pyromorphite-mimetite-turneaureite-chlorapatite system suggests the existence of complete solid solution among pyromorphite, mimetite, hedyphane, and phosphohedyphane. No stable solid solutions appear to exist between the joins phosphohedyphane-hedyphane and chlorapatite-turneaureite in natural systems. Keywords: Phosphohedyphane, new mineral, crystal structure, chemical analysis, solid solution, pyromorphite, apatite group

X-band electron paramagnetic resonance (EPR) spectra of gamma-irradiated crystals of carbonate-bearing fluorapatite from the Levant mine, Cornwall, England, revealed the presence of two previously characterized centers (i.e., an [O.sup.-] defect and an [O.sup.-][[].sub.F] defect, where [[].sub.F] represents a vacancy at the F site), a C[O.sup.-.sub.2] radical, and a new oxygen-associated hole-like center in the anion column. The [O.sup.-][[].sub.F] center in carbonate-bearing fluorapatite is stable at room temperature, whereas in carbonate-free fluorapatite the stability of this radical is shifted to lower temperatures ( Keywords: EPR spectroscopy, fluorapatite, C[O.sup.-.sub.2] radical, new [O.sup.-] center, carbonate ions

Rietveld refinements using 12 sets of X-ray diffraction powder data from milligram samples of human dental enamel provide detailed information about the structure and composition of enamel apatite. The principal difference in atomic parameters between enamel apatite and Holly Springs hydroxylapatite is in O2, which is reflected in a reduction in the P-O2 bond length of 0.085 [Angstrom] and P[O.sub.4] volume by 3.6%. Modeling the hexad axis scattering with a single OH- ion gives a 0.089 [Angstrom] shift of the ion further away from the mirror plane at z = 1/4. The known distributed electron density along the hexad axis in enamel has been confirmed by direct comparison with synthetic hydroxylapatite. Although the [Mathematical Expression Omitted] ion position could not be determined directly, evidence for partial replacement of [Mathematical Expression Omitted] by [Mathematical Expression Omitted] ions came from an 8% diminution of the P site occupancy compared with that in stoichiometric hydroxylapatite. The observed reduction in the P-O2 bond length and P[O.sub.4] volume in enamel is also consistent with this substitution. The loss of negative charge caused by [Mathematical Expression Omitted] replacing [Mathematical Expression Omitted] ions and loss of OH- ions is compensated by loss of [Ca.sup.2+] ions from Ca2 sites. The calculated density from the X-ray results is 3.021 g/[cm.sup.3], in agreement with deductions from previous chemical analyses.

Apatite crystals from two types of samples were analyzed by electron microprobe for 15 major and trace elements: (1) apatite in [H.sub.2]O- and S-saturated experimental charges of the 1982 El Chichon trachyandesite and (2) apatite in volcanic rocks erupted from 20 volcanoes. The S[O.sub.3] contents of the experimental apatite increase with increasing oxygen fugacity ([f.sub.[o.sub.2]]), from [less than[0.04 wt% in reduced charges buffered by fayalite-magnetite-quartz (FMQ), to 1.0-2.6 wt% in oxidized charges buffered by manganosite-hausmanite (MNH) or magnetite-hematite (MTH). The S[O.sub.3] contents of MNH- and MTH-buffered apatite also generally increase with increasing pressure from 2 to 4 kbar and decreasing temperature from 950 to 800 [degrees] C. The partition coefficient for S[O.sub.3] between apatite and oxidized melt increases with decreasing temperature but appears to be independent of pressure. Apatites in volcanic rocks show a wide range of S[O.sub.3] contents (

Single crystals of natural F-rich apatite and 800 keV [Kr.sup.2+] ion-beam-amorphized apatite were irradiated by an electron beam in a transmission electron microscope over a range of beam energies and beam currents. Irradiation of crystalline apatite using a high current density (16 A/[cm.sup.2]) caused the precipitation of cubic CaO from the crystalline apatite matrix. Using a lower beam current (1.6 A/[cm.sup.2]), the formation of nanometer-sized voids was observed, but CaO did not crystallize even after prolonged irradiation. Amorphous apatite crystallized to a coarse-grained polycrystalline assemblage of apatite crystallites at 85-200 keV. Increasing the beam current through the sample caused the formation of fine-grained cubic CaO, and the crystallization of apatite was not observed, even at high doses. In each case, many beam-induced bubbles formed and were typically larger at the edge of the beam. Thermal annealing at 450 [degrees]C resulted in epitaxial crystallization from the thick portions of the TEM foil and resulted in a single crystal with a high defect density. Electron-beam irradiations at 300 [degrees]C confirmed that the difference in microstructural evolution as a function of current density is driven by dose-rate effects. In fact, temperature and dose rate are competing effects in the precipitation of CaO.

Crystals of binary (LREE+HREE)-bearing fluorapatite [La,Gd-FAp, Ce,Dy-FAp, Pr, ErFAp, Eu,Lu-FAp; [Ca.sub.10-x-2y][Na.sub.y][REE.sub.x + y][([P.sub.1-x][Si.sub.x][O.sub.4]).sub.6][A.sub.2], with x = 0.12-0.20, y = 0.260.42; [P6.sub.3]/m] have been grown from [H.sub.2]0-rich phosphate-fluoride melts, and their structures refined at room temperature with single-crystal X-ray intensities to R = 0.017-0.022. These binary-REE-substituted fluorapatite samples have REE site-occupancy ratios (REECa2/REE-Cal) of 2.32, and 2.32, 2.03, 1.71, respectively, which are 0.47-0.16 smaller than corresponding ratios calculated using data for reference single-REE-substituted fluorapatite. Discrepancies in intracrystalline partitioning between multiple-REE-substituted apatites and single-REE-substituted fluorapatite decrease with a decrease in REE concentration, becoming negligible at 0.2-0.3 total REE cations pfu in synthetic binary-REE-substituted fluorapatite and at trace abundances of REE in natural apatites. However, quantitative transference of laboratory REE site preferences to natural apatites is frustrated by the compositional complexity in nature. In the synthetic fluorapatite, there is a profound change in the spatial accommodation of REE in the apatite structure at about the position of Nd in the 4f transition-metal series, corresponding to the peak in the experimental uptake curve. Discrepancies in intracrystalline partitioning between binary-REE- and single-REE-substituted fluorapatite are attributed to non-ideal mixing of LREE and HREE that results in contraction of the Cal coordination sphere.

Natural apatites used in fertilizer industries often contain trace amounts of Cd, which may reach concentrations of several tens to a few hundred parts per million. Cd is not eliminated during the production of phosphate fertilizers, and its. concentration in the final product can exceed environmental norms. Knowledge of the chemical state of Cd in apatite ores is a prerequisite for the design of technical processes of extraction. In the present study, Cd K-edge EXAFS spectroscopy was used to investigate the structural environment of Cd present in sedimentary apatite ores from West Africa. These apatites are fluorinated and contain goethite, quartz, and crandallite as ancillary phases detected by X-ray diffraction or EXAFS spectroscopy. Cd K-edge EXAFS spectra for two natural samples were analyzed and compared with those for Cd-containing reference minerals, including hydroxylapatite, goethite, otavite, and crandallite. A good spectral resemblance was observed between natural products and synthetic apatite containing small amounts of Cd. This spectral likeness indicates that the majority of Cd atoms are diluted in the apatitic framework and do not form [Cd.sub.10][(P[O.sub.4]).sub.6][(OH, F).sub.2] clusters. This finding was confirmed by quantitative analysis of the EXAFS spectra, which indicated that Cd atoms are surrounded by nearest O atoms at 2.33 [angstrom], next-nearest P atoms at [approximately]3.53 [angstrom], and a third-nearest shell of Ca atoms at [approximately]4.02 [angstrom]. A comparison of these data with those obtained for synthetic apatites allowed us to assess that Cd occupies both Ca crystallographic sites with a slight preference for the Ca2 site.

Synchrotron x-ray fluorescence microanalysis reveals the heterogeneous distribution of trace elements in apatite crystals that show intrasectoral and sectoral zoning. The sharp compositional differences agree with face symmetry. During growth, the surface structure exhibits a significant influence on the differential partitioning of trace elements into apatite and on the ordering of atoms generating dissymmetrization in apatite. Llallagua apatite samples exhibit optical anomalies that correspond directly to the different sectors and subsectors.

Quantum mechanical calculations and 13C NMR spectroscopic and Fourier transform infrared spectroscopic studies of the microenvironment of CO(sub 3)(super 2-) ion present in synthetic and natural sedimentary carbonate-fluorapatite reveal that the central C atom in the ion is surrounded by an sp2-hybridized planar arrangement of three O atoms. Addition of F- to the ion leads to modulation in NMR isotopic shifts, revealing the changed environment around the central C atom and the absence of direct bonding between CO(sub 3)(super 2-) and F-.

Geochemical study of hydrothermal apatite and scheelite pairs from different kinds of W ore deposits to explore rare earth elements (REE) partitioning between these minerals revealed the development of only five of the 18 examined pairs under equilibrium conditions. Three pairs possessed extremely different REE trends, suggesting crystallization from distinct fluids.

Carlson's proposed model for the kinetics of apatite fission-track annealing is criticized as it is an empirical model fitted to laboratory data which fails to account for observed temperature and time dependence of annealing under natural physical and experimental conditions. The fanning model of Laslett et al, on the other hand, fits experimental conditions and is able to extrapolate with natural geological observations. The model of Laslett et al is the most appropriate approach to all facets of apatite fission-track annealing.

The criticism of Green et al on the proposed model for apatite fission-track annealing in answered. The main objective of the criticized model is to contribute to the mineralogical understanding of the atomic-scale phenomena of annealing. Thus, the approach is based from the postulates underlying the physical model and not from experimental data. It is conceded that the model lacks refinement such that it would replicate experimental data in a more detailed manner.

The kinetic model of apatite-fission track annealing criticized by Kevin D. Crowley is based on sound and valid postulates and can be used to predict experimental measurements accurately. In addition, contrary to Crowley's proposal, the theories of Fletcher and Brown cannot be applied to fission-track annealing since they are concerned with isolated, random defects arising from electron bombardment. Despite being rudimentary, the zero-order rate law can adequately account for experimental constraints accurately. The goodness of fit statistic alone does not determine the validity or value of an empirical model.

Eu [L.sub.3] X-ray absorption near edge structure (XANES) spectroscopy was conducted with a wavelength dispersive (WDS) spectrometer to determine the valence state of europium in a natural, Mn-and REE-rich apatite from Llallagua, Bolivia. Europium exists in both the divalent and trivalent states in the Llallagua apatite with a [Eu.sup.2+]/[Eu.sup.3+] ratio between 0.12 and 0.22. With the enhanced energy resolution of WDS the EuL[[Alpha].sub.1] fluorescence line can be resolved from the Mn[K[Alpha] line, allowing for significant reduction of background in the Eu[L.sub.3] absorption edge region and resolution of the Eu XAS, which is difficult by conventional methods because of fluorescence peak interference. The anomalous partitioning behavior of Eu in these samples (Rakovan and Reeder 1996) can be explained by the observed presence of [Eu.sup.2+] and [Eu.sup.3+] and is consistent with the suggested size effect on intrasectoral zoning of REEs in apatite.

In annealing experiments on natural crystals of apatite and titanite, two types of fission track were observed that had not been identified previously: (1) 'stable' tracks, and (2) tracks through fluid inclusions. Stable tracks do not shorten significantly even when heated to temperatures well above those normally sufficient for complete annealing of fission tracks. Tracks through fluid inclusions owe their excessive length to the fact that one fission fragment traveled through a fluid inclusion without appreciable energy loss. If they go undetected, both types of track can distort the temperature-time paths constructed on the basis of confined fission-track-length measurements. Simple measures are proposed to avoid these pitfalls.

A new model for examining fission-track data from natural specimens has been developed on the basis of new laboratory data describing fission-track annealing in a wide variety of apatites and the empirical correction for fission-track length anisotropy presented in earlier papers. Using revised and simplified statistical methods, we examine how well various empirical equations are able to fit the laboratory data and reproduce expected behavior on geological time scales. Based on the latter criterion, we find that so-called 'fanning Arrhenius' models of mean track length are not the best-suited for our data. Instead, we find that fitting c-axis projected lengths with a model that incorporates some curvature on an Arrhenius plot produces results that are in better agreement with the available geological benchmarks. In examining the relative annealing behavior of apatites with different resistance to annealing, we find that the laboratory-time-scale behavior of any two apatites can be reproduced well by a simple one- or two-parameter equation. This function is used to convert the reduced fission-track length of one apatite that has undergone a certain time-temperature history into the length that would be measured in a second, less-resistant apatite that has undergone the same history. Using this conversion, we create a single model that encompasses the annealing behavior of all of the apatites we studied. The predictions made by this model match closely those made by fits to data for individual apatites. We therefore infer that, although the conversion equation is imperfect, it presents an excellent practical solution to characterizing the range of kinetic variability for annealing of fission tracks in apatite.

A study conducted by Regnier et al. confirms the absence of a hypothetical ion in carbonate-fluorapatite during substitution reactions. However, there are several issues that need to be clarified. The synthetic samples used for the study are inappropriate since they do not possess the qualities of most natural carbonate-fluorapatites. The study also contradicts the existence of the correlation between nonstoichiometric fluorine and structured carbonate and carbon dioxide in carbon-fluorapatite.

Annealing rates for fission tracks in apatite vary markedly as a complex function of composition, based on an experimental study of 15 well-characterized, compositionally diverse apatites. Extensive annealing data were obtained in 69 experiments (durations of 1, 10, 100, and 1000 h at temperatures from 75 to 400 [degrees] C) on each of four apatites, three with near end-member occupancy of the halogen site by F, Cl, and OH, plus the well-known apatite from Durango, Mexico. These results were supplemented by less-comprehensive annealing data from 12 experiments over the same range of time and temperature on each of the remaining 11 apatites. Measurements of initial fission-track length, a parameter of considerable importance to the derivation of time-temperature paths from fission-track data, reveal substantial variations from one apatite to another; initial lengths are best predicted from etch figures. Interlaboratory comparisons of data on annealing kinetics highlight discrepancies that appear to result largely from differences in the precision and accuracy of experimental temperatures. None of the factors previously proposed as the dominant compositional controls on annealing rates can account completely for annealing behavior over the full range of compositions studied. Nevertheless, relative rates of annealing among all apatites are highly systematic, which allows this data set to be used in its entirety to constrain multikinetic annealing models that predict fission-track lengths as a function of time and temperature.

A method is presented that permits the length of any horizontal, confined fission-track inclined at a specified angle to the crystallographic c axis in apatite to be converted to an equivalent track length parallel to the crystallographic c axis. The model is based on the results of annealing experiments for six selected apatites (five calcian fluorapatites and Durango apatite) representing a subset of the 15 total apatite specimens studied. An iterative process of calculation is required to project fission-track lengths onto the c axis and computer source code implementing the solution to this problem is presented. This method of projecting apatite fission-track lengths onto the crystallographic c axis is shown to remove effectively fission-track length variation within single fission-track populations due to anisotropic track-length reduction for all 15 apatites studied. In addition, a model is developed that offers predictions that closely reproduce published experimental data concerning the relationship between fission-track density (etched fission tracks per unit area of apatite surface) and the arithmetic mean fission-track length. Finally, it is shown that natural fission-track populations exhibit fission-track length anisotropy similar to that of fission-track populations created and annealed in the laboratory. This observation implies that the same process by which apatite fission tracks anneal in the laboratory is responsible for annealing of apatite fission tracks in the geological environment.

An experiment has been conducted to study the mechanisms of carbonate substitution in carbonate-fluorapatite. The synthetic samples used for the study possess all the characteristics of natural samples and contain large amounts of monovalent anions which allow for the charge balance despite the low carbonate substitution. The study also supports the existence of a correlation between nonstoichiometric fluorine and structural carbonate and carbon dioxide in carbonate-fluorapatite. However, the formation of a structural ion during substitution is questioned.