The complex asymmetric stretch (v3) region infrared (IR) spectrum of synthetic sodium- and carbonate-bearing hydroxylapatites (CHAP) has been interpreted using overlapped Gaussian distributions for individual carbonate ion species. There is now good agreement for the distribution of carbonate ions between phosphate (type B) and c-axis channel (type A) positions using three independent methods: X-ray structure site occupancies, out-of-plane bend (v(2)) band areas, and asymmetric stretch (v(3)) band areas; B/A ratios for a well-crystallized CHAP sample being 0.77, 0.78, and 0.75, respectively. The reported dominance of type B carbonate ions in bone mineral and dental enamel is attributed to the anomalous shift of type A band frequencies into the spectral region of type B, resulting from the substitution of Ca2+ by Na+ in the nearest-neighbor cation shell of the channel carbonate ions. The infrared spectra show that the hydrogencarbonate (bicarbonate) ion in apatite crystals is a channel species, as are its room-temperature decomposition products, type A carbonate and labile (type L) carbonate. The research suggests that bone mineral crystals may actively communicate with body fluids through the apatite channel, pointing to a possible role for the apatite channel in mediating acid-base reactions in the body.

25 The complex asymmetric stretch (ν3) region infrared (IR) spectrum of synthetic sodium26 and carbonate-bearing hydroxylapatites (CHAP) has been interpreted using overlapped Gaussian 27 distributions for individual carbonate ion species. There is now good agreement for the 28 distribution of carbonate ions between phosphate (type B) and c-axis channel (type A) positions 29 using three independent methods: X-ray structure site occupancies, out-of-plane bend (ν2) band 30 areas, and asymmetric stretch (ν3) band areas; B/A ratios for a well-crystallized CHAP sample 31 being 0.77, 0.78 and 0.75, respectively. The reported dominance of type B carbonate ions in 32 bone mineral and dental enamel is attributed to the anomalous shift of type A band frequencies 33 into the spectral region of type B, resulting from the substitution of Ca by Na in the nearest34 neighbor cation shell of the channel carbonate ions. The infrared spectra show that the 35 hydrogencarbonate (bicarbonate) ion in apatite crystals is a channel species, as are its room36 temperature decomposition products, type A carbonate and labile (type L) carbonate. The 37 research suggests that bone mineral crystals may actively communicate with body fluids through 38 the apatite channel, pointing to a possible role for the apatite channel in mediating acid-base 39 reactions in the body. 40 41

We report results of electron-microprobe analyses for detrital grains of chromian spinel (n approximate to 800) and associated minerals recovered from modern fluvial and buried paleochannel (Pt-)Au placer deposits (n = 30) of mostly Holocene age distributed throughout British Columbia. The majority of analyzed spinel-group minerals are members of the chromite-magnesiochromite solid-solution series, and extend to Fe3+-rich compositions of chromian spinel. The values of mg# [100Mg/(Mg + Fe2+)] and cr# [100Cr/(Cr + Al)] in grains from all 30 placer samples occupy a relatively narrow range, 58-82 and 68-92, respectively, consistent with a mafic to ultramafic source. Four types of core-to-rim patterns of zoning have been identified: 1) a weak decrease in Cr coupled with a strong decrease in mg# at typically constant cr# and fe(3+)#, as a result of limited chromian spinel liquid fractionation and subsolidus Fe2+ Mg exchange with ferromagnesian silicates; 2) a sharp decrease in Cr and cr# and increase in fe(3+)# [100Fe(3+)/(Al + Cr + Fe3+)] with limited decrease in mg#, reflecting magmatic fractionation; 3) a significant increase in Cr and cr# with weak to strong decrease in mg#, reflecting late-stage interaction with a Cr-rich magma followed by variable enrichment in Fe2+ during cooling; and 4) a strong decrease in Cr and cr# with a small increase in mg# and fe(3+)#, likely due to interaction with a more oxidized melt and subsolidus re-equilibration with an aluminous phase (Al-rich residual liquid or amphibole). Values of mg# in placer silicate grains (olivine, clinopyroxene, either Na-Al-poor or Na-Al-Ti-rich, rare aluminous orthopyroxene, edenitic amphibole, among others) and in micro-inclusions in chromian spinel attain 90-95. We evaluated the provenance of chromian spinel grains in the placers using the global spinel database and a new one focusing on Alaskan-type intrusions. Chromian spinel in the Atlin area and a majority of Dease Lake placers exhibits a pronounced Cr-Al trend indicative of an ophiolitic affinity (Slide Mountain and Cache Creek oceanic terranes). In certain placers in the Tulameen area, the chromian spinel defines a Cr-Fe3+ trend indicating derivation from the Tulameen Alaskan-type intrusion in the Quesnellia island-arc terrane. Placer deposits in central British Columbia (Manson Creek - Cariboo region) display a mixed heritage involving both ophiolitic and Alaskan-type sources. The recognition of an Alaskan-type mafic to ultramafic source for some of the Cr-PGE-bearing placers in central British Columbia underscores the potential for bedrock mineralization in an area where such intrusions are presently poorly represented.

The location and orientation of the carbonate ion in the channel (A) and phosphate (B) positions of hydroxyapatite (CHAP) have been investigated by single- crystal X-ray structure and Fourier transform infrared (FTIR) spectroscopy, using crystals synthesized at high pressure. The type A carbonate ion is oriented in the apatite channel with two oxygen atoms close to the c-axis and the B carbonate ion is located near a sloping face of the substituted phosphate tetrahedron. Close comparison of FTIR and X-ray structure results shows that a Na-bearing CHAP containing approximately equal amounts of A and B carbonate ions is a realistic model for the overall crystal structure of biological apatite. However, the absence of distinct OH stretch and OH libration bands indicates that the hydroxyl content of biological apatite is disordered in respect to its orientation and precise location both in the channel and elsewhere in the structure.

The influence of Mn content on the stability of the high-pressure CaCO3 phases CaCO3-I, CaCO3II, and CaCO3-III at 300 K has been investigated up to 40 mol% MnCO3 using Raman spectroscopy recorded in situ with a diamond-anvil cell at pressures up to 14 GPa. Beyond about 5 mol% MnCO3, there is a progressive linear upward shift in the pressure of the CaCO3-I → CaCO3-II and CaCO3-II → CaCO3-III transitions, and expansion of the field of the CaCO3-II phase, with increase in MnCO3 content. The shifts in transition pressure are 0.19 GPa/mol% for I → II and 0.26 GPa/mol% for II → III over the 5 to 40 mol% MnCO3 composition interval, results fully consistent with elevation of transition pressure by the introduction of a smaller cation. However, minor and trace amounts of Mn appear to have a relatively insignificant influence on the pressure of these transitions.

X-ray structure and FTIR spectroscopy evidence is reviewed for two separate orientations of carbonate ions in the c-axis channel of carbonate apatite (CHAP) synthesized at high pressure and temperature: Al carbonate has two 0 atoms close to the c-axis, whereas A2 carbonate has only one. The A2 orientation is reevaluated and its local structure refined using a rigid body model. A2 is the high-pressure configuration, but the A1 --> A2 transformation is also dependent on bulk composition, especially the presence of type B carbonate. In the dry CaO-P2O5-CO2 system, the A1 --> A2 transformation is initiated beyond about 4 GPa in type A CHAP compared with 1-2 GPa in A-B CHAP. Also, A2 carbonate is only weakly present in Na-bearing A-B CHAP synthesized at 0.5-1 GPa, which is assumed to be close to the threshold pressure for the transformation. The pressure stability of A2 is believed to be related to its central location in the channel and equitable distribution of bond distances to Ca2 cations in the channel wall (2.25 to 2.54 angstrom).

Lead bromapatite [Pb10(PO4)6Br2] has been synthesized via solid-state reaction at pressures up to 1.0 GPa, and its structure determined by single-crystal X-ray diffraction at ambient temperature and pressure. The large bromide anion is accommodated in the c-axis channel by lateral displacements of structural elements, particularly of Pb2 cations and PO4 tetrahedra. The compressibility of bromapatite was also investigated up to about 20.7 GPa at ambient temperature, using a diamond-anvil cell and synchrotron X-ray radiation. The compressibility of lead bromapatite is significantly different from that of lead fluorapatite. The pressure–volume data of lead bromapatite (P < 10 GPa) fitted to the third-order Birch-Murnaghan equation yield an isothermal bulk modulus (K T ) of 49.8(16) GPa and first pressure derivative (\( K_{T}^{\prime } \)) of 10.1(10). If \( K_{T}^{\prime } \) is fixed at 4, the derived K T is 60.8(11) GPa. The relative difference of the bulk moduli of these two lead apatites is thus about 12%, which is about two times the relative difference of the bulk moduli (~5%) of the calcium apatites fluorapatite [Ca10(PO4)6F2], chlorapatite [Ca10(PO4)6Cl2] and hydroxylapatite [Ca10(PO4)6(OH)2]. Another interesting feature apparently related to the replacement of F by Br in lead apatite is the switch in the principle axes of the strain ellipsoid: the c-axis is less compressible than the a-axis in lead bromapatite but more compressible in lead fluorapatite.

The incorporation of the carbonate ion into the crystal structure of hydroxylapatite results in the creation of vacancies, oxygen-loss, and disorder, with consequent changes in physical and chemical properties. High-pressure experimental investigation up to 10 GPa of two synthetic carbonated hydroxylapatite samples with up to 11 wt% CO3, using a diamond-anvil cell and synchrotron powder X-ray diffraction, provides the first rigorous assessment of the mechanical behavior of the carbonated hydroxylapatite. The pressure-volume data suggest that the isothermal bulk modulus of these carbonated hydroxylapatites has been significantly decreased by the presence of the carbonate (up to about 15%), which in turn will affect all the carbonated apatite-related reactions in the geosystem. Since hydroxylapatite is one of the major components of the bones and teeth, the incorporation of the carbonate in the hydroxylapatite weakens teeth and bones not only chemically, but also physically.

Blue and mauve ultramarine artists' pigments and their heat-treated products have been investigated by sulfur K-edge X-ray absorption. X-ray absorption near-edge structure spectra are dominated by features of reduced sulfur and sulfate species. There is also a pre-peak at about 2468.0 eV which reflects the presence of the unpaired electron on the polysulfide radical anion (S3−). Pre-peak intensity is directly proportional to the depth of blue coloration, and provides a new, independent method for estimating the proportion of ultramarine cage sites occupied by the blue chromophore. The occupancy of the polysulfide radical anion S3− is estimated to be 33% in an intense ultramarine blue pigment, 22% in a dark blue ultramarine pigment, and 1% in deep royal blue lazurite from Afghanistan. The more efficient development of color in lazurite is attributed to extensive annealing of the mineral structure in the natural environment.