Apatite Crystals Research Articles

Geochemistry and Geophysics of Iron Oxide-Apatite Deposits and Associated Waste Piles with Implications for Potential Rare Earth Element Resources from Ore and Historical Mine Waste in the Eastern Adirondack Highlands, New York, USA

Abstract The iron oxide-apatite (IOA) deposits of the eastern Adirondack Highlands, New York, are historical high-grade magnetite mines that contain variable concentrations of rare earth element (REE)-bearing apatite crystals. The majority of the deposits are hosted within sodically altered Lyon Mountain granite gneiss, although some deposits occur within paragneiss, gabbro, anorthosite, or potassically altered Lyon Mountain granite gneiss. The IOA deposits and the waste and/or tailings piles associated with them have potential as an unconventional resource for REEs. Reprocessing of these piles would have the advantage of partial recycling of the waste material to produce a set of critical elements. Thirty-four ore, nine rock, 25 waste-pile, and four tailings-pile samples were collected and analyzed for major, minor, and trace elements. At the tailings- and waste-pile sites, composite samples were collected by combining 30 to >50 subsamples randomly distributed over each pile. The total REE content of the waste and tailings piles varied from approximately 10 to 22,000 ppm, whereas the ore sample concentrations ranged from approximately 15 to 48,000 ppm total REEs. A positive correlation exists between the total REE content of ore and its associated waste pile. Median light REE/heavy REE values were 2.14 for waste/tailings piles and 2.25 for ore, which is a substantial relative enrichment in the heavy REEs in comparison to many developed REE mines, such as the mined carbonatites of Bayan Obo, China, and Mountain Pass, California. Importantly, the ore and waste samples are significantly enriched in both Y and Nd compared to other REEs in the samples. Other minor components such as Th are also elevated. Airborne radiometric surveys show large positive eTh and eU anomalies corresponding to tailings piles. Although it is a limited data set, geochemical data of unaltered and altered host rocks suggest a speculative new model for IOA ore formation in the Adirondack Highlands that is consistent with the geology and previously published data. The ferroan ore-hosting Lyon Mountain granite gneiss underwent localized potassic alteration that enriched the altered rock in Fe, REEs, Th, and other metals. A later sodic alteration event affected the previously potassically altered Lyon Mountain granite gneiss, which increased rock porosity and remobilized Fe, REEs, and other elements from the host rock into the iron ore seams. The sodic fluids responsible for ore formation were enriched in F and Cl.

Fluoride: Double-Edged Sword Implicated in Caries Prevention and in Fluorosis

How fluoride acts to prevent caries as well as cause demineralization is not well understood. Tunnels located inside apatite crystals are acting as ions exchangers favoring diffusion pathways, and substitution of ions such as fluoride inside the crystal lattice. Cariostatique effects prevent the carious decay, increasing the mineral solubility and remineralization. Given systemically or topically, fluorides added to water or used as salt, varnish, gel or paste, decrease the number and severity of carious lesions on deciduous and/or permanent erupted teeth. It comes out from fluoride supplementation that dental fluorosis may be induced.

Bioprinted 3D calcium phosphate scaffolds with gentamicin releasing capability

We investigated the suitability of 3D printed calcium phosphate scaffolds as drug carriers. The 3D powder printing process utilized α-tricalcium phosphate (α-TCP) as a solid phase and deionized water with 2.5% disodium hydrogen phosphate as a setting accelerator. The antibiotic gentamicin sulfate was incorporated by mixing it into α-TCP powder before printing. Two different concentrations of gentamicin (3 wt%, 7 wt%) were used to study the correlation between drug release kinetics and gentamicin content in the scaffolds. The scaffolds were hardened at 100% humidity. The synthesized scaffolds were characterized in terms of morphology, composition, mechanical strength, in vitro bioactivity and drug release kinetics. X-ray diffraction (XRD) analysis revealed that the α-TCP converted into calcium deficient hydroxyapatite (CDHA) during the printing process. Scanning electron microscopy (SEM) showed the typical needle-like structure of CDHA. Gentamicin release was investigated for a period of two weeks with an initial burst release. The produced scaffolds formed calcium enriched apatite crystals on their surface after three days of incubation in simulated body fluid.

Apatite Chemical Compositions from Acadian-Related Granitoids of New Brunswick, Canada: Implications for Petrogenesis and Metallogenesis

The geochemistry of apatite crystals from fifteen fertile and infertile Acadian-related granitoids of New Brunswick (Canada) was studied in situ, using electron microprobe and laser ablation-inductively coupled plasma-mass spectrometry to further investigate petrogenesis and fertility index among these intrusions. The results indicate a clear geochemical contrast between barren and mineralized samples where apatite grains from barren intrusions are the most hydrous (OH > 0.3 wt. %), with lowest Mn (<1700 ppm), Fe (<800 ppm), and Sn (<0.01 ppm). In contrast, apatite grains from Cu-Mo related intrusions are distinguished by higher Cl (>0.1 wt. %), (La/Yb)N ratios of 21.17, (Eu/Eu*)N ratios of 0.30, and LREE/HREE ratios of 6.03. Apatites from Sn-W related magmatic suites have the highest F (>3 wt. %), Mn (>5350 ppm), Fe (>2200 ppm), Y (>4900 ppm), Sn (>2 ppm), and the lowest Cl (<0.01 wt. %), Sr (<60 ppm), U (<18 ppm), Th (<29 ppm), (Eu/Eu*)N ratios (<0.01), and (La/Yb)N ratios (<0.88). Lastly, apatite grains from Mo-bearing systems have the lowest SiO2 (<0.4 wt. %), Sr (<33 ppm), Th (<28 ppm), a moderate Mn (~3800 ppm), Y (~3500 ppm), and highest FeOt (<0.9 wt. %). However, the results indicated apatite Mn, Sr, LREE/HREE, and (Eu/Eu*)N ratios as the best fertility indices used for discriminating barren from fertile granite intrusions.

U/Pb and (U-Th-Sm)/He “double” dating of detrital apatite by laser ablation: A critical evaluation

Paired U/Pb and (U-Th-Sm)/He dating of individual detrital apatite crystals has potential as a valuable tool for constraining the high- and low-temperature thermal evolution of sediment source terrains. Here we present the results of exploratory applications of the laser ablation double-dating (LADD) method – originally developed for detrital zircon research – to the widely available Durango fluorapatite standard in order to evaluate the practicality of the method. Sixty-two laser-ablation analyses of a single large crystal of Durango fluorapatite yielded an inverse-variance weighted mean 206Pb/238U date of 31.46 ± 0.48 Ma and an inverse-variance weighted mean (U-Th-Sm)/He date of 31.75 ± 0.60 Ma, both of which are in good agreement with previously published conventional dates. While these results are encouraging, several factors suggest that LADD may be less useful for detrital apatites than for detrital zircons given widely available analytical instrumentation. These principally reflect the propensity for apatites to have comparatively lower U + Th concentrations, and thus lower radiogenic He and Pb concentrations, as well as high concentrations of common Pb. These factors contribute to substantially higher analytical imprecision for most LADD U/Pb dates for apatite, occasionally too high for the dates to be geologically useful. Reasonably precise laser ablation (U-Th-Sm)/He dating of detrital apatites requires relatively large crystal sizes (≥ 100 μm in the shortest dimension), with the minimum useful size increasing with decreasing (U-Th-Sm)/He apparent age. In contrast to the geological interpretation of LADD datasets for detrital zircons, the interpretation of datasets for detrital apatites is less straightforward. In particular, researchers should consider carefully the possibility that (U-Th-Sm)/He apparent age distributions are biased by the need to analyze only larger apatites.

Integrating the PILP-mineralization process into a restorative dental treatment

The addition of charged polymers, like poly-aspartic acid (pAsp), to mineralizing solutions allows for transport of calcium and phosphate ions into the lumen of collagen fibrils and subsequent crystallization of oriented apatite crystals by the so-called Polymer-Induced Liquid Precursor (PILP) mineralization process, leading to the functional recovery of artificial dentin lesions by intrafibrillar mineralization of collagen. ObjectiveTo evaluate the feasibility of applying the PILP method as part of a restorative treatment and test for effectiveness to functionally remineralize artificial lesions in dentin. Materials and MethodsTwo methods of providing pAsp to standardized artificial lesions during a restorative procedure were applied: (A) pAsp was mixed into commercial RMGI (resin modified glass ionomer) cement formulations and (B) pAsp was added at high concentration (25mg/ml) in solution to rehydrate lesions before restoring with a RMGI cement. All specimens were immersed in simulated body fluid for two weeks to allow for remineralization and then analyzed for dehydration shrinkage, integrity of cement-dentin interface, degree of mineralization, and changes in the nanomechanical profile (E-modulus) across the lesion. ResultsAfter the remineralization treatment, lesion shrinkage was significantly reduced for all treatment groups compared to demineralized samples. Pores developed in RMGI when pAsp was added. A thin layer at the dentin-cement interface, rich in polymer formed possibly from a reaction between pAsp and the RMGI. When analyzed by SEM under vacuum, most lesions delaminated from the cement interface. EDS-analysis showed some but not full recovery of calcium and phosphorous levels for treatment groups that involved pAsp. Nanoindentations placed across the interface indicated improvement for RMGI containing 40% pAsp, and were significantly elevated when lesions were rehydrated with pAsp before being restored with RMGI. In particular the most demineralized outer zone recovered substantially in the elastic modulus, suggesting that functional remineralization has been initiated by pAsp delivery upon rehydration of air-dried demineralized dentin. In contrast, the effectiveness of the RMGI on functional remineralization of dentin was minimal when pAsp was absent. SignificanceIncorporation of pAsp into restorative treatments using RMGIs promises to be a feasible way to induce the PILP-mineralization process in a clinical setting and to repair the structure and properties of dentin damaged by the caries process.

Calcium release and physical properties of modified carbonate apatite cement as pulp capping agent in dental application

BackgroundCarbonate apatite (CO3Ap) and silica-calcium phosphate composite (SCPC) are bone substitutes with good prospect for dental application. SCPC creates a hydroxyapatite surface layer and stimulate bone cell function while, CO3Ap induce apatite crystal formation with good adaptation providing good seal between cement and the bone. Together, these materials will add favorable properties as a pulp capping material to stimulate mineral barrier and maintain pulp vitality. The aim of this study is to investigate modification of CO3Ap cement combined with SCPC, later term as CO3Ap-SCPC cement (CAS) in means of its chemical (Calcium release) and physical properties (setting time, DTS and pH value).MethodsThe study consist of three groups; group 1 (100% calcium hydroxide, group 2 CO3Ap (60% DCPA: 40% vaterite, and group 3 CAS (60% DCPA: 20% vaterite: 20% SCPC. Distilled water was employed as a solution for group 1, and 0.2 mol/L Na3PO4 used for group 2 and group 3.Samples were evaluated with respect to important properties for pulp capping application such as pH, setting time, mechanical strength and calcium release evaluation.ResultsThe fastest setting time was in CO3Ap cement group without SCPC, while the addition of 20% SCPC slightly increase the pH value but did not improved the cement mechanical strength, however, the mechanical strength of both CO3Ap groups were significantly higher than calcium hydroxide. All three groups released calcium ions and had alkaline pH. Highest pH level, as well as calcium released level, was in the control group.ConclusionThe CAS cement had good mechanical and acceptable chemical properties for pulp capping application compared to calcium hydroxide as a gold standard. However, improvements and in vivo studies are to be carried out with the further development of this material.

Expression and function of Slc34 sodium-phosphate co-transporters in skeleton and teeth.

Under normal physiological condition, the biomineralization process is limited to skeletal tissues and teeth and occurs throughout the individual's life. Biomineralization is an actively regulated process involving the progressive mineralization of the extracellular matrix secreted by osteoblasts in bone or odontoblasts and ameloblasts in tooth. Although the detailed molecular mechanisms underlying the formation of calcium-phosphate apatite crystals are still debated, it is suggested that calcium and phosphate may need to be transported across the membrane of the mineralizing cell, suggesting a pivotal role of phosphate transporters in bone and tooth mineralization. In this context, this short review describes the current knowledge on the role of Slc34 Na+-phosphate transporters in skeletal and tooth mineralization.

In situ preparation and osteogenic properties of bionanocomposite scaffolds based on aliphatic polyurethane and bioactive glass nanoparticles

Bionanocomposite scaffolds based on aliphatic polyurethane (PU) and bioactive glass nanoparticles were produced by using a one-step in situ polymerization method. Bioactive glass nanoparticles (nBG) or mesoporous BG nanospheres (nMBG) were incorporated during the polymerization reaction to produce simultaneous formation and foaming of porous nanocomposite scaffolds. The in vitro bioactivity of the scaffolds was assessed in simulated body fluid (SBF), and through cytocompatibility and osteogenic differentiation assays with stem cells. Bone regeneration properties of the scaffold materials were in vivo assessed by using a critical-sized femoral defect model in rat. The scaffold nanocomposites showed excellent cytocompatibility and ability to accelerate the crystallization of bone-like apatite in vitro. nBG/PU bionanocomposite scaffold exhibited the higher capacity to stimulate osteogenic cell differentiation as judged by an increased ALP activity and the presence of mineralized nodules associated with the stem cells. nBG (5%)/PU scaffold significantly also produces in vivo a denser and more significant amount of new bone after 8 weeks of implantation, which is attributed to the more rapid dissolution rate of nBG into osteogenic ionic products compared to nMBG. The results of this work show that the in situ polymerization method combined with the use of nanodimensional BG particles enable the production of PU – based scaffolds with enhanced bioactive properties to stimulate the bone tissue regeneration.

Chemical, microstructural and chronological record of phosphates in the Ksar Ghilane 002 enriched shergottite

The enriched basaltic (martian) shergottite Ksar Ghilane (KG) 002, discovered in 2010, is exceptionally rich in coexisting but discrete apatite and merrillite crystals. It has been selected to better constrain the formation conditions and post-crystallization processes, and thus the evolution of martian rocks based on Ca-phosphates. A petrological, chemical, chronological and microstructural approach using a series of high-spatial resolution techniques including Raman spectroscopy, electron microscopy (SEM, EPMA, CL-imaging) and secondary ion mass spectrometry (SIMS) analysis has been applied to a representative number of Ca-phosphate grains. Analytical results for apatite and merrillite reveal: (i) zoning in F, Cl, Br and I concentrations, (ii) elevated Cl concentrations in the range of ∼11,900–35,300 µg/g and halogen ratios, i.e., Cl/Br and Cl/I, as well as stable chlorine isotope composition, reported as δ37Cl values rel. to Standard Mean Ocean Chloride (SMOC, defined as 0‰) with a value of +0.67 ± 0.14‰ (1σ), distinguishing KG 002 phosphates from that of other enriched and depleted shergottites. The halogen and heavier δ37Cl record indicate a slightly higher degree of ∼3.5% assimilation of Cl-rich and isotopically heavier crustal reservoir on Mars when compared to other enriched shergottites. (iii) Structural investigations together with the chemical and petrological context of the grains confirm the occurrence of hydroxyl-poor merrillite, indicate weak if any alteration effects induced by metamictization, only minor structural modifications due to shock metamorphism, and absence of replacement reactions. Therefore, igneous crystallization of Ca-phosphates from a fractionated, hydrous and ferrous mantle source, rich in volatiles including the halogens and Na and lithophile rare earth-elements, and absence of interaction with crustal fluids/brines of the sample is deduced. (iv) The Pb isotopic composition of six apatite and three merrillite grains is highly unradiogenic and the 238U-206Pb record yields a phosphate crystallization time at 395 ± 240 Ma (2σ), which is similar to those of other enriched shergottites.

Chemical Composition and Petrogenetic Implications of Apatite in the Khibiny Apatite-Nepheline Deposits (Kola Peninsula)

Khibiny, one of the largest of the world’s peralkaline intrusions, hosts gigantic apatite deposits. Apatite is represented by F-apatite and it contains exceptionally high concentration of SrO. (4.5 wt % on average) and increased amounts of rare earth elements (REEs; up to 8891 ppm). Such enrichment of apatite ores in REEs defined Khibiny deposit as world-class deposit with resources reaching several millions tons REE2O3. Apatite from the Khibina alkaline complex is characterized by the significant enrichment in light REEs relative to the heavy REEs (with average Ce/Yb ratio of 682) and the absence of a negative Eu anomaly. The obtained geochemical signature of apatite suggests a residual character of the Khibiny alkaline magma and it indicates that the differentiation of the primary olivine-melanephelinitic magma developed without fractionation of plagioclase which is the main mineral-concentrator of Sr and Eu in basaltic magmatic systems. The compositional evolution of the Khibiny apatite in the vertical section of the intrusion reflects primary fractionation processes in the alkaline magma that differentiated in situ. The main mechanism for the formation of the apatite-nepheline deposits was the gravitational settling of large nepheline crystals in the lower part of the magma chamber, while very small apatite crystals were suspended in a convective magma, and, together with the melt, were concentrated in its upper part of the magmatic chamber.

Phosphogenesis in the immediate aftermath of the Great Oxidation Event: Evidence from the Turee Creek Group, Western Australia

Phosphatic peloids and pebble-sized microbially-bound sediment clasts have been identified within dolomitic sediment that separates centimetric columnar stromatolites in a 20–150 cm thick horizon from the 2.4–2.2 Ga Turee Creek Group in Western Australia.Petrographic examination of the phosphatic clasts reveals complex internal textures that are defined by different mixtures of fine-grained apatite, medium-grained dolomite, irregular domains of partly euhedral micro-quartz, and carbonaceous matter.Phosphatic sand-sized peloids observed within microbially-bound sediment clasts are similar in size, texture, and composition to the individual peloids in the intercolumn sediment. Both occurrences have finely dispersed kerogen throughout and zoned cores that are packed with both microscopic apatite granules and larger, euhedral quartz crystals. The percentage of apatite vs quartz vs kerogen varies between studied peloids.Similar-sized, white peloids in the intercolumn sediment and in the microbially-bound sediment clasts contain illite and are lined internally by euhedral apatite crystals, 20–100 µm in size. Some peloids are kerogenous in one domain and illite-bearing in another.The phosphatic microbially-bound sediment clasts and peloids are interpreted to represent fragments of a peritidal phosphorite, redeposited in an offshore setting during high energy events. Peloids formed within the microbially-bound sediment and were subsequently eroded out and redeposited in the intercolumn sediment. White peloids are interpreted to represent peloids affected by surficial weathering during temporary exposure, when the fine-grained kerogenous interior was replaced by clays. These weathered peloids were then re-sedimented and captured both within microbially-bound sediment and in the intercolumn sediment.Euhedral apatite crystals that line the insides of illite-bearing white peloids were dated to 2104 ± 70 Ma and 2041 ± 33 Ma, using U-Pb and Th-Pb dating methods, respectively. This age is interpreted to represent the time of metamorphic fluid circulation during a period of known regional metamorphism, and provides a minimum age of the reef complex.

Transformation of amorphous calcium phosphate to bone-like apatite

Mineralisation of calcium phosphates in bone has been proposed to proceed via an initial amorphous precursor phase which transforms into nanocrystalline, carbonated hydroxyapatite. While calcium phosphates have been under intense investigation, the exact steps during the crystallisation of spherical amorphous particles to platelet-like bone apatite are unclear. Herein, we demonstrate a detailed transformation mechanism of amorphous calcium phosphate spherical particles to apatite platelet-like crystals, within the confined nanodomains of a bone-inspired nanocomposite. The transformation is initiated under the presence of humidity, where nanocrystalline areas are formed and crystallisation advances via migration of nanometre sized clusters by forming steps at the growth front. We propose that such transformation is a possible crystallisation mechanism and is characteristic of calcium phosphates from a thermodynamic perspective and might be unrelated to the environment. Our observations provide insight into a crucial but unclear stage in bone mineralisation, the origins of the nanostructured, platelet-like bone apatite crystals.

Compressive Strength Evaluation and Phase Analysis of Pulp Capping Materials Based on Carbonate Apatite-SCPC Using Different Concentration of SCPC and Calcium Hydroxide

The mechanical strength of pulp capping material based on carbonate apatite and silica calcium-phosphate composite (CO3Ap-SCPC) is one of the key factors for the success of the material in protecting the vitality of the pulp during the formation of apatite and dentin reparative. Modifying the material in the powder phase was known to increase the mechanical strength. The purpose of this study was to determine whether the addition of SCPC and calcium hydroxide in pulp capping materials based on CO3Ap-SCPC would affect the compressive strength of this pulp capping material. In this study, three cement groups were used, each group consisted of dicalcium phosphate anhydrous and vaterite which added by SCPC concentration 0%, 5% and 10% and calcium hydroxide concentration 0%, 5% and 10%, respectively. All groups were tested by a compressive strength test and X-Ray diffraction (XRD) for phase analysis. The mean value of compressive strength with addition of 0% SCPC and 10% Ca(OH)2 was 16.54 ± 1.35 MPa, addition of 5% SCPC and 5% Ca(OH)2 of 18.55 ± 2.81 MPa, addition of 10% SCPC and 0% Ca(OH)2 was 9.22 ± 1.21 MPa. The addition of SCPC and Ca(OH)2 show statistically significant difference in compressive strength (p<0.05). The XRD analysis of the highest compressive strength revealed that the apatite crystal was successfully formed. It can be concluded that incorporated specific amount of SCPC and Ca(OH)2 could improve the mechanical strength and the apatite formation of the CO3Ap-SCPC pulp capping material.

Hydrothermal-to-metasomatic overprint of the neovolcanic rocks evidenced by composite apatite crystals: a case study from the Maglovec Hill, Slanské vrchy Mountains, Slovakia

Abstract The apatite assemblage from Maglovec hill (Slanské vrchy Mountains near the city of Prešov) from fissures of hydrothermally altered neovolcanic rocks (andesites and related lithologies) was studied. The assemblage consists of two different morphological apatite types (apatite in cores of prismatic crystals and fibrous apatite mantling these cores). The assemblage was investigated by a multi-analytical approach to reveal its unique chemical composition and structure. Both types of apatite display zoning visible in back-scattered electron (BSE) images. Core apatite is relatively homogenous with porous rims appearing darker in the BSE images at the contact with fibrous apatite, and occasionally with darker regions along fractures. These parts are depleted in trace elements, mostly in LREE. Fibrous apatites display concentric and/or patchy zoning. Dark regions in fibrous apatite occasionally display a porous structure. In part of fibrous crystals, substitution of (CO3)2− for phosphorus is confirmed by Raman spectroscopy by the presence of a band at ~ 1071 cm−1. This method also confirmed the presence of OH in different populations in the structure of all apatite types. The three most important observed peaks are caused by vibrations of hydroxyls influenced by different adjacent anions: hydroxyl (band at ~ 3575 cm−1); fluorine (band at ~ 3535–3540 cm−1); chlorine (band at ~ 3494 cm−1). In REE-depleted parts of both apatite types, fine inclusions of monazite and rarely Th-rich silicate are observed. The acquired data suggest a hydrothermal origin of this assemblage and indicate a formation sequence of distinct apatite types. Moreover, minerals from the epidote group were identified, which have not been described from this locality before as well as vanadium-rich magnetites that form exsolution lamellae in ilmenite grains.

Substantial effect of magnesium incorporation on hydroxyapatite/carbon nanotubes coatings on metallic implant surfaces for better anticorrosive protection and antibacterial ability

This work elucidates the antibacterial and corrosion protection efficiency of Mg substituted hydroxyapatite/multiwalled carbon nanotube composite coated 316L SS implant. A novel spray deposition method was adopted for the composite coating. The fabricated HA/MWCNT and Mg-HA/MWCNT films were characterized by X-ray diffraction, Fourier transform infrared spectroscopy and Field emission scanning electron microscopy. Mg addition played a significant role to decrease the crystallinity and increase the surface area of the HA/MWCNT composite. The vibrational spectra show shift in the PO stretching vibration of PO43− group at 1127 cm-1 after Mg substitution. The morphology of Mg-HA/MWCNT coating revealed spherical shaped apatite crystals with interconnected network like microstructure which is beneficial for bone tissue engineering. The Mg-HA/MWCNT nanocomposite revealed a better antibacterial activity against E.coli with a high zone of inhibition of 16 mm. The electrochemical behaviour of the Mg-HA/MWCNT/SS in simulated body fluid demonstrated an increased polarization resistance with decreased current density (icorr). Compared to bare HA/CNT coated implant, the Mg ions substituted implant revealed superior corrosion resistance with a lower corrosion penetration rate. Thus, the bioactive Mg-HA/MWCNT composite coated 316L SS implant developed with superior antibacterial and anticorrosion properties is considered as the potential aspirant for orthopaedic applications.

Alkali Counterions Impact Crystallization Kinetics of Apatite Nanocrystals from Amorphous Calcium Phosphate in Water at High pH

Crystallization from amorphous precursors emerges as a major principle in biomineralization, but the mechanisms and kinetics involved remain unclear for many materials. Herein, we used in situ synchrotron X-ray diffraction to study crystallization of apatite from amorphous calcium phosphate at a pH where the solution is dominated by phosphate to circumvent the formation of acidic calcium phosphates. The initial crystals were almost isometric. Highly anisotropic growth resulted in needle shaped crystals at later times. Changing the counterion from sodium to potassium lead to changes in nucleation time, growth speed, and nanocrystal shape but preserved the overall crystallization mechanism.

Crystallographic and Colorimetric Analysis of Dental Enamel

Tooth color and the correlation of the compostion of dental enamel with color were investigated in samples of teeth from two medieval Serb cemeteries. Differences in the composition of apatite crystals in the dental enamel of the two samples were found. Color ranges of teeth from the two samples differ in hues and chromas. This result suggests that enamel composition may have an influence on the color of teeth. The prevalence of chlorapatite in enamel causes tooth color to be closer to red and of higher chroma than teeth whose enamel consists of hydroxylapatite. No evidenc indicated that soil ingredients were incorporated into the dental enamel of either sample.

X-ray Linear Dichroism in Apatite.

The recent observation in parrotfish teeth of X-ray linear dichroism motivated an in-depth investigation into this spectroscopic effect in various apatite crystals, including geologic hydroxyapatite (Ca5(PO4)3OH), fluorapatite (Ca5(PO4)3F), and their biogenic counterparts in human bone, mouse enamel, and in parrotfish bone, dentin, and enameloid, the equivalent of dental enamel in certain fish. These data are important because they now enable visualization of the nano- to microscale structure of apatite crystals in teeth and bone. Polarization-dependent imaging contrast (PIC) maps of lamellar bone, obtained with a new method that minimizes space-charge and charging effects, show the expected rotating apatite crystal orientations. PIC maps of mouse enamel reveal a complex arrangement of hydroxyapatite crystals perpendicular to the dentin-enamel junction, with rods arranged in a decussation pattern in inner enamel and nearly parallel to one another in outer enamel. In both inner and outer enamel crystal c-axes are not always aligned with the rod elongation direction.

Ghosts of Apatite Past: Using Hyperspectral Cathodoluminescence and Micro-Geochemical Data To Reveal Multi-Generational Apatite in the Gifford Creek Carbonatite Complex, Australia

Apatite can host significant levels of trace elements, including REE, within its crystal lattice, making it particularly useful for deciphering geological events and processes. This study employs hyperspectral cathodoluminescence (CL) and in situ microchemical techniques to identify and characterize various generations of apatite occurring in the phoscorites, carbonatites, and fenites of the Gifford Creek Carbonatite Complex (GCCC), Western Australia. Hyperspectral CL revealed that apatite crystals in all samples have complex internal zoning, including multiple distinct generations, with zones of relatively bright CL generally having more complex spectra compared to darker CL zones. Most of the CL spectra have prominent sharp peaks at similar to 1.4 eV and similar to 2.l eV as well as a broad peak between 2.3 eV and 3.5 eV. We relate these different peaks to individual REE activators and groups of activators, in particular Nd3+, Eu3+, Sm3+, and Ce3+. Trace element analyses of apatite confirm the relative enrichment of REE in the CL brighter zones. Most apatite generations exhibit concave-down to sinusoidal REY patterns lacking Eu anomalies, but often feature distinct negative Y anomalies. The depletion in LREE is interpreted to be due to LREE sequestration into monazite, which is relatively abundant in most of the samples. Most apatite samples contain very low Si contents, but appreciable Na, so REE incorporation into apatite was primarily via a coupled substitution of REE + Na replacing 2Ca, which is consistent with the highly alkaline, low SiO(2 )environment under which the apatite formed. Based on the combined trace-element signatures and CL textures, we interpret the multiple generations of apatite to reflect magmatic growth from alkaline magmas followed by recrystallization during subsequent metamorphic/hydrothermal events. The notable exception is the apatite core domains from a fenite sample that contain relatively high Si and Mn contents, low Sr, and relatively HREE-enriched REY patterns with distinct negative Eu anomalies. This apatite is interpreted to be relict from the granitic precursor to fenitization. The apatite samples also show systematic compositional variations across the GCCC, with apatite from phoscorite samples from the southeast part of the complex containing higher Sr, lower Gd/Ce, and lower lambda(3 )values (normalized REE pattern inflections) compared to apatite from the northwest part of the complex. Recognition of these spatial variations in apatite compositions from the intra-grain micro-scale through to the district scale demonstrates the utility of combining advanced petrographic methods, such as hyperspectral CL, with micro-chemical analysis to reveal complex geological records preserved in apatite. As apatite is a common accessory mineral, these techniques may be more broadly applicable to igneous source tracing, understanding metamorphic and/or metasomatic processes, provenance studies from detrital mineral records, and studies of the evolution of ore systems.