Carbonate-ion substitution in francolite: A new equation

doi:10.1016/0016-7037(90)90058-S

Geochimica et Cosmochimica Acta

Volume 54, Issue 8, August 1990, Pages 2323-2328

https://doi.org/10.1016/0016-7037(90)90058-S Get rights and content

Abstract

The physical and chemical effects of variable carbonate-ion substitution in francolite (carbonate fluorapatite) are of considerable geochemical interest. A new empirical equation defines the relation between CO₃²⁻ content and Δ2θ for the (004) and (410) X-ray diffraction (XRD) reflections of francolite. The equation is based on analyses of 37 synthetic samples with a compositional range of 0.0–9.0 wt% CO₃²⁻. Reported concentrations of CO₃²⁻ in the synthetic francolites are accurate to within ±0.30 wt% CO₃²⁻, and the standard deviation of the reported values with respect to the empirical equation is ±0.49 wt% CO₃²⁻. In contrast, the standard chemical procedure for determining the CO₃²⁻ content of natural francolite may produce absolute errors of 1–3 wt% CO₃²⁻, especially for samples associated with small amounts of dolomite. Within these constraints, data from a variety of natural samples agree closely with the new empirical relation and imply that the new equation can be used to calculate the CO₃²⁻ content of natural francolites to within ±0.61 wt%, provided that Δ2θ for the (004) and (410) pair of XRD reflections is measured to within ±0.005°2θ.

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Cited by (71)

Post-phosphogenesis processes and the natural beneficiation of phosphates: Geochemical evidence from the Moroccan High Atlas phosphate-rich sediments
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The Upper Cretaceous-Paleogene sedimentary series of the High Atlas hosts underexplored phosphate-rich sediments and presents an excellent example of how complex interactions between various geological processes control the accumulation and distribution of phosphates. Genetically-linked phosphatic lithofacies from this series were investigated for their mineralogical and geochemical compositions with the purpose of assessing the impact of post-phosphogenesis sedimentary processes on the geochemical behavior of genetically-linked phosphatic lithofacies and understanding the distribution of chemical elements. Sampled phosphatic horizons from five representative sections along the Marrakesh High Atlas borders were investigated using optical and scanning electron microscopies (OM and SEM), X-ray diffraction (XRD), inductively coupled plasma optical emission spectroscopy (ICP-OES), and inductively coupled plasma mass spectrometry (ICP-MS) techniques. Results indicated that the studied lithofacies are composed of a mixture of phosphatic and non-phosphatic particles in variable proportions depending on the facies type. However, the phosphatic fraction is dominated by carbonate fluorapatite (CFA), showing an average CO₃²⁻ content of 8.40 ± 0.37 wt%. Geochemical results revealed variations in the chemical composition of the different phosphate facies. Low-grade pristine phosphatic sediment (P₂O₅ < 12%wt.) can be turned by the effects of storms and bottom currents into high-grade granular phosphate beds containing up to 24 wt% P₂O₅. This natural enrichment is mirrored in the bulk rock by increasing P₂O₅ concentrations against decreasing detrital phase-associated chemical elements (K, Al, Si, and Ti). Among the investigated lithofacies, karst-filling phosphate show relatively lower uranium and cadmium contents as it experienced a polyphase evolution, including winnowing, submarine reworking, transport, and subaerial weathering, traced using some redox-sensitive proxies such as cerium and uranium. Despite this polyphase sedimentary differentiation, the apatite resisted chemical changes and preserved its original geochemical signature, specifically the rare earth elements signal that reflects oxic seawater conditions. However, the differentiation processes modified the bulk rock composition of phosphates through the preferential leaching of gangue phases (e.g., carbonates). Furthermore, these processes triggered the oxidation of organic matter and sulfides, removing some associated elements of environmental concern, such as uranium and cadmium. Conclusively, the combination of mechanical winnowing/reworking and subaerial chemical weathering is the most effective way of the natural beneficiation of phosphate, both economically and environmentally.
Highly variable content of fluorapatite-hosted CO<inf>3</inf><sup>2−</sup>in the Upper Cretaceous/Paleogene phosphorites (Morocco) and implications for paleodepositional conditions
2022, Chemical Geology
Citation Excerpt :
The studied bulk XRD profiles thus indicate the presence of CO32−-rich fluorapatites in the Gantour phosphorites and CO32−-poor fluorapatites in the Oued Eddahab phosphorites (Fig. 4a, c; Figs. S3, S4). In addition, measurement of CO32−content in CFA can be achieved using specific XRD reflections (Gulbrandsen, 1971;Schuffert et al., 1990). Schuffert et al. (1990)introduced an empirical equation between the concentration of CO32−and the difference in °2θ (Δ2θ) of the (004) and (410) CFA peaks.
Phosphorus (P) usually occurs as microcrystalline minerals of carbonate fluorapatite (CFA) in sediments of upwelling-related areas. The Upper Cretaceous/Paleogene marine sedimentary rocks in Morocco host the world's largest economic reserves in phosphorus, which were deposited on the eastern passive margin of the central Atlantic Ocean. However, a full petrographic, mineralogical and geochemical characterization of the Moroccan phosphate minerals is yet to be determined. Here, we use optical and scanning electron microscopies, X-ray diffraction, and infrared spectroscopy, in combination with a microprobe analyzer to document carbonate (CO₃²⁻) substitutions in CFA from two distinct phosphate basins and their possible implications for paleoenvironmental conditions. The Moroccan phosphorus-rich deposits predominantly comprise porous phosphatic peloids and coated grains with a maximum size of 500 μm, which in turn consist of high-density nano- to micron-sized rod-shaped and sphere-like crystallites. Bulk and in situ analyses reveal heterogeneous CO₃²⁻content in CFA at the regional- and grain-scale. Fourier transform infrared spectra indicate that CO₃²⁻accommodates only in the phosphate site within the fluorapatite structure, as found in other natural phosphate minerals. Bulk CO₃²⁻variations in CFA between phosphate basins, ranging from 1.97 to 8.24%, are likely related to depositional conditions – and thus of supersaturation with respect to CFA – during mineral formation, transport, and redeposition. Moreover, some phosphatic coated grains have revealed alternations of CO₃²⁻-rich and CO₃²⁻-poor CFA laminae, pointing out local differences in CO₃²⁻concentrations in sediment pore waters. Interestingly, the change in shape and diminution in size of CFA crystallites caused by high content of CFA-hosted CO₃²⁻could have possibly increased the intragranular porosity, highlighting an underestimated role of CO₃²⁻on grain morphology. Considering the higher solubility of CO₃²⁻-rich CFA than fluorapatite, the application of such phosphate rocks from specific Moroccan areas as rock phosphate powders may be of deep interest for long-term soil fertilization. These results further evidence the importance of studying phosphorites for economic, environmental, and scientific interests.
Nanocomposite of starch-phosphate rock bioactivated for environmentally-friendly fertilization
2018, Minerals Engineering
Citation Excerpt :
The francolite is a carbonate-fluorapatite, Ca10[(PO4)6-X(CO3, SO4)X]F2, therefore it contains some phosphate groups (PO43−) that can be substituted by carbonate (CO32−) or sulfate (SO42−) groups in its crystalline reticulum. This substitution results in the change of physicochemical properties of the material, such as its reactivity and solubility (Chien, 1993; Loureiro et al., 2008; McArthur, 1985; Schuffert et al., 1990; Zapata, 2004). It was not possible to identify other minor phases in the XRD pattern of BPR (Fig. 1).
The use of natural phosphate rock as a source of phosphorus for plant growth is considered more environmentally friendly, compared to conventional chemical fertilizers. However, the very low solubility of the phosphate present in natural rocks limits its practical application. To overcome this limitation, a composite was developed based on the dispersion of nanoparticulate phosphate rock in a polysaccharide matrix of starch, as an integrated strategy to facilitate application and provide a supporting substrate for an acidulant microorganism, Aspergillus niger. The bioactivation of the nanocomposite resulted in a remarkable solubilization of up to 70% of the total available phosphate in very low solubility phosphate rocks (Bayóvar and Itafós) and 100% of the total available phosphate in a reference mineral (hydroxyapatite), in only 96 h. Such approach employing bioactivation of starch-phosphate rock nanocomposites significantly contributed to improve P-solubilization, opening new routes for the development of smart fertilizers based on polysaccharide matrix.
Geochemistry and depositional environments of Paleocene–Eocene phosphorites: Metlaoui Group, Tunisia
2017, Journal of African Earth Sciences
The Late Paleocene–Early Eocene phosphorites of the Metlaoui Group in Tunisia are a world-class phosphate resource. We review the characteristics of phosphorites deposited in three areas: the Northern Basins; Eastern Basins; and Gafsa-Metlaoui Basin. Comprehensive new bulk rock elemental data are presented, together with complementary mineralogical and mineral chemical results. Carbonate fluorapatite (francolite) constitutes the dominant mineral phase in the deposits. Phosphorite samples are enriched in Cd, Sr, U, rare-earth elements and Y, together with environmentally diagnostic trace elements that provide detrital (Cr, Zr), productivity (Cu, Ni, Zn) and redox (Mo, V) proxies. Suboxic bottom-water conditions predominated, with suboxic to anoxic porewaters accompanying francolite precipitation. Phosphorite deposition occurred under increasingly arid climate conditions, accompanying global Paleocene–Eocene warming. The Northern Basins show the strongest Tethys Ocean influence, with surface seawater rare-earth element signatures consistently developed in the phosphorites. Bed-scale compositional variation indicates relatively unstable environmental conditions and episodes of sediment redeposition, with varying detrital supply and a relatively wet local climate. Glauconitic facies in the Northern Basins and the more isolated evaporite-associated phosphorites in the dryer Eastern Basins display the greatest diagenetic influences. The phosphorite – organic-rich marl – diatom-bearing porcelanite facies association in the Gafsa-Metlaoui Basin represents the classic coastal upwelling trinity. Modified Tethyan waters occurred within the Basin during phosphorite deposition, with decreasing marine productivity from NW to SE evidenced by systematically falling enrichment factors for Cu, Ni, Cd and Zn in the phosphorites. Productivity declined in concert with increasing basin isolation during the deposition of the commercial phosphorite beds in the latest Paleocene to earliest Eocene. This isolation trend was temporarily reversed during an episode of maximum flooding associated with the earliest Eocene Paleocene–Eocene Thermal Maximum (PETM).
Coprolites of marine vertebrate predators from the Lower Triassic of southern Poland
2015, Palaeogeography, Palaeoclimatology, Palaeoecology
Citation Excerpt :
The quantitative phase content and crystallographic parameters were calculated using the Rietveld Module in HighScore + software. The CO32 − content in apatite structure was calculated using the equations provided by Schuffert et al. (1990). X-ray diffraction analyses were conducted for both the rock residuum dissolved in 10% HCl and for the coprolites dissolved in 3% HNO3.
Numerous coprolites are described for the first time herein from the Lower Triassic (Olenekian) shallow marine sedimentary rocks in southern Poland. X-ray Diffraction and geochemical analyses show that they are preserved as calcium phosphate with small admixtures of quartz and calcite. Additionally, SEM and thin section studies revealed that they contain highly fragmented faunal remains (crinoids, molluscs and vertebrates). The size, shape, geochemistry, biostratigraphic distribution and co-occurrence with vertebrate skeletal remains imply that the coprolites at hand were likely produced by nothosaurids, the durophagous sauropterygian reptiles and actinopterygian (ray-finned) fish. The large number of recorded coprolites implies that durophagous predation has been intense during the Early Triassic and suggests that the so-called Mesozoic Marine Revolution probably started soon after the end-Permian extinction. Furthermore, discovery of sinusoidal trails attributable to nematodes in some coprolites implies that the intestinal parasitic associations with these predators had already evolved by at least the Early Triassic.
Coprolites of Late Triassic carnivorous vertebrates from Poland: An integrative approach
2015, Palaeogeography, Palaeoclimatology, Palaeoecology
Vertebrate coprolites derived from Upper Triassic terrestrial deposits of southern Poland have been subjected to various analytical methods in order to retrieve information about their composition, producer’s diet and nature of the microscopic structures preserved in the groundmass. Morphologically, the coprolites have been classified into four morphotypes, of which only three were further analysed due to their good state of preservation. Their groundmass are composed of francolite, a carbonate-rich apatite, in which abundant coccoid structures are preserved. Based on various microscopic and organic geochemical techniques, they are interpreted as fossilized bacteria which could have mediated the phosphatization of the faeces. The thin sectioning revealed that the coprolites consist of those containing exclusively bone remains, and those preserving both bone and plant remains. Those coprolites preserving only vertebrate remains are suggestive for exclusive carnivorous diet of the producers. However, the interpretation of coprolites consisting of both vertebrate and plant remains is more debatable. Although they may attest to omnivory, it cannot be excluded that potential producers were carnivorous and occasionally ingested plants, or accidentally swallowed plant material during feeding. The latter may involve predation or scavenging upon other herbivorous animals. The potential producers may have been animals that foraged in or near aquatic habitats, such as semi-aquatic archosaurs and/or temnospondyls. This is supported by the presence of ostracode and other aquatic arthropod remains, and fish scales within the coprolites, as well as by the presence of specific biomarkers such as phytanic and pristanic acids, which are characteristic constituents of fish oil. The preservation of such labile organic compounds as sterols, palmitin, stearin or levoglucosan attests for rapid, microbially-mediated mineralization of the faeces at very early stages of diagenesis.

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LetterCarbonate-ion substitution in francolite: A new equation

Abstract

Mar. Geol.

Chem. Geol.

Geochim. Cosmochim. Acta

Earth Planet. Sci. Lett.

Deep-Sea Res.

Carbon and oxygen isotopic composition of some sedimentary apatites from Iraq

Econ. Geol.

X-ray evidence of the nature of carbonate-apatite (abstr.)

Geol. Soc. Amer. Bull.

Origin of sedimentary francolite from its sulphur and carbon isotope composition

Nature

Isotopic composition of carbon in the minerals of phosphorite ores

Geochem. Intl.

Free energy of formation of carbonate apatites in some phosphate rocks

J. Soil Sci. Soc. Amer.

Relation of carbon dioxide content of apatite of the Phosphoria Formation to regional facies

USGS Prof. Paper 700-B

Letter
Carbonate-ion substitution in francolite: A new equation