Phosphate Grains Research Articles

Oriented growth of surface grains in sintered β tricalcium phosphate bioceramics

Preferential orientation of sintered surface grains of beta tricalcium phosphate (β TCP) ceramic was studied by X-ray diffraction (XRD) method and scanning electron microcopy (SEM). The β TCP powder was prepared by calcining the precipitate at 800°C, obtained by precipitation method involving calcium nitrate and ammonium dihydrogen orthophosphate. The powder was compacted and sintered at 1150°C to ∼95% density. The surface grains of the above ceramic were found to preferentially orient with respect to a–b plane by XRD study. High grain growth was observed to the surface grains compared to the bulk grains.

Le cadmium associé aux dépôts phosphatés en Tunisie méridionale

Cadmium values varying between ppm to tens of ppm have been noted in phosphorites from the Early Eocene of Tunisia. The heavy fraction (d≥3.1) (< 2%) with higher Cd contents is composed of pellets with sulphide inclusions of free sphalerite and pyrite. The sphalerite XRD indicates a crystallographic parameter with a value of 5.4492 Å. Analytical data give the following structural formula: (Zn .825 Cd ,109 Fe .002 Pb .001) S 1.012 for J. M'Rata; (Zn .959 Cd ,056) S .995 for J.M'Dilla; (Zn .952 Cd .029 Fe .004 Cu .006 Pb .005) S .954 for J. Abdallah. SEM photographs show a heterogeneous Cd distribution within sphalerite. One transverse using 10 linear points across a grain of sphalerite gave a composition of S (48.47– 49.80 at.%), Zn (44.39–46.19 at.%), Cd (4.85–7.16 at.%) and Fe (0.02–0.19 at.%). Such high Cd values have not been found in natural sphalerite. A geochemical process involving sulphate-reducing bacteria could explain the presence of sphalerite inside and outside the phosphate grains. Zinc and Cd are present in sediment abnormally rich in exclusively planktonic organic matter, which is known to specifically concentrate them.

Cratonic and foreland Early Cambrian phosphorites of West Africa: palaeoceanographical and climatical contexts

Abstract In Western Gondwana, two types of Pan-African-Brasiliano fold belts have been distinguished. The first type is characterized by an old long-lived tectonic cycle (1000-600 Ma) and is generally oceanic. The closure of these oceans resulted in the aggregation of Western Gondwana in late Neoproterozoic time. The second type is characterized by a young short-lived cycle (about 600-520 Ma) and is mostly intracontinental. Fold belts of the second type have various geodynamic significances and represent folded foreland, intermontane or impactogen-type basins formed during, or slightly after, the deformational closure of the old long-lived cycle. Phosphorites of West Africa are associated with the young short-lived tectonic cycle. They occur together with the upper member of a distinctive formation known all over West Africa. This formation is made up of three members, and consists, from base to top, of diamictites generally tillites with detrital intercalations, carbonate rocks with barite, and bedded cherts with local shale or siltstone intercalations. In Eastern Senegal, in the cratonic Taoudeni basin, the carbonate member is palaeontologically dated as Early Cambrian. Deposited during a glacio-eustatic transgression, it can be considered as a chronostratigraphic marker for the entire West Africa. Phosphorites, associated with or immediately following the eustatic transgression, are also possibly of Early Cambrian age. They are located either in the lower part of the chert member, in the upper part of the carbonate member, or sandwiched between these two units. In Fersiga and Namel, in the Taoudeni basin, phosphorites are associated with stromatolites characterizing the initial stage of the transgression. They are overlain by evaporites which can be related to an isostatic rebound. In the Volta foreland basin, phosphorites are associated with the chert member. There, they were deposited on a shelf ramp, either on its proximal portion, where they are strongly reworked (Kodjari and Arli occurrences), or in its distal part where they are interbedded with sediments deposited under anoxic conditions (Tapoa, Mekrou and Aloub Djouana occurrences). The depth of marine sedimentation, as indicated by the sedimentological characteristics of phosphorites and associated sediments, was controlled by the combined effect of glacio-eustatic sea-level rise, subsidence rate and isostatic rebound. Numerous textural features are similar in the phosphorites from the two basins. Phosphatic grains occurring as oncolites and stromatolitic fragments, together with intraclasts show a noticeable biogenic contribution of microbial origin. Microbial build-ups were formed in submergent/emergent environments in the Taoudeni basin, and in the outer shelf of the Volta basin. Apatites from these basins have similar REE contents, which suggest a continental origin for the phosphorus. Early Cambrian phosphorites of West Africa were deposited soon after glacially derived sediments. According to the available palaeocontinental maps, both were laid down at palaeolatitudes of about 40°S. West African Early Cambrian phosphorites thus formed at higher latitudes than their modern analogues, which are confined within the inter-tropical zone.

Rare-earth geochemistry of `old' phosphorites and probability of syngenetic precipitation and accumulation of phosphate

`Old' (Vendian–Early Cambrian) phosphorites of the World demonstrate `seawater-like' patterns in their rare-earth element (REE) distributions but are depleted in heavy REE. They also show a broad range of variation in total and elemental abundances. Primary bedded structureless phosphorites contain only 15 ppm total REE (ΣREE) while granular (pelletal) varieties contain up to 650 ppm. REE were incorporated into phosphate grains diagenetically during prolonged periods of transport and redeposition. A higher surface/mass ratio in pelletal phosphorites contributed to their enrichment in the REE's. In primary bedded structureless phosphorites immediate burial of phosphate syngenetically precipitated and accumulated in situ at the sediment–water interface prevented the later incorporation of the REE. Large deposits of Vendian–Early Cambrian phosphorites have originated by both processes. Primary bedded varieties occur in deeper-water facies and granular phosphorites accumulated closer to the shore. In terms of their REE geochemistry, old granular phosphorites are almost indistinguishable from their Phanerozoic counterparts, indicating that both were deposited by the same mechanisms. Primary bedded structureless phosphorites were only deposited on a large scale at the beginning of the Phanerozoic.

Geochemistry of cadmium in the Negev phosphorites

Cadmium (Cd) in the Negev phosphorites, as in many other phosphorites, is the most enriched trace element relative to ‘average’ shale. Nevertheless, similar to many other phosphorites, Cd is either not correlated to P 2O 5, or has a slightly negative correlation with it. There is a clear correlation between the Cd content, the textural and the compositional characteristics of the phosphorites, and the structural position of the phosphate deposits. Well developed ‘synclinal’ sections, comprised of many thin strata of laminated (non-disturbed) pelletal phosphorites, have much higher Cd concentrations than the condensed ‘anticlinal’ sections which consist of a few amalgamated thick beds of massive (reworked) phosphorites. Cd is in general poorly related to the present organic matter content, but has a good correlation with organic matter in unoxidized beds of the ‘synclinal’ sections. Cd is strongly correlated to Zn in the relatively undisturbed beds of the ‘synclinal’ sections but shows no correlation in the highly reworked beds of ‘anticlinal’ sections. The source of Cd is upwelled seawater that is enriched in both P and Cd. This is the basis of the relationship between phosphogenesis and Cd concentration. Reducing conditions during decomposition of organic matter immobilize Cd in the pore fluids of the bottom sediments. Precipitation of Cd-rich sphalerite and adsorption on organic matter appear to be the main processes for its initial speciation and concentration in the Negev phosphorites. Cd is concentrated mainly in laminated (non-disturbed) microbially generated phosphorites where authigenic apatites are also found. Cd does not as a rule replace Ca in the structure of authigenic (primary) sedimentary apatites. This finding has practical applications since it makes possible the production of a phosphate concentrate with a much lower Cd concentration than the original phosphorite. Exposure to oxic conditions leads to mobilization of the sulfide-bound Cd. Scouring activity, bioturbation, and recycling and transportation of stirred-up phosphate corpuscles in an oxidizing environment appear to be effectual for removing Cd. Oxidation alone, without fabric disruption and mobilization of the phosphate grains does not significantly lower Cd concentrations, since the released Cd may be almost immediately readsorbed on the various components of the phosphorite such as iron oxyhydroxides, organo-clay fractions and apatites. The cadmium concentration and the Cd Zn ratio of a phosphorite may help to determine whether a phosphorite is pristine or not and to unravel its diagenetic history.

Sr Isotopic Age Analysis of Co-Occurring Miocene Phosphate Grain Types on the North Carolina Continental Shelf

ABSTRACT Four phosphate grain types and associated foraminifera were analyzed for their 87Sr/86Sr ratios to test the hypothesis that different authigenic grain types have specific times and processes of formation in response to changing conditions during one sea-level transgression. The samples were collected from one phosphate-rich depositional sequence (FPS-1) of the Miocene Pungo River Formation in Onslow Bay, North Carolina continental shelf. The hand-picked grains were from three cores representing upsection changes from lowstand, to transgressive, and highstand systems tracts. The phosphate grain types carried significantly different strontium isotopic signatures and included the following: peloidal grains (spherical to oval, highly rounded and polished grains with inor inclusions and common coatings), intraclastic grains (irregular-shaped rip-up grains from preexisting crusts and nodules), microsphorite (laminated crusts and bored hardgrounds associated with unconformities and containing common inclusions), and skeletal grains (formed directly by organisms). Four different phosphate skeletal grains were analyzed (brachiopods, vertebrae, teeth, and bone splinters); however, the results were not significantly different and therefore they were all included as one grain type. Analyses of phosphate grain types and associated foraminifera support the interpretation that they formed in response to specific chemical and physical processes and at different times during phosphogenesis, as follows. (1) Peloids cluster tightly and have Sr isotope ages contemporaneous with the associated foraminifera. They are interpreted to represent the time of phosphogenesis and primary sedimentation during early- to mid-stage transgression. (2) Intraclasts are significantly older than other associated phosphate grain types with Sr isotope ages similar to the microsphorites. They are interpreted to have formed as microsphorite on unconformity surfaces during the lowstand, were ripped up from these surfaces during the subsequent transgression, and reworked into the overlying sedi ent sequence. (3) Skeletal grains have quite variable Sr isotope ages, but in general they are younger than the other phosphate grain types. Most skeletal grains are interpreted to be contemporaneous with the peloids and foraminifera. However, their younger ages are interpreted to be in response to secondary geochemical processes associated with (a) recrystallization from carbonate hydroxyapatite to carbonate fluorapatite and (b) subsequent precipitation of apatite in the pore spaces characteristic of porous vertebrate skeletal material. Some skeletal material is older and probably has been reworked from the underlying unconformity surfaces.

Lower Cambrian Apatitic Stromatolites And Phospharenites Related To The Glacio-Eustatic Cratonic Rebound (Sahara, Algeria)

ABSTRACT At the end of the Early Cambrian glacial event occurring on the West African craton, the tillite surface was marked, especially in the northeastern part of the Taoudenni basin, by noticeable topographic relief. After deglaciation, the eustatic rise in sea level led to deposition over all the West African Craton of a thin but widespread carbonate unit containing Lower Cambrian shelly fossils. At the ice-cap periphery, isostatic rebound of the craton resulted in uplift of the topographic highs and weathering of the carbonate. During fluctuations of relative sea level, before the return to marine transgressive conditions, a succession of events took place. First, in the submerged areas, stromatolitic phosphorite in flat-domal to columnar buildups accreted, followed by growth of dolomitic ministromatolites. Laterally, phospharenitic beds (oncolites and phosphatic grains) accumulated under water in the depressions. The stromatolites and oncolites are made of carbonate-fluorapatite, characteristic of coastal environments. Glauconite is closely related to apatite and suggests conditions of a sea-level rise in respect to phosphatization. Precipitation of dolomite, calcite and evaporite postdates phosphatization and glauconitization, indicating a regression and a chemical concentration due to a warm and perhaps arid climate. Apatitic stromatolites and oncolites resulted from in situ precipitation within bacterial mats. This phosphogenesis corresponds to the major Early Paleozoic phosphogenetic event.

A Neogene Mixed Siliciclastic and Carbonate Foundation for the Quaternary Carbonate Shelf, Florida Keys

ABSTRACT New core holes and marine seismic profiles show the distribution and paleoenvironments of a siliciclastic foundation sandwiched between the much-studied Quaternary carbonates of the Middle and Upper Florida Keys and Oligocene to Miocene carbonates of the Arcadia Formation, a distance of some 160 km. Throughout this area the Quaternary carbonates are generally no more than 30 m thick and are underlain by up to 145 m of Plio-Pleistocene mixed limestones and sands, and marine siliciclastics (Peace River Formation). Quartz sands, days, and minor reworked phosphatic grains of the Peace River are the distal deposits of an almost peninsula-wide blanket that interrupted prolonged Tertiary carbonate deposition in southernmost Florida. In the Lower Keys the Peace River siliciclastics change fac es to very fine- to fine-grained carbonates composed of bioclastic, planktonic-foraminiferal grainstones and packstones. The position of the present southern Florida shelf margin, with its discontinuous coral reefs, is determined by the seaward paleoslope of siliciclastics and/or carbonates of the Plio-Pleistocene section defined in the Florida Keys. Lithostratigraphy of five continuous cores on the Keys and southeastern mainland, each about 300 m long, combined with numerous well logs and sample descriptions from the Keys and mainland, establish the southern limit of Peace River siliciclastics to at least the Middle Keys. The well data show intervals of quartz of coarse sand to granule size localized as a north-south pathway of maximum paleocurrents that extend from the central part of the peninsula approximately 350 km to the Middle Keys. This pathway of maximum paleocurrents, about 100 km wide, is interpreted as the palimpsest record of unusually strong, southward-moving shoreline and channeled deposition or a giant prograding spit. The ultimate source of these siliciclastics is from the Appalachians, approximately 1000 km to t e north. We propose that the Plio-Pleistocene marine siliciclastics and carbonates (including the Peace River Formation) recovered in wells on the Keys extend seaward beneath the present southern Florida outer shelf and are present in seismic profiles acquired across the shelf-margin slope. A series of profiles forming an offshore traverse from the Lower to Upper Keys show seaward-dipping clinothems that locally dip obliquely to the arcuate strike of the Florida Keys. These clinothems downlap on a prominent reflector that can be traced to an exposed submarine surface of upper Tertiary carbonates, the Pourtales and Miami Terraces. Some reflectors show configurations similar to contourites, and others, to channels. We contend that southward entry of the siliciclastics into southernmost Florida w s deflected eastward by west-to-east channeled flow of an ancestral Florida Current.

Nannostructure and significance of fish coprolites in phosphorites

Phosphatic coprolites (0.5–2 cm long) occurring in Cretaceous-Eocene phosphorites of Tunisia, Morocco, Mauritania, Senegal, and Egypt were investigated for their petrology by using SEM. They exhibit a homogeneous porous apatite structure with a few fish remains. The nannostructures of the coprolites consist of inframicron-sized and botryoid-type apatite microparticles. Spherical cavities and cavity-infill cavity-infilled inframicron-sized apatite globules are typical in the coprolites and are apparently formed by extracellular precipitation of phosphate around microbial organisms. We presume that the coprolites studied here may belong to fishes, whose excrements contain abundant organic matter and phosphate. Phosphatization of excrements appears to be a microbial process controlled by the microenvironment. The nannostructures observed in the coprolites investigated here differ significantly from the nannostructures of other studied phosphatic grains (pellets and coated grains) in the same samples which are mostly heterogeneous and consist of ovoid-type apatite particles. In the case they are not reworked, the centimetre to decimetre thick coprolite beds in phosphorites indicate a lack of detrital input and strong bottom water currents at the sediment-water interface during deposition and subsequent phosphatization of the excrements.

Iron phosphate precipitation by epilithic microbial biofilms in Arctic Canada

On Ellesmere Island, in the Canadian Arctic, dark-colored biofilms proliferate on moist surfaces, including exposed granodiorite outcrops. Transmission electron microscopy of these biofilms indicates that complex epilithic microbial communities developed, consisting of cyanobacteria and fungi symbiotically associated in a lichen, along with a consortium of free-living algae and gram-negative bacteria. The epilithic cyanobacteria and bacteria were shown to extensively precipitate phosphatic minerals, ranging from relatively large polyphosphate granules (approximately 250 nm in diameter) within their cytoplasmic membranes to smaller iron phosphate grains (generally less than 50 nm in diameter) associated with the periplasmic space and encompassing capsule. Complete encrustation of some bacterial cells by the iron phosphates was observed. Energy-dispersive X-ray spectroscopy suggested that these grains are compositionally similar to the mineral strengite (FePO4∙2H2O). This study clearly indicates that the Arctic supports a thriving microbial community that influences the biogeochemical cycling of PO4 in an environment of low nutrient availability. Nutritional requirements by the microorganisms were actively maintained through a relatively closed recycling mechanism, which restricted the immediate loss of phosphorus from the biofilm.

I-Xe studies of the Acapulco meteorite: Absolute I-Xe ages of individual phosphate grains and the Bjurböle standard

We have measured Kr and Xe by laser volatilization of seventeen individual neutron-irradiated phosphate grains (nine apatite and eight merrillite) separated from the (reclassified) A-chondrite Acapulco. Radiogenic 129Xe from now-extinct 129I is only observed in the apatites, which all formed simultaneously (8.1 ± 1.2 Ma) after Xe closure in the Bjurböle (L4) standard. This relative closure time, when coupled with the Pb-Pb age of Acapulco phosphates (4.557 ± 0.002 Ga) provides an absolute I-Xe age for these apatites, for Bjurböle and for all samples previously referenced to the Bjurböle standard, subject to the assumptions implicit in I-Xe dating. We have also measured Kr and Xe by stepwise heating of irradiated and unirradiated whole-rock samples of Acapulco and of unirradiated phosphate concentrates. Iodine-derived xenon in the irradiated whole-rock sample does not yield an I-Xe isochron, consistent with the presence of multiple iodine host phases previously observed. Xenon from the spontaneous fission of now-extinct 244Pu is observed in the single phosphate grains and the inferred initial ( 244Pu/ 238U) 0 ratios reflect fractionation of the actinides favoring plutonium in the merrillites and uranium in the apatites.

Analyse expérimentale et théorique des mécanismes de transfert de matière au cours du séchage des grains d'un engrais phosphaté dans un lit fixe

The aim of this work is to make a detailed study of the drying kinetics of diammonium phosphate (DAP) grains disposed in a fixed bed in order to determine the mechanisms restricting the transfer of water vapor in the grain. The drying rate is measured using a microbalance. The solid sample consisting of a pile of spherical grains of the humid DAP is placed on the scale and is subject to a constant flux of heat. Theoretical analysis of the matter's internal transfer is carried out using a vapor diffusion model. The temperature gradient within the granules and the external mass transfer resistance are neglected. Experiments show that the drying rate depends enormously on the temperature and is inversely proportional to the radius squared. The agreement between the calculated results and the laboratory measurements is very satisfactory. The postulated mechanism is thus fully compatible with the observations.

Sr and Nd isotopic analysis of phosphorite sedimentation through one Miocene high-frequency depositional cycle on the North Carolina continental shelf

A well-developed seismic-, litho-, and biostratigraphic data base with good models for the formation and deposition of phosphate-rich sediments exist for the Upper Cenozoic sediments on the North Carolina continental margin. This provides a unique opportunity to evaluate the potential of utilizing strontium and neodymium isotopic analyses to help unravel a complex genetic history of phosphate formation and deposition. Thirty one samples from three vibracores in Onslow Bay supplied hand-picked concentrate subsamples of five phosphate grain types and thirty one concentrate subsamples of phosphate peloids for stratigraphic analysis. All subsamples were analyzed for their Sr isotope composition, while only thirteen were analyzed for Nd isotopes. The 31 peloid samples represent three different stratigraphic units and include: (1) Holocene surficial sands with highly variable concentrations and types of reworked phosphate; (2) Pleistocene moldic carbonates with minor concentrations of black peloidal phosphate; and (3) Miocene (mid-Burdigalian), Frying Pan Sequence unit 1 (FPS-1) of the Pungo River Formation with very high concentrations of multiple types of phosphate grains that decrease upsection and change grain types with changing lithofacies. The latter unit represents deposition during one high-frequency sea-level cycle. Stratigraphic analysis utilized one grain type, phosphate peloids, for comparative purposes with the depositional model of Riggs and Mallette (1990). The Sr and Nd data cluster into significant categories that are coincident with these three major stratigraphic units.Sr and Nd isotope compositions of selected phosphate grains from three phosphate-rich depositional sequences are utilized to: (1) date the initial Miocene transgression of unit FPS-1 of the Pungo River Formation; (2) decipher processes of formation and deposition of major phosphate grain types through one Miocene sea-level cycle including the processes of in situ formation versus reworking; (3) develop a Sr and Nd chronostratigraphy for evaluating changing patterns of continental margin sedimentation through high-frequency depositional cycles; (4) determine effects of reworking and weathering on the isotope signals between Miocene, Pleistocene, and Holocene sediments; (5) evaluate the suitability of utilizing Sr and Nd isotopes for detailed chronostratigraphic analysis of phosphorite sedimentation; and (6) refine our understanding of the Upper Cenozoic evolutionary history of North Carolina's continental margin.

Primary bedded phosphorites in the Campanian Mishash Formation, Negev, southern Israel

Thin, lentiform beds (up to 1 cm thick) of primary (non-pelletal) phosphate have been found sporadically in the highly condensed “anticlinal” sections of the late Campanian Mishash Formation of the central Negev (southern Israel). These primary phosphate beds are typically associated with iron-rich montmorillonites of probable authigenic origin, and occur at the top of upward-fining phosphate-dominated sequences. The clay-rich and primary phosphate levels are commonly truncated by amalgamated beds of high-energy coarse-grained phosphorites which form the basal part of these sequences, with ripped-up clasts of the primary phosphate commonly re-incorporated in these rocks. SEM-EDS study of both the primary phosphate levels and the reworked phosphate clasts shows that they are built by a crudely stratified network of closely packed and gently compressed, capsule-like phosphate cells 2.5 μm in external diameter and with walls of 0.5 μm thick. These structures are interpreted as being the synsedimentarily phosphatized remnants of former mat-forming coccoid prokaryotes (now represented as extracellular envelopes), which intermittently colonized the Mishash sea bottoms at the end of the high-energy episodes. Differential degradation of the periphytic mat-community, possibly coupled with Fe—P redox cycling across the surficial mat (iron was trapped as iron-rich montmorillonites in the anoxic zone below the living mat), probably resulted in an excess of dissolved phosphate immobilized as solid phases on decay-resistant microbial remains which escaped total degradation. Development of the mat-forming microbes on the Mishash bottoms, and their mineralization to form primary phosphorite beds, occurred during periods of halted deposition, which resulted in a high residence time of the mat on the seafloor and hence, in intensified interface-linked biogeochemical reactions ending in phosphate precipitation. The restriction of primary phosphate occurrences to the condensed Mishash sections of marginal (palaeohigh) positions, strengthens previous assumptions that these zones were probably the main locus of Mishash phosphate production. It also points to a phosphate grain supply from marginal to basinal sites, leading to the accumulation of pelletal phosphorites in the latter areas.

Variations in Phosphatic Grain Types and Facies Variations in the Oliog-Miocene San Gregoria Formation of Baja California Sur, Mexico

Sediments from the San Gregorian Formation, including phosphorites,were deposited during a period of active volcanism in a fore-arc basinal setting. Three major types of phosphatic grains are present in the phosphorites: multiple coated grains, concentrically coated grains with multi-generations of phosphate accumulation around a nucleus and commonly bored, possibly by bacteria, single-coated grains with one generation of phosphatization surrounding a detrital nucleus, and non-nucleated peloidal grains that also represent single events of phosphatization. Phosphatized microstructures of probably microbial origin have been identified by electron microscopy in the multiple coated grains, suggesting possible involvement in the phosphatization process. In the northern sections of the study area, transport and deposition of phosphatic grainstones into an oxygen-poor basin occurred via turbidity currents. Here, phosphorites occur interbedded with deep-water porcelanites, cherts, and thinly laminated organic-rich mudrocks; the dominant type of phosphatic grain is the multiple coated variety, suggesting original formation on a sediment-starved outer shelf or isolated bank prior to redeposition. In the south, phosphorites are also allochthonous, but here they are associated with more detrital-rich, shallow-water lithologies (including coquinas and tidal laminities). Dominant phosphatic grains are single-coated and non-nucleated peloids, suggesting fewer episodes of phosphatization. In contrast to the northern occurrences, these southernmore » phosphorites appear to have been redeposited on a shallow shelf.« less

Comparative study by infrared spectroscopy of the organic matter of phosphate-rich (Oulad Abdoun basin) and black shale (Timahdit basin) series (Morocco)

Fourier transform infrared spectroscopy has been used to get quantitative information on the oxidation state of the organic matter of stable residues extracted from strata of the phosphate-rich basin of Oulad Abdoun and from oil shales of the Timahdit basin, Morocco. Throughout the respective filling of both basins the redox conditions have been more oxidizing in the top part of the sediments in Oulad Abdoun basin than in the Timahdit basin. This is interpreted as the result of the sedictural evolution of the basin Oulad Abdoun basin having evolved in stable platform conditions while the Timahdit basin evolved in subsiding ones. The phosphate-rich beds contain the more oxidized organic stable residues. Sedimentological evidence shows that the oxidation was associated reworking processes. These processes are responsible for the progressive concentration of the phosphatic grains and prove to be necessary for the P 2O 5 content to become higher than 10% this value being the highest encountered in the sediment within which the phosphatic grains form.

Role of microbial processes in the genesis of Jordanian Upper Cretaceous phosphorites

Abstract Samples representing granular and laminated phosphorites from the main Upper Cretaceous deposits of Jordan, have been examined by SEM for microbial structures. Globular clusters and sheaths, postulated to have formed by bacterial activity or possibly as inorganic precipitates, have been found only within cavities and in the cementing phosphatic material around phosphate grains of the granular phosphorites. These structures are interpreted to have originated in association with percolation of ground-water late in the history of the sediments, rather than as a consequence of primary grain-forming processes. Organic-rich phosphate grains are believed to have acted as a food source for the bacteria and are, therefore, the cause of the microbial structures and not the result of them. On the other hand, empty cylindrical and/or spherical sheaths, interpreted to be of primary, cyanobacterial (blue-green algae) origin, were readily observed in the laminated phosphorites. These phosphorites are interpreted to have been deposited as algal mats in shallow, subtidal to intertidal environments.

Ultrastructure of a Middle Cambrian Primary Nonpelletal Phosphorite and its Early Transformation into Phosphate Vadoids: Georgina Basin, Australia

ABSTRACT Mudstone phosphorites (collophane mudstones) and mini-profiles composed of phosphate-coated grains from the Duchess phosphate deposit of early Middle Cambrian age in the Georgina Basin, Australia, are described in detail. The mudstone phosphorites form thin beds and laminae, interlayered with transported peloidal phosphorites. In thin section the mudstone phosphorites are characterized by predominately homogeneous micritic phosphate; under the scanning electron microscope (SEM) they are mostly composed of globose structures of 10-25 µm diameter and tubules. These structures are interpreted as the preserved (phosphatized) remnants of solitary and colonial unicells and sheaths that once formed part of a larger microbial mat community. Intermittent subaerial exposure of the microbi lly constructed mudstone phosphorites resulted in their in situ vadose transformation into mini-profiles composed of coated phosphate grains. These profiles are characterized by poorly laminated coated phosphate grains, inverse grading, hollow rims, gravitational cements, and a fitted fabric. Transition zones between the mudstone phosphorites and the overlying mini-profiles allow the steps involved in the transformation to be understood. The mudstone phosphorites and mini-profiles are characterized by distinctive structures, identical to those found in many transported phosphate grains. As these surfaces are frequently laterally discontinuous and eroded at their margins, they are probably important contributors of grains to the deposits.

Two close associations : Si-P-Fe and Si-P-C in the Upper Campanian and Lower Maastrichtian sediments of the Negev, Israel. Deux paragenèses associées : Si-P-Fe et Si-P-C dans les sédiments du Campanien supérieur et du Maastrichtien inférieur du Néguev, Israël

Two geochemical-mineralogica ! associations, Si-P-Fe and Si-P-C, are found in the Upper Cretaceous strata of the Negev, southern Israel. The Si-P-Fe association occurs only within a very short time span, represented by the uppermost part of Mishash Formation and the lower part of the Ghareb Formation (Upper Campanian-Lower Maastrichtian). The Si-P-C association occurs in the same time interval, and also lower in the section within the Mishash Formation. There appear to be a structural control (water depth) on the occurrence of these two associations with Si-P-Fe occurring in higher structural positions than Si-P-C. The two associations occur in close proximity, in places within less than 500 m of each other, but they are not genetically related since the Si-P-C association has a depositional origin while the Si-P-Fe is diagenetic. In the Si-P-C association organic carbon was preserved along the synclinal axes together with carbonate, silica and phosphate accumulations. The thickness and P2O5 content of the «main phosphorite » bed increases downslope, and in places this trend is enhanced by the presence of Campanian faults, with the down-faulted margins serving as traps for the accumulation of phosphate. In the Si-P-Fe association, iron oxide, mostly goethite, associated with apatite nodules, is located on the upper slopes, occurring as concretions, grains and as ooliths consisting of an intergrowth of goethite and apatite. Iron and phosphate grains are in places replaced by silica ; the replacement probably took place before the final burial, since the phosphate host rock is in most cases not affected.

Ironstones in the Mesozoic passive margin sequence of the Tethys Himalaya (Zanskar, Northern India): sedimentology and metamorphism

Summary The sedimentary succession of the Zanskar continental terrace comprises Triassic (Quartzite Series), Jurassic (Ferruginous Oolite) and Cretaceous (Giumal Sandstone) shelf siliciclastic units, all of which contain either chamositic ironstones or glauconitic greensands associated with reworked phosphorites. Ironstones are found above major unconformities at the top of shoaling arenaceous sequences, and generally mark the rapid transition from shallow-marine sands to highstand offshore pelites or pelagic foraminiferal limestones. Petrographical and sedimentological features of condensed intervals indicate deposition by transgressive fronts. During sea-level rise, high-energy waves mixed ferruginous and phosphatic grains formed in the course of earlier starved stages at low sedimentation rates, with detritus from shoreline and paralic sources reworked during stepwise coastal retreat. The timing of ironstone deposition apparently coincides with breakup stages affecting the Indian continental margin. Ironstones are also important metamorphic markers in the Tethyan sedimentary zone, which has undergone intense fold-thrust deformation at very low to low metamorphic grade during the Tertiary Himalayan orogeny. In the Triassic iron oolites of central Zanskar, the occurrence of stilpnomelane suggests upper anchimetamorphic conditions ( c. 300° C). In the Cretaceous greensands, however, K-rich glauconite is only peripherally replaced by incipient stilpnomelane growth, pointing to metamorphic conditions comparable to lower prehnite-pumpellyite facies ( c. 260° C).