Iron phosphate minerals

Nitrogen pollution has received far more attention than that of phosphorus for two reasons. First, it has been considered as the nutrient-limiting primary production in estuaries and coastal waters. Second, its loading into the coastal zone has been far greater than that of phosphorus (Figure 24.21). It is also more efficiently exported into the ocean due in part to formation of iron phosphate minerals in anoxic estuarine sediments. 

Fig. 18.6. The iron phosphate mineral cacoxenite. FeOs octahedra are shown in gray and AIO4 and PO4 tetrahedra are represented in light gray. Water molecules are omitted for clarity.

The minerals of the burangaite, Na[Fe Al5(P04)4(OH)6(H20)2], group contain a trimer of face-sharing octahedra that is a feature of several basic iron-phosphate minerals (Moore 1970). An (Fe ( )6) octahedron shares two trans faces with (Al( )6) octahedron to form a trimer of the form (the h cluster of Moore 1970). This trimer is corner... 

Moore PB, Kampf AR (1977) Schoonerite, a new zinc-manganese-iron-phosphate mineral. Am Mineral 62 246-249... 

S.2.3 Aluminum and Iron Phosphates - A total P2O5 analysis of a potential ore is not a dependable criterion for evaluation of a phosphate deposit. With the weathering of apatitic phosphates and associated minerals, a suite of other secondary phosphate minerals may form (Table 5.7). These minerals are most com--monly-aluminurn and.irorhConteining phosphates. These minerals cannot be processed by the methods used to beneficiate apatitic calcium phosphates. Aluminum and iron phosphate minerals may form in both sedimentary and igneous deposits. 

Considering the substantial fraction of total iron that is present in this second phase ( 40 %), it is puzzling that this phase was not detected in the X-ray dififractogram. A possible explanation could be that the involved impurity concerns an amorphous iron phosphate. It should be noted that recently Fehr et al. [266] reported the Mossbauer spectrum of a naturally occurring Mn-substituted heterosite (Sandamab, Namibia). In addition to the heterosite doublet with A = 1.63 mm/s, the authors found an inner doublet having A = 0.69 mm/s. They concluded that this inner doublet is due to an associated iron-phosphate mineral. 

Rose (1916) states HaaXElsenblau ( ironblue ) mdBlaueisenerde ( blue iron earth ) were terms for the iron phosphate mineral, vivianite (q.v.). 

Phosphorus is the eleventh element in order of abundance in crustal rocks of the earth and it occurs there to the extent of 1120 ppm (cf. H 1520 ppm, Mn 1060 ppm). All its known terrestrial minerals are orthophosphates though the reduced phosphide mineral schrieber-site (Fe,Ni)3P occurs in most iron meteorites. Some 200 crystalline phosphate minerals have been described, but by far the major amount of P occurs in a single mineral family, the apatites, and these are the only ones of industrial importance, the others being rare curiosities. Apatites (p. 523) have the idealized general formula 3Ca3(P04)2.CaX2, that is Caio(P04)6X2, and common members are fluorapatite Ca5(P04)3p, chloroapatite Ca5(P04)3Cl, and hydroxyapatite Ca5(P04)3(0H). In addition, there are vast deposits of amorphous phosphate rock, phosphorite, which approximates in composition to fluoroapatite. " These deposits are widely... 

The iron formed in a blast furnace, called pig iron, contains impurities that make the metal brittle. These include phosphorus and silicon from silicate and phosphate minerals that contaminated the original ore, as well as carbon and sulfur from the coke. This iron is refined in a converter furnace. Here, a stream of O2 gas blows through molten impure iron. Oxygen reacts with the nonmetal impurities, converting them to oxides. As in the blast furnace, CaO is added to convert Si02 into liquid calcium silicate, in which the other oxides dissolve. The molten iron is analyzed at intervals until its impurities have been reduced to satisfactory levels. Then the liquid metal, now in the form called steel, is poured from the converter and allowed to solidify. 

During the lifetime of a root, considerable depletion of the available mineral nutrients (MN) in the rhizosphere is to be expected. This, in turn, will affect the equilibrium between available and unavailable forms of MN. For example, dissolution of insoluble calcium or iron phosphates may occur, clay-fixed ammonium or potassium may be released, and nonlabile forms of P associated with clay and sesquioxide surfaces may enter soil solution (10). Any or all of these conversions to available forms will act to buffer the soil solution concentrations and reduce the intensity of the depletion curves around the root. However, because they occur relatively slowly (e.g., over hours, days, or weeks), they cannot be accounted for in the buffer capacity term and have to be included as separate source (dCldl) terms in Eq. (8). Such source terms are likely to be highly soil specific and difficult to measure (11). Many rhizosphere modelers have chosen to ignore them altogether, either by dealing with soils in which they are of limited importance or by growing plants for relatively short periods of time, where their contribution is small. Where such terms have been included, it is common to find first-order kinetic equations being used to describe the rate of interconversion (12). 

The impurities in pig iron, the iron formed in a blast furnace, that make it brittle include four elements phosphorus and silicon, two elements that came from the silicate and phosphate minerals that contaminated the original ore, and carbon and sulfur that came from the coke. 

Phosphate is remineralized during the oxidation of organic matter and dissolution of hard parts, such as bones and teeth, that are composed of the minerals hydroxyapatite and fluoroapatite. Unlike the other products of remineralization, pore-water phosphate concentrations are regulated only by mineral solubility, such as through vivianite (iron phosphate) and francolite (carbonate fluoroapatite). Redox reactions are not significant because phosphorus exists nearly entirely in the h-5 oxidation state. 

The sodium phosphates can be made on a large scale from calcium phosphate or bone ash, the mineral phosphates, pr Thomas slag. The calcium phosphate or bone ash can be digested with sulphuric acid and the resulting phosphoric acid (g.v.) treated with sodium carbonate or hydroxide. J. Neustadtl treated the bone ash with hydrochloric acid, and mixed the soln. with Glauber s salt, and neutralized the filtered soln. with soda. R. Holver-Scheit digested iron phosphate from Thomas slag with sodium sulphide under press. L. Blum,... 

J. T. Way, and T. Twynam heated Thomas slag with soda, and extracted the alkali phosphate with M ater. N. A, Helouis and M. Rychonnet, L. Imperatori, and F. Jean calcined a mixture of the phosphate with sodium sulphate and carbon C. Schwarz, and M. Boblique heated the mineral with iron so as to make ferric phosphate, which was then heated with sodium sulphate and carbon. In each of these cases the alkali phosphate was leached from the mass. M. Drevermann treated the iron phosphate with sodium sulphide, C. Clemm with potassium sulphide. 

By far the most important activators in mineral luminescence are the iron group ions which exhibit transitions between partly filled d-orbitals. These will dominate the discussion that follows. Luminescence arising from the trivalent rare earth ions occurs in some phosphate minerals but is dealt with elsewhere in this volume (Wright). The filled d-shell ions are activators for cathodoluminescence phosphors such as ZnS, however, most sulfide mineral phases contain too many luminescence poisons for the transitions from these ions to be observed. 

Another group of As-bearing minerals contains arsenic in the 5+ valence state as arsenate commonly substituting for the phosphate group. An unidentified As-bearing iron phosphate, usually associated with banded iron oxyhydroxide as veins or masses, has a P/As atomic ratio on the order of 4 (Belkin et al., 1997). Jarosite... 

Phosphorus extracted from sediment by NaOH has been related to non-occluded, surface-exchangeable, bioavailable forms (22). Hydrochloric acid extraction yields occluded phosphorus incorporated in hydrous metal oxides, carbonate and phosphate minerals of sediment. Hydroxylamine reagent specifically removes hydrous manganese oxides, while amorphous hydrous oxides of iron and aluminijm are removed by the oxalate reagent. Total available sediment phosphorus analyses includes sediment organic phosphorus components in addition to the inorganic portion determined by the selective extraction procedures. 

Dissolved metals other than calcium have a minor effect on the distribution of phosphorus between the water column and sediment in this fluvial system. The two principal metals of potential interest, iron and aluminum, are present in Genesee River water almost entirely in the particulate phase ( ). Dissolved concentrations of these metals are below the detection limit (less than 50 ug/1). Iron and aluminum minimum detectable dissolved concentrations were used to estimate the saturation levels of the corresponding phosphate minerals. These calculations suggest that both iron and aluminum phosphate minerals are substantially below saturation levels. The solid surfaces exhibited by iron and aluminum hydrous oxides (as particulate material in the water column) undoubtedly serve as sites for phosphorus adsorption and incorporation in the fluvial system. Data presented for the oxalate extraction procedure in Table III demonstrate the importance of phosphorus binding by hydrous metal oxides. 

Differences in river basin morphology, soil characteristics, rainfall, and land use in a watershed Influence phosphorus transport in a fluvial system. However, the dominance of iron oxides as an inorganic phosphate sink and the discharge dependent behavior of calcium carbonate-phosphate minerals found in this study would be expected to exist in other calcareous agricultural regions of New York State as well. Mountainous terrain and areas of sand and muck soil would probably not exhibit the same behavior. It would seem that the results of this study could also apply to other agricultural watersheds adjacent to the North American Great Lakes. 

Sediment phosphorus extraction analyses show that hydrous iron oxides (extracted by (NH4)2C204) play a major role in the transport of sediment phosphorus. In northern areas of the Genesee River watershed calcium carbonate formation also appears to be Involved in phosphorus fixation. Ion activity product calculations for water column samples from the Genesee River consistently exhibit subsaturation with respect to the stable calcium phosphate phase, hydroxyapatite. Calcium carbonate, which can serve as a substrate for phosphate mineralization, shows an ion activity product below the solubility product in the Genesee River except during the summer low-rainfall season. 

Past depletions in dissolved inorganic phosphate may have also limited nitrogen-fixation and primary production due to high adsorption of phosphate by iron-rich minerals. Indeed, based on adsorption isotherms in banded iron formations, sea-water phosphate concentrations in the Archean have been suggested to have been more than ten fold lower than the present values (0.15-0.6 pM versus the modem value of 2.3 pM) (Bjermm and Canfield, 2002). Thus, present restrictions of phosphate on N2 fixers in some oligotrophic areas such as the western subtropical Atlantic (Mills et ai, 2004) and in the tropical Pacific (Moutin et al., 2007) may have correlations in the paleoceanographic records of the late Archean eon. 

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