Mineralization apatite precipitate

As discussed in Chapters 5 and 7, the use of lime to precipitate calcium arsenates is a common method for removing inorganic As(V) from water or flue gases. Calcium arsenates were also once extensively used in pesticides (Chapter 5). The compositions of some calcium arsenates, such as johnbaumite (Ca5(As04)3(0H) Table 2.5), resemble the very common phosphate mineral, apatite (Ca5(P04)3(F,Cl,0H)), where arsenate replaces phosphate. Some lead arsenates, such as mimetite (Pb5(As04)3Cl Table 2.5), also have crystalline structures that are related to apatite. Mimetite may occur in oxidized lead-rich hydrothermal deposits. 

Phosphoric acid and phosphate salts are produced either by oxidation of elemental phosphorous or by extraction of the phosphate mineral Ca3(P04)2 (apatite) with sulfuric acid. Production from elemental phosphorous is energetically more demanding and therefore capacities are increasingly shifting towards the extraction process. However, the production of food grade phosphoric acid from the natural mineral apatite requires additional separation and purification steps to remove heavy metals (such as e.g., Cu or As) from the crude phosphoric acid. Precipitation techniques and countercurrent extraction with organic solvents, such as n-butanol or diisopropyl ether, are applied for this purpose. 

From the data in fig. 21.19 it can be seen that most minerals do not remove lanthanides effectively from silicate liquids (i.e., lanthanide D values are mostly less than unity). Exceptions are apatite (a lanthanide-concentrating mineral that is a common late-stage product of crystallization of mafic liquids) and garnet (which concentrates heavy lanthanides but not light lanthanides). In acid magmas, lanthanide minerals may precipitate these are discussed in later sections. Ca-rich clinopyroxenes and hornblendes (related to pyroxenes, but more compositionally complex) compete for lanthanides successfully against somewhat acidic lavas (Nagasawa and Schnetzler, 1971). 

Hydroxyapatite, Ca2Q(PO (OH)2, may be regarded as the parent member of a whole series of stmcturaHy related calcium phosphates that can be represented by the formula M2q(ZO X2, where M is a metal or H O" Z is P, As, Si, Ga, S, or Cr and X is OH, F, Cl, Br, 1/2 CO, etc. The apatite compounds all exhibit the same type of hexagonal crystal stmcture. Included are a series of naturally occurring minerals, synthetic salts, and precipitated hydroxyapatites. Highly substituted apatites such as FrancoHte, Ca2Q(PO (C02) (F,0H)2, are the principal component of phosphate rock used for the production of both wet-process and furnace-process phosphoric acid. 

Inorganic reactions in the soil interstitial waters also influence dissolved P concentrations. These reactions include the dissolution or precipitation of P-containing minerals or the adsorption and desorption of P onto and from mineral surfaces. As discussed above, the inorganic reactivity of phosphate is strongly dependent on pH. In alkaline systems, apatite solubility should limit groundwater phosphate whereas in acidic soils, aluminum phosphates should dominate. Adsorption of phosphate onto mineral surfaces, such as iron or aluminum oxyhydroxides and clays, is favored by low solution pH and may influence soil interstitial water concentrations. Phosphorus will be exchanged between organic materials, soil inter-... 

The study of the basaltic dykes in evaporites demonstrates that dissolution and precipitation of phosphate minerals is a key process for the control of REE mobility and REE fractionation. In the present case, all REE found in secondary apatite in the basalt and in the salt are derived from the dissolution of primary magmatic apatite during basalt corrosion. This loss of REE from the basalt to the salt was not sufficient to lower significantly the REE concentrations of the basalt and it could only be detected by the analysis of the salt. The absolute quantity of REE transferred from the basalt into the salt, however, cannot be quantified because we have no three-dimensional control on the REE concentrations around the basalt apophy sis. 

Solid solutions within the apatite family are readily synthesized in the laboratory. Some examples include (Ca,Zn,Pb)5(P04)30H (Panda et al. 1991), (Ca,Cd,Pb)5(P04)30H (Mahapatra et al. 1995), (Ca,Sr,Cu)5(P04)30H (Pujari Patel 1989), or other apatite solid solutions containing various quantities of Cd, Mg, Zn, Cd or Y (Ergun etal. 2001). They are also found naturally (Botto etal. 1997). Unlike well-ordered naturally occurring minerals, these solid solutions may actually be the more typical form of the mineral in stabilized ash systems given the system complexity, rapid precipitation kinetics, and wide prevalence of available divalent cations. 

However, methane-diphosphonate could not prevent the growth of apatite crystals in vitro on prepared sinews of rats tail out of a metastable solution with calcium and phosphate ions. On the contrary, the precipitated crystalline particles were bigger and better crystallized than those from control solutions. This is in surprising contrast to most of the information from the literature. No other calcium phosphate minerals besides apatite have been found by X-ray diffraction, whereas under comparable conditions brushite and octacalcium phosphate grow on collagenous sinews549. ... 

The solvent extraction of rare-earth nitrates into solutions of TBP has been used commercially for the production of high-purity oxides of yttrium, lanthanum, praseodymium and neodymium from various mineral concentrates,39 as well as for the recovery of mixed rare-earth oxides as a byproduct in the manufacture of phosphoric acid from apatite ores.272 273 In both instances, extraction is carried out from concentrated nitrate solutions, and the loaded organic phases are stripped with water. The rare-earth metals are precipitated from the strip liquors in the form of hydroxides or oxalates, both of which can be calcined to the oxides. Since the distribution coefficients (D) for adjacent rare earths are closely similar, mixer—settler assemblies with 50 or more stages operated under conditions of total reflux are necessary to yield products of adequate purity.39... 

A topic of considerable controversy is the question of carbonate incorporation in the apatite lattice since carbonate apatite does not precipitate from aqueous solutions28, 395 398 Carbonate apatites (phosphorites) forming in marine environments134 are considered metasomatic alteration products of calcium carbonate, and as a result carbonate content decreases and phosphate content increases with time399. In biophosphates, the situation appears to be just reversed in that carbonate content increases with bone maturation and it was argued that the similarity between bone mineral and naturally occuring C03-apatite ends before it begins 397. ... 

Carbonate band assignment has been more difficult in Raman spectra than in the infrared [15] because of near overlap of the major carbonate Vi mode at 1070cm-1 with a component of phosphate V3 at 1076cm-1 in carbonated apatites [16]. These bands have earlier been reported as coincident [17] or have been assumed to be a single broad carbonate band [18]. Most investigators have used the ratio of the carbonate Vi band/phosphate Vi band as a measure of the carbonate/phosphate ratio. It is likely that in many cases this error is small, but for lightly carbonated mineral, typically freshly precipitated mineral, the error may be important. Remeasurement or reinterpretation of some Raman spectroscopic data may be needed. 

Phosphorites are sedimentary rocks that contain at least 15-20 wt % P2O5 (Boggs, 1995), 266. The phosphate in phosphorites primarily occurs as apatite (Ca5(P04)3(F,Cl,0H)). Typically, phosphorites chemically precipitate in deep, cold marine waters. Due to chemical similarities, arsenate may partially substitute for phosphate and the arsenic concentrations of phosphorites may exceed 100 mg kg-1 ((Matschullat, 2000), 299 Table 3.23). However, arsenic concentrations in some phosphorites (e.g. southeast Jordan) are relatively low (7-9 mg kg-1) and the arsenic is mostly associated with clay and carbonate minerals rather than phosphates (Al-Hwaiti, Matheis and Saffarini, 2005). 

While apatite and other calcium-based and substituted solid phases of phosphate minerals precipitated in the primary formation of cmstal sediments, secondary reactions of phosphate with... 

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