Phosphate Minerals
Readings
Lindsay, W.L., P.L.G. Vlek, and S.H. Chien. 1989. Phosphate
Minerals. Ch. 22, p. 1089-1130. In: J.B. Dixon and S.B. Weed (ed.),
Minerals in Soil Environments, 2nd Edition.
Phosphates
By one count, 172 naturally-occurring phosphate minerals have
been identified. A number of phosphate mineral classification schemes exist but
we will follow that chosen by Lindsay et al. (1989), which is that of
Povarennykh (1972; Crystal chemical classification of the minerals. Vol.
1 & 2, Plenum Publ. Corp., N.Y.) and which is similar to that used by
silicate minerals--framework, chain, and layer phosphates. However, unlike the
silicate polyanion, SiO44-, the ortho-phosphate
polyanion, PO43-, does not generally polymerize in the
formation of crystal structure and so the analogy to silicates is necessarily
that, an analogy. [When phosphates do polymerize, polyphosphates are formed,
starting with pyrophosphate,
PO2(=O)-O-P(=O)O24-, in which a bridge O links
to phosphate tetrahedra. Such condensation reactions to prepare polyphosphates
require oven temperatures when conducted industrially but cellular organisms
routinely conduct low-temperature synthesis of adenosine triphosphate (ATP),
which consists of an ester bond between tripolyphosphate and adenosine and
which is dephosphorylated to adenosine diphosphate (ADP) to release energy.
Polyphosphates are produced from phosphoric acid by the fertilizer industry as
a phosphorus source.]
With so many phosphate minerals identified and organized into
14 groups in four classes, it seems practical to limit discussion to those
typically found in the soil environment. Apatite constitutes 95% of the P in
igneous rocks and, due to its persistence, almost certainly constitutes a
similar percentage of the P in sedimentary rocks and soils. Apatites have the
general formula A10(XO4)6Z2, where
A is most often Ca2+, X is P(5), and Z is most often OH and/or F.
Other divalent cations may substitution in A, among them Sr, Mn, Pb, Mg, Ba,
Zn, Cd, as well as monovalent cations (Na, K, and Rb) and trivalent cations
(Sc, Y, Bi). For X, many elements that form tetrahedrally-coordinated oxides,
among them Si, S, As, V, Cr, and Be, can substitute for P.
Apatite is very insoluble and there is relatively little or no
fertilizing potential for rock phosphate in unprocessed form. Therefore,
phosphorus fertilizers are produced by acidulating rock phosphate, ore
containing apatite of either the hydroxyapatite, fluorapatite, or francolite
(carbonate) varieties, by one of two general schemes: Either sulfuric acid
sufficient to produce a mixture of monocalcium phosphate and gypsum [the
mixture marketed as "superphosphate"], or sulfuric acid sufficient to produce
phosphoric acid, which is separated from gypsum by centrifugation and either
reacted with more apatite to form monocalcium phosphate [marketed as "triple
superphosphate"] or compounded with ammonia to produce mono- or di- ammonium
phosphates or, less commonly, roasted to form polyphosphates. These processes
produce water-soluble phosphate fertilizers; partially acidulated rock
phosphate routinely underperforms as a fertilizer material.
Among solution chemists, discussion of phosphate chemistry in
soils often considers the presence of strengite,
FePO4·2H2O, and variscite,
AlPO4·2H2O, as controlling the solubility of
phosphate in acid and slightly acid soils. However, early reports (based
largely on x-ray identification) of strengite and variscite as reaction
products of phosphate fertilizers with acid soils have been challenged and
presumed solubility control by strengite and variscite is not sufficient
evidence of their presence in soils. Among surface chemists, discussion
revolves around the adsorption of phosphate on iron and aluminum oxides, aging
into less labile forms with time. Current approaches to considering phosphorus
movement in the environment, principally that of A. Sharpley and co-workers, is
to propose a "phosphorus capacity" equal to the sum of ammonium
oxalate-extractable iron and aluminum oxides.
Addition of phosphate fertilizers to calcareous soils is
thought to produce a series of calcium phosphates:
monocalcium phosphate (fertilizer) ==> dicalcium
phosphate dihydrate ==> octacalcium phosphate ==>
hydroxyapatite
| Proposed reaction series for phosphate
fertilizers in calcareous and neutral soils: |
|
| Name |
Chemical Formula |
Ca/P |
| monocalcium phosphate |
Ca(H2PO4)2 |
1/2 |
= 0.5 |
| brushite, dicalcium phosphate dihydrate |
CaH(PO4)·2H2O |
1/1 |
= 1.0 |
| octacalcium phosphate |
Ca8H2(PO4)6·5H2O |
8/6 |
= 1.3 |
| hydroxyapatite |
Ca10(PO4)6(OH)2 |
10/6 |
= 1.7 |
In reality, the existence of 49 known calcium
phosphates, 14 magnesium phosphates, 49 aluminum phosphates, and 51 iron
phosphates (and some contain one or more of these cations) points to the
complexity of the chemistry of phosphate minerals in soils and it is dubious
that any fertilizer reaction species or series is more than
illustrative.
Methods
Phosphate minerals comprise only a fractional percentage of
the soil mass, perhaps averaging 0.02% or so, which impedes their direct
identification by optical microscopy and x-ray analysis unless their
concentration has been locally increased by addition of phosphate fertilizers.
In some cases, beneficiation of native phosphate minerals is possible by
dissolving ancillary minerals with HF; yet others separate phosphate minerals
based on heavy liquid separations. Advances in microscopy allow both
semi-quantitative elemental analysis and electron diffraction of individual
particles.
Another phosphate mineral of some interest is struvite,
NH4MgPO4. That this ammonium salt is insoluble is
somewhat surprising given that ammonium phosphates, both mono- and di, are
quite soluble, as are most other ammonium salts. Because this mineral contains
nitrogen, an element associated with biota, it is to be found in conjunction
with organisms, either alive or dead: urinary stones, manure, canned fish, or
cadavers immersed in seawater. In the past, struvite formation has been used as
an analytical technique for gravimetric determination of Mg2+, by
addition of ammonium phosphate to the test solution.
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