Minerals pyromorphite

About 1770 phosphorus was recognised as an essential ingredient of animal bones and teeth by C.W. Scheele, when he prepared the element from bone ash, carbon and sand. By 1779 the first phosphorus-containing mineral, Pyromorphite, had been identified by J.G. Gahn. 

Various apatites of lead are knowni56 i with the general stoichiometry 3[Pb3(P04)2].PbX2 where X = F, Cl, Br, I and OH. These apatites of lead form a homogeneous crystallographic group in the same way as the better-known apatites of calcium. The chloroapatite 3[Pb3(P04)2].PbCl2 occurs in nature as the mineral pyromorphite (density 7-01 g/cc, dielectric constant 47 5 at 2 = 75 cm). 

In sediments of the Mezica mining and smelting area, mineral phases such as cerussite (PbC03), galena (PbS) (Fig. 2), pyromorphite (Pb5(P04)3CI), descloizite (PbZn(V04)(0H)), bindheimite... 

The principal lead oxide minerals include pyromorphite, wulfenite, mimetite and plum-bojerusite. Some galena is also present in this ore type. The principal gangue minerals include silicate, dolomite, siderite, ferohydrooxides and clay minerals. About 20% of the ore is represented by ultra-fine slime with an average size of K%0 = 12 pm. These slimes are composed of Fe-hydroxides, kaolin and slimes of plumbojerusite, rich in silver. 

Pyromorphite belongs to the apatite group. This group is divided into two series, the apatite series and pyromorphite series. These minerals are isostruc-tural, however the unit cell volume of pyromorphite is about one-fifth larger... 

As was described earlier, evidently Ce " emission in pyromorphite is characterized by a very long decay component, which is not typical for this luminescence center. The possilile reason may be energy migration from Pb ions, which are the major elements in these minerals. It is well known that Pb is a very effective sensitizer for many luminescence centers in minerals, including Mn " and REE (Marfunin 1979). The excitation spectrum of Ce luminescence in esperite revealed a UV band at 280 nm, which is not typical for Ce luminescence and is possibly connected with Pb excitation with energy migration. 

At some distance close to the mineral surface, the saturation index (SI, IAP/Xsp) of pyromorphite must be greater than zero to allow local super saturation and precipitation. The question is whether this is a heterogeneous nucleation/ surface precipitation reaction or a dissolution homogeneous nucleation reaction. 

Wondratschek, H. Z. Kristallchemie des Blei-Apatits (Pyromorphit). Neues Jb. Miner. 

PYROMORPHITE. The mineral pvroinorphite is lead chlorophosphate with a formula corresponding to Pb PO Cl. The phosphorus is sometimes replaced by arsenic and the lead by calcium. It occurs in prismatic, sometimes hollow, hexagonal crystals or may appear m massive forms. It is brittle hardness, 3.5-4 specific gravity, 7.04 luster, resinous color, green, yellow-green, yellow, brown, and less often gray or white translucent to opaque. 

The most common apatite is Ca5(P04)30H and is called hydroxyapatite. Other forms include chloroapatite (Ca5(P04)3Cl), fluoroapatite (Ca5(P04)3F) and carbonate apatite or dahllite (Ca5(P04)3C03). These minerals are in pure forms, but it is also possible to generate them by partial replacement of one anion by another or one cation by another. For example, Ca may be replaced by Pb by ionic substitution, yielding pyromorphites [Pb5(P04)3(0H,Cl,F)]. As we shall see in Chapter 16, this mineral is very important in stabilizing the hazardous metal Pb. Also as discussed in Chapter 2 and shall be seen in later chapters, Mg-based CBPCs have many applications, and hence minerals such as Mg5(P04)3(0H,Cl,F) are also very common. 

Lead is a heavy soft bluish-gray metal (symbol Pb atomic no. 82) that is widespread as different salts in minerals such as chromite (oxide), crocoisite (chromate), Jamesonite and zinkenite (sulfides), mimetite (arsenate and chloride), pyromorphite (phosphate and chloride), and vanadinite (vanadate and chloride). 

A reduction in system pH enhances the solubility of PR, making the precipitation of pyromorphite minerals possible. However, the sorption of Pb decreases sharply as the system pH decreased, producing a sigmoidal function, usually referred to as an adsorption edge, which reflects the affinity of a metal species for a mineral surface (Sposito, 1984). The ability of Pb to form inner-sphere surface complexes is related to the ability of a species in solution to form hydroxides. In fact, it has been shown that surface affinity of metal cations for Fe-oxide and Fe-hydroxide surfaces agrees with their hydrolysis values (Hayes and Katz, 1996). An analogy between solution complexation and surface complexation is represented in the following reactions (Hayes and Katz, 1996) ... 

In a system where a P mineral (e.g., PR or HA) reacts with aqueous Pb, both Ca and P concentrations are controlled mainly by the dissolution of the P mineral [Eq. (15.1)]. This is because the precipitation of pyromorphite from... 

The effectiveness of P in decreasing water-soluble Pb in aqueous solution as well as in soils has been well demonstrated (Ma et al., 1993, 1995 Ma and Rao, 1999). Consistent with the mechanism of dissolution of P minerals and subsequent or concurrent precipitation of pyromorphite minerals, the effectiveness of such a decrease depends on the solubility of the P mineral used. For example, at a rate of 4 g HA L HA effectively decreased aqueous Pb from 500 mg... 

Templeton et al. (2002a) used a combination of Pb Lm-XAFS and pXANES spectroscopy and transmission electron microscopy to show that B. cepacia causes biomineralization of Pb(II) in the form of highly insoluble pyromorphite at ( ) concentrations well below supersaturation with respect to pyromorphite. The phosphate in these minimal medium experiments is though to be provided by B. cepacia, and the pyromorphite forms on the outer cell membrane of B. cepacia. These types of studies are beginning to provide unique information on how microbial biofilms affect metal sorption processes at mineral surfaces, which is essential for understanding the transport and bioavailability of toxic metal ions in natural systems where such biofilms exist. They are also allowing quantitative evaluation of the competition between NOM (or biofilms) and the mineral substrates they coat for metal ion binding. 

Conway BE (1981) Ionic hydration in chemistry and biophysics. In Studies in Physical and Theoretical Chemistry 12, Elsevier Scientific Publishing Co., Amsterdam, p 59-74 Cotter-Howells JD, Champness PE, Chamock JM (1999) Mineralogy of lead-phosphorous grains in the roots of Agrostis capillaries L. by ATEM and EXAFS. Mineral Mag 63 777-789 Cotter-Howells JD, Champness PE, Chamock JM, Pattrick RAD (1994) Identification of pyromorphite in mine-waste contaminated soils by ATEM and EXAFS. J Soil Sci 45 393-402 Cowan PL, Golovchenko JA, Robins MF (1980) X-ray standing wave at crystal surfaces. Phys Rev Lett 44 1680-1683... 

Dai Y, Hughes JM (1989) Crystal structure refinements of vanadinite and pyromorphite. Can Mineral 27 189-192... 

Fleischer M, Altschuler ZS (1986) The lanthanides and yttrium in minerals of the apatite group - an analysis of the available data. N Jahrb Mineral Monatsh 467-480 Fleischer M, Mandarine JA (1995) Glossary of Mineral Species (7th ed). Mineral Record, Tucson, Arizona. Fortsch E, Freiburg IB (1970) Untersuchungen as mineralien der pyromorphit grappe. N Jahib Mineral Abh 113 219-250... 

Wondratschek H (1963) Untersuchungen zur kristallchmie der blei-apatite (pyromorphite). N Jahrb Mineral Abh 99 113-160... 

Curry NA, Jones DW (1971) Crystal structure of brashite, calcium hydrogen orthophosphate dihydrate A neutron-diffraction investigation. J Chem Soc 3725-3729 Dai Y, Hughes JM (1989) Crystal-structure refinements of vanadinite and pyromorphite. Can Mineral 27 189-192... 

Relatively little work has been carried out on fission track dating of phosphate minerals other than apatite, and merrillite, P-Ca3(P04)2, is the only other example to have received any significant attention. Two early studies examined the potential of monazite, (Ce,La,Y,Th)P04, and pyromorphite Pb5(P04)3Cl, but these minerals have not been studied further. Pyromorphite was shown by Haack (1973) to contain numerous spontaneous fission tracks, but these were very unevenly distributed, making its use in dating very difficult. 

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