Sedimentary Apatite

The active participation of the So level is indirectly accredited by the simultaneous observation of UV and visible emission in cathodoluminescence 

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Fig. 4.5. a-c Laser-induced time-resolved luminescence spectra of sedimentary apatite (francolite) (a) and francolite activated by Pr (b,c) after heating demonstrating uranyl and Pr +, Eu + and Sm + centers... 

Sedimentary carbonate-fluor-apatite Cas(P04,003)3 is not luminescent under X-rays, and under UV lamp excitation it is characterized by broad structureless bands which are very similar to those encountered in many sedimentary minerals. It was concluded that this luminescence is due to different kinds of water-organic complexes (Tarashchan 1978). For this reason the luminescence properties of the sedimentary apatites is much less informative compared to magmatic apatite and attracted not much attention. 

The spectral-kinetic parameters of the green laser-induced luminescence of the sedimentary apatites allow its association with emission. The spectra... 

The spectra of the green laser-induced luminescence represented in Fig. 4.4a, together with their decay time, also allows its association with These luminescence spectra strongly differ from the spectral parameters of all known uranyl minerals. For this reason it is not possible to connect this type of green luminescence with finely dissipated uranyl phases. On the other hand, this luminescence is very similar in such different host minerals as sedimentary apatites, opalites, chalcedony, chert, quartz and barites. Luminescence independence from the minerals structure evidences that it may be connected with uranyl adsorption on the minerals surface, supposedly in the form of (UO2 X nH20)2+. 

X-ray excited steady-state luminescence has been checked as a distinctive feature for the phosphorites, Karatau (Gorobets et al. 1997a). Sedimentary apatite (francolite) is not luminescent under X-ray excitation and red Mn " luminescence of dolomite was employed. As a result, concentration of MgO in the accepted fraction was lowered from 5.0% to 2.4%. The use of this method is limited because in many cases the X-ray luminescence of dolomite is very low or non-existent. Besides that, other minerals without luminescence come together with the accepted phosphate fraction, including sihceous shale, which is harmful for the following deep beneficiation. Thus new methods have to be developed for detection and content evaluation of dolomite in Florida phosphates on moving belt conveyors. 

Our study of sedimentary apatite from Israel proved that laser-induced time-resolved luminescence is a perspective tool for evaluation of sedimentary phosphate ores with high dolomite content (Gaft et al. 1993b). The idea was based on the fact that natural apatite contains several characteristic luminescence centers, which enables us to differentiate it from dolomite. The most widespread characteristic luminescence center in sedimentary apatite is uranyl (U02) with a typical vibrational green band luminescence under nitrogen laser excitation (Fig. 8.13a,b). Nevertheless, it appears that such luminescence is absent in phosphate rock samples from Florida, evidently because of extremely low uranium concentration (Fig. 8.13c,d). hi order to find potential liuninescence centers, ICP-MS analyses of Florida phosphates was accompHshed. From discovered REE, theoretically Dy + is the best candidate... 

The most common and widely distributed phosphate minerals are the apatite group, with the general formula Ca10(PO4)6(X)2. The apatite is designated as fluorapatite, hydroxyapatite, or chlorapatite, when X = F, OH, or Cl, respectively. The most abundant sedimentary apatite is carbonate fluorapatite (ffancolite). Relative to pure fluorapatite, francolite is characterized by the substitution of Na and Mg for Ca and of carbonate and fluoride for phosphate. An empirical formula for francolite... 

The primary marine sedimentary apatite mineral is CFA (or francolite McClellan, 1980 Kolodny, 1981 Nathan, 1984 McClellan and Van Kauwenbergh, 1990 Follmi, 1996). CFA is... 

Guidry M. W. and Mackenzie F. T. (2003) Igneous and sedimentary apatite dissolution and the long-term phosphorus cycle. Geochim. Cosmochim. Acta 67(16), 2949-2963. 

Phosphate ores, especially sedimentary apatites, are widely distributed. There are massive beds particularly in the USA and Africa. The estimated phosphate content of these reserves is many tens of millions of tons, which at the rate of present exploitation is sufficient for over 100 and up to 1000 years. An estimate of the reserves, the exploitation of which, according to current standards, is still not yet economic, is given in Table 1.5-1. 

Sedimentary apatite deposits are in quantity much more important than the magmatic ones. 

Although the number of studies of fluorapatite growth kinetics is smaller than for hydroxylapatite, they have particular relevance to sedimentary apatite formation. Amjad et al. (1981) repeated the experiments of Koutsoukos et al. (1980) (pH = 7.40, T = 37°C, S = 10-30) for fluorapatite and similarly found that the rate equation held for FAP with an apparent reaction order of 1.25 indicating a spiral growth mechanism. Van Cappellen and Berner (1991) conducted seeded fluorapatite growth rate experiments in a carbonate-free simulated seawater solution at different degrees of saturation, S. They fit their growth rate data with an empirical rate law of the form... 

McClellan GH, Van Kauwenbergh SJ (1990) Mineralogy of sedimentary apatites. In Phosphorite research and development. Notholt AJG, Jarvis I (eds) Geol Soc Spec Pubs 52 23-31 McConnell D (1938) A stractural investigation of the isomorphism of the apatite group. Am Mineral 23 1-19... 

Shemesh A (1990) Crystallinity and diagenesis of sedimentary apatites. Geochim Cosmochim Acta 54 2433-2438... 

Schweitzer MH, Johnson C, Zocco TG, Homer JR, Starkey JR (1997) Preservation of biomolecules in cancellous bone of Tyrannosaurus rex. J Vert Paleo 17 349-359 Shaw HF, Wasserburg GJ (1985) Sm-Nd in marine carbonates and phosphates Implications for Nd isotopes in seawater and crastal ages. Geochim Cosmochim Acta 49 503-518 Shemesh A (1990) Ciystallinity and diagenesis of sedimentary apatites. Geochim Cosmochim Acta 54 2433-2438... 

UV at 248 and 278 nm, and sets of narrow bands centered at about 488 and 615 nm (Mitchell et al. 1997) yielding a brick-red emission color. Natural samples of hydroxyl-apatite and fluorapatite show similar CL bands (Gaft et al. 1999). Time-resolved emission spectra (Fig. 19) show well defined Pr emission in natural magmatic apatite at 485 and 607 nm (Gaft et al. 1999). Studies using polarized emission measurements on this sample were interpreted as representative of only Cal site occupation by Pr (Reisfeld et al. 1996). In contrast, a sedimentary apatite annealed in air showed a different Pr spectrum, with a set of bands centered at about 630 nm (Fig. 19). This spectrum was interpreted to be due to Pr in Ca2 (Reisfeld et al. 1996). Pr appears to be an efficient sensitizer for Sm, as many of its transition energies are almost identical to Sm, and in general Pr probably is more important as a sensitizer of other REE than for its own emission (Mitchell et al. 1997). 

It is practically impossible to find pure samples of sedimentary apatites. This fact has plagued apatite mineralogists since apatite studies were first performed. In order to evaluate these sedimentary apatites, each chemical analysis was recalculated to a 100% apatite basis. Based on previous studies, it was assumed that the excess-fluorine francolites contained CaO, Na20, MgO, P2O5, CO3, SO4, and F. 

The determination of the type and amount of apatite, in a phosphate rock is very important. Sedimentary phosphate rocks containing the most highly carbonate-substituted apatites can only be beneficiated to maxi-mumrgradesrofr33-34 wt % P2O5. Depending on the amount of carbonate substitution, the grade can increase to a maximum of about 42 wt % P2O5 for sedimentary apatites with essentiaDy no substitution. In order to reach... 

Mineralogy of Sedimentary Apatites and the Relationship to Phosphate Rock Reactivity, Paper presented at the National Workshop on Fertilizer Efficiency. Cisarua, Indonesia, November 12-13, 1990.. ... 

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