Min er al ogy of Mio cene phos phatic nod ules in SE Sic ily (It aly)
Giuseppe CULTRONE, Giorgio ANFUSO and Edu ar do SEBASTIÁN
Cultrone G., Anfuso G. and Sebastián E. (2008) — Min er al ogy of Mio cene phos phatic nod ules in SE Sic ily (It aly). Geol. Quart., 52 (1):
61–70. Warszawa.
This pa per de scribes the geo chem is try and pe trog ra phy of three phos phatic nod u lar de pos its in Ragusa Prov ince, south-east Sic ily (It - aly). Phos phate nod ules of late Burdigalian age are dis persed in a soft, fri a ble packstone ma trix within the Irminio Mem ber of the Ragusa For ma tion. Min er al og i cal anal y ses re vealed large amounts of cal cite (64 to 89 wt. %) and smaller quan ti ties of car bon ate-fluorapatite (CFA). P2O5 con tent was less than 18%. Microtextural ob ser va tions dem on strated that phos phate pre cip i ta tion oc curred in the microenvironments in the sed i ment in con fined spaces (i.e. cav i ties, per fo ra tions) and not in the shells of mi cro or gan isms. The crys tals were all of a sim i lar size (~1 mm), pre sented im per fec tions, and the
{
0001}
CFA and the{
1010}
CFA forms were clearly ap par ent. These hex - ag o nal prisms showed re duced growth along their c axis. Our data sug gest that bac te ria were in volved in the pre cip i ta tion of CFA.Giuseppe Cultrone, Edu ar do Sebastián, De part ment of Min er al ogy and Pe trol ogy, Uni ver sity of Granada, Avda. Fuentenueva s/n, 18002 Granada, Spain, e-mail: cultrone@ugr.es; Giorgio Anfuso, De part ment of Ge ol ogy, Fac ulty of Ma rine and En vi ron men tal Sci - ences, Uni ver sity of Cadiz, Polígono Río San Pedro s/n, 11510 Puerto Real, Spain (re ceived: April 12, 2007; ac cepted: No vem ber 10, 2007).
Key words: Late Burdigalian, francolite, crys tal lites, bac te ria.
INTRODUCTION
Geo log i cal in ter est in phos phatic sed i ments has been grow - ing dur ing re cent de cades be cause of their eco nomic sig nif i - cance, es pe cially in the fer til izer in dus try which con sumes about 90% of world pro duc tion (Zapata and Roy, 2004), and be cause they are use ful in palaeoenvironmental and palaeooceanographic re con struc tions (Carbone et al., 1987;
Martín Algarra and Sánchez Navas, 2000; Chen et al., 2004).
Phos phatic de pos its have been ob served all around the world (Baturin, 1971, 2000; Reimers et al., 1990; Ilyin, 1997;
Knight, 1999; Soudry, 2000) and have been re cov ered from the bot tom sed i ments of oceans, e.g. off the coasts of South Af rica (Birch, 1979), Peru and Chile (Froelich et al., 1988; Kim and Bur nett, 1988), south-east In dia (Purnachandra Rao et al., 2000), south ern Cal i for nia (Schuffert et al., 1994) and Florida (Foun tain and McClellan, 2000). The larg est phos phate de pos - its in Eu rope are the Up per Cre ta ceous phos phatic chalks that ex tend across parts of north ern France, south ern Eng land and Bel gium (Jarvis, 1992).
In palaeo geo graphi cal terms, autochthonous phos phatic sed i - ments, as so ci ated with con densed neritic and pe lagic de pos its, are used to iden tify time-gaps in strati graphic se quences (Mar shall
and Ruffell, 2004). Their com mon min eral phase is francolite, a car bon ate-fluorapatite (CFA), in which sub sti tu tion for and struc - tural dis tor tion of the crys tal lat tice are usual (Matthews and Na - than, 1977; Sánchez Navas and Martín Algarra, 2001).
As for their gen e sis, Van Cappellen and Berner (1991) and Gunnars et al. (2004) stated that phos phatic par ti cles can be pro duced by in or ganic pre cip i ta tion in sea wa ter so lu tions, by high rates of or ganic mat ter deg ra da tion and, prob a bly, by the dis so lu tion of fish de bris (Schenau et al., 2000; Soudry and Na - than, 2000). Fur ther more, bac te ria may in duce the pre cip i ta tion of a phos pho rus-en riched or ganic gel and act as a pre cur sor to an amor phous cal cium phos phate which in turn evolves into francolite (Soudry, 1993; Krajewski et al., 1994; Martín Algarra and Sánchez Návas, 1995; Schwennicke et al., 2000).
Lucas and Prévôt (1984) ob served how the type of wa ter in - volved (nat u ral fresh wa ter and sea wa ter) in flu ence the crystallinity of ap a tite syn the sis by bac te rial ac tiv ity. Phos phate authigenesis rep re sents an im por tant oce anic sink for re ac tive phos pho rus (Kim et al., 1999). The hy poth e ses that have been sug gested for the for ma tion of ma rine phosphorites are not nec - es sar ily ex clu sive, but they prob a bly rep re sent dif fer ent stages of the same phosphogenic pro cess (Jarvis, 1992).
This pa per aims to pro vide new in for ma tion about the phos - phatic de pos its in south-east Sic ily (It aly), es pe cially with re -
gard to their pe trog ra phy and gen e sis, by ana lys ing the char ac - ter is tics of three out crops in the prov ince of Ragusa. Ini tial re - search on these phos phate nod ules and their lithostratigraphic char ac ter is tics was car ried out by Ragusa (1902), De Stefani (1912) and Cortese (1929). Later, a dis tri bu tion map was ob - tained by Bommarito and La Rosa (1962) and con firmed by Tedesco (1966) who es ti mated that the phos phatic oc cur rences were of Aquitanian-Langhian age. Di Grande et al. (1978) stated that the phos phatic nod ules ap peared in dis con tin u ous lenses, with the phos phatic ce ment be ing made up of francolite, which re placed an ear lier car bon ate. Fur ther stud ies on such top ics were car ried out by Pedley and Bennett (1985) and Carbone et al. (1987) who found a great sim i lar ity be tween Si - cil ian and Mal tese phosphorites.
GEOLOGICAL SETTING
The Hyblean Pla teau (south-east Sic ily, It aly) con sists of a thick, non-de formed plat form of Tri as sic to Qua ter nary cal car e - ous rocks (Grasso, 1997). The phos phatic nod ules of the Hyblean Pla teau are con tained within the bed ded packstones at the base of the Irminio Mem ber of the Ragusa For ma tion, which is made up of an al ter nat ing suc ces sion of marly lime stones, coarse grained car bon ate packstones and mi cro-grainstones
span ning Late Oligocene–Mid dle Mio cene range. Ac cord ing to Carbone et al. (1987), autochthonous phos phatic nod ules be long to the same depositional en vi ron ment, which is char ac ter ised by the pro gres sive tran si tion from car bon ate sed i men ta tion to new ero sive hardground con di tions.
PHOSPHATIC NODULE OUTCROPS
We ana lysed the char ac ter is tics of three out crops ob served at the coastal area on the Biddemi River, the Irminio River and at an ac tive cliff at Donnalucata (Fig. 1). The area of in ves ti ga - tion is char ac ter ized by a flat to pog ra phy be cause of flu vial and ma rine ter races. The most im por tant out crops be long to:
— the afore men tioned Irminio Mem ber (Lower–Mid dle Mio cene Age), char ac ter ized by non-de formed, hor i zon tal strata;
— flu vial/ma rine de pos its, at the mouth of the Irminio River (Qua ter nary Age);
— marls of the Tellaro For ma tion (Mid dle Mio cene Age);
— slope con ti nen tal de pos its, in clud ing blocks of dif fer ent sizes within a silt ma trix;
— qua ter nary grainstones rich in shell frag ments, de vel - oped above all on tops of most important hills.
The three phosphorite out crops are autochthonous (Carbone et al., 1987) and are lo cated (Fig. 1): (1) close to the mouth of the Biddemi River, at the top of a 5 m high cliff ly ing
Fig. 1. Geo log i cal map and cross-sec tion of Ma rina di Ragusa and sur round ing ar eas (south-east Sic ily)
di rectly on top of a crossed strat i fi ca tion; (2) at the mouth of the Irminio River, on a sec tion cut by a coastal road; (3) on the cliff lo cated be tween Plaja Grande and Donnalucata vil lage. The three out crops have sim i lar fea tures (Fig. 2): phos phate nod ules are dis persed in a soft, fri a ble, light-col oured packstone ma trix in very dis tinct lay ers about 20 cm thick. Their brown col our cre ates a strong con trast with the yel low ish, cream col our of the im me di ately un der ly ing cal car e ous strata. Weath ered sur faces are darker brown and have a rough tex ture. Nod ules are well ce mented and the most com mon shapes are subrounded and elon gated. Their ma jor axis ranges ap prox i mately from 2 to 10 cm and their mi nor one from 2 to 5 cm. The core area of the phos phatic nod ules is quite uni form and dense with a dis tinc - tive, un vary ing brown col our.
ANALYTICAL METHODS
Tex ture, min er al og i cal and chem i cal com po si tion of phosphorite nod ules from Biddemi River (la belled as BR), Irminio River (IR) and Donnalucata (DL) were char ac ter ized.
Anal y ses were car ried out in the fresh brown cores of nod ules in or der to avoid al ter ation phases that could mod ify the min er - al ogy and chem is try of the phos phates. Bulk chem i cal anal y ses of nod ules were per formed by means of X-ray flu o res cence (XRF; Philips PW-1480). 1 g per sam ple was finely ground and well mixed in ag ate mor tar be fore be ing com pressed into an Al holder for com pressed XRF disk prep a ra tion. ZAF cor - rec tion was per formed sys tem at i cally (Scott and Love, 1983).
In ter na tional stan dards (Govindaraju, 1989) were fol lowed through out. The es ti mated de tec tion limit for ma jor el e ments was 0.01 wt. %.
The min er al ogy of the raw ma te rial was stud ied by pow - der X-ray dif frac tion (PXRD) us ing a Philips PW-1710
diffractometer with graph ite monocromator, au to matic slit and CuKa ra di a tion (l = 1.5405 ). The data were col lected in step-scan ning mode with 0.02° goniometer rate and 2q from 3 to 60°. PXRD goniometric cal i bra tion was per formed us ing a sil i con stan dard. Phosphorite nod ules were well-ground in a wol fram disk mill to <10 mm par ti cle size and then ana lysed. The ra zor tamped sur face (RTS) method (Zhang et al., 2003) was used to at tain ran dom par ti cles ori en - ta tion. The re sult ing data was in ter preted us ing XPowder soft - ware (Martín, 2004). We per formed a quan ti ta tive anal y sis of min eral phases con vert ing the data to the con stant sam ple vol - ume and us ing the PDF2 data base and Nor mal ized RIR method. No in ter nal stan dard was added to the pow der sam - ples. XPowder soft ware was used to make quan ti ta tive stud -
ies us ing meth ods of non-lin ear squared min i mums on the full-pro file fit ting of the diffractograms mak ing use of all the in for ma tion in the PDF2 data base. Weight ing was cal cu lated us ing the Nor mal ized RIR Method de scribed by Chung (1974). RIR co ef fi cients were ob tained from the PDF2 da ta - base. The lat tice pa ram e ters of CFA were re fined be cause the length of the a axis var ies de pend ing on the car bon ate con tent in crys tals (LeGeros, 1965).
To char ac ter ize the tex ture and the microstructure of the nod ules op ti cal light mi cros copy (OLM; Olym pus BX-60) and field emis sion scan ning elec tron mi cros copy (FESEM; Leo Gem ini, 1530) were em ployed. Two thin sec tions per sam ple type were pre pared, one of which was pol ished. FESEM sec - ond ary elec tron (SE) and back-scat tered elec tron (BSE) im ages were ob tained us ing small sam ple pieces (5 ´ 5 ´ 3 mm in size) or pol ished car bon-coated thin sec tions. We also ob tained Ca, P and F X-ray en ergy maps with the use of EDX Ox ford INCA-200 microanalysis.
Fig. 2. Pho to graphs of the stud ied out crops
A — Biddemi River, phos phatic nod ules lie on a cross strat i fied lime stone; B — de tail of the out crop in A; C — 25 cm thick phos phatic layer at Irminio River; D — out crop on the cliff be tween Plaja Grande and Donnalucata
RESULTS
X-RAY FLUORESCENCE
The re sults of this anal y sis in di cate the high CaO con tent in all the sam ples (from 50.15% in DL3 to 55.23% in IR1). The P2O5
con tent was higher in the BR sam ples than in the other two groups of sam ples (Ta ble 1). The SiO2 con tent is very low and only slightly ex ceeds 2% in BR1, BR2, BR3 and DL3 sam ples. Ac -
cord ing to Slansky (1979), stud ied sam ples can be clas si fied as phos phatic lime stones nod ules (P2O5 con tent is lower than 18%).
POWDER X-RAY DIFFRACTION
Diffractometric anal y ses show sig nif i cant amounts of cal - cite and smaller amounts of CFA (Fig. 3). CFA val ues range from 10 to 36 wt. % (Ta ble 2) and the high est CFA con tent ap - pears in BR sam ples. Carbone et al., (1987) in di cated that the
Sam ple SiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O TiO2 P2O5 LOI
BR1 2.03 0.65 1.07 0.01 0.66 51.41 0.34 0.24 0.02 16.21 26.24
BR2 2.37 0.57 0.97 0.01 1.00 52.51 0.96 0.16 0.08 15.38 25.97
BR3 2.35 0.60 0.96 0.01 1.03 51.70 0.93 0.14 0.19 16.75 25.31
BR4 1.47 0.32 0.65 0.01 0.85 51.12 0.88 0.12 0.10 13.60 30.90
IR1 0.17 0.22 0.35 0.01 0.47 55.23 0.04 0.09 0.01 7.96 35.76
IR2 1.43 0.31 0.66 0.01 0.85 51.58 0.84 0.12 0.07 13.40 30.75
IR3 1.05 0.25 0.37 0.01 0.74 52.92 0.57 0.07 0.09 10.24 33.69
IR4 1.93 0.45 0.81 0.01 0.86 50.91 0.74 0.15 0.09 12.87 31.16
IR5 1.25 0.35 0.36 0.01 0.77 53.19 0.45 0.06 0.06 7.65 35.85
DL1 0.49 0.34 0.41 0.02 0.73 53.37 0.75 0.13 0.00 4.37 39.85
DL2 0.84 0.26 0.28 0.02 0.93 53.56 0.46 0.06 0.03 3.02 40.49
DL3 2.12 0.50 1.25 0.02 1.10 50.15 1.04 0.18 0.04 4.62 38.97
DL4 1.20 0.36 0.37 0.02 0.99 51.18 1.13 0.08 0.07 4.73 39.89
BR — Biddemi River; IR — Irminio River; DL — Donnalucata
T a b l e 1 Con cen tra tion of ma jor el e ments (wt. %) of phosphorite nod ules
Fig. 3. PXRD diffractograms of Irminio River (a), Biddemi River (b) and Donnalucata (c) phos phatic nod ules C — cal cite; CFA — car bon ate-fluorapatite; CuKa X-ray ra di a tion, l = 1.5405 ; main hkl Bragg peaks and dhkl val ues are in cluded
darker col our of the phosphatized clasts they stud ied was a re - sult of the Fe ox ides, glauconite and of the high francolite con - tent. In these nod ules be sides cal cite, only CFA was de tected and there were no traces of ei ther glauconite or Fe ox ides, prob - a bly be cause these phases were be low the de tec tion limit of pow der dif frac tion method. The re fine ment of CFA lat tice con - stants on 20 re flec tions gave the fol low ing pa ram e ters: a = b = 9.3218 ; c = 6.9146 ; Z = 2; unit-cell vol ume = 520.35 3. Con sid er ing that in CFA the unit-cell a di men sion can de crease ap prox i mately from 9.37 to 9.32 , be cause of car bon ate sub sti tu tion for phos phate (Zapata and Roy, 2004), the cal cu - lated fig ure, 9.3218 , in di cates the high level of CO32- sub sti - tu tion for PO43-. The high CO32- con tent in CFA struc ture is con firmed mea sur ing the 2q dis tance be tween the (410) and (004) CFA peaks. The cal cu lated CO32- in the lat tice var ies from 6.20 to 7.57 us ing the equa tion of Gulbrandsen (1970) and from 7.95 to 10.45 us ing the one pro posed by Schuffert et al. (1990). A con tent of 1.4 CO32- moles per for mula wt. is ob - tained when this value is plot ted on Fig ure 4 of Zapata and Roy (2004). This re sult is log i cal con sid er ing that older (i.e.
Palaeozoic) phos phatic rocks gen er ally con tain a lim ited amount of sub sti tuted car bon ate, while youn ger (i.e.
Mio-Pliocenic) ones per mit higher lev els of CO32- sub sti tu tion for PO43- (McClellan and Van Kauwenbergh, 2004).
OPTICAL LIGHT MICROSCOPY
OLM ob ser va tion re vealed the pres ence of abun dant benthonic and plank tonic foraminifera and smaller amounts of echinoid plates and bryo zoans well-ce mented to gether by sparitic cal cite. The fauna ob served in thin sec tions is al most the same as that de scribed by Carbone et al. (1987), i.e., Miogypsina sp., Amphistegina sp., Globigerinoides trilobus, Globigerinoides siakensis and Praeorbulina glomerosa, as
Sam ple C CFA D2q CO3
2-
(1) CO3 2-
(2)
BR1 67.4 ± 5.8 32.6 ± 3.8 1.218 6.68 8.82
BR2 67.9 ± 3.9 32.1 ± 4.7 1.193 7.05 9.49
BR3 64.3 ± 4.4 35.7 ± 5.8 1.186 7.16 9.68
BR4 70.0 ± 5.1 30.0 ± 4.2 1.205 6.88 9.16
IR1 72.6 ± 5.0 27.4 ± 4.6 1.247 6.27 8.05
IR2 70.2 ± 5.4 29.8 ± 4.3 1.185 7.17 9.70
IR3 71.5 ± 4.9 28.5 ± 5.0 1.158 7.57 10.45
IR4 70.7 ± 4.6 29.3 ± 4.4 1.249 6.23 8.00
IR5 72.8 ± 4.2 27.2 ± 5.2 1.194 7.04 9.46
DL1 84.2 ± 3.6 15.8 ± 6.0 1.251 6.20 7.95
DL2 89.6 ± 2.8 10.4 ± 5.4 1.164 7.48 10.28
DL3 83.9 ± 5.1 16.1 ± 4.8 1.192 7.07 9.51
DL4 83.3 ± 5.3 16.7 ± 4.1 1.223 6.61 8.68
XRD D2q(004)–(410) and wt. % us ing Gulbrandsen (1) and Schuffert et al. (2) equa tions are also re ported; other ex pla na tions as in Ta ble 1 and Fig ure 3 T a b l e 2 PXRD quan ti ta tive anal y sis (wt. %) of the three phosphorite groups
Fig. 4. OLM pho to graphs of phosphorite nod ules where mi cro-or - gan isms well-ce mented to gether by sparitic cal cite can be ob served
(crossed nicols)
A — BR nod ule with abun dant ben thic foraminifers; B — IR nod ule with a sea ur chin spine sec tion and frag ments of la melli bran ches; C —
DL nod ule with var i ous num mu lites
well echinoid plates and coralline al gae, in di cat ing that these phos phates are of the Late Burdigalian age. Be cause of their sub mi cro scopic di men sions, it was not pos si ble to iden tify any CFA crys tals with this tech nique. We could only iden tify a brown ish micritic ma trix which can sug gest the pres ence of glauconite and/or Fe ox ides (Fig. 4A, B and C). Intraparticle and interparticle po ros ity is very lim ited (~10%) and is gen er - ally ob served in side hol low parts of bioclasts or be tween frag - ments within the ma trix where sec ond ary cal cite has not to tally filled the empty spaces.
FIELD EMISSION SCANNING ELECTRON MICROSCOPY
Sec ond ary elec tron mi cro pho to graphs show ir reg u lar sur - faces that were mainly com posed of mi crom e ter cal cite crys - tals. Hex ag o nal crys tal lites were iden ti fied as CFAs be cause EDX microanalysis con firmed the pres ence of Ca and P (Fig. 5A). The afore men tioned crys tals were ~600 nm in di am - e ter and showed re duced growth along their c axis. The
{
0001}
CFA and the{
1010}
CFA forms could be seen. The short c axis in CFAs has been at trib uted by Sánchez Navas and Martín Algarra (2001) to co ales cence or coars en ing mech a nisms of ear lier crys tals that were lon ger and thin ner and of amor phous phos phates. The short c axis would also seem to be a re sult of the high car bon ate sub sti tu tion which lim its the size of the CFAs and makes them more equiaxial in shape (Na than, 1984).At low mag ni fi ca tions the gen eral ap pear ance of these crys tals sug gests the pres ence of euhedral shapes (Fig. 5A), but when mag ni fied they are clearly subhedral (Fig. 5B). In fact, all the crys tals show im per fec tions. In some cases the cen tre of the crys tal has not de vel oped in the same way as the outer parts pro duc ing a hol low, skel e tal shape. Sim i lar shapes have been ob served in phos phates by other au thors and have been in ter - preted as the re sult of mi cro bial ac tiv ity which fa vours ac cre - tion mech a nisms (Lucas and Prévôt, 1984; Soudry, 1994, 2000; Krajewski, 2000), even if there is no clear ev i dence that cel lu lar bod ies are nec es sary for the for ma tion of CFAs (Krajewski et al., 1994). The ab sence of euhedral CFAs is com mon in many sed i men tary phos phate de pos its and they can also form di rectly from supersatured pore so lu tions. In most of the crys tals, it is clear that the sur face of the
{
0001}
CFA face isnot flat and is in fact slightly oval-shaped, which could sug gest that pre-ex ist ing mi cro-or gan isms (i.e. bac te ria) have been coated by francolite crys tals (Fig. 5B and D). The crys tals are ran domly po si tioned with no pref er en tial ori en ta tion and it is clearly pos si ble to ob serve a po rous space be tween them (Fig. 5A). The CFAs ac cu mu late in con fined microspaces such as pores, fis sures or for ex am ple in side microborings (Fig. 5E and F). Fig ures 5G and H con firm these re sults as the CFAs do
not re place skel e tal ma te rial and ap pear, in stead, in more shel - tered ar eas nearby.
Fig ure 6A shows a BSE im age of the phos phatic nod ules.
The un ion among grains is al most com plete, po ros ity seems to be very low and pore in ter con nec tion is lim ited. Frag ments of var i - ous microfossils are eas ily rec og niz able and subangular quartz grains can also be de tected. They are min ute (just a few mi crom - e ters) and could not there fore be seen with OLM. The spa tial dis - tri bu tion of Ca, P and F was de ter mined with the use of EDS by el e men tal map ping. In a map show ing the dis tri bu tion of Ca it was im pos si ble to dis tin guish the bioclasts from the ma trix be - cause of the high con cen tra tion of Ca in the en tire nod ule; only iso lated black ar eas, those with ir reg u lar-shaped pores, could be seen (Fig. 6B). P (Fig. 6C) and F con tent (Fig. 6D) were rather low com pared to Ca and were dis trib uted al most en tirely in the ma trix rather than in the shells of bioclasts and con firm the pre vi - ous ob ser va tions (Fig. 5G and H). How ever, it was im pos si ble to see a more con cen trated dis tri bu tion of F and P (in the con fined microspaces where CFAs crys tal lize, Fig. 5) be cause the mag ni - fi ca tion of SE and BSE im ages is dif fer ent.
DISCUSSION
Phos phatic lime stones out crops are as so ci ated with the es - tab lish ment of hardground con di tions dur ing the early Mio cene pe riod (Carbone et al., 1987). This work com ple mented and par tially mod i fied the in ter pre ta tions of the pre vi ous au thors by pro pos ing that the crys tal li za tion of CFA be gan in the Ragusa For ma tion af ter an ini tial pe riod of lithification of the sea-floor (car bon ate ce men ta tion) and glauconite re place ment in pore sed i ment. Dur ing this time, the hardground sur face was char ac - ter ized by an ac cu mu la tion of bioclasts (benthonic and plank - tonic com mu ni ties).
Phosphogenesis, which started within the pore-space of sed i ment, could have been the re sult of a bac te ri ally-me di ated pre cip i ta tion of min ute non-ori ented ovoidal phos phate par ti - cles and it is pre sum able that bac te ria were in volved at the be - gin ning of CFAs for ma tion. The phos phate ce men ta tion fol - lowed the car bon ate ce men ta tion fill ing up the pore-space in nod ules (see the low po ros ity in Fig. 6). The lack of euhedral CFA crys tals in the stud ied nod ules can not be con sid ered a di - rect proof of bac te rial phosphatization of sed i ment and in or - ganic pre cip i ta tion of fluorapatite has been dem on strated in the lab o ra tory (Van Cappellen and Berner, 1991). How ever, a supersaturation of this phase in the so lu tion is re quired and it is known that the av er age con cen tra tion of P in sea wa ter is only 0.09 mgL–1 (Ehrlich, 2002). Even if di rect pre cip i ta tion oc curs, ki netic growth is very slow and the fluorapatite pre cip i ta tion
Fig. 5. FESEM mi cro pho to graphs of phos phatic nod ules
A — gen eral view of CFA crys tals in BR nod ules, no tice that the crys tals are al most of the same size; B — mag ni fi ca tion of a por tion of the pre vi ous fig ure (the area in side the white rect an gle) in which can be seen the im per fec tions on the faces of the CFA crys tals, the microanalysis of these crys tals can be seen in the in set; C — for ma tion of skel e tal CFA crys tals in a DL nod ule; D — de vel op ment of a rounded shape in
{
0001}
CFA faces; E — elon gated tubes par - tially empty com posed of CFA crys tals in RI nod ules; F — de tailed im age of the pre vi ous pho to graph (the area in side the white rect an gle) in which hex ag - o nal CFA crys tals can be rec og nized, the added spec trum shows the com po si tion of these crys tals; G — dasycladal alga com posed of cal cite (see the microanalysis); H — de tailed im age of the pre vi ous fig ure in which CFA crys tals can be seen (white ar row), no tice how these crys tals de velop only in the pro tected area of the ma trix, not in the shell of the algarates are low (Krajewski et al., 1994). Fi nally, a marked ac cel e - r a tion of cur rent ve loc ity within the sea-floor sed i ment was the re spon si ble for the emer gence of phosphatized nod ules above the hardground (Carbone et al., 1987).
Hence, bac te ria could have pro vided the ve hi cle for phos - phate min er al iza tion due to the fol low ing con sid er ations:
1. It was ob served that the crys tal li za tion of CFA did not oc cur in microfossils shells. It usu ally de vel ops in con fined spaces in side the ma trix (cav i ties, per fo ra tions, etc.) where or - ganic mat ter may be eas ily ac cu mu lated. In these micro - environments mi cro bial ac tiv ity causes the pro duc tion and break down of or ganic mat ter, the re lease of in or ganic com - pounds and met a bolic prod ucts, and it pro vides sites for the nu - cle ation and growth of solid phases. The walls of bac te rial cells are chem i cally re ac tive which fa cil i tates the nu cle ation and growth of min eral phases (Fer ris, 1997).
2. FESEM im ages high light the ab sence of a pre ferred ori en - ta tion of CFA crys tals as well as the high po ros ity of these microenvironments where mi crobes could have been de vel oped.
In these ar eas, which are su per sat u rated in P, sol u ble phos phate, lib er ated by bac te ria, re acted with cal cium ions form ing in sol u - ble cal cium phos phate com pounds (Ehrlich, 2002).
3. No tice how all the CFA crys tal lites are the same size (~1 mm). If bac te ria pro moted phosphogenesis, it would be rea - son able to as sume that CFAs would have equal di men sions.
They al low the francolite to crys tal lize but, at the same time, their cells cre ate a limit to the ex pan sion of the crys tal lites.
4. All CFA crys tals show some de fects such as in com plete face growth, skel e tal morphologies de vel op ment and round ing of
{
0001}
CFA faces. It seems that in the wa ter-sed i ment sur face there was a high phos phate supersaturation as a re sult of mi cro - bial pro lif er a tion and a rapid pre cip i ta tion of CFAs in an en vi - ron ment full of im pu ri ties that causes such im per fec tions to oc - cur (Krajewski, 2000). The ma jor ar gu ment in fa vour of bac te - rial ac tion in phosphogenesis should be the pres ence of abun - dant bac te ria-like par ti cles (Foun tain and McClellan, 2000).The non-ap pear ance of bac te rial morphologies may be re lated to the fact that bac te rial bod ies were too del i cate and ephem eral
Fig. 6. BSE and X-ray im ages of a phos phatic nod ule from the BR site
A — gen eral view of the sam ple (BSE im age), black ar eas cor re spond to pores and dark grey ar eas to small quartz grains; B–D — X-ray im ages of Ca, P and F, re spec tively, note the nu mer ous nonphosphatized bioclastic frag ments
to sur vive the diagenetic pro cesses (Baturin, 2000). Cel lu lar bod ies are not nec es saries for the for ma tion of CFA, but it is known that their abun dance leads to the com mon de po si tion of ap a tite (Krajewski et al., 1994).
Ac cord ing to Ruttenberg and Berner (1993) and Schenau et al. (2000), it is pos si ble to state that dis sem i nated CFA crys tals acted as a sed i men tary sink for re moval of the re ac tive phos - pho rus and flu o rine in the sea so fa vour ing the de vel op ment of phos phatic nod ules. High car bon ate sub sti tu tion for phos phate oc curred dur ing this time, as in di cated by the low unit-cell a value. The phosphatised lay ers we stud ied were no tice ably thin. This is be cause the pre cip i ta tion of phos phates is gen er - ally re stricted to the up per most part of the sed i ment, and the in - crease of car bon ate al ka lin ity in pore wa ter be low a depth of a few centi metres pre cludes fur ther de vel op ment of phos phatic min er als (Glenn and Ar thur, 1988; Schenau et al., 2000).
CONCLUSIONS
The for ma tion of francolite (car bon ate-fluorapatite) was in - ves ti gated in south-east Si cil ian phos phatic lime stones. Such de pos its were formed dur ing long pe ri ods of hardground con - di tions. Phosphogenesis was mainly due to francolite pre cip i ta -
tion within the pores sed i ment form ing francolite crys tals al - most all of which mea sured ~1 mm in di am e ter. These crys tals, of ir reg u lar mor phol ogy, pre cip i tated in microenvironments su per sat u rated in P which prob a bly had been pro duced by bac - te ria. Francolite crys tal li za tion only af fected the ma trix of the phos phatic nod ules and did not af fect the car bon ate shells of ben thic and plank tonic foraminifera. It oc curred in con fined spaces where or ganic mat ter tended to ac cu mu late. The re - duced growth of these crys tals along the c axis sug gests two hy - poth e ses: 1) they rep re sent a sec ond stage of recrystallization of ear lier, thin ner, more elon gated crys tals or 2) a high CO32- sub - sti tu tion for PO43- fa voured the de vel op ment of tab u lar-shaped crys tals. In fact, the low value of the unit-cell a of francolite crys tals in di cates that they were sub jected to high rates of car - bon ate for phos phate sub sti tu tion, which nor mally oc curs in young sed i men tary phos phatic de pos its.
Ac knowl edge ments. This re search has been sup ported by the Re search Groups RNM179 and RNM328 of the Junta de Andalucía. We thank N. Walkington for the trans la tion of the manu script and A. Martín Algarra, F. Martínez Ruiz and J. D.
Martín for fruit ful dis cus sions and com ments. The manu script has bene fited from re views by K. Krajewski, M. Pedley and an anon y mous ref eree.
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