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Geo log i cal Quar terly, 2008, 52 (1): 61–70

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 RagusaFor 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 themicroenvironments 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 talswere 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: [email protected]; 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 sumesabout 90% of world pro duc tion (Zapata and Roy, 2004), andbe cause they are use ful in palaeoenvironmental andpalaeooceanographic 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 theworld (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 andBur 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 thatex tend across parts of north ern France, south ern Eng land andBel 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, areused to iden tify time-gaps in strati graphic se quences (Mar shall

and Ruffell, 2004). Their com mon min eral phase is francolite, acar 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) andGunnars et al. (2004) stated that phos phatic par ti cles can bepro duced by in or ganic pre cip i ta tion in sea wa ter so lu tions, byhigh rates of or ganic mat ter deg ra da tion and, prob a bly, by thedis 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 toan amor phous cal cium phos phate which in turn evolves intofrancolite (Soudry, 1993; Krajewski et al., 1994; MartínAlgarra 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 thecrystallinity 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 tivephos pho rus (Kim et al., 1999). The hy poth e ses that have beensug 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 stagesof 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 lithostratigraphicchar 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 byTedesco (1966) who es ti mated that the phos phatic oc cur renceswere of Aquitanian-Langhian age. Di Grande et al. (1978)stated that the phos phatic nod ules ap peared in dis con tin u ouslenses, 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 suchtop ics were car ried out by Pedley and Bennett (1985) andCarbone 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 athick, 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 theHyblean Pla teau are con tained within the bed ded packstones atthe base of the Irminio Mem ber of the Ragusa For ma tion, whichis 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 toCarbone et al. (1987), autochthonous phos phatic nod ules be long to the same depositional en vi ron ment, which is char ac ter ised bythe pro gres sive tran si tion from car bon ate sed i men ta tion to newero sive hardground con di tions.

PHOSPHATIC NODULE OUTCROPS

We ana lysed the char ac ter is tics of three out crops ob servedat the coastal area on the Biddemi River, the Irminio River andat 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 andma rine ter races. The most im por tant out crops be long to:

— the afore men tioned Irminio Mem ber (Lower–Mid dleMio cene Age), char ac ter ized by non-de formed, hor i zon talstrata;

— flu vial/ma rine de pos its, at the mouth of the IrminioRiver (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 themouth of the Biddemi River, at the top of a 5 m high cliff ly ing

62 Giuseppe Cultrone, Giorgio Anfuso and Eduardo Sebastián

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 clifflo cated be tween Plaja Grande and Donnalucata vil lage. Thethree 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 trixin very dis tinct lay ers about 20 cm thick. Their brown col ourcre 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 facesare darker brown and have a rough tex ture. Nod ules are wellce mented and the most com mon shapes are subrounded andelon gated. Their ma jor axis ranges ap prox i mately from 2 to10 cm and their mi nor one from 2 to 5 cm. The core area of thephos 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 ofphosphorite 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 ulesin 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 groundand well mixed in ag ate mor tar be fore be ing com pressed intoan 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 lowedthrough out. The es ti mated de tec tion limit for ma jor el e mentswas 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 slitand CuKa ra di a tion (l = 1.5405 �). The data were col lectedin step-scan ning mode with 0.02° goniometer rate and 2qfrom 3 to 60°. PXRD goniometric cal i bra tion was per formedus ing a sil i con stan dard. Phosphorite nod ules werewell-ground in a wol fram disk mill to <10 mm par ti cle sizeand 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 ofmin 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 RIRmethod. 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 thefull-pro file fit ting of the diffractograms mak ing use of all thein for ma tion in the PDF2 data base. Weight ing was cal cu latedus 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 thelength of the a axis var ies de pend ing on the car bon ate con tentin crys tals (LeGeros, 1965).

To char ac ter ize the tex ture and the microstructure of thenod ules op ti cal light mi cros copy (OLM; Olym pus BX-60) andfield emis sion scan ning elec tron mi cros copy (FESEM; LeoGem ini, 1530) were em ployed. Two thin sec tions per sam pletype 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 fordINCA-200 microanalysis.

Mineralogy of Miocene phosphatic nodules in SE Sicily (Italy) 63

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 cmthick 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 allthe 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 groupsof sam ples (Ta ble 1). The SiO2 con tent is very low and onlyslightly 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 asphos 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 rangefrom 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 tectedand 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 ofpow 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 atesub 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 CO3

2- 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 CO3

2- in the lat tice var iesfrom 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 etal. (1990). A con tent of 1.4 CO3

2- 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 itedamount of sub sti tuted car bon ate, while youn ger (i.e.Mio-Pliocenic) ones per mit higher lev els of CO3

2- sub sti tu tionfor PO4

3- (McClellan and Van Kauwenbergh, 2004).

OPTICAL LIGHT MICROSCOPY

OLM ob ser va tion re vealed the pres ence of abun dantbenthonic and plank tonic foraminifera and smaller amounts ofechinoid plates and bryo zoans well-ce mented to gether bysparitic cal cite. The fauna ob served in thin sec tions is al mostthe 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 CO32 -(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 ulewith 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 thesephos phates are of the Late Burdigalian age. Be cause of theirsub mi cro scopic di men sions, it was not pos si ble to iden tify anyCFA crys tals with this tech nique. We could only iden tify abrown ish micritic ma trix which can sug gest the pres ence ofglauconite and/or Fe ox ides (Fig. 4A, B and C). Intraparticleand 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 tallyfilled 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 causeEDX 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 caxis in CFAs has been at trib uted by Sánchez Navas and MartínAlgarra (2001) to co ales cence or coars en ing mech a nisms ofear lier crys tals that were lon ger and thin ner and of amor phousphos phates. The short c axis would also seem to be a re sult ofthe high car bon ate sub sti tu tion which lim its the size of theCFAs 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 talssug gests the pres ence of euhedral shapes (Fig. 5A), but whenmag ni fied they are clearly subhedral (Fig. 5B). In fact, all thecrys tals show im per fec tions. In some cases the cen tre of thecrys tal has not de vel oped in the same way as the outer partspro duc ing a hol low, skel e tal shape. Sim i lar shapes have beenob 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 thatcel 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 iscom 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 ofthe 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 gestthat pre-ex ist ing mi cro-or gan isms (i.e. bac te ria) have beencoated by francolite crys tals (Fig. 5B and D). The crys tals areran domly po si tioned with no pref er en tial ori en ta tion and it isclearly 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 suchas pores, fis sures or for ex am ple in side microborings (Fig. 5Eand 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 quartzgrains 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 byel e men tal map ping. In a map show ing the dis tri bu tion of Ca itwas 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; onlyiso lated black ar eas, those with ir reg u lar-shaped pores, could beseen (Fig. 6B). P (Fig. 6C) and F con tent (Fig. 6D) were ratherlow com pared to Ca and were dis trib uted al most en tirely in thema 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 finedmicrospaces 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 andpar 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 RagusaFor 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 poresed 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 ofsed i ment, could have been the re sult of a bac te ri ally-me di atedpre 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 innod ules (see the low po ros ity in Fig. 6). The lack of euhedralCFA 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 thelab o ra tory (Van Cappellen and Berner, 1991). How ever, asupersaturation of this phase in the so lu tion is re quired and it isknown that the av er age con cen tra tion of P in sea wa ter is only0.09 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 seenin 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 themicroanalysis); 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 thepro tected area of the ma trix, not in the shell of the alga

rates 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 there spon si ble for the emer gence of phosphatized nod ules abovethe 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 notoc cur in microfossils shells. It usu ally de vel ops in con finedspaces 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 andbreak 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 cellsare chem i cally re ac tive which fa cil i tates the nu cle ation andgrowth 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 thesemicroenvironments 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 pleteface growth, skel e tal morphologies de vel op ment and round ingof { }0001 CFA faces. It seems that in the wa ter-sed i ment sur facethere 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 latedto 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 larbod ies are not nec es saries for the for ma tion of CFA, but it isknown that their abun dance leads to the com mon de po si tion ofap 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 talsacted 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 ofphos phatic nod ules. High car bon ate sub sti tu tion for phos phateoc curred dur ing this time, as in di cated by the low unit-cell avalue. The phosphatised lay ers we stud ied were no tice ablythin. 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 afew centi metres pre cludes fur ther de vel op ment of phos phaticmin 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. Suchde 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 microenvironmentssu 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 thephos phatic nod ules and did not af fect the car bon ate shells ofben thic and plank tonic foraminifera. It oc curred in con finedspaces 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 CO3

2- sub -sti tu tion for PO4

3- fa voured the de vel op ment of tab u lar-shapedcrys tals. In fact, the low value of the unit-cell a of francolitecrys 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 inyoung sed i men tary phos phatic de pos its.

Ac knowl edge ments. This re search has been sup ported bythe Re search Groups RNM179 and RNM328 of the Junta deAndalucía. We thank N. Walkington for the trans la tion of themanu script and A. Martín Algarra, F. Martínez Ruiz and J. D.Martín for fruit ful dis cus sions and com ments. The manu scripthas bene fited from re views by K. Krajewski, M. Pedley and ananon y mous ref eree.

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