03 segovic.indd 19 � 1. INTRODUCTION In car bo na te ter rai ns goet hi te is a com mon pa ra ge ne tic mi ne- ral in soi ls, par ti cu lar ly in ter ra ros sa, and is al so fou nd as prin- ci pal con sti tue nt of Fe-cre tes or it can be tra ced as the ma jor pha se in so me mi ne ral de po si ts in kar sti fi ed areas. The se kin ds of goet hi te ap pea ran ce are ob vious ly con trol led by weat he ri ng whi ch cause an en vi ron me nt fa vou ri ng for ma tion of aci dic so lu tio ns. The type and exte nt of goet hi te mi ne ra li za tion is de pen de nt on the geoc he mi cal abi li ty and amou nt of sub stra te avai lab le du ri ng the weat he ri ng pro ces ses. The mi ne ra li za tion has col lo mor ph textu res; goet hi te is low Al-sub sti tu ted and poor ly crystal li zed. This wo rk pro vi des evi den ce of a com ple te ly dif fe re nt ki nd of goet hi te mi ne ra li za tion in the Ad ria tic seg me nt of the Ad ria ti c-Di na ri de car bo na te plat fo rm, wi th the type lo ca li ty oc cur ren ce in the Br bin šći ca co ve on Du gi otok (Cen tral Ad ria- tic). He re, ve ry fi ne ag gre ga tes of we ll crystal li zed, Al-sub sti- tu ted and stoi chiomet ric goet hi te, ac com pa nied by coar se grained cal ci te in fi ll sha rp tec to nic cre vi ces and in te r-bed di ng pla nes in the Up per Cre ta ceous li mes to nes. The mi ne ra li za tion sug ges ts pre ci pi ta tion from a ba sic so lu tion and a deep pla ced sour ce, ri ch in Fe, Al, Si and P. He re, we pre se nt a set of X- ray dif frac tion (XRD), scan ni ng elec tron mic ros co pe (SEM) and elec tron pro be mic roa na lyses (EPMA) mi ne ra lo gi cal and geoc he mi cal da ta on the goet hi te mi ne ra li za tion. The ori gin of the mi ne ra li za tion is dis cus sed and its sig ni fi can ce on a re gio nal sca le is stres sed in re ga rd to the nor the rn sub mer ged seg me nt of the Ad ria mic rop la te. 2. GEO LO GI CAL SET TI NG AND MI NE RA LI ZA TION TEXTU RES Du gi otok, as pa rt of the Ad ria tic car bo na te plat fo rm, be lon gs to the Di na ri dic oro ge nic moun tain chain whi ch was fi nal ly sha ped by exten si ve sea le vel ri se du ri ng the Ho lo ce ne deg la- Bra ni mir Šegvić1, Boš ko Lugović1, Ne ven Tadej1, Vla di mir Bermanec2, and Lov ro Panjkota1 AB STRA CT At the we stern seawa rd coa st of the is la nd of Du gi otok, unu sual goet hi te mi ne ra li za tion oc cu rs as the in fi l li ng of a system of thin cre vi ces in the Up per Cre ta ceous li mes to nes of the Ad ria tic car bo na te plat fo rm. Goet hi te is excep tio- nal ly we ll or de red and shows a va rie ty of che mi cal com po si tio ns ran gi ng from al mo st pu re FeO·OH to Al-sub sti tu ted com po si tion ac com pa nied by hi gh Si and P im pu ri ty. The most cha rac te ris tic textu ral type as sem bla ges are goet hi te and coar se grained cal ci te in com po si te ag gre ga tes. These ag gre ga tes con tain Al-sub sti tu ted goet hi te wi th me ta mor- phic si li ca te mi ne ra ls em bed ded in the co re (bla ck goet hi te), ar mou red by the XR D-pu re, al mo st stoi chiomet ric goet- hi te (brown goet hi te). The geoc he mi cal and mi ne ra lo gi cal fea tu res of the mi ne ra li za tion sug ge st an as cen di ng so lu- tion from a hid den me ta mor phic sour ce. We spe cu la te that the sour ce cou ld ha ve been ac ti va ted by the ther mal in fl ux lin ked to the Plio ce ne in trap la te mag ma ti sm in the eas te rn seg me nt of Ad ria. Keywor ds: Al-goet hi te, stoi chiomet ric goet hi te, crysta loc he mis try, en do ge nic ori gin, Du gi otok, Adriatic Sea, Ad ria mic rop la te, Croatia 1 Fa cul ty of Mi ni ng, Geo lo gy and Pet ro leum En gi nee ri ng; Uni ver si ty of Zag reb, Pie rot ti je va 6, HR-10 000 Zag reb, Croa tia (bseg vic@r gn.hr; bseg vic @geos.uni-heidelberg.de, bosko.lugovic@rgn.hr, neven.tadej@ rgn.hr, lovro.panjkota@rgn.hr) 2 Institute of Mineralogy and Petrology, De par te me nt of Geology, Fa cul ty of Scien ce, Uni ver si ty of Zag reb, Hor va to vac bb, HR-10 000 Zag reb, Croa tia (vberman@public.carnet.hr) Si-P im pu re Al-goet hi te mi ne ra li za tion on the is la nd of Du gi otok (Cen tral Ad ria tic, Croa tia) Geologia Croatica 61/1 19–26 7 Figs. 3 Tabs. Zagreb 2008 Geologia CroaticaGeologia Croatica � 03 segovic.indd 1903 segovic.indd 19 24.6.2008 11:06:2024.6.2008 11:06:20 Geologia Croatica Geologia Croatica 61/1 20 cia tion (Fig. 1). The is la nd con sis ts en ti re ly of Up per Cre ta- ceous do lo mi tes and li mes to nes showi ng va rious kar sti fi ca- tion mor pho lo gies (DŽAJA, 2003). Goet hi te mi ne ra li za tion oc cu rs exclu si ve ly alo ng the wes te rn seawa rd coa st of the is la nd, as the in fi l li ng of in ter bed- di ng pla nes (Fig. 2A) and sub ver ti cal ly, up to 10 cm thi ck tec to nic cre vi ces in the ho st li mes to nes (Fig. 2B) whi ch abun- dan tly con tain chon dro don ts and rudists of the Cenomanian age. Goet hi te mi ne ra li za tion shows three textu ral types. Mi ne- ra li za tion of textu ral type 1 is con fi ned to the vei ns in si de the ho st li mes to nes fi l led wi th a para ge ne tic as sem bla ge con sis- ti ng of ag gre ga tes of sub mic ro nic goet hi te and eu hed ral coar se grai ned cal ci te (Fig. 3A). Due to the mic ros co pic to sub mic- ros co pic si zed si li ca te mi ne ral im pu ri ties, goet hi te ag gre ga tes show a bla ck in ner pa rt (bla ck goet hi te co re), sur roun ded by the brown goet hi te wit hout XRD de tec tab le in clu sio ns (brown goet hi te man tle). The textu ral type 2 (Fig. 3B) com pri ses brown goet hi te ag gre ga te [plus a mi nor quan ti ty of sub mic ros co pic quar tz] fi l lin gs of mic ro frac tu res in si de the ho st li mes to ne. Al thou gh si mi lar to the pre vious type 2, the textu ral type 3 (Fig. 3C) is cha rac te ri sed by an ad mixtu re of the vir tual waxy quar tz im preg na tio ns alo ng the ho st li mes to ne bed di ng pla nes. Fi gu re 1: Map of the Ad ria tic Sea and sur roun di ng main la nd showi ng the lo ca tion of goet hi te mi ne ra li za tion on Du gi otok. DOF = Du gi otok fau lt; IAF = In ter nal Ad ria tic frac tu re (Geop hysi cal ano ma lies ac cor di ng to BRDAREVIĆ & OLUIĆ, 1979 and KOŠĆEC, 1986) Fi gu re 2: Pho tog rap hy of an out crop of goet hi te mi ne ra li za tion at Br bin šći ca co ve, in fi l li ng the system of in ter bed di ng pla nes (A) and sub ver ti cal tec to nic cre vi ces (B) in the ho st Up per Cre ta ceous li mes to ne from the shal low sea and hi gh-ti dal zo ne. The oc cur ren ce com pri ses goet hi te mi ne ra li za tion of textu ral type 1. Fi gu re 3: Pho tog rap hs of ha nd spe ci me ns of goet hi te mi ne ra li za tion of textu ral type 1 (A), textu ral type 2 (B) and textu ral type 3 (C). On textu ral type 3 com ple te ly ge ne tic and tem po ral in de pen de nt to do ro ki te mi ne ra li sa tion is ob ser ved whi ch is the sub je ct of anot her pa per (LUGOVIĆ et al., 2008). For goet hi te mi ne ra li za tion de tai ls see the text. 03 segovic.indd 2003 segovic.indd 20 26.6.2008 9:17:5426.6.2008 9:17:54 Geologia Croatica 21 Bra ni mir Šegvić: Si-P im pu re Al-goet hi te mi ne ra li za tion on the is la nd of Du gi otok (Cen tral Ad ria tic, Croa tia) 3. ANA LYTI CAL METHODS Sam ples rep re sen ti ng all three textu ral grou ps we re trea ted wi th pH 4.5 buf fe red NaAc/HAc so lu tion to dis sol ve cal ci te and di sag gre ga te the goet hi te. Brown and bla ck goet hi te of textu ral type 1 se pa ra ted as vir tual ly pu re frac tio ns by ha nd pic ki ng, usi ng a bi no cu lar mic ros co pe af ter crus hi ng the com po si te sam ple. The XRD ana lyses of powde red se pa ra tes from the goet- hi te mi ne ra li za tion textu ral types 1 and 2 we re per for med at the Fa cul ty of Mi ni ng, Geo lo gy and Pet ro leum En gi nee ri ng, Uni ver si ty of Zag reb, usi ng a Phil li ps dif frac to me ter 1820 and CuKα ra dia tion, grap hi tic mo noc hro ma tor (U = 40 kV, I = 35 mA). The sam ples we re scan ned at a ra te of 0.5°/min over the ran ge of 2–70° 2Θ. The mi ne ral pha ses we re iden ti fi ed usi ng the Powder Dif frac tion Fi le (1996)(*) da ta system (goet hi te – JCPDF car ds num ber 00-029-0713; cal ci te – JCPDF car ds num ber 00-005-0586; quar tz – JCPDF car ds num ber 00-005- 0490). The XRD ana lyses of textu ral type 3 we re do ne at the Fa cul ty of Scien ce, Uni ver si ty of Zag reb usi ng a Phil li ps dif frac to me ter X’PERT PRO (U = 45 kV, I = 40 mA). The sam ples we re scan ned at the ra te of 2°/min over the ran ge of 4–64° 2Θ and the dif frac tion pat ter ns we re iden ti fi ed usi ng the JCPDS system da ta ba se (quar tz – JCPDF car ds num ber 00-046-1045). The che mi cal com po si tion of goet hi te from textu ral types 1 and 2 was mea su red by SEM EDS and EPMA at the Mi ne- ra lo gi cal In sti tu te of the Rup rec ht-Kar ls Uni ver si ty of Hei del- be rg usi ng the CAMECA SX51 elec tron mic rop ro be equip ped wi th fi ve wa ve len gth-dis per si ve spec tro me te rs. Ope ra ti ng pa ra- me te rs we re 15 kV ac ce le ra ti ng vol ta ge, 20 nA beam cur re nt, ~1 µm beam si ze and 10 s coun ti ng ti me for all ele men ts. Na tu- ral mi ne ra ls, oxi des (co run dum, spi nel, haema ti te and ru ti le) and si li ca tes (al bi te, or thoc la se, anor thi te and wol las to ni te) we re used for ca lib ra tion. Raw da ta for all ana lyses we re cor rec ted for mat rix ef fec ts wi th the PAP al go rit hm (POUCHOU & PICHOIR, 1984, 1985) im ple men ted by CAMECA. For mu lae cal cu la tio ns we re do ne usi ng a sof twa re pac ka ge de sig ned by Ha ns-Pe ter Meyer (Mi ne ra lo gi cal In sti- tu te of the Rup rec ht-Kar ls Uni ver si ty of Hei del be rg). 4. RESULTS 4.1. XRD mi ne ra lo gy Goet hi te of textu ral type 1 cha rac te ri zes stro ng pea ks at 4.1698 Å, 2.4499 Å and 1.7195 Å for bla ck ag gre ga te, and 4.1730 Å, 2.4469 Å and 1.7166 Å for brown aggre ga te (Tab le 1). The XRD pat ter ns of bo th ag gre ga tes sug ge st re la ti ve ly hi gh crystal li ni ty, par ti cu lar ly that of the bla ck va rie ty. Quar tz was iden ti fi ed as the mixi ng pha se wit hin the bla ck ag gre ga te, whe reas the XRD pat te rn of the brown ag gre ga te iden ti fi ed only goet hi te. Unit ce ll crystal log rap hic pa ra me te rs for brown and bla ck goet hi te from the mi ne ra li za tion textu ral type 1 are dis played in Tab le 2. The b axis di men sio ns are al mo st iden ti- cal whi le the a and c axes of bla ck goet hi te, i.e. Al-sub sti tu ted va rie ty (see be low), are slig htly lar ger re sul ti ng in its lar ger unit ce ll vo lu me. Sin ce Al3+ is smal ler than Fe3+ the se da ta ap pear crystalloc he mi cal ly il lo gi cal and wi ll be re vi si ted in the dis cus sion. Goet hi te of textu ral types 2 and 3 is cha rac te ri zed by stro ng pea ks at 4.176 Å, 2.444 Å, 1.717 Å and 4.915 Å, 2.577 Å, 1.720 Å res pec ti ve ly. Quar tz is a mixi ng pha se in bo th types as we ll as MnO·OH in the textu ral type 3. 4.2. EPMA and SEM mi ne ra lo gy and che mis try Scan ni ng elec tron mic ros co pe (SEM) ob ser va tio ns (Fi gs 4A and 5) re vea led that goet hi te mi ne ra li za tion of textu ral type 1 com pri ses a he te ro ge neous co re, wi th va rious em bed ded pet ro- ge ne tic mi ne ra ls (bla ck goet hi te), and a ho mo ge nous man tle con si sting of goet hi te free of vir tual in clu sio ns (brown goet- hi te). K-fel dspar (Or91.4–94.9Ab5.1–8.4), al man di ne-ri ch me ta mor- phic gar net (Py9.7Alm62.5Sp5.0And4.5Gro17.9), cal ci te, quar tz and (*) JCPDF In ter na tio nal cen ter for dif frac tion da ta (1996) Table 1 Va lues for d-spa ci ng (d/Å) and in ten si ty (I/%) for the re fe re nt goet hi te and for brown and bla ck goet hi te from the Du gi otok mi ne ra li za- tion textu ral Type 1 Hin dlow. UK (00-029-0713) Sam ple Do-1d brown goet hi te Sam ple Do-1c bla ck goet hi te d I dhkl Ihkl dhkl Ihkl 4.9800 12 4.9661 14 4.9599 12 4.1830 100 4.1730 100 4.1698 100 3.3830 10 3.3799 16 3.3749 9 2.6930 35 2.6900 40 2.6899 27 2.5270 4 2.5219 5 2.5212 8 2.4890 10 2.4857 8 2.4863 9 2.4500 50 2.4469 63 2.4449 39 2.2530 14 2.2498 14 2.2499 7 2.1900 18 2.1836 15 2.1876 14 2.0890 1 2.0078 3 2.0888 3 1.9200 5 1.9176 6 1.9172 3 1.7728 1 1.7166 21 1.7196 20 1.5637 10 1.5638 10 1.5649 10 1.4541 5 1.4536 11 1.4552 7 Table 2 Crysta log rap hic pa ra me te rs of the brown and bla ck goet hi te of the Du gi otok mi ne ra li za tion textu ral Type 1 Ce ll Pa ra me te rs brown (Do-Id) bla ck (Do-Ic) a (Å) 4.609(1) 4.616(2) b (Å) 9.961(2) 9.963(3) c (Å) 3.015(8) 3.022(1) Vo lu me 138.44(5) 138.99(6) 03 segovic.indd 2103 segovic.indd 21 24.6.2008 11:06:2724.6.2008 11:06:27 Geologia Croatica Geologia Croatica 61/1 22 ru ti le we re iden ti fi ed amon gst the inclu sio ns in the bla ck goet- hi te (Fig. 4B). Textu ral type 2 goet hi te showed no su ch re gu- la ri ties in the sen se of struc tu ral and che mi cal di su ni ty. Al thou gh the EPMA mea su re men ts we re per for med usi ng a hig hly fo cu sed beam (~1 µm), all goet hi te ana lyses ap pear to be con ta mi na ted by ac ces so ry pha ses, par ti cu lar ly quar tz (Tab le 3). Howe ver, the mea su red che mi cal com po si tio ns from the bla ck and brown seg men ts of type 1 mi ne ra li za tion are clear ly dis tin guis hed in the Fe2O3/(Fe2O3+Al2O3) vs. Fe2O3 plot (Fig. 6). The ma jo ri ty of the ana lyses from the bla ck seg me nt fo rm a nar row com po si tio nal fi e ld (70–76 wt % Fe2O3, Fe-Al ra tio 0.91–0.94), and clo sely rep re se nt the com po- si tion of Al-sub sti tu ted, i.e. bla ck goet hi te. Brown man tle goet- hi te shows com pa ra ti ve ly low Al2O3 and SiO2 (Tab le 3) and wi th the Fe2O3/(Fe2O3+Al2O3) ra tio of ~ 1 re sem bles the stoi- chiomet ric com po si tion. The dis cre te knob by va rie ties it se lf show si mi lar po la ri zed com po si tio ns in de pen de nt of the textu- ral po si tion of the ana lyzed spo ts (Fig. 6). Ran dom SEM EDS ana lyses of goet hi te of textu ral type 1 are dis played in Fig. 7 as four ma jor cor re la tion tren ds. The cor re la tion diag ram Al2O3-SiO2 (Fig. 7A) de pic ts the po si ti ve cor re la tion of si li ca wi th Al, i.e. it con strai ns si li ca exclu si ve ly alo ng wi th the Al-goet hi te in the bla ck co res (r2=0.69). The es tab lis hed Al2O3-SiO2 cor re la tion is plot ted again st the Fe2O3, MgO and P2O5 (Fi gs. 7B–7D) abun dan ces. A nega ti ve Fe2O3 cor re la tion in res pe ct to the Al2O3-SiO2 (r2=0.52) (Fig. 7B) amou nt shows the wa ne of goet hi te Fe con te nt towar ds the mi ne ral co re. A posi ti ve MgO cor re la tion in res pe ct to the Al2O3-SiO2 amou nt (r2=0.37) (Fig. 7C) cor ro bo ra tes an amou nt of crypto-crystal li ne si li ca te pha ses in the goet hi te co re. SEM EDS ana lyses re vea led unu sual ly hi gh P2O5 con cen tra tion in the ana lyzed goet hi te (up to 2.8 wt %, Fig. 7D). Hig her phos- pho rus va lues are as so cia ted wi th hi gh Fe2O3 values (80–100 wt %) sug ges ti ng its com po si tio nal en ric hme nt in the goet hi te bla ck co re ag gre ga tes. 5. DISCUSSION 5.1. Mi ne ra li za tion as sem bla ge and goet hi te che mis try Due to the slow goet hi te growth, the ana lysed mi ne ra lization con si sts of nu me rous me dium to hi gh crystal li ni ty sub mic ro- so pic goet hi te crystal li tes. The mi ne ra li za tion is com po sed al mo st en ti re ly of goet hi te. Im pu ri ties wit hin goet hi te (e special ly bla ck goet hi te of textu ral type 1), com pri se si li ca te pha ses (quar tz, K-fel dspar, me ta mor phic gar net and ru ti le) and cal ci te. Howe ver, apa rt from coar se cal ci te and ubiqui tous quar tz, their amou nt is always be low the XRD de tec tion li mit (~5 wt %). It is known that quar tz and P, (in the fo rm of PO4 3–), bi nd to the reac ti ve pla ces of goet hi te ter mi nal fa ces and do not en ter the crystal struc tu re (GLASAUER et al., 1999; PARFITT, 1979, 1989). In synthe tic goet hi te, si li ca is con strai ned to be ad sor bed on sur fa ces and lo ca ted wit hin the po res be tween struc tu ral do mai ns (FISHER, 1999; GLASAUER et al., 1999). Fi gu re 4: A. SEM ima ge ry of goet hi te mi ne ra li za tion textu ral type 1 showi ng the lo ca tion of the EPMA WDS mea su red com po si tio nal pro fi le. Bla ck goet hi te in the co re (grey) in clu des va rious mi ne ral im pu ri ties. Clean brown goet hi te (brig ht) for ms a man tle arou nd the bla ck goet hi te. B. BSE ima ge ry of goet hi te mi ne ra li za tion textu ral type 1. Kfs = K-fel dspar, Grt = gar net, Rt = ru ti le, Qtz = quar tz and Cc = cal ci te in clu sio ns. Fi gu re 5: SEM ima ge ry of goet hi te mi ne ra li za tion type 1 showi ng in mo re de tail the re la tio nshi ps from Fi gs. 4A and B. Bla ck goet hi te (lig ht grey) exhi bi ts va rious si li ca te mi ne ral in clu sio ns (bla ck spo ts). Clean brown goet hi te (da rk) for ms a rim arou nd the bla ck goet hi te. 03 segovic.indd 2203 segovic.indd 22 24.6.2008 11:06:2824.6.2008 11:06:28 Geologia Croatica 23 Bra ni mir Šegvić: Si-P im pu re Al-goet hi te mi ne ra li za tion on the is la nd of Du gi otok (Cen tral Ad ria tic, Croa tia) This va rie ty is col loquial ly na med Si-goet hi te. We pro po se to avoid this na me sin ce si li ca is not struc tu red in to the goet hi te ce ll. In the ba sic and slig htly acid en vi ron me nt, phos pha te wil lin gly ma kes com plexes wi th Fe3+ de pen di ng on reac ti ve sur fa ce FeO·OH grou ps and the crystal li ni ty of the oxi de. The reac tion pro bab ly in vol ves a rapid, stro ng li ga nd exchan ge, fol lowed by wea ker li ga nd exchan ge and phos pha te pe net ra- tion at de fe ctive si tes and po res (PARFITT, 1989). The re fo re, the ran dom phos pho rus con te nt of ana lysed bla ck goet hi te (Fig. 7D) refl ec ts its com plex po ro si ty. The measu red goet hi te che mi cal com po si tion was in fl uen- ced by the che mis try of ir ra dia ted sub mic ro nic im pu ri ties, cau si ng a sig ni fi ca nt num ber of ana lyses to be con ta mi na ted (Fig. 6, Fe203 < 70 wt %). Resul ts wi th Fe2O3 > 70 wt % may be ac cep ted as the lea st con ta mi na ted or even un con ta mi na ted (Fe2O3 > 80 wt %). The re sul ts be tween 70 and 76 wt % Fe2O3 ob tai ned for bla ck goet hi te co res of textu ral type 1 show a hi gh abun dan ce of Al, (Tab le 3, Fig. 6) and Si, whi ch is mos tly al lo ca ted to quar tz. The amou nt of Al can not be stoic hio met- rically com bi ned wi th ot her ana lyzed ca tio ns to yie ld any known (si li ca te) mi ne ral pha se. Sin ce Al (oxy)hydroxi des are exclu ded as pos sib le pa ra ge ne tic mi ne rals, we pre su me that Al is sub sti tu ted for Fe in the goet hi te ag gre ga ted bla ck co re of the textu ral type 1, (Fig. 6, Fe2O3/(Al2O3+Fe2O3) ra tio va ries from 0.90 to 0.95). Goet hi te from the brown man tle of the same textu ral type is Al poor (Fig. 6, Fe2O3/(Al2O3+Fe2O3) ra tio ~1), and re sem bles a near stoic hio met ric com po si tion. In na tu ral goet hi te, Fe3+ is rea di ly sub sti tu ted by Al oc ca- sio nal ly reac hi ng up to 0.33 mo le frac tion (e.g. FITZPATRICK & SCHWERTMANN, 1982; TARDY & NAHON, 1985; FABRIS et al., 1986; CARLSON, 1995). The deg ree of sub sti- tu tion of Al for Fe is one of the pa ra me te rs fa vou ri ng hi gh P abun dan ce in goet hi te (AINSWORTH & SUMNER, 1985). Al sub sti tu ted goet hi te from the ana lysed goet hi te mi ne ra li za- tion stron gly sup por ts this sta te me nt by a po si ti ve cor re la tion of P and Al abun dan ces (Fig. 7D). Refer ri ng to the unit ce ll pa ra me ter con tra dic tion, i.e. lar ger ce ll vo lu me of Al-sub sti tu ted goet hi te (bla ck goet hi te) com pa red to the stoic hio met ric goet hi te (brown goet hi te), the phe no me non can be explai ned wi th ran dom Al-sub sti tu tion as a plau sib le cau se of this crystalloc he mi cal dis cre pan cy. Inves- ti ga tio ns of Al-sub sti tu ted goet hi te pre ci pi ta ted at re la ti ve ly Table 3 Se lec ted mic rop ro be spot ana lyses (in wt.%) of the Du gi otok goet hi te mi ne ra li za tion Mi ne ra li za tion type Ic Ic Im Im II II III III Ana lyses no. 9a 27 02 17 01 04 04b 02a SiO2 7.54 11.31 1.79 1.68 6.49 2.34 6.42 15.36 TiO2 0.17 0.31 0.00 0.00 0.05 0.02 0.09 0.35 Al2O3 5.97 11.46 0.04 0.05 4.77 0.00 4.59 9.12 Cr2O3 0.00 0.02 0.00 0.00 0.02 0.00 0.02 0.04 Fe2O3 71.22 60.59 81.12 81.08 71.57 78.26 72.59 58.36 MnO 0.00 0.06 0.08 0.03 0.06 0.03 0.14 0.12 MgO 0.55 0.48 0.12 0.18 0.99 0.09 0.76 1.46 CaO 0.25 0.21 0.33 0.36 0.38 0.34 0.52 0.82 Na2O 0.16 0.20 0.05 0.02 0.21 0.10 0.12 0.18 K2O 0.42 0.13 0.02 0.00 0.19 0.01 0.52 1.88 To tal 86.29 84.77 83.55 83.40 84.73 81.19 85.77 87.69 Al sub st. (mol. %) 13.10 29.60 0.10 0.10 10.40 0.00 9.90 24.50 � mi ne ra li za tion types I, II and III mat ch diff e re nt textu ral types (see text for de tail in for ma tion). � c=co re, m=man tle of the goet hi te ap pea ran ce from the textu ral type 1. � Al sub st. (mol. %) cor res pon ds to goet hi te for mu la cal cu la ted on the ba sis of 1.5 oxygen and 1.0 to tal ca tion af ter sub trac tion of SiO2 and al ka lies from the EPMA poi nt ana lyses. Fi gu re 6: Plot of Fe2O3/(Fe2O3+Al2O3) ver sus Fe2O3 for EPMA WDS spot ana lyses of goet hi te mi ne ra li za tion from all three textu ral types. In the mi ne ra li za tion of textu ral type 1, three diff e re nt com po si tio ns are clear ly dis tin guis hed: SiO2 + si li ca pha ses hig hly mixed com po si tion and Al- sub sti tu ted com po si tion (vir tual ly goet hi te) and near stoi chiomet ric goet hi te (vir tual brown goet hi te). The other two textu ral types show si mi lar com po si tio nal re la tio ns. 03 segovic.indd 2303 segovic.indd 23 24.6.2008 11:06:3024.6.2008 11:06:30 Geologia Croatica Geologia Croatica 61/1 24 low tem pe ra tu res, al so indi ca te that unit ce ll pa ra me te rs in crea se wi th dec rea si ng synthe sis tem pe ra tu re (CORNELL & SCHWERTMANN, 2003). This re la tion ship is at tri bu ted to the incor po ra tion of struc tu ral OH in the goet hi te lat ti ce whi ch aug men ts in crea si ng Al-sub sti tu tion (SCHWERT- MANN et al., 1985). 5.2. Che mi cal con straints of goet hi te for ma tion Goet hi te is a com mon and abun da nt con sti tue nt of ma ny se con- da ry iron ore de po si ts and va rious types of soi ls, par ti cu lar ly of ter ra ros sa (DURN et al., 1999). For ma tion of the se pro duc ts is stron gly con trol led by weat he ri ng, i.e. by des cen di ng so lu- tio ns. Su ch goet hi te-bea ri ng oc cur ren ces com pri se leac hi ng of Fe from a pri ma ry sub stra te and mo bi li sa tion in the fo rm of fer ri hydri te (SINGER et al., 1998). La ter, goet hi te is for med di rec tly by the tran sfor ma tion of fer ri hydri te and/or even tual ly via hae ma ti te in an as sem bla ge al so con sis ti ng of Al-hydra ted oxi des and clay mi ne ra ls as the pro duc ts of in com ple te la te ri- ti za tion (TARDY & NAHON, 1985). An as sem bla ge for med fol lowi ng this ge ne tic pat te rn shows typi cal col lo mor ph- ban ded textu re, poor crystal li ni ty of all pha ses and pu re ly Al- sub sti tu ted goet hi te. In con tra st, goet hi te from the ana lysed mi ne ra li za tion is we ll crystal li zed and may show excep tio nal ly coar se ag gre ga- tes (Fig. 3A). Ear ly crystal li zed Al-sub sti tu ted goet hi te em be ds K-fel dspar, al man di ne-ri ch me ta mor phic gar net, cal ci te, quar tz and ru ti le and is fol lowed by al mo st pu re goet hi te (Fig. 4). This na tu ral ca se sup por ts the re sul ts of ther mo dyna mic expe- ri men ts whi ch show that the ra te of Al-sub sti tu tion in goet hi te is stron gly de pen de nt on the avai la bi li ty of Al and the che mis- try of as so cia ted mi ne ra ls (TARDY & NAHON, 1985). The Al-substituted goethite (black goethite) from Dugi otok is characterized by 13.1–29.6 mol % of Al (Table 3, Fig. 6), and high amounts of silica impurities and bonded phos- phate. Silica binds to the goethite terminal faces suppressing the growth rate and consequently enhances its crystallinity (CORNELL & GIOVANOLI, 1990). The adsorption of silica is minimal at low pH due to formation of protonated or neutral silica polymers which make a stable FeOSi(OH)3 2+ complex (WEBER & STUMM,1965). At pH values between 8 and 11, goethite forms at the expense of hydrolyzed Fe(OH)4 −, whereas silica interferes either by hindering nucleation and crystal growth of goethite or by reacting with metastable ferrihydrite, thus retarding the supply to growing units (GLASAUER et al., 1999). Phosphate enrols the goethite surface rapidly with strong ligand exchange both at the surface and at defect sites and pores. The rate of goethite formation is greatly reduced in the presence of phosphate, due to an increase in the entropic component of the free energy of activation. Analogous to the silica effect, the overall goethite crystallinity is enhanced due to the slower crystal growth (SHAW et al., 2005). The en han ced crystal li ni ty of goet hi te com po si tio nal types, the com po si tion of Al-sub sti tu ted goet hi te wi th si li ca su b-mic ro do mai ns, the pre sen ce of P but la ck of Fe-phos pha te and stab le, coar se grai ned cal ci te in the pa ra ge ne sis stron gly sug ge sts al ka li ne con di tio ns of for ma tion. The re fo re, for ma- tion of Du gi otok goet hi te mi ne ra li za tion di rec tly lin ked to kar sti fi ca tion cou ld ha ve been exclu ded sin ce kar sti fi ca tion Fi gu re 7: Ma jor ele me nt (a ll in wt.%) cor re la tion diag ra ms of SEM EDS spot ana lyses from goet hi te mi ne ra li za tion type 1. No te the almo st stoi chiomet- ric com po si tion of goet hi te from the brown man tle (ar row). A) Al2O3 vs. SiO2 diag ram showi ng a po si ti ve cor re la tion. B) FeO vs. (SiO2+Al2O3) diag ram showi ng a ne ga ti ve cor re la tion. C) MgO vs. (SiO2+Al2O3) showi ng a slig ht po si ti ve cor re la tion. D) P2O5 vs. (SiO2+Al2O3) showi ng a slig ht po si ti ve cor re la tion. 03 segovic.indd 2403 segovic.indd 24 24.6.2008 11:06:3124.6.2008 11:06:31 Geologia Croatica 25 Bra ni mir Šegvić: Si-P im pu re Al-goet hi te mi ne ra li za tion on the is la nd of Du gi otok (Cen tral Ad ria tic, Croa tia) fa vou rs an acid en vi ron me nt. This sta te me nt is cor ro bo ra ted wi th the mo de of mi ne ra li za tion oc cur ren ce ob ser ved at the out cro ps. 5.3. Geo tec to nic en vi ron me nt of goet hi te mi ne ra li za tion Simi lar goet hi te mi ne ra li za tion of po st Eo ce ne age was re por- ted near No vi Vi no dol ski and on the is lan ds of Krk and Cres si tua ted so me 125 km NNW from Du gi otok (Fig. 1) (MAR KOVIĆ, 2002) sug ges ti ng that the in ves ti ga ted goet hi te mi ne ra li za tion shou ld be con si de red on the re gio nal sca le of the Nor th Ad ria tic. The pre se nt sur fa ce lit ho lo gy of Du gi otok does not of fer any sub stra tum from whi ch Fe, Al, Si and P may ha ve been de ri ved. Hen ce, we pos tu la te their sour ce as bei ng in the sub mer ged Ad ria tic cru st. There is a hi gh po si ti ve gra vi met ric ano ma ly si tua ted la nd-wa rd of Du gi otok, (KOŠĆEC, 1986). Fur ther mo re, se ve ral geo mag ne tic ano ma lies are lo ca ted seawa rd of the is la nd (BRDAREVIĆ & OLUIĆ, 1979). All cou ld be as sig ned to ig neous roc ks, al thou gh ig neous ma te- ria ls we re not pro ven (Fig. 1). Howe ver, ul tra-al ka li ne in trap- la te vol ca nic roc ks da ted to the Plio ce ne we re re cog ni zed in se ve ral co res dril led in the dep res sion of Du gi otok (MILETIĆ & LUGOVIĆ, 2000). The ig neous oc cur ren ces tra ce the sou th eas te rn seg me nt of the Schio fau lt system whi ch con ti nuous ly stret ches NW-SE from Vi cen za to Pa lag ru ža (GRANDIĆ et al., 1997). The Du gi otok fau lt (DOF), be lon gs to this fau lt system (Fig. 1), and tra ces a lithos phe ric frac tu re (In ter nal Ad ria tic frac tu re – IAF) wit hin the Ad ria mic rop la te (OLDOW et al., 2002). The Schio fau lt system cau sed ma jor ver ti cal dis pla ce men ts in the Ad ria tic cru st and is tran sec ted by dextral stri ke slip W-E tren di ng li tos phe ric faul ts (DI BUCCI & MAZZOLI, 2003). Bo th fau lt syste ms we re reac ti va ted du ri ng the Plio ce ne (5–2 Ma BP) on ac cou nt of an ancie nt struc tu ral weak ne ss in the Ad ria tic cru st. Mag ma tic oc cur ren ces are lo ca- ted at or near the in ter sec tio ns of the se fau lt syste ms (Fig. 1). We li nk the Du gi otok goet hi te mi ne ra li za tion to the Plio ce ne exten sio nal tec to nic ac ti vi ty whi ch pro du ced sub ma ri ne mag ma tic ac ti vi ty du ri ng an exten si ve ma ri ne tran sgres sion af ter the Mio ce ne emer sion. The tem pe ra te as cen da nt pa ren tal so lu tions ri ch in Fe2+, Al, Si and P, yie lded goet hi te mi ne ra li- za tion, repre sen ti ng the af ter ma th of Plio ce ne mag ma ti sm. 6. CONCLUSIONS In our mo del, goet hi te crystal li zed from as cen da nt so lu tio ns, pro bab ly slig htly war med due to in crea sed hea th fl ux cau sed by Plio ce ne ig neous ac ti vi ty. The Fe2+, Al, Si and P ri ch so lu- tio ns to get her we re tec to ni cal ly mo bi li zed from a hid den Fe- en ric hed me ta mor phic si li ca te sour ce, and ro se up throug hout the com plex sub mer ged cre vi ces for med by Mio ce ne kar sti fi - ca tion and Plio ce ne tec to nic ac ti vi ty. It is li ke ly that du ri ng the up li ft, ve ry sma ll quan ti ties of the me ta mor phic sub stra te we re in clu ded in the so lu tion repre se nted by in com ple te dis sol- ved si li ca te mi ne ra ls. Ini tial crystal li za tion took pla ce un der hi gh oxygen fu ga ci ty, and slig htly ba sic pH, lea di ng to the fa st re mo val of Al, Si and P throu gh the pre ci pi ta tion of mixtu res com pri si ng Al-sub sti tu ted goet hi te and quar tz wi th syntem po- ral de po si tion of sus pen ded me ta mor phic pha ses. The refo re, due to the com ple te re mo val of si li ca and Al, the re si dual so lu- tion crystal li zed as al mo st pu re, stoic hio met ric goet hi te. ACKNOWLEDGMENT This pub li ca tion is the re su lt of con tri bu tion from the 0195025 and 195-1951126-3205 to BL and 0119420 to VB gran ts fo rm the Croa- tian Mi nis try of Scien ce, Edu ca tion and Spor ts. Than kful re gar ds go to R. Al the rr and H.-P. Meyer from the Mi ne ra lo gi cal In sti tu te of the Rup rec ht-Kar ls Uni ver si ty in Hei del be rg, who pro vi ded the in stru- men tal fa ci li ties. V. Garašić and an ano nymous re viewer sig ni fi can- tly im pro ved an ear ly ver sion of the ma nus cri pt. The aut ho rs are great ly in deb ted to G. Du rn and G. Si ja ric for their de tai led re view. We exte nd our ap pre cia tion to Ž. Ži go več ki Go bac for her he lp du ri ng the XRD mea su re men ts and in ter pre ta tion as we ll as to N. Če gec, B. Pr ša and M. Va le nt for their as sis tan ce in the la bo ra to ry. Mrs. Kras na Panj ko ta pro vi ded us wi th her ki nd hos pi ta li ty du ri ng fi e ld wo rk. REFERENCES: AINSWORTH, C. C. & SUMNER, M. E. (1985): Ef fe ct of alu mi nium sub sti tu tion in goet hi te on phos pho rus ad sor ption. II Ra te of ad sor- ption. – Soil Scien ce So cie ty of Ame ri ca Jour nal, 49, 1149–1153. BRDAREVIĆ, N. & OLUIĆ, M. (1979): Pri log poz na va nju tek ton ske gra đe jad ran skog pod mor ja [A con tri bu tion to un der stan di ng of the Ad ria tic sea bed tec to ni cs – in Croa tian]. – Zbor nik RGN fa kul te ta u po vo du 40 go di na ra da (1939–1979), 318–332. CARLSON, L. (1995): Alu mi nium sub sti tu tion in goet hi te in la ke ore. – Bul le tin of the Geo lo gi cal So cie ty of Fin la nd, 67, 19–28. CORNELL, R. M. & GIOVANOLI, R. (1990): Tran sfor ma tion of aka ga- nei te in to goet hi te and hae ma ti te in al ka li ne me dia. – Clays and Clay Mi ner., 38, 469–476. CORNELL, R. M. & SCHWERTMANN, U. (2003): The iron oxi des: struc tu re, pro per ties, seac tio ns, oc cu ren ces and uses, 2nd edn. – Wi- ley-Vch, Wein heim, 664 p. DI BUCCI, D. & MAZZOLI, S. (2003): The Oc to be r-No vem ber 2002 Mo li se seis mic sequen ce (sout he rn Ita ly): an expres sion of Ad ria in trap la te de for ma tion. – J. Geol. Soc., 160, 503–506. DURN, G., OTTNER, F. & SLOVENEC, D. (1999): Clay mi ne ra ls as an in di ca tor of po lyge ne tic ori gin of Ter ra Ros sa in Is tria, Croa tia. – Geo der ma, 91, 125–150. DŽAJA, K. (2003): Geo mor fo loš ke zna čaj ke Du gog oto ka [Geo mor pho- lo gic fea tu res of the Du gi otok Is la nd – in Croa tian]. – Geoad ria, 8/2, 5–44. FABRIS, J. D., RESENDE, M., ALLAN, J., & COEY, J. M. D. (1986): Mössbauer ana lysis of Bra zi lian oxi so ls. – Hyper fi ne In te rac tio ns, 29, 1093–1096. FISHER, L. (1999): In ves ti ga tio ns of the as-/desorption and dif fu sion pro ces ses of hea vy me ta ls and alu mi nium wi th goet hi tes of dif fe re- nt crystal li ni ty – expe ri men tal re sul ts and pro ce ss mo del li ng. – Ph.D. The sis, Univ. of Bo nn, 205 p. FITZPATRICK, R. W. & SCHWERTMANN, U. (1982): Al-sub sti tu ted goet hi te-an in di ca tor of pe do ge nic and ot her weat he ri ng en vi ron- men ts in Sou th Af ri ca. – Geo der ma, 27, 335–347. GLASAUER, S., FRIEDL, J. & SCHWERTMANN, U. (1999): Pro per- ties of goet hi tes pre pa red un der aci dic and ba sic con di tio ns in the pre sen ce of si li ca te. – Jour nal of Col loid and In ter fa ce Scien ce, 216, 106–115. 03 segovic.indd 2503 segovic.indd 25 24.6.2008 11:06:3124.6.2008 11:06:31 Geologia Croatica Geologia Croatica 61/1 26 GRANDIĆ, S., BOROMISA-BALAŠ, E. & ŠUŠTERČIĆ, M. (1997): Explo ra tion con ce pt and cha rac te ris tic of the Di na ri des stra tig rap- hic and struc tu ral mo del in the Croa tian of fsho re area, Pa rt 1. – Naf- ta, 48, 117–128. KOŠĆEC, B. (1986): Prim je na da ljin skih me to da u naf tno geo loškim is- traživa nji ma Is tre i pod mor ja sje ve rois točnog Jad ra na [The ap pli ca- tion of re mo te sen si ng met ho ds in pet ro leum geo lo gy in ves ti ga tion of Is tra and NE Ad ria tic sea bed – in Croa tian]. – Ph.D. The sis, Univ. of Bel gra de, 277 p. LUGOVIĆ, B., ŠEGVIĆ, B. & ŠEGVIĆ, T (2008): Mn-cru st to do ro ki te mi ne ra li za tion on SW bac ksho re Cre ta ceous li mes to nes from the is la nd of Du gi otok (Cen tral Ad ria tic, Croa tia). – Ac ta Ad ria ti ca, 49, 55–64. MARKOVIĆ, S. (2002): Hr vat ske mi ne ral ne si ro vi ne. [Croa tian mi ne- ral re sour ces – in Croa tian]. – Hr vat. geol. in sti tut, Zag reb, 544 p. MILETIĆ, D. & LUGOVIĆ, B. (2000): Con tras ti ng ul traal ka li lit hoc las- ts in Plio ce ne se di men ta ry roc ks from the dep res sion of Du gi otok Is la nd, Ad ria tic o ff-sho re (Croa tia). – 2nd Croat. Geol. Con gre ss Ab- stra ct book (I. Vla ho vić & R. Biondić, Eds.), Hr vat. geol. in sti tut, Zag reb, 327–331. OLDOW, J. S., FERRANTI, L., LEWIS, D. S., CAMPBELL, J. K., D’ ARGENIO, B., CATALANO, R., PAPPONE, G., CARMIGNANI, L., CONTI, P. & AIKEN, C. L. V. (2002): Ac ti ve frag men ta tion of Ad ria, the nor th Af ri can pro mon to ry, cen tral Me di ter ra nean oro gen. – Geo lo gy, 30, 779–782. PARFITT, R. L. (1979): The avai la bi li ty of P from phos pha te-goet hi te brid gi ng com plexes. De sor ption and up ta ke by ryeg ra ss – Pla nt and Soil, 53, 55–65. PARFITT, R. L. (1989): Phos pha te reac tio ns wi th na tu ral al lop ho ne, fer- ri hydri te and goet hi te. – Jour nal of Soil Scien ce, 40, 359–369. POUCHOU, J. L. & PICHOIR, F. (1984): A new mo del for quan ti ta ti ve ana lyses. I. Ap pli ca tion to the ana lysis of ho mo ge neous sam ples. – La Rec her che Aéros pa tia le, 3, 13–38. POUCHOU, J. L. & PICHOIR, F. (1985): »PAP« (ϕ−ρ−Ζ) cor rec tion pro ce du re for im pro ved quan ti ta ti ve mic roa na lysis. – Mic ro beam Ana lysis (J. T. Ar mstro ng Eds.), San Fran cis co Pre ss, 104–106. SCHWERTMANN, U., CAMBIER, P. & MURAD, E. (1985): Pro per- ties of goet hi tes of va ryi ng crystal li ni ty. – Clays and Clay Mi ner., 33, 369–378. SHAW, S., PEPPER, S., BRYAN, N. D. & LIVENS. F. (2005): The ki- ne ti cs and mec ha nis ms of goet hi te and he ma ti te crystal li za tion un- der al ka li ne con di tio ns, and in the pre sen ce of phos pha te. – Am. Mi ner., 90, 1852–1860. SINGER, A., SCHWERTMANN, U. & FRIEDL, J. (1998): Iron oxi de mi ne ra lo gy of Ter re Ros se and ren dzi nas in re la tion to their mois tu- re and tem pe ra tu re re gi mes. – Eur. Jour. of Soil Sci., 49, 385–395. TARDY, Y. & NAHON, D. (1985): Geoc he mis try of the la te ri tes, sta bi- li ty of Al-goet hi te, Al-he ma ti te, and Fe3+-kao li ni te in bauxi tes and fer ric re tes: An ap proa ch to the mec ha ni sm of con cre tion for ma tion. – Am. J. Sci., 285, 865–903. WEBER, W. J. & STUMM, W. (1965): For ma tion of a si li ca to-i ron (III) com plex in di lu te aqueous so lu tion. – Jour nal of Inor ga nic and Nuc- lear Che mis try, 27, 237–239. Manuscript received June 6, 2007. Revised manuscipt accepted January 18, 2008. 03 segovic.indd 2603 segovic.indd 26 24.6.2008 11:06:3224.6.2008 11:06:32