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Iodine in karst-type phosphorites from the Lahn region, Germany
Chemical Geology, 25 (1979) 305--316
305
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
IODINE IN KARST-TYPE PHOSPHORITES FROM THE LAHN REGION, GERMANY
I~ GERMANN I, J.-M. PAGEL 1 and P.P. PAREKH 2' *
Institut f~r Angewandte Geo!ogie, Freie Universit~t Berlin, Berlin Hahn-Meitner-Institut fiir Kernforschung, Berlin (Received July 10, 1978; accepted for publication October 20, 1978)
ABSTRACT Germann, K., Pagel, J.-M. and Parekh, P.P., 1979. Iodine in karst-type phosphorites from the Lahn region, Germany. Chem. Geol., 25: 305--316. The concentration of iodine in 31 Tertiary continental phosphorites from the Lahn region in Germany ranges from about 0.01% to 0.18%, the mean value of 0.044% being quite compatible with that of typical commercial-grade iodine deposits. Microscopic and X-ray diffractometric studies manifested three main mineral phases in these phosphorites viz. francolite, clay minerals (kaolinite, iUite), and ferric oxides (goethite, hematite). These mineral phases were investigated for iodine distribution, mode of its fixation and possible source. Sample heating at 600 ° C evinced no appreciable organic matter in the sample and further showed that the iodine was non-volatile in character. Regression analysis revealed a good correlation between I and Al (Ti), practically no correlation with Fe and negative correlation with P~Os. It was thus deduced that the iodine content is inorganic in nature and bound principally and probably internally to Al-bearing minerals. An iodine content of about 150 ppm is believed to be associated with the apatite phase of the phosphorite. Devonian volcanic rocks are assumed to be the ultimate source of both phosphorus and iodine. Those elements were released during a Tertiary weathering period, their accumulation being controlled by chemical environment and karst topography of a Devonian reef limestone.
INTRODUCTION
As a biophile element, iodine is known to concentrate preferentially in rocks and softs rich in organic matter (Cosgrove, 1970; Gulyayeva and Itkina, 1962; Price and Calvert, 1973) and in subsurface brines related to them (Collins, 1969; Collins et al., 1971). Besides these brines, inorganic nitrate deposits in Chile are the only commercially viable source of iodine known to date (Jan, 1975; Anonymous, 1977). *Present address: Department of Chemistry, Brookhaven National Laboratory, Associated Universities, Inc., Upton, New York (U.S.A.)
306
S a m p l i n g l o c a l i t i e s in the Lahn Phosphorite District
,., . ~ _ woltersDerg
JWeilburg ~-W ¢~ Edelsberg ~I
lOOkm
. . . . . . . . . . .
Dehrn Steeden
DIETZ
LIMBURG
Villmar
_c,--
1
00ss,,0o , LEGEND
~
quarry
I'~
I~ I Katzenelnbogen
~
,oom
'
1
1
\~
Kob,, , .. f~'-~
Museum
I ~' I collection from ~ " old mines
.~X~J I ,.o~-C~ "\ ~ X
'
I
'
Frankfurt
Fig. 1. I n d e x m a p s h o w i n g s a m p l i n g l o c a t i o n s in t h e L a h n area f r o m w h i c h p h o s p h o r i t e s were analyzed.
Middle Devonian: Schalstein Pyrocrastics
Tertiary: Limestone ~ (Massenkalk)
Phosphonte ~
Clays;weathered Sc lalsleln Fe Mn or~!s
Fig. 2. S c h e m a t i c r e p r e s e n t a t i o n o f p h o s p h o r i t e d i s t r i b u t i o n o n t h e surface o f D e v o n i a n l i m e s t o n e s . Cross s e c t i o n , n o t t o scale ( t h i c k n e s s o f p h o s p h o r i t e lenses a n d p o c k e t s varies b e t w e e n 0.1 a n d 10 m e t r e s ) .
307 The early works of Wilke-DSrfurt et al. (1928) and Hill and Jacob (1933} document anomalous iodine contents of phosphate rocks too. However, the reported iodine levels (Chilean Educational Bureau, 1956; Swaine, 1962) are generally below those found in the typical commercial-grade resources. Nevertheless, with the exception of the Chilean nitrates and some Recent marine sediments in which iodine concentrations of up to 1990 ppm have been analysed (Price and Calvert, 1973), "the iodine content of phosphate rock is considerably higher than that of other rock deposits" (Hill and Jacob, 1933). Even (acidic) artesian ground waters in contact with phosphorites reveal significantly high concentrations of iodine which, according to Brown (1958), are indicative of the presence of buried phosphorites. The highest iodine contents ever reported in phosphorites are those from deposits in the Lahn region, Germany (Wilke-DSrfurt, 1927; Wilke-DSrfurt et al., 1928; values which have appeared in the literature thereafter are actually quoted from these early studies). In the course of a detailed geological, mineralogical and geochemical investigation of these deposits, we tried to examine the concentration and distribution of iodine in the phosphorites to get general information on factors and processes responsible for iodine enrichment. The crystallochemical problem of iodine entering the Ca-apatite lattice (Wondratschek, 1963) was another reason that gave impetus to the present investigation. GEOLOGY AND COMPOSITION OF THE LAHN PHOSPHORITES
Geological setting The phosphorite deposits of the Lahn region, which were mined up to 1927, are situated in the Lahn-Syncline of the Southeastern Rhenish Schiefergebirge (Fig. 1). They rest on the karstic surface of a Middle Devonian reef limestone ("Massenkalk"), the topography of which developed during Tertiary weathering periods and peneplanation. The phosphorite ores are embedded in and covered by clays and are often accompanied by Lower Tertiary iron and manganese oxide ores. Both clays and iron or manganese ores are interpreted as weathering products of a Devonian rock suite in which pyroclastic rocks of the "schalstein" type predominate. Since the deposits are no longer accessible, their structural properties must be reconstructed from the detailed work of Kegel (1922) and some small outcrops in limestone quarries (Fig. 2). Sample material has been collected from these outcrops and from old mine dumps (Fig. 1).
Mineralogy and petrography Two types of phosphorite ores occur: hard concretionary lumps (the highgrade "Sti~ckstein"-ore) and low-grade earthy material with high amounts of admixed clays.
308
X-ray diffraction studies proved the prevailing apatite phase in these ores to be a well-crystallized carbonate-fluor-apatite (francolite), the CO2 content of which lies between 1.5 and 4.5% as determined by the method of Gulbrandsen (1970). Fluorine measured with an ion-sensitive electrode gave an average value of 1.45%. Compared with the composition of francolites from marine phosphorites from the Phosphoria Formation (Gulbrandsen, 1966), these values do agree as far as the CO2 is concerned but are lower by a factor of about two for fluorine. Other phosphate phases occasionally detected in these phosphorites are wavellite, dehrnite and crandallite (Larsen and Shannon, 1930). The non-phosphate phases which could be detected in X-ray diffractograms are kaolinite, illite, goethite/hematite and small amounts of quartz. The primary ore, an extremely dense mixture of francolite, silicate minerals and goethite, shows clear evidence of repeated cracking and cementation. Breccia fragments are cemented by fibrous layers of pure francolite (Fig. 3). These tufa-like apatite crusts cover most of the outer and inner surfaces of the lumpy ores; they are products of solution and redeposition of primary ore material. ANALYTICAL METHODS
Analytical techniques used in this work were instrumental neutron activa-
Fig. 3. Photomicrograph of a typical phosphorite breccia cemented b y fibrous francolite. Sample (Ser. no. 17) from limestone quarry between Limburg and Dietz, Lahn.
309
tion analysis using high-resolution -/-spectrometry (INAA) and X-ray fluorescence analysis (XRF). For the INAA analysis of I, A1, Ti and Mn, samples and standards were individually irradiated in " r a b b i t " at a flux of 1013 n cm -2 s -~ for 10 s, using a fast pneumatic carrier facility (Br~itter et al., 1976) in the BerII reactor, Berlin. Delay interval prior to counting on a V-spectrometer was 6--8 min. The irradiated samples were counted at a geometry which limited the dead-time to ~ 15%. The Fe content o f the samples was measured in an independent study in which longer irradiation and cooling periods were employed. The detection limit for iodine was between 12 and 45 ppm, according to the particular composition of the sample. For the X R F analysis of P (and also A1 and Ti) samples were prepared by fusing them with anhydrous lithium tetraborate in a wt. ratio of 1:19. The dilution sufficed to reduce the matrix effect so that variations in the contents of major elements in phosphorites did not appreciably influence the analytical result. Pure apatite was used as a standard for P2Os determination. For iodine analysis the finely powdered samples were pressed into pellets without the use o f any binding material. Iodine was determined from the intensity of its characteristic Ks X-rays, using a W-tube and LiF-200 crystal. The iodine values determined b y the t w o independent analytical methods INAA and XRF, agree within +15% of the mean which indirectly hints at the good precision of our methods. RESULTS AND DISCUSSION
Analytical data obtained on samples of phosphorites are presented in Table I (for iodine given as the mean of the t w o methods). From the compilations of iodine values b y the Chilean Iodine Educational Bureau (1956) and Swaine (1962), one finds that the iodine contents (averages) reported in marine rock phosphates o f the world lie anywhere between 0.8 p p m and 130 ppm. On the other hand, the iodine content of the phosphorites from the Lahn region were found to vary in the range of approximately 100--1800 p p m with an average of a b o u t 440 p p m (31 samples). These values are higher than those found b y Wilke-DSrfurt (1927) and Wilke-DSrfurt et al. (1928) for phosphorites from the same area and represent the highest values ever reported for rock phosphates. The marine phosphorites analysed b y us yielded values b e t w e e n < 16 (Tunisia) to 113 (Morocco) ppm which compare well with the range of values found in such phosphorites b y other authors. The observed average of 0.044% I in Lahn phosphorites competes with the typical values reported b y Jan (1975) for the world's known and potential sources of iodine (0.04% (average) in Chilean nitrate deposits, 0.01% in natural gas well brines in Japan and 0.0035--0.05% in various subsurface brines in the United States).
310 TABLE I C o n t e n t s o f I, Al, Ti, Fe, Mn, P 2 0 s a n d loss o n i g n i t i o n ( L O I ) in L a h n p h o s p h o r i t e s Ser.
Locality*
no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
St St St St St St St St De De De De De De Di Di Di Di Ed Ed Ed Ed Ed Wo Wo Wo Wo Sf Sf Vi Ka
I
Al
Ti,
Fe
Mn
P2Os
LOI
(ppm)
(%)
(%)
(%)
(ppm)
(%)
(%)
680 330 430 540 1780 370 620 550 130 220 220 160 180 650 570 680 320 440 430 370 380 370 300 170 90 240 300 680 610 560 260
4.5 3.5 6.2 2.8 4.0 3.8 3.5 3.5 1.0 0.07 1.3 0.09 0.75 5.2 4.8 3.5 2.5 4.1 3.6 2.9 2.8 2.1 2.3 2.8 0.09 2.0 1.9 4.2 2.0 4.2 1.4
0.33 0.15 0.39 0.31 0.13 0.28 0.18
0.79 1.81 0.52 2.24
8 332 172 101 95
30.4 32.7 23.1 24.8
0.8
0.02 0.02 0.04 0.004 0.084 0.46 0.26 0.18 0.12 0.17 0.31 0.24 0.27 0.22 0.26 0.26 0.01 0.18 0.12 0.21
2.45 0.09 0.94 0.16 4.99 0.63 2.26
1.21
35 1 56
4.05
74
0.11 0.13
0.45 1.14
307
27.3 29.0 26.3 40.7 5 25 13 167 47 93
1.64
47 27 36
1.66
1.59
26.2 27.9 23.7 29.9 27.3 29.1 24.4 30.2 28.9 31.9 33.4 31.8 28.0 28.1 30.1 31.1
0.8 0.9 0.9 0.9
1.1
0.9 1.1 0.7
0.7 0.9
0.7 1.1
* S t = S t e e d e n , De = D e h m , Di = Dietz, Ed = Edelsberg, Wo = Wolfersberg, Sf = Staffel, Vi = Yillmar, K a = K a t z e n e l n b o g e n .
Correlation o f iodine with phosphorite constituents Excluding the presence of any known individual iodine mineral phases in our samples, we tried to correlate iodine with the inorganic and organic (if any) constituents of the phosphorites. Because of the well-documented biogeochemical affinity of iodine its possible association with organic matter in the phosphorites was first investigated. As should be expected from the higher oxidation states of iron and manganese compounds (Fe 3+ and Mn 4÷) in our samples, macroscopic
311
and microscopic examination revealed no indications of organic matter. Sample heating at 600°C gave values for loss on ignition (see Table I) in reasonable agreement with that calculated from the kaolinite contents in the samples. The iodine contents in b o t h the heated and unheated samples were the same within experimental errors. Iodine therefore cannot be associated with an organic phase which in any case is practically negligible. To study the main mineral phases, viz. apatite, goethite/hematite, clays (illite, kaolinite) and quartz, with respect to fixation of iodine in them, we had to resort to an indirect m e t h o d to study the correlation between iodine and the various mineral phases. A1 and Ti were taken as representative of the non-phosphate portion (clay minerals etc.) of the rock, while the concentrations of Fe and P2Os were considered as direct measures of the ferric oxide (goethite/hematite) and apatite fractions, respectively. The iodine concentrations were plotted as functions of wt.% P:Os, Fe, A1 and Ti in Fig. 4. As m a y be clear from this figure and the correlation matrix (Table II), the iodine c o n t e n t is found to increase with increasing values of A1 and Ti of the samples (correlation coefficient r = 0.75 and 0.61, resp.) whereas no correlation seems to exist between iodine and the Fe contents of the samples. As expected from the negative correlation between AI and P2Os (r = --0.59, Table II), the iodine contents decrease with increasing P2Os contents (Fig. 4). On the other hand, according to the negative A1/P2Os correlation, the presence of Al-rich apatites (McConnell, 1973) or significant amounts of Al-phosphates can be ruled o u t and we are justified in attributing the bulk of A1 to the clay mineral and/or free alumina phases. Thus it may be concluded that iodine is mainly fixed in these Al-rich weathering products. As the iodine contents in the surrounding clays devoid of apatite were found below the detection limit, iodine accumulation, however, seems to be restricted to an environment in which apatites occur. Regression analysis (Fig. 4) gave values of 152 and 202 ppm iodine for the intercept corresponding to zero wt. % A1 and Ti, respectively. This amount, we believe, is associated with the apatite phase of the phosphorites and should be the maximum c o n t e n t fitting into the apatite lattice. Strikingly, the iodine contents of rather pure francolite from cementing crusts in the T A B L E II Correlation m a t r i x for I, A1, Ti, Fe, Mn, P205 in Lahn phosphorites
I Al Ti Fe Mn P~O s
I
A1
Ti
Fe
Mn
P20s
1
0.75 1
0.61 0.77 1
0.02 --0.06 0.11 1
0.22 0.36 --0.01 0.17 1
--O.61 --0.59 ---0.64 0.15 0.17 1
312
I [pp~]
I [ppm]
800
800
600
• •
600
400
4OO
00 •
200
•
•
200 40,7-,~ e
P o [N 22
2'7
3'2
3~/
115
I [ppm] 800
800
600
600
400
400
20O e ~
200
2:o
420
6,o
3,'0
4,5
o,lo
o,45
I [pp~]
Ti[%] o)5
Fig. 4. Regression lines for iodine versus P205, Fe, AI and Ti c o n t e n t s o f L a h n p h o s p h o r i t e s : [I] = - - 2 7 [ P 2 0 ~ ] + 1240; [I] = 87[A1] + 152; [I] = 9 4 3 [ T i ] + 202. I and P2Os, Al, Ti are in p p m and wt%, respectively.
Lahn phosphorites average at 157 p p m (sample nos. 10, 12 and 25 in Table
I). We thus conclude t h a t iodine in Lahn phosphorites is inorganic in nature and is bound principally to Al-bearing minerals and to a much lesser e x t e n t to the Ca-apatite phase.
313
Binding type The question that still remains open is the form in which iodine is bound to the Al-rich minerals. From the four hypothetical binding types of iodine in sediments and sedimentary rocks investigated by Walters and Winchester (1971), the "internally-bound" type seems to be the most fitting to our samples. Obviously, the failure to see any loss of iodine in our heating experiments precludes any "surface-bound" iodine. In soils only trace amounts of iodine are sorbed by the clay minerals (Raja and Babcock, 1961; Hamid and Warkentin, 1967), whereas the sesquioxides ferric oxide and "free" aluminium oxide are known to be responsible for the retention of iodine (Whitehead, 1973, 1974). Concerning the composition of clays in the karst relief of the Devonian "Massenkalk" in the Lahn region, the investigations of Blanck et al. (1942) have clearly demonstrated that Fe203, A1203 and TiO2 are enriched and the clays are of the "terra rossa" type. This leads one to believe that iodine in the Lahn phosphorites might be fixed mainly to hydroxides or oxyhydroxides of A1, where it should be able to replace the hydroxyl ion (Goldschmidt, 1954). During our heating experiments, however, the Al-hydrates should have been transformed to corundum (7-A1203) releasing iodine, which is indeed in sharp contrast to our observations.
Sources of iodine Confinement of iodine anomalies to the phosphorus-rich sediments suggests a common geological history for both elements, iodine and phosphorus; their possible sources can therefore be discussed in close connection. The source from which such high concentrations of iodine (and phosphorus) are derived may be either biological or inorganic. Earlier genetical concepts for the phosphorite deposits of the Lahn either assumed initial biogenic accumulation of phosphorus by reef-building organisms or by vertebrate excretion, or speculated on hydrothermal and weathering solutions as the inorganic sources. Preconcentration of biogenic phosphorus (and iodine) in the Devonian reef limestones and secondary enrichment during weathering, as proposed by some earlier authors, seem unlikely considering the extremely low P2Os contents (< 0.01% according to Kegel, 1922) of the limestones. Accumulation of biogenic iodine via vertebrate excretion (Stutzer, 1911) can also be ruled out, since during the assumed periods of weathering and phosphorite accumulation in the Lahn region any marine influence and hence sea-fowl activity was lacking and furthermore no relics of cave-living vertebrates are known. Basaltic volcanism of the region could be a plausible source of inorganic phosphorus (Ahlburg, 1918). The definite spatial correlation between phosphorites and supergene weathering products (karst relief, clays, Fe--Mn
314
ores), which for the most part are older than the Miocene volcanic events, however, seems to exclude post-volcanic hydrothermal solutions as main suppliers o f phosphorus and iodine. Coincidence of phosphorus--iodine and iron -manganese accumulations oh a limestone weathering surface strongly points to the decomposition by weathering of phosphorus- and iron-rich rocks as sources for the elements. From their wide distribution in the vicinity of the limestones and their favourable chemical composition (Hentschel, 1952) the Devonian "schalstein" pyroclastics seem to be the most probable source at least of phosphorus. Even though the iodine contents of the "schalsteins '~ we analysed were found to be below the detection limit of our method, it is well k n o w n that iodine can be derived from volcanic sources (Horn and Adams, 1966; Becket et al., 1972) and that the concentration of iodine in volcanic materials is higher than the general average for igneous rocks; in soils from volcanic ash iodine can be considerably enriched (on an average 2.8 ppm according to the Chilean Educational Bureau, 1956). Thus, lower Tertiary weathering of the Devonian '~schalstein" could have resulted in the release of both phosphorus and iodine which, under acid conditions, could be transported in solution. Underlying or neighbouring limestone horizons might have acted as barriers to iodine and phosphorus migration, inducing precipitation of apatite, metasomatic replacement of preexisting rocks and fixation of iodine in Al-rich products of weathering and neomineralization. CONCLUSIONS
Anomalous iodine accumulation (100--1800 ppm) in the continental phosphorites of the Lahn area seems to be governed b y the inorganic nonapatite phases of the sediment. An iodine fraction of just around 150 p p m is believed to be associated with the francolite apatite phase. This value is well in agreement with the maximum concentration of iodine observed in marine phosphorites. The rest of the iodine, which -- with an average of 440 ppm -makes the Lahn phosphorites the most iodine-rich inorganic sedimentary rocks known to date (except of course the Chilean nitrate), is fixed to A1rich products of weathering and neomineralization. Sources of b o t h phosphorus and iodine could have been Devonian volcanic rocks which were dec o m p o s e d during Tertiary weathering periods. Place and extent of phosphorus and iodine accumulation are controlled b y chemical environment and karst topography of an underlying Devonian reef limestone. Similar deposits with supergene phosphorite accumulation controlled b y karst topography are known in southern France (G~ze, 1938) and in the A l t a y - S a y a n region, USSR (Tsykin, 1967; Zanin, 1967). Both deposits are characterized b y the enrichment of A1203 (and Fe203) due to lateritic weathering, and at least in one of them (Quercy, France) an iodine anomaly has long been k n o w n (Daubr~e, 1871; Kuhlmann, 1872). It thus seems that the "inherent faculty" o f inorganic phosphorites for the collection of iodine
315
(Goldschmidt, 1954) could be explained by their genetical coexistence with karstic limestones and Al-rich weathering products. ACKNOWLEDGEMENTS
The authors wish to thank Mr. W. Gatschke and Dr. F. Kubanek (HahnMeitner-Institut) for their help in experimental work. Special thanks are due to Dr. G. Schneider {Institut ftir Mineralogie, Freie Universit~it Berlin) for his assistance in XRF analysis. F. Meyer (Heimat- und Bergbaumuseum, Weilburg/Lahn) kindly provided some phosphorite samples. The financial support of this work by the Bundesministerium ffir Forschung und Technologie, Bonn {Grant 303-7291-NTS60) is gratefully acknowledged.
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© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
IODINE IN KARST-TYPE PHOSPHORITES FROM THE LAHN REGION, GERMANY
I~ GERMANN I, J.-M. PAGEL 1 and P.P. PAREKH 2' *
Institut f~r Angewandte Geo!ogie, Freie Universit~t Berlin, Berlin Hahn-Meitner-Institut fiir Kernforschung, Berlin (Received July 10, 1978; accepted for publication October 20, 1978)
ABSTRACT Germann, K., Pagel, J.-M. and Parekh, P.P., 1979. Iodine in karst-type phosphorites from the Lahn region, Germany. Chem. Geol., 25: 305--316. The concentration of iodine in 31 Tertiary continental phosphorites from the Lahn region in Germany ranges from about 0.01% to 0.18%, the mean value of 0.044% being quite compatible with that of typical commercial-grade iodine deposits. Microscopic and X-ray diffractometric studies manifested three main mineral phases in these phosphorites viz. francolite, clay minerals (kaolinite, iUite), and ferric oxides (goethite, hematite). These mineral phases were investigated for iodine distribution, mode of its fixation and possible source. Sample heating at 600 ° C evinced no appreciable organic matter in the sample and further showed that the iodine was non-volatile in character. Regression analysis revealed a good correlation between I and Al (Ti), practically no correlation with Fe and negative correlation with P~Os. It was thus deduced that the iodine content is inorganic in nature and bound principally and probably internally to Al-bearing minerals. An iodine content of about 150 ppm is believed to be associated with the apatite phase of the phosphorite. Devonian volcanic rocks are assumed to be the ultimate source of both phosphorus and iodine. Those elements were released during a Tertiary weathering period, their accumulation being controlled by chemical environment and karst topography of a Devonian reef limestone.
INTRODUCTION
As a biophile element, iodine is known to concentrate preferentially in rocks and softs rich in organic matter (Cosgrove, 1970; Gulyayeva and Itkina, 1962; Price and Calvert, 1973) and in subsurface brines related to them (Collins, 1969; Collins et al., 1971). Besides these brines, inorganic nitrate deposits in Chile are the only commercially viable source of iodine known to date (Jan, 1975; Anonymous, 1977). *Present address: Department of Chemistry, Brookhaven National Laboratory, Associated Universities, Inc., Upton, New York (U.S.A.)
306
S a m p l i n g l o c a l i t i e s in the Lahn Phosphorite District
,., . ~ _ woltersDerg
JWeilburg ~-W ¢~ Edelsberg ~I
lOOkm
. . . . . . . . . . .
Dehrn Steeden
DIETZ
LIMBURG
Villmar
_c,--
1
00ss,,0o , LEGEND
~
quarry
I'~
I~ I Katzenelnbogen
~
,oom
'
1
1
\~
Kob,, , .. f~'-~
Museum
I ~' I collection from ~ " old mines
.~X~J I ,.o~-C~ "\ ~ X
'
I
'
Frankfurt
Fig. 1. I n d e x m a p s h o w i n g s a m p l i n g l o c a t i o n s in t h e L a h n area f r o m w h i c h p h o s p h o r i t e s were analyzed.
Middle Devonian: Schalstein Pyrocrastics
Tertiary: Limestone ~ (Massenkalk)
Phosphonte ~
Clays;weathered Sc lalsleln Fe Mn or~!s
Fig. 2. S c h e m a t i c r e p r e s e n t a t i o n o f p h o s p h o r i t e d i s t r i b u t i o n o n t h e surface o f D e v o n i a n l i m e s t o n e s . Cross s e c t i o n , n o t t o scale ( t h i c k n e s s o f p h o s p h o r i t e lenses a n d p o c k e t s varies b e t w e e n 0.1 a n d 10 m e t r e s ) .
307 The early works of Wilke-DSrfurt et al. (1928) and Hill and Jacob (1933} document anomalous iodine contents of phosphate rocks too. However, the reported iodine levels (Chilean Educational Bureau, 1956; Swaine, 1962) are generally below those found in the typical commercial-grade resources. Nevertheless, with the exception of the Chilean nitrates and some Recent marine sediments in which iodine concentrations of up to 1990 ppm have been analysed (Price and Calvert, 1973), "the iodine content of phosphate rock is considerably higher than that of other rock deposits" (Hill and Jacob, 1933). Even (acidic) artesian ground waters in contact with phosphorites reveal significantly high concentrations of iodine which, according to Brown (1958), are indicative of the presence of buried phosphorites. The highest iodine contents ever reported in phosphorites are those from deposits in the Lahn region, Germany (Wilke-DSrfurt, 1927; Wilke-DSrfurt et al., 1928; values which have appeared in the literature thereafter are actually quoted from these early studies). In the course of a detailed geological, mineralogical and geochemical investigation of these deposits, we tried to examine the concentration and distribution of iodine in the phosphorites to get general information on factors and processes responsible for iodine enrichment. The crystallochemical problem of iodine entering the Ca-apatite lattice (Wondratschek, 1963) was another reason that gave impetus to the present investigation. GEOLOGY AND COMPOSITION OF THE LAHN PHOSPHORITES
Geological setting The phosphorite deposits of the Lahn region, which were mined up to 1927, are situated in the Lahn-Syncline of the Southeastern Rhenish Schiefergebirge (Fig. 1). They rest on the karstic surface of a Middle Devonian reef limestone ("Massenkalk"), the topography of which developed during Tertiary weathering periods and peneplanation. The phosphorite ores are embedded in and covered by clays and are often accompanied by Lower Tertiary iron and manganese oxide ores. Both clays and iron or manganese ores are interpreted as weathering products of a Devonian rock suite in which pyroclastic rocks of the "schalstein" type predominate. Since the deposits are no longer accessible, their structural properties must be reconstructed from the detailed work of Kegel (1922) and some small outcrops in limestone quarries (Fig. 2). Sample material has been collected from these outcrops and from old mine dumps (Fig. 1).
Mineralogy and petrography Two types of phosphorite ores occur: hard concretionary lumps (the highgrade "Sti~ckstein"-ore) and low-grade earthy material with high amounts of admixed clays.
308
X-ray diffraction studies proved the prevailing apatite phase in these ores to be a well-crystallized carbonate-fluor-apatite (francolite), the CO2 content of which lies between 1.5 and 4.5% as determined by the method of Gulbrandsen (1970). Fluorine measured with an ion-sensitive electrode gave an average value of 1.45%. Compared with the composition of francolites from marine phosphorites from the Phosphoria Formation (Gulbrandsen, 1966), these values do agree as far as the CO2 is concerned but are lower by a factor of about two for fluorine. Other phosphate phases occasionally detected in these phosphorites are wavellite, dehrnite and crandallite (Larsen and Shannon, 1930). The non-phosphate phases which could be detected in X-ray diffractograms are kaolinite, illite, goethite/hematite and small amounts of quartz. The primary ore, an extremely dense mixture of francolite, silicate minerals and goethite, shows clear evidence of repeated cracking and cementation. Breccia fragments are cemented by fibrous layers of pure francolite (Fig. 3). These tufa-like apatite crusts cover most of the outer and inner surfaces of the lumpy ores; they are products of solution and redeposition of primary ore material. ANALYTICAL METHODS
Analytical techniques used in this work were instrumental neutron activa-
Fig. 3. Photomicrograph of a typical phosphorite breccia cemented b y fibrous francolite. Sample (Ser. no. 17) from limestone quarry between Limburg and Dietz, Lahn.
309
tion analysis using high-resolution -/-spectrometry (INAA) and X-ray fluorescence analysis (XRF). For the INAA analysis of I, A1, Ti and Mn, samples and standards were individually irradiated in " r a b b i t " at a flux of 1013 n cm -2 s -~ for 10 s, using a fast pneumatic carrier facility (Br~itter et al., 1976) in the BerII reactor, Berlin. Delay interval prior to counting on a V-spectrometer was 6--8 min. The irradiated samples were counted at a geometry which limited the dead-time to ~ 15%. The Fe content o f the samples was measured in an independent study in which longer irradiation and cooling periods were employed. The detection limit for iodine was between 12 and 45 ppm, according to the particular composition of the sample. For the X R F analysis of P (and also A1 and Ti) samples were prepared by fusing them with anhydrous lithium tetraborate in a wt. ratio of 1:19. The dilution sufficed to reduce the matrix effect so that variations in the contents of major elements in phosphorites did not appreciably influence the analytical result. Pure apatite was used as a standard for P2Os determination. For iodine analysis the finely powdered samples were pressed into pellets without the use o f any binding material. Iodine was determined from the intensity of its characteristic Ks X-rays, using a W-tube and LiF-200 crystal. The iodine values determined b y the t w o independent analytical methods INAA and XRF, agree within +15% of the mean which indirectly hints at the good precision of our methods. RESULTS AND DISCUSSION
Analytical data obtained on samples of phosphorites are presented in Table I (for iodine given as the mean of the t w o methods). From the compilations of iodine values b y the Chilean Iodine Educational Bureau (1956) and Swaine (1962), one finds that the iodine contents (averages) reported in marine rock phosphates o f the world lie anywhere between 0.8 p p m and 130 ppm. On the other hand, the iodine content of the phosphorites from the Lahn region were found to vary in the range of approximately 100--1800 p p m with an average of a b o u t 440 p p m (31 samples). These values are higher than those found b y Wilke-DSrfurt (1927) and Wilke-DSrfurt et al. (1928) for phosphorites from the same area and represent the highest values ever reported for rock phosphates. The marine phosphorites analysed b y us yielded values b e t w e e n < 16 (Tunisia) to 113 (Morocco) ppm which compare well with the range of values found in such phosphorites b y other authors. The observed average of 0.044% I in Lahn phosphorites competes with the typical values reported b y Jan (1975) for the world's known and potential sources of iodine (0.04% (average) in Chilean nitrate deposits, 0.01% in natural gas well brines in Japan and 0.0035--0.05% in various subsurface brines in the United States).
310 TABLE I C o n t e n t s o f I, Al, Ti, Fe, Mn, P 2 0 s a n d loss o n i g n i t i o n ( L O I ) in L a h n p h o s p h o r i t e s Ser.
Locality*
no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
St St St St St St St St De De De De De De Di Di Di Di Ed Ed Ed Ed Ed Wo Wo Wo Wo Sf Sf Vi Ka
I
Al
Ti,
Fe
Mn
P2Os
LOI
(ppm)
(%)
(%)
(%)
(ppm)
(%)
(%)
680 330 430 540 1780 370 620 550 130 220 220 160 180 650 570 680 320 440 430 370 380 370 300 170 90 240 300 680 610 560 260
4.5 3.5 6.2 2.8 4.0 3.8 3.5 3.5 1.0 0.07 1.3 0.09 0.75 5.2 4.8 3.5 2.5 4.1 3.6 2.9 2.8 2.1 2.3 2.8 0.09 2.0 1.9 4.2 2.0 4.2 1.4
0.33 0.15 0.39 0.31 0.13 0.28 0.18
0.79 1.81 0.52 2.24
8 332 172 101 95
30.4 32.7 23.1 24.8
0.8
0.02 0.02 0.04 0.004 0.084 0.46 0.26 0.18 0.12 0.17 0.31 0.24 0.27 0.22 0.26 0.26 0.01 0.18 0.12 0.21
2.45 0.09 0.94 0.16 4.99 0.63 2.26
1.21
35 1 56
4.05
74
0.11 0.13
0.45 1.14
307
27.3 29.0 26.3 40.7 5 25 13 167 47 93
1.64
47 27 36
1.66
1.59
26.2 27.9 23.7 29.9 27.3 29.1 24.4 30.2 28.9 31.9 33.4 31.8 28.0 28.1 30.1 31.1
0.8 0.9 0.9 0.9
1.1
0.9 1.1 0.7
0.7 0.9
0.7 1.1
* S t = S t e e d e n , De = D e h m , Di = Dietz, Ed = Edelsberg, Wo = Wolfersberg, Sf = Staffel, Vi = Yillmar, K a = K a t z e n e l n b o g e n .
Correlation o f iodine with phosphorite constituents Excluding the presence of any known individual iodine mineral phases in our samples, we tried to correlate iodine with the inorganic and organic (if any) constituents of the phosphorites. Because of the well-documented biogeochemical affinity of iodine its possible association with organic matter in the phosphorites was first investigated. As should be expected from the higher oxidation states of iron and manganese compounds (Fe 3+ and Mn 4÷) in our samples, macroscopic
311
and microscopic examination revealed no indications of organic matter. Sample heating at 600°C gave values for loss on ignition (see Table I) in reasonable agreement with that calculated from the kaolinite contents in the samples. The iodine contents in b o t h the heated and unheated samples were the same within experimental errors. Iodine therefore cannot be associated with an organic phase which in any case is practically negligible. To study the main mineral phases, viz. apatite, goethite/hematite, clays (illite, kaolinite) and quartz, with respect to fixation of iodine in them, we had to resort to an indirect m e t h o d to study the correlation between iodine and the various mineral phases. A1 and Ti were taken as representative of the non-phosphate portion (clay minerals etc.) of the rock, while the concentrations of Fe and P2Os were considered as direct measures of the ferric oxide (goethite/hematite) and apatite fractions, respectively. The iodine concentrations were plotted as functions of wt.% P:Os, Fe, A1 and Ti in Fig. 4. As m a y be clear from this figure and the correlation matrix (Table II), the iodine c o n t e n t is found to increase with increasing values of A1 and Ti of the samples (correlation coefficient r = 0.75 and 0.61, resp.) whereas no correlation seems to exist between iodine and the Fe contents of the samples. As expected from the negative correlation between AI and P2Os (r = --0.59, Table II), the iodine contents decrease with increasing P2Os contents (Fig. 4). On the other hand, according to the negative A1/P2Os correlation, the presence of Al-rich apatites (McConnell, 1973) or significant amounts of Al-phosphates can be ruled o u t and we are justified in attributing the bulk of A1 to the clay mineral and/or free alumina phases. Thus it may be concluded that iodine is mainly fixed in these Al-rich weathering products. As the iodine contents in the surrounding clays devoid of apatite were found below the detection limit, iodine accumulation, however, seems to be restricted to an environment in which apatites occur. Regression analysis (Fig. 4) gave values of 152 and 202 ppm iodine for the intercept corresponding to zero wt. % A1 and Ti, respectively. This amount, we believe, is associated with the apatite phase of the phosphorites and should be the maximum c o n t e n t fitting into the apatite lattice. Strikingly, the iodine contents of rather pure francolite from cementing crusts in the T A B L E II Correlation m a t r i x for I, A1, Ti, Fe, Mn, P205 in Lahn phosphorites
I Al Ti Fe Mn P~O s
I
A1
Ti
Fe
Mn
P20s
1
0.75 1
0.61 0.77 1
0.02 --0.06 0.11 1
0.22 0.36 --0.01 0.17 1
--O.61 --0.59 ---0.64 0.15 0.17 1
312
I [pp~]
I [ppm]
800
800
600
• •
600
400
4OO
00 •
200
•
•
200 40,7-,~ e
P o [N 22
2'7
3'2
3~/
115
I [ppm] 800
800
600
600
400
400
20O e ~
200
2:o
420
6,o
3,'0
4,5
o,lo
o,45
I [pp~]
Ti[%] o)5
Fig. 4. Regression lines for iodine versus P205, Fe, AI and Ti c o n t e n t s o f L a h n p h o s p h o r i t e s : [I] = - - 2 7 [ P 2 0 ~ ] + 1240; [I] = 87[A1] + 152; [I] = 9 4 3 [ T i ] + 202. I and P2Os, Al, Ti are in p p m and wt%, respectively.
Lahn phosphorites average at 157 p p m (sample nos. 10, 12 and 25 in Table
I). We thus conclude t h a t iodine in Lahn phosphorites is inorganic in nature and is bound principally to Al-bearing minerals and to a much lesser e x t e n t to the Ca-apatite phase.
313
Binding type The question that still remains open is the form in which iodine is bound to the Al-rich minerals. From the four hypothetical binding types of iodine in sediments and sedimentary rocks investigated by Walters and Winchester (1971), the "internally-bound" type seems to be the most fitting to our samples. Obviously, the failure to see any loss of iodine in our heating experiments precludes any "surface-bound" iodine. In soils only trace amounts of iodine are sorbed by the clay minerals (Raja and Babcock, 1961; Hamid and Warkentin, 1967), whereas the sesquioxides ferric oxide and "free" aluminium oxide are known to be responsible for the retention of iodine (Whitehead, 1973, 1974). Concerning the composition of clays in the karst relief of the Devonian "Massenkalk" in the Lahn region, the investigations of Blanck et al. (1942) have clearly demonstrated that Fe203, A1203 and TiO2 are enriched and the clays are of the "terra rossa" type. This leads one to believe that iodine in the Lahn phosphorites might be fixed mainly to hydroxides or oxyhydroxides of A1, where it should be able to replace the hydroxyl ion (Goldschmidt, 1954). During our heating experiments, however, the Al-hydrates should have been transformed to corundum (7-A1203) releasing iodine, which is indeed in sharp contrast to our observations.
Sources of iodine Confinement of iodine anomalies to the phosphorus-rich sediments suggests a common geological history for both elements, iodine and phosphorus; their possible sources can therefore be discussed in close connection. The source from which such high concentrations of iodine (and phosphorus) are derived may be either biological or inorganic. Earlier genetical concepts for the phosphorite deposits of the Lahn either assumed initial biogenic accumulation of phosphorus by reef-building organisms or by vertebrate excretion, or speculated on hydrothermal and weathering solutions as the inorganic sources. Preconcentration of biogenic phosphorus (and iodine) in the Devonian reef limestones and secondary enrichment during weathering, as proposed by some earlier authors, seem unlikely considering the extremely low P2Os contents (< 0.01% according to Kegel, 1922) of the limestones. Accumulation of biogenic iodine via vertebrate excretion (Stutzer, 1911) can also be ruled out, since during the assumed periods of weathering and phosphorite accumulation in the Lahn region any marine influence and hence sea-fowl activity was lacking and furthermore no relics of cave-living vertebrates are known. Basaltic volcanism of the region could be a plausible source of inorganic phosphorus (Ahlburg, 1918). The definite spatial correlation between phosphorites and supergene weathering products (karst relief, clays, Fe--Mn
314
ores), which for the most part are older than the Miocene volcanic events, however, seems to exclude post-volcanic hydrothermal solutions as main suppliers o f phosphorus and iodine. Coincidence of phosphorus--iodine and iron -manganese accumulations oh a limestone weathering surface strongly points to the decomposition by weathering of phosphorus- and iron-rich rocks as sources for the elements. From their wide distribution in the vicinity of the limestones and their favourable chemical composition (Hentschel, 1952) the Devonian "schalstein" pyroclastics seem to be the most probable source at least of phosphorus. Even though the iodine contents of the "schalsteins '~ we analysed were found to be below the detection limit of our method, it is well k n o w n that iodine can be derived from volcanic sources (Horn and Adams, 1966; Becket et al., 1972) and that the concentration of iodine in volcanic materials is higher than the general average for igneous rocks; in soils from volcanic ash iodine can be considerably enriched (on an average 2.8 ppm according to the Chilean Educational Bureau, 1956). Thus, lower Tertiary weathering of the Devonian '~schalstein" could have resulted in the release of both phosphorus and iodine which, under acid conditions, could be transported in solution. Underlying or neighbouring limestone horizons might have acted as barriers to iodine and phosphorus migration, inducing precipitation of apatite, metasomatic replacement of preexisting rocks and fixation of iodine in Al-rich products of weathering and neomineralization. CONCLUSIONS
Anomalous iodine accumulation (100--1800 ppm) in the continental phosphorites of the Lahn area seems to be governed b y the inorganic nonapatite phases of the sediment. An iodine fraction of just around 150 p p m is believed to be associated with the francolite apatite phase. This value is well in agreement with the maximum concentration of iodine observed in marine phosphorites. The rest of the iodine, which -- with an average of 440 ppm -makes the Lahn phosphorites the most iodine-rich inorganic sedimentary rocks known to date (except of course the Chilean nitrate), is fixed to A1rich products of weathering and neomineralization. Sources of b o t h phosphorus and iodine could have been Devonian volcanic rocks which were dec o m p o s e d during Tertiary weathering periods. Place and extent of phosphorus and iodine accumulation are controlled b y chemical environment and karst topography of an underlying Devonian reef limestone. Similar deposits with supergene phosphorite accumulation controlled b y karst topography are known in southern France (G~ze, 1938) and in the A l t a y - S a y a n region, USSR (Tsykin, 1967; Zanin, 1967). Both deposits are characterized b y the enrichment of A1203 (and Fe203) due to lateritic weathering, and at least in one of them (Quercy, France) an iodine anomaly has long been k n o w n (Daubr~e, 1871; Kuhlmann, 1872). It thus seems that the "inherent faculty" o f inorganic phosphorites for the collection of iodine
315
(Goldschmidt, 1954) could be explained by their genetical coexistence with karstic limestones and Al-rich weathering products. ACKNOWLEDGEMENTS
The authors wish to thank Mr. W. Gatschke and Dr. F. Kubanek (HahnMeitner-Institut) for their help in experimental work. Special thanks are due to Dr. G. Schneider {Institut ftir Mineralogie, Freie Universit~it Berlin) for his assistance in XRF analysis. F. Meyer (Heimat- und Bergbaumuseum, Weilburg/Lahn) kindly provided some phosphorite samples. The financial support of this work by the Bundesministerium ffir Forschung und Technologie, Bonn {Grant 303-7291-NTS60) is gratefully acknowledged.
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