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Clay Minerals (1998) 33, 3-13 Source and grain-size influences upon the clay mineral distribution in the Skagerrak and northern Kattegat H. BENGTSSON AND R. L. S T E V E N S Department of Geology, Earth Sciences Centre, G6teborg University, Box 460, S-40530 G6teborg, Sweden (Received 12 August 1996; revised 27 January 1997) A B S T R A C T : Bottom sediments from 78 sites in the Skagerrak-Kattegat area have been used to determine the semi-quantitative distribution of clay minerals, and to interpret sediment provenance and the transport pathways. The sediment reflects incorporation of four sources, affected both by earlier glacial processes and on-going marine mixing. The southern North Sea, characterized by dioctaheadral illite, kaolinite and smectite, is the dominating source. The central North Sea provides a limited, but significant, source for chlorite. The sediment from the Swedish west coast has an unweathered character, containing trioctaheadral illite, vermiculite, illite-vermiculite mixed-layer minerals and chlorite. The contribution from the southern Kattegat (southern Sweden and Denmark) is dominated by kaolinite and smectite. Correlation between clay content and the contents of smectite, kaolinite and chlorite in the Skagerrak indicate that the clay mineral distribution is influenced by sorting. This grain-size dependency must be considered when interpreting sources. The Skagerrak is an eastern extension of the North Sea, and has a maximum depth of 700 m in the central, Norwegian Trench (Fig. 1). The shallower Kattegat (maximum depth of 100 m) lies south-east of Skagerrak and is connected in the south to the Baltic Sea. The Skagerrak provides a natural sediment trap for the material transported from the Baltic, the Scandinavian rivers and the comparatively shallow North Sea. The dominant sediment transport pathways from the North Sea are well established in general (Svansson, 1975; Eisma et al., 1979; Eisma & Kalf, 1987; van Weering, 1981, 1982; Kuijpers et al., 1993; Lepland & Stevens, 1996). Drainage from the Baltic, Scandinavian rivers from adjacent land, and seabottom erosion are also considered significant sediment sources. Recent volumes of collected papers dealing with the Skagerrak-Kattegat are found in Liebezeit et al. (1993) and Rosenberg (1996). Several clay mineral investigations have been published for parts of the Skagerrak and the Kattegat, although usually as an extension of samplings in the North Sea proper or limited to a local region with particular interest (Ronningsland, 1976; Jorgensen et al., 1981; Pederstad, 1982; Pederstad et al., 1993; Wirth & Wiesner, 1988; Meyenburg & Liebezeit, 1993; Z611mer & Irion, 1993). Considering the importance of fine-grained sedimentation, therefore, we have extended the geographic coverage with the objective of interpreting transport pathways and sources. This investigation also evaluates the association between grain size and clay mineralogy, which involves important modifications in some areas due to sediment sorting processes. SAMPLES AND METHODOLOGY The samples consist of the upper 2 cm of the sediment, subsampled from box cores and multicore tubes c o l l e c t e d d u r i n g three cruises ( 1 9 8 8 - 1 9 9 3 ) . Additional samples have been collected in co-operation with the Geological Survey of Sweden during marine geological mapping along the Swedish west coast (Fig. 1). 9 1998 The Mineralogical Society H. Bengtsson and R. L. Stevens '~ Norway .... :~.:i:"2".. ':.'~5-. ~.h)"i 9"i.:.:.:. " " " " " " "%"" " ""i: ",,~ .;:.: i :.'. )!.:::. =!'.i:' ':':.... goe 200 m ..... r ..,.?.i:.:i~=:i.~i-)'.::..: :2.,.! / " 9..-:~..:.:. r?.::- .'-::)i':::>.: North Sea ::'~'.~.,'.,'.=:::}, 12~ +59~ ~nd ,"~Germany , ,-,.." X t ~;;=;;::: X .: :.: ~ >.x, /~.~ Netheda~nds .-.'.=i::':).i;.:.~ 9,.. ...:.?:;i;~:=-;-.:.:..:..-:~ Skagerrakx xx~Xxx ':'"'!"!i': 'i!i': !i';';~;i~i: :': "':': I ":.::" ..j.:,.:::-:!;...;-..,,..:.;.,.~ 9 ) ' ~ " ~ " " ; :" :' i :i::?::::!)::ti::i~;:i~:::: ~ . . .,m".!~ 20 40 ~ | I ~ x I X • x ~ x X X [] X x~'---"~' x ~ x •215215/ Z,,'V'• * x 5703'N+8~ x x ~ X - 9,,. -.-. ~. ,,.. x • ~ 9 X xx , ~,?)..?..- •215 . / x x x~ / 4 ~ ~ | ~x x ' | Kattegat i k :' FIG. 1. The Skagerrak and Kattegat area with crosses showing the positions of the sampled sites. The main surface currents are indicated with thick arrows for the dominant currents and thin arrows for episodic currents (after Svansson, 1975; Rodhe, 1987). The sites marked with a square and with circles are referred to in Fig. 3. The samples were treated with hydrogen peroxide to remove organic matter, and dispersed with sodium diphosphate. Grain-size analyses combined wet-sieving and pipette methodologies (Krumbein & Pettijohn, 1938). The mineralogical analyses were performed by X-ray diffractometry (XRD) using a Siemens D5000 diffractometer, Ni-filtered Cu-Kc~ radiation (40 kV and 25 mA), and a scanning speed of 1~ 20 per minute. After removal of the organic material, the clay fraction (<2 p.m) was separated from the coarser sediment by repeated gravitational settling. Oriented preparations of the clay were first collected onto a 0.45 p.m membrane filter and then transferred to a glass slide (Drever, 1973). The clay samples were all routinely investigated with two preparations: Mg-saturated and, separately, Mg-saturated with glycerol saturation. For selected samples, additional pretreatments included ethylene glycol saturation, K saturation followed by heating to 350~ and, separately, to 550~ and partial dissolution with 1 M hydrochloric acid for 6 h in an 80~ water bath. Non-oriented powder samples were run to identify the predominance of dioctahedral (060 = c. 1.50 A) or trioctahedral (060 = c. 1.54 .~) clay mineral varieties. In addition, the dioctahedral and trioctahedral characterization of illite could also be estimated using the intensity ratio of the 001 and 002 peaks, where 001/002 <4 is consistent with a predominance of dioctahedral illite. Criteria for clay mineral identification were based upon information in Brown (1961), Brown & Brindley (1980), and Reynolds (1985). Illite has a strong 10 A peak with MgC12 saturation. A comparison of XRD curves for various pretreatments is given in Fig. 3. The Grain-size influence on clay mineral distribution 5 :;.?:..-. ;-,%,,, .: -'...:..~..:. .'.:':.'..':.:::.~ ~i-i~}i:..i.Swe d e n :?.'.".:":"i ~" ": S":' "i":'; "" 9 ,.'.:'.:'?~',:'.-'.). 9 ?"" i.:-:".::-','. ; .. ,, .7" ...:-:-:::':.'q.:-.. ~. :'.:'.k~,.~ / y _,, : 9::'~"3." . ." : ..' :.~...... .,.'" ...... m /jX-:::.:::{=::i.~::i}:'.-::!: " / / Denmark . . . . . 0 20 ....:.:.;...} "..'' .i.:'.='.k ~..2-.~ / / / / / / / / / r?ii.!:;.:..=i.,k/ ~. :-;,.::{.: ":8! km 40 Clay content I / I, ' II >50% -- 40-50% / <40% FIG. 2. The bottom-sediment clay content in the Skagerrak-Kattegat area (after van Weering, 1982; Pederstad et al., 1993; Stevens et al., 1996). The increasing clay content corresponds closely to increasing depth. The bathymetry is displayed with 100 m contour lines. patterns from the MgC12 and glycerol saturated pretreatment were used for the semi-quantitative calculations. The computer program DIFFRAC/AT version 3.06 was used to measure the integrated peak areas in a consistent manner. Clay mineral percentages were obtained by resolving overlapping peaks when necessary and multiplying the peak areas with weighting factors to compensate for their relative intensities (Table 1). RESULTS The grain-size character of the bottom sediments in the Skagerrak and the Kattegat varies from sandy deposits along the west coast of Denmark (Jutland), with a few percent clay, to the clay-rich (>50%) deposits of the Norwegian Trench (Fig. 2). The clay fractions include the clay minerals illite, kaolinite, smectite, chlorite, mixed-layer minerals and vermiculite (Fig. 3A); amphiboles, quartz, feldspars and calcite are also present. With the exception of vermiculite, all clay minerals are present at all sites, although the proportion of each mineral species varies (Table 1). The highest contents of chlorite are in western Skagerrak (>15%, Fig. 4A). Low contents (<5%) are only found in restricted areas west of the north end of Denmark and at the border between the 6 H. Bengtsson and R. L. Stevens (A) Chlorite Illite (B) to 20 ~o 3o to 20 Jo ;o FIG. 3. Typical XRD patterns from the Skagerrak-Kattegat region, showing the different preparations (A) that were selectively used for mineral identification (example from north of Jutland, Denmark, marked with a square in Fig. 1). Mixed-layer minerals (I-S and I-V) and vermiculite are only present in very small amounts in this example. The peak at 12 ~. in the air-dried treatment is not a mixed-layer mineral, as it disappears with MgCI2 saturation. Rather, this peak is a degraded illite presumably expanded by the laboratory dispersion treatment. (B) X-ray diffraction patterns for samples after treatment with MgCI2 and glycerol saturation from regions of different composition. The sites used as examples are marked with circles in Fig. 1. Cu-K~x radiation. Skagerrak and Kattegat. Large amounts of kaolinite are found north east and north west of Jutland, Denmark (>20%, Fig. 4B). Smaller amounts are found in western Skagerrak. The ratio between kaolinite and chlorite is based on the 3.58 A peak for kaolinite and the 3.54 A. peak for chlorite (Biscaye, 1964). This ratio ranges from 0.8-1.9 (Table 1). It is at its greatest off the Jutland coast, and smallest in western Skagerrak and near the Swedish coast (Fig. 4C). The lowest contents of illite are found in the southern part of the Skagerrak (Fig. 4D), where the illite is predominantly a dioctahedral mica. The highest illite contents are near-shore to the Grain-size influence on clay mineral distribution TABLE 1. Clay mineral percentages calculated using the peak area of the glycerol-saturated sample, recalculated to 100% intensity if necessary, and then normalized relative to the other minerals using an intensity factor, I.F. (after Snail, 1973; Pederstad & J~rgensen, 1985; Reynolds, 1989). Mineral Chlorite Kaolinite Kaolinite/chlorite ratio Illite Mixed-layer minerals Vermiculite Smectite Peak 4.74/k 7.18 A 3.58 ,~/3.54 ]~ 10.0 A 12-13 ,~ 14/k 17 ,~ I.F. Calc. formulae Statistical parameters (%) Av. Min. Max. S.D. 2 3 (4.74 ~ x 2.5)/2, (7.18 A-(4.74 Ax2.5))/3 1 2 3 3 10.0 ~,/1 11-13 A/2 (14 ~ - ( 4 . 7 4 A x 1.5))/3 17 A/3 Scandinavian mainland, where the illite is of mainly trioctahedral character. Although these regional trends are seen in Fig. 3B (cf. 001/002 ratio of illite), the separate distributions of dioctahedral and trioctahedral mica are not presented because of the difficulties in quantifying their proportions. The mixed-layer minerals consist of illite-smeetite (I-S) and illite-vermiculite (I-V) varieties. The I-S composition predominates in all samples except for a few sites near Sweden. The large variation of mixed-layer minerals ( 1 - 2 4 % ) reflects the extremely high contents in a few samples near Swedish river outlets (Fig. 4E). The individual distributions of I-S and I-V are not presented because of their low average percentage and irregular distribution. Vermiculite is not present at all sites, and in many cases only as a trace mineral (Fig. 4F). The largest contents are near the Grta ~ilv River outlet on the central coast of western Sweden (Fig. 1). The greatest smectite contents are in the deepest part of the Skagerrak (Fig. 4G), and the smallest are just north of Denmark. The mineralogical variation is sufficient to distinguish different sub-areas by comparison of the XRD patterns from key sites (Fig. 3B). The average XRD pattem in the Kattegat is characterized by trioctahedral illite and chlorite. The southern Skagerrak primarily contains dioctahedrat illite, kaolinite and smectite. The mixed-layer minerals in the southern Skagerrak are mainly l-S, but I-V is also present. Eastern Skagerrak has a composition intermediate between those found in southern Skagerrak and the Kattegat, characterized by dioctahedral illite, kaolinite, smectite, chlorite and feldspars. In north western Skagerrak, dioctahe- 9 15 1.3 62 4 1 10 3 7 0.8 43 1 0 3 18 25 1.9 74 24 5 27 +_4 +5 +0.3 +7.5 _+4.5 _ 1 -+5 dral illite and chlorite predominate, while the kaolinite, smectite and feldspar contents are relatively low. In western Skagerrak, smectite and chlorite increase, while kaolinite and illite decrease compared to north western Skagerrak. For the entire sample set, there are no significant correlations between grain size and the individual clay minerals. For samples w i t h i n western Skagerrak, however, there is a weak, positive correlation between the contents of clay (<2 gm) and smectite (Fig. 5A). Also, mixed-layer minerals in this area have a very weak, positive correlation with clay content, whereas a weak, negative relationship exists between clay and both kaolinite and chlorite (not shown). Elsewhere, the only correlation that was evident was the very weak, negative relationship between smectite and clay content in the Kattegat (Fig. 5B). DISCUSSION General mineralogy The illite-rich clay composition of the Skagerrak and Kattegat sediments (Table 1, Fig. 4D) is in common with most marine deposits of the North Atlantic and adjacent seas (cf. Griffin et al., 1968). Illite, particularly the trioctahedral form, reflects the limited weathering of micaceous source material within a temperate, terrestrial climate, complemented in northern latitudes with the recent reworking of widespread glacial deposits of immature mineralogy. Although in lesser amounts, the presence of chlorite in all samples also requires a relatively unweathered source, as does the H, Bengtsson and R. L. Stevens (A) (C) Ratio kaolinite/chlorite (F) (B) Chlorite (D) Illite Vermiculite Kaolinite (E) (G) Mixed-layer minerals Smectite 05 FIG. 4, Distribution of clay minerals (%) in the Skagerrak-Kattegat area, based on semi-quantitative calculations from the XRD patterns. The illite content (D) includes both dioctahedral and l~ioctahedral mineral varieties, and the I-S and I-V mixed-layer minerals (E) are combined. survival of a feldspar component in the clay fraction of most samples. At the same time, the smectite and kaolinite contents suggest the incorporation of significant materials produced by recent or ancient weathering processes. For the Quaternary clays in southwestern Sweden, a similar reflection of dual source character has been explained by the glacial erosion of a terrain with areas of both pre- weathered mantle and non-weathered sediments and crystalline bedrock (Stevens et al., 1987). However, in the Swedish deposits, the predominant products of weathering are vermiculite and I-V mixed-layer clay minerals, which are only poorly represented in the Skagerrak-Kattegat sediments. Therefore, kaolinite and smectite are probably derived from areas where older deposits or sedimentary bedrock occur, e.g. south of the Grain-size influence on clay mineral distribution (A) ~ 9 (B) R=0.43 15 9 : "5 ~1o 10 V 9 "" E oO 9" " E 03 5 ," 5 V G) R=0.82 . 10 . 20 . . 30 . 40. 50 60~176 Clay content ;o ~o 1o d0 Clay content F~G. 5. Bivariant plots for smecfite and clay contents in the Skagerrak along the Jutland Current (A), and the Kattegat (B). Tomqvist zone separating the crystalline bedrock of the Fennoscandian segment from the younger sedimentary formations in Denmark, southernmost Sweden and most of the North Sea and westem Europe. The non-glaciated areas to the south also have greater preservation of older and, in places, more extensive soil development. The regional clay mineral variations are evaluated below in terms of possible sources. It is also necessary to consider the grain-size dependency that, although weak, may have influenced the patterns of mineral concentration. Since the grain size to mineral content relationships are weak and only regionally defined, normalization of mineral contents is not feasible. The weak correlations themselves may suggest that the sediments in most areas have been mixed from more than one source and modified by transport and depositional processes that were variable over time. Sediment sorting may also have been limited by the prevalence of clay particles within larger aggregates, although the timing of aggregate formation is not clearly established (Eisma & Kalf, 1987; Stevens et al., 1996). Therefore, the mineral trends are apparently not controlled solely by grain-size influences, and provenance interpretations based upon multiple sites are believed to be generally reliable when due consideration is given to the sedimentological conditions. North Sea p r o v e n a n c e Chlorite contents in the Skagerrak increase westward along the deep Norwegian Trench (to >15%), suggesting a North Sea origin. This interpretation is strengthened by the weak, negative correlation between chlorite and clay content which would favour lower chlorite contents in the very fine-grained trench sediments, where they are highest, and therefore not a grain-size effect. The most likely sources for chlorite are the exposed Pleistocene deposits with immature mineralogy in the central North Sea (Wirth & Weisner, 1988; Zrllmer & Irion, 1993). Transport into the Norwegian Trench, along which this chlorite trend is aligned, is consistent with the inflow of the North Atlantic Water Current from the west. Although volumetrically the largest in the Skagerrak, this deep water current has not previously been attributed a major role in the sediment supply. Variations of approximately 5% in the chlorite content are, nevertheless, a reasonable effect of diffuse (low concentration) transport of a relatively chlorite-rich sediment in suspension. The high content of kaolinite along the west coast of Denmark and eastward to the Swedish coast (Fig. 4B) is consistent with its transport from the southern North Sea by the Jutland Current along the Danish west coast, as has been suggested by several authors (Wirth & Wiesner, 1988; Zrllmer & [rion, 1993). The tendency for kaolinite particles to be larger and more stable than other clay minerals could help to concentrate kaolinite in the coarser, near-shore sediments. However, this grain-size relationship was very weak when tested, and the southern source supply is interpreted to be primarily responsible for the distribution pattern. Since the western Skagerrak has well defined distribution patterns and low grain-size dependency for both chlorite and kaolinite, the mapped ratio of these two minerals (Fig. 4C) represents the relative contributions from two sources: a mineralogically immature source in the central North Sea for chlorite-rich sediment, and a more southern North 10 H. Bengtsson and R. L. Stevens Sea source with kaolinite reflecting its history of weathered alteration. The K/C ratio is lowest in the Norwegian Trench, but increases eastward in both deep and shallow portions of south-western Skagerrak, indicating increasing sedimentation in this direction from the coastal Jutland Current relative to that of the North Atlantic Deep Water Current. The trends documented here extend and add detail to the same ratio mapped in the North Sea by Wirth & Wiesner (1988). Further eastward, beyond a north-south boundary that corresponds to the relatively deep trough 'Djupa r'annan', the K/C relationship decreases rapidly, indicating increasing amounts of chlorite eroded from exposures of less mature glacial deposits in the Kattegat and along the Swedish west coast (see below). Furthermore, the diminished importance of the Jutland Current in this area of interaction with the northward surface outflow of Baltic waters may selectively limit the transport of relatively coarse kaolinite from the west. The grain-size dependency associated with smectite is presumably due to its smaller and less stable particles, which would tend to be enriched within the areas of very low turbulence, in particular the Norwegian Trench (Fig. 4G). Similar to kaolinite, smectite has been attributed to southem North Sea sources (Wirth & Wiesner, 1988; Z611mer & Irion, 1993). We concur with this interpretation, noting that the different distribution patterns of kaolinite and smectite from the same source can be explained by the selective transport of fine-grained smectite offshore. The minor amounts of mixed-layer minerals demonstrate a distribution similar to smectite, likely due to a southern North Sea supply that is modified by the tendency for the fine-grained mixed-layer minerals to also be transported offshore. Other sources There are two different sottrce types that are combined in the Kattegat area. Together with trioctahedral illite and chlorite, the widespread occurrence of feldspars and amphibole in the clay fraction indicates a relatively unweathered mineralogical source material (Fig. 3B, 4A, 4D). Kaolinite and smectite reflect, on the other hand, the concurrent erosion of weathered sources, especially in the southern part of the Kattegat (Fig. 3B, 4B, 4G). The relatively low contents of chlorite near the border between the Skagerrak and the Kattegat (Fig. 4A) are partly influenced by the Jutland Current, which transports chlorite-poor sediment into the area, masking the local input. This effect is not so evident for illite, as it predominates in the sediment from the local sources as well as from the Jutland Current. However, the illite from the North Sea has a stronger dioctahedral character than do the sites from western Sweden. High illite contents of mainly trioctahedral character along the Swedish west coast strongly suggest a local source, presumably derived by terrigenous or near-shore erosion (Fig. 4D). This is in agreement with the composition of Pleistocene surface deposits in south-western Sweden (Bengtsson, 1991; Lfing & Stevens, 1996). The illite distribution agrees with the results by Pederstad et al. (1993), documenting an illite-rich sediment in the fjords of western Sweden and southern Norway. They noted a decrease in illite content towards the Skagerrak. Large chlorite contents have also been recorded in the Norwegian fjords (Ronningsland, 1976; Pederstad, 1982; Pederstad et al., 1993), but a significantly higher chlorite content was not recorded at the few fjord sites in this study. Vermiculite and I-V mixed-layer minerals are interpreted to be principally supplied by the G6ta ~ilv River and other coastal sources in western Sweden (Fig. 4E, 4F) where these minerals are the principal products of moderate podzol weathering in this area (Melkerud, 1984; Lhng & Stevens, 1996). The smectite, kaolinite and mixed-layer minerals of I-S composition in the Kattegat originate either from the southern North Sea (via the Jutland Current) or, more likely, from erosion of Mesozoic and Tertiary sediments in Denmark and southern Sweden (Bondam, 1967; Nielsen, 1974; Pederstad et al., 1993). CONCLUSIONS The clay mineral composition in the SkagerrakKattegat area reflects the incorporation of four regional sources (Fig. 6). A smectite and kaolinite source in the southern North Sea and the subsequent north-east transport of these minerals along the Jutland Current is interpreted from the distribution patterns, with consideration for the mineral sorting effects related to grain size (Fig. 4B, 4G). The distribution of chlorite in western Skagerrak indicates an influx transported by the North Atlantic Deep Water, although of limited proportions. This component probably Grain-size influence on clay mineral distribution 11 e'--Illite Vermiculite I-V Chlorite Chlorite Smectite Kaolinite Vermiculite I-S FIG. 6. Interpreted clay-mineral contributions from the principal sources. Smectite, kaolinite and I-S mixed-layer minerals are derived from the southern North Sea. The central North Sea source is characterized by a chlorite component. Illite, vermiculite, I-V mixed-layer minerals, and chlorite are supplied from the Swedish west coast. Southern Sweden and Denmark are sources for smectite, kaolinite, vermiculite and I-S mixed-layer minerals. originates from sea-floor erosion of chlorite-rich Pleistocene deposits of immature (unweathered) mineralogy. In the Kattegat and along the Swedish west coast in the Skagerrak, the relatively large amounts of trioctahedral illite, chlorite, amphiboles and feldspar minerals originate from unweathered, local sources. The relatively large amounts of vermiculite and mixed-layer minerals of I-V composition in the Kattegat, especially near the G6ta ~ilv River mouth, indicate input from terrigenous sources. The presence of smectite, kaolinite and mixed-layer minerals of I-S composition in the Kattegat indicate the erosion of a weathered source within the area or transport by bottom currents from the Skagerrak. The suspension deposition within marine environments is not conducive to simple correlations between fine-grained deposits and their sources. The complex source character, related to hinterland weathering and erosion variability, as well as the mixing involved during transport and sedimenta- tion, will result in a mineralogical continuum most appropriate for relative interpretations. On the other hand, fine-grained samples provide a time-integrated record that is representative for the combined processes within an environment, which may be hard to approximate when sampling coarser deposits. Also, clay mineralogy allows identification of rather subtle changes in the components that are most sensitive to provenance differences in weathering history, in this case, the areas with immature glacial deposits compared with southern, less glacially influenced provenances with greater amounts of weathering products. ACKNOWLEDGMENTS This work is part of the Swedish West Coast Project, organized by a Swedish interdisciplinary project group and financed by the Swedish Environmental Protection Agency. Hierta-Retzeius foundation and Wilhelm and Martina Lundgrens foundation provided financial 1t. Bengtsson and R. L. Stevens 12 support for analyses. We also thank the marine geological division at the Swedish Geological Survey for samples complementing our basin coverage. REFERENCES Bengtsson H. (1991) Mineralogiska unders6kningar av jordm~nsprover fr~n Vgstergrtland. GU/CTH publikationsserie, B 362, 1-42. Biscaye P.E. (1964) Distinction between kaolinite and chlorite in recent sediments by X-ray diffraction. Am. Miner. 46, 1281-1289. Bondam J. (1967) Undersogekser vedr~rende de geokemiske forhold i kaolinforekomsten ved Ranne ph Bomholm. Dan. Geol. Foren. Medd. 17, 297-365. Brown G., editor (1961) X-ray Identification and Crystal Structures of Clay Minerals. Second edition. 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