Non-pelletal glauconite from the Campanian Qusseir Formation, Egypt: Implication for glauconitization

Sedimentary Geology 249–250 (2012) 1–9

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Non-pelletal glauconite from the Campanian Qusseir Formation, Egypt: Implication for glauconitization Hassan Baioumy ⁎, Sabah Boulis Central Metallurgical R & D Institute, P.O. Box 87 Helwan, Cairo, Egypt

a r t i c l e

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Article history: Received 29 March 2011 Received in revised form 14 December 2011 Accepted 10 January 2012 Available online 18 January 2012 Editor: G.J. Weltje Keywords: Glauconite Egypt Mineralogy Geochemistry Origin Glauconitization

a b s t r a c t Glauconitization processes proposed in the literatures include the “layer lattice” theory in which glauconite formed through K uptake by a detrital smectite precursor and the “neoformation theory” that does not require a smectite-like precursor. The non pelletal glauconite bed in the uppermost part of the Campanian Qusseir Formation at the Abu Tartur area and the underlying smectite-rich beds provided a potential example for the glauconitization process. The glauconite occurs as fine (clayey), moderately hard, and homogeneous bed ranges in thickness from 2 to 3 m. Under the optical microscope, glauconite occurs in a unique morphology as very fine green clayey matrix, which differs from the typical pelletal shape of many glauconites. Under the scanning electron microscope, it appears as dense, uniform, and very fine flakes that are composed of Si, Al, Fe, and K. X-ray diffraction analysis of both bulk samples and clay fractions indicates that the glauconite bed is composed entirely of glauconite. Geochemical analysis suggested that the investigated glauconites belong to the 2nd stage of the maturity scale (i.e. moderate maturity); or they can be alternatively ranked as evolved. Chondritenormalized REE patterns exhibit LREE enrichment relative to HREE ((La/Yb)N ratios vary from 5 to 8) and slightly negative Eu anomalies (Eu/Eu* from 0.7 to 0.8). Clay beds that underlie the glauconite bed are composed of two types of smectites; smectite of low K2O and Fe2O3 contents and smectite of high K2O and Fe2O3 contents. Smectite of low K2O and Fe2O3 contents is considered in this study as the initial precursor of glauconite, while smectite of high K2O and Fe2O3 represents the transitional phase between smectite and glauconite. This, in turns, supports the “layer lattice” theory in which glauconite formed by K diffusion to a detrital smectite precursor from the sea-water due to transgression. Absence of mixed-layer minerals suggests that the diffusion process was active and continuous until reaction completion was reached, i.e. formation of glauconite. Maturity and REE patterns suggest the possible formation of the studied glauconite probably at shelf environment at approximately 100 m water depth. The occurrence of glauconite indicates a marine invasion of the upper part of the Qusseir Formation, which was considered previously as non-marine sediments. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Odom (1984) reviewed the various hypotheses related to the mode or modes of glauconite formation. Odom (1984) refers to the theories of glauconite formation through K uptake by a detrital smectite precursor as the “layer lattice theory”, which contrasts to the “neoformation theory” that does not require a detrital smectite or smectite-like precursor. Odom (1984) favors the neoformation theory using evidence reported by Odin and Matter (1981), in which smectite might be a precursor of glauconite, but the smectite involved is apparently a neoformation product. Therefore, the formation mechanism of glauconite (glauconitization process) is still controversial. The glauconite beds in the uppermost part of the Campanian Qusseir Formation

⁎ Corresponding author. E-mail address: [email protected] (H. Baioumy). 0037-0738/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2012.01.003

directly underlying the phosphorite deposits of the Duwi Formation at the Abu Tartur area, Egypt (Fig. 1) and the underlying smectiterich beds provide a potential example for the glauconitization process. The term “Qusseir variegated shales” is used in Egypt to denote the succession of shale beds which exhibit a wide variation in color and constitute the upper part of the Nubia Formation. Variegated shales are mainly encountered in Kharga Oasis, Dakhla Oasis, the Nile Valley, Qusseir and Safaga areas of the Red Sea area, and in central Sinai. Ghorab (1956) considered the variegated shales overlying the Nubia Formation and underlying the lowermost phosphate bed in the Qusseir area as a separate formation and named it “Qusseir Formation” with the type locality at Duwi Mountain, Qusseir. Youssef (1957) suggested the name “Kosseir Variegated Shale” for the same unit. Due to the fact that there is no unconformity between the variegated shales and the underlying nonfossiliferous Nubia Sandstone Formation and also there is hardly any change of dip between them, most authors consider that these two formations (Qusseir and

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Fig. 1. Location and geology of the Abu Tartur Plateau, Western Desert, Egypt with the location of the studied glauconite beds.

Nubia Formations) are of the same age or possibly the Nubia Formation is slightly earlier (Said, 1961). Nakkady (1951) described vertebrate fauna from the Qusseir Formation Stephanodus libycus Dams, Corax bosaniii (Gamellaro) Agassiz, Lamma appendiculata Agassiz, Schizoria stromeri Stromer, and turtle and crocodile remains, which are considered as being indicative of Early to Late Campanian age. The presence of plant remains, the lenticular character of the beds, and the nature of detrital minerals, let Bowman (1926) to conclude that the sediments of the Qusseir Formation were deposited under deltaic conditions based on the absence of marine fossils and presence of plant remains. As to composition of these shales, only little has been published on their mineralogy and geochemistry probably due to the lack of geological and economic significances of these shales. Abu Zeid (1974) concluded that the clay minerals in these shales are Ca-montmorillonite and kaolinite. Baioumy (2004) indicated that the Qusseir Formation is dominated by smectite and kaolinite. No glauconite was reported in these studies. During the field excursion to the Abu Tartur area (Fig. 1) in early 2010 a thickness of 2 to 3 m green claystone was reported by the authors in the upper part of the Qusseir Formation, just below the phosphorite beds of the Duwi Formation and above the thick smectite-rich clay beds. Several samples representing the glauconite beds both inside the fresh underground phosphorite mine and weathered outcrop as well as the underlying smectite-rich clays were collected to examine their mineralogical and geochemical compositions. The investigations indicated that this green bed is composed entirely of glauconite and the underlying smectite-rich clays could represent the precursor of these glauconites. This paper presents detailed petrographical, mineralogical, and geochemical characteristics of this glauconite as well as its origin. 2. Materials and methods Samples were collected from the Abu Tartur Plateau to represent the fresh and weathered glauconite as well as the underlying smectite-rich beds for the petrographical, mineralogical, and geochemical investigations. Thin sections were prepared and investigated under the Olympus optical microscope. Bulk samples and clay fractions were analyzed for their mineralogical composition by X-ray diffraction (XRD) technique using a Bruker D8 using Cu Kα radiation. Tube voltage and current were 40 kV and 30 mA, respectively. Morphology and chemistry of glauconite were investigated with SEM-EDX (JEOL-JSM

5410). Petrographical and mineralogical analyses were performed at the Central Metallurgical R & D Institute (CMRDI), Cairo, Egypt. Fused disks prepared from representative samples were analyzed for their major oxides (SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, K2O, Na2O, and P2O5) by XRF using a Philips PW 2400 X-ray spectrometer at Tohoku University, Japan. Tube voltage and current for W target were 40 kV and 60 mA, respectively. Loss on ignition (L.O.I.) was obtained by heating sample powders to 1000 °C for 6 h. REE concentrations of three glauconite samples were determined by a SCIEX-ELAN DRC II ICP-MS at Tohoku University, Japan. Samples powders were digested with 2 ml conc. HF in capped TEFLON bombs on an electrical hot plate (~150 °C) for 24 h. The solution was evaporated to near dryness, and re-dissolved in 2 ml 6 N HNO3 in capped TEFLON at 150 °C for two days. The samples were then evaporated near to dryness, then 1 ml of 6 N HNO3 was added, and the solutions were further diluted for analysis.

3. Geology and stratigraphy of the studied area The Abu-Tartur Plateau lies approximately 650 km southwest of Cairo in the Western Desert between Dakhla Oasis to the west and Kharga Oasis to the east (Fig. 1). The stratigraphic sequence at the Abu Tartur Plateau starts with the Nubia Formation, which rests unconformably on the igneous and metamorphic basement complex. The term Nubia Formation is well established in the stratigraphic column of Egypt and is used to describe the extensive clastic series (e.g. Said, 1962; El-Khoriby, 2003). The age of the Nubia Formation ranges from Early Cretaceous to Late Turonian–Coniacian time (Van Houten et al., 1984). It consists of basal conglomeratic sandstone, cross-bedded sandstone and siltstone with local oolitic iron beds in the middle part, and cross-laminated siltstone in the upper part (Said, 1962). Thickness of the Nubia Formation ranges from 30 to 120 m. This formation is conformably overlain by the variegated colored shales of the Qusseir Formation. The Qusseir Formation is overlain by the Duwi Formation of Late Campanian to Early Masstrichtian in age (e.g. Glenn and Arthur, 1990). These phosphorites are exploited from the Abu Tartur area both from underground and open cast mines for domestic uses and exportation. Many authors (e.g. Baioumy and Tada, 2005) considered that the marine environment of late Cretaceous transgression of Tethys is marked by the appearance of phosphorite bed. In addition Baioumy and Tada (2005) considered the occurrence of Y-shaped Thalassinoides trace fossils as well as the lag deposits as another evidence for Upper Cretaceous marine transgression in Egypt.

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The Duwi Formation is overlain comformably by the Dakhla Formation that is composed of hemipelagic sedimentary sequence representing continuous deposition (Nakkady, 1950) of green to gray shales, marl, and chalk. The thickness of the Dakhla Formation ranges from 46 to 100 m. The lower part of the Dakhla Formation is Middle to Late Maastrichtian based on the occurrence of Globotruncana gensseri and G. ensahensis, whereas the upper part belongs to the Danian based on the occurrence of Globorotalia trinidadensis and G. uncinata (Said, 1961).

4. Results 4.1. Petrology and petrography of glauconite bed The variegated shales in the Abu Tartur area consist of gray, greenish, gray, and steel gray shale together with thin bands of brown, yellowish, and purple clays, rich in gypsum bands and/or salt veinlets. The shales range in thickness from 50 to 80 m and are generally non-fossiliferous, although few plant remains can be observed. Thinner sandstone beds are occasionally intercalated with these shale beds (Fig. 2A). The uppermost glauconite bed, which directly underlies the phosphatic horizon of the Duwi Formation (Fig. 2B), is dark green, fine-grained, homogeneous, moderately hard in the fresh underground mine (Fig. 2C), while it is pale green and friable in the weathered outcrop (Fig. 2D). Smectite-rich beds underlying the glauconite beds consist of yellowish to greenish gray clays with yellowish iron-rich bands (Fig. 2E). Under the optical microscope, the glauconite appears as very fine pale green to yellowish green clayey material. No coarse particles/ grains have been observed inside this clayey matrix (Fig. 3A and B). Only in few instances brownish green relatively coarse cluster (few microns in diameter) of iron oxides can be observed (Fig. 3A) probably formed as a result of oxidation during thin section preparation.

Fig. 3. Photomicrographs of glauconite that appears as very fine green clayey material in plane light (A) and polarized light (B).

Few of very fine detrital quartz grains can also be observed (Fig. 3B). Under SEM, glauconite occurs as dense, uniform, and very fine flakes (Fig. 4A). The EDX analysis (Fig. 4B) shows the

Fig. 2. Stratigraphic column of the Qusseir Formation at the Abu Tartur Plateau (A) with field photos of the Qusseir Formation and the overlying Duwi Formation (B) as well as the fresh (C) and weathered (D) glauconite beds and the underlying smectite-bearing beds (E). Hammer length is about 30 cm.

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typical composition of glauconite in which it is dominated by Si, Al, Fe, and K. 4.2. Mineralogy of glauconite bed Figs. 5 and 6 showed the XRD patterns of the bulk samples and clay fractions of fresh and weathered glauconites from the Qusseir Formation at the Abu Tartur area, respectively. X-ray patterns of the bulk samples and clay fractions of both fresh and weathered glauconites display the characteristic (001), (020) and (003) reflections of glauconitic minerals (Bayliss et al., 1986) at ~ 10 Å, ~ 4.48 Å, and 3.33 Å, respectively. According to Hower (1961), glauconites exhibit a continuous series from b5% to approximately 40% expandable layers. Ordered glauconites contain ≤ 10% expandable layers and its 001 peak appears at ~ 10 Å, disordered glauconites contain 10–20% expandable layers and its 001 peak appears at ~9.9 Å, and interlayered glauconites contain ≥20% expandable layers and shows two peaks at ~ 12.5 and 9.7 Å. Following this classification, glauconites from the Qusseir Formation are interpreted as ordered glauconites. Fresh glauconite shows symmetric and stronger basal (001) peaks compared to that of the weathered samples suggesting structural deformation of glauconite due to weathering.

Fig. 5. X-ray diffraction patterns of the bulk sample (A) and clay fraction (B) of fresh glauconites from the Qusseir Formation. Both are composed of glauconite (G) with traces of quartz (Q) in the bulk samples.

4.3. Geochemistry of glauconite bed

Fig. 4. General SEM photomicrograph (A) as well as a close up (B) of glauconite that occurs as dense, uniform, and very fine flakes. EDX analysis shows a typical composition of glauconite, which is dominated by Si, Al, Fe, and K (C).

Distributions of major oxides in the fresh and weathered glauconite samples from the Qusseir Formation are shown in Table 1. The average K2O content in the fresh glauconite is 6.7% and 5.2% in the weathered samples. Accordingly, the investigated glauconites belong to the 2nd stage of the maturity scale of Birch et al. (1976) (i.e. moderate maturity); or they can be alternatively ranked as evolved (Odin and Matter, 1981). The average Fe2O3 content in the fresh glauconite is 18.4% and 8.8% in the weathered samples. The Fe2O3 displays a significant positive relationship with K2O (Fig. 7A) in both samples. Such a behavior has been documented by Valeton et al. (1982) and Nishimura (1994) among many others. The significant positive relationship between potassium and iron could reflect a genetic relationship between the two cations as has been suggested by Kohler (1980). This relationship implies that the starting materials were transformed to evolved and highly evolved glauconite through the replacement of the octahedral Al 3 + by Fe 3 + and a concomitant fixation of K in the interlayers. Furthermore, the substitution of Al for Si in the tetrahedral sites as well as the octahedral substitution of divalent for trivalent cations contributes to the interlayer charge. The average Al2O3 content in the fresh glauconite is 5.8% and 16.5% in the weathered samples. The Al2O3 shows significant inverse relationships with Fe2O3 (Fig. 7B). The strong negative correlation between Al2O3 and Fe2O3 is in accordance with the predominance of Al-ferric substitutions in glauconitic minerals (e.g. Velde, 1985; Dasgupta et al., 1990). SiO2 contents do not vary significantly between fresh (average of 55%) and weathered (average of 54.7%) glauconites. The average value of MgO in the fresh glauconite is 3.9% and 4.5% in the weathered samples. These are almost in accordance with the values reported in the literature, which fall mainly between 1.5 and 5% (e.g., Valeton et al., 1982; Odom, 1984). MgO displays a significant negative

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elements also did not show any correlations with the K2O contents indicating no genetic correlations between these elements and glauconite evolution and probably reflect detrital phase especially for TiO2 and/or diagenetic phases, especially for CaO and Na2O. Rare earth element distributions in the fresh and weathered glauconite samples from the Qusseir Formation are shown in Table 2. The average ΣREE is 62 ppm in the fresh sample and 174 ppm in the weathered samples. Light rare earth elements (La, Ce, Pr, Nd, Sm, and Eu) represent the major fraction of the ΣREE. The REE patterns were chondrite-normalized according to Boynton (1984). The Eu anomaly is calculated as E/E* = EuN / (SmN · GdN) and Ce anomaly is calculated as Ce/Ce* = (3Ce/ CeCH) / (2La/LaCH + Pr/PrCH). Chondrite-normalized REE patterns for the fresh and weathered samples are shown in Fig. 8. The samples especially weathered samples, have higher REE contents compared to the REE concentrations of the chondrite. The fresh samples exhibit LREE enrichment relative to HREE as shown by the average (La/Yb)N ratio, of 14 and slightly negative Eu anomalies (0.7 to 0.8) as well as slightly negative Ce anomalies (Ce/Ce*= 0.7–0.8). The weathered samples also exhibit LREE enrichment relative to HREE as shown by the average (La/Yb)N ratio, of 8 and slightly negative Eu anomaly (0.8). No Ce anomaly was observed in these samples (Ce/Ce* = ~1). Compared to the concentrations of REE in the seawater of all depths reported by Elderfield and Greaves (1982), the studied glauconites have higher REE concentrations. Chondrite-normalized REE patterns of the studied glauconites more or less match the chondrite-normalized REE pattern of the seawater at 100 m depth of Elderfield and Greaves (1982). 4.4. Mineralogy and geochemistry of the underlying smectite-rich beds

Fig. 6. X-ray diffraction patterns of the bulk sample (A) and clay fraction (B) of weathered glauconites from the Qusseir Formation. Both are composed of glauconite (G) with traces of quartz (Q) in the bulk samples.

relationship with K2O (Fig. 7C). This relationship implies the occurrence of Mg cation in the preexisting material of the glauconites and it was substituted either by potassium and/or iron during the glauconite evolution. The average MnO content is low in the investigated glauconites (0.01 and 0.03% for fresh and weathered glauconites, respectively), which locates very close to that of the previous research results indicating concentrations only up to 0.05% (e.g., Kohler and Köster, 1976). TiO2, CaO, and Na2O contents (averages of 0.3, 0.2, and 1.1%, respectively in the fresh samples and 1.1, 0.7, and 1.2%, respectively in the weathered samples) are considerably higher than those in glauconites reported in Jarrar, et al. (2000) and others. These

Table 1 Major oxide compositions (%) of fresh and weathered glauconites from the Qusseir Formation. Fresh glauconite F1

F2

F3

Weathered glauconite Average W1

55.44 54.58 55.01 55.01 SiO2 TiO2 0.31 0.34 0.33 0.33 Al2O3 6.10 5.60 5.80 5.83 Fe2O3 17.82 18.94 18.10 18.29 MnO 0.01 0.01 0.01 0.01 MgO 3.43 3.85 3.50 3.59 CaO 0.24 0.22 0.23 0.23 Na2O 1.08 1.19 1.13 1.13 K2O 6.52 6.93 6.73 6.73 P2O5 0.03 0.03 0.03 0.03 L.O.I. 8.32 8.16 8.24 8.24 Sum 99.27 99.86 99.57 99.57

W2

W3

W4

Average

54.00 55.34 54.83 56.97 55.28 1.05 1.15 0.92 1.11 1.06 16.47 17.50 17.61 17.96 17.39 8.97 8.69 8.85 8.54 8.76 0.03 0.04 0.05 0.04 0.04 4.57 3.35 3.49 2.50 3.48 0.74 0.60 0.59 0.43 0.59 1.28 1.13 0.16 0.13 0.68 5.34 5.18 5.19 4.92 5.16 0.12 0.03 0.31 0.19 0.16 7.63 6.99 7.30 6.40 7.08 100.20 100.00 99.30 99.19 99.67

Several clay beds are found underlying the glauconite bed in the Qusseir Formation at the Abu Tartur Plateau (Fig. 2). Smectite-rich beds are dark green, fine-grained, homogeneous, moderately hard in the fresh underground mine while it is pale green and friable in the weathered outcrop. XRD analysis of bulk samples (Fig. 9A) and clay fractions (Fig. 9B) from these beds showed that they are composed entirely of smectite that was identified by its characteristic reflection at ~ 12.8 Å in the untreated oriented specimen, which expands to ~16.6 Å through glycolation by ethelene glycole and collapses to ~9.9 Å due to heating to 550 °C. No glauconite, illite, or mixed layer minerals were identified in the XRD patterns of the smectite-rich beds. Under SEM, smectite-rich clays occur as irregular flakes with undefined edges (Fig. 10). The EDX analysis (Fig. 10) shows the composition of smectite, dominated by Si, Al, O with some K, Fe, Ca, and Mg. Chemical analysis of smectite-rich beds by XRF (Table 3) distinguished two types of smectites. Smectite of low K2O and Fe2O3 contents (average of 0.8% and 1.9%, respectively) and smectite of high K2O and Fe2O3 contents (average of 2.6% and 6.3%, respectively). Behaviors of major oxides in the K2O- and Fe2O3-rich smectites are more or less similar to those in the glauconite bed. For example K2O in the smectite-rich beds shows positive correlation with Fe2O3 and MgO and negative correlation with Al2O3, which is similar to that in the glauconite samples (Fig. 7). On the other hand, the behaviors of major oxides in the low K2O and Fe2O3 smectites are completely different where no correlations were observed between these oxides. 5. Discussion 5.1. Origin of glauconite The term glauconite in the sense of Odin and Matter (1981) is used to designate a dark green, pelletal, and Fe-rich with K2O contents higher than 6% mica-type mineral of marine origin. In addition, most of the published work on glauconites indicated that the glauconites occur in the pelletal morphology (e.g., Jarrar et al., 2000; Kim

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Fig. 7. Binary plots of major oxides in the glauconite and smectite beds from the Qusseir Formation. Positive correlations between K2O and Fe2O3 (A) and MgO (B) and negative correlations between F2O3 and Al2O3 (C) in the glauconite samples as well as smectite beds with high K2O and Fe2O3 contents. No correlations were observed in the smectites samples with low K2O and Fe2O3 contents.

and Lee, 2000; Chang et al., 2008; Mei et al., 2008; Huggett et al., 2010; Baioumy and Boulis, Submitted for publication). Probably the occurrence of glauconite in the upper part of the Qusseir Formation (2–3 m) as very fine and clayey materials (non-pelletal texture) lets the geologists and researchers to consider these beds as green clays in the previous research works. To the best knowledge of the authors, no body used the term glauconite to describe these beds. The mineralogical analysis of bulk samples and clay fractions of these beds displayed the characteristic (001), (020), and (003) reflections of glauconite mineral (Bayliss et al., 1986) at ~10 Å, ~ 4.48 Å, and 3.33 Å, respectively. The geochemical analysis also showed that K2O

and Fe2O3 contents are 6.7% and 18.4%, respectively, which match the composition of typical glauconite cited in Odin (1988) in which Fe2O3 ranges between 19 and 25%, while K2O varies from 3 to 9%. These data are also supported by EDX analysis. Therefore, the green beds in the upper part of the Qusseir Formation, Abu Tartur Plateau, which previously considered as green clays dominated by smectite and kaolinite, are in fact composed of glauconite. Three main mechanisms have been published to explain the formation of glauconite granules. (1) The layer lattice theory (Burst, 1958a, 1958b; Hower, 1961) is based on the transformation of a

Table 2 Rare earth elements distributions (ppm) of fresh and weathered glauconites from the Qusseir Formation. Fresh glauconite

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Sum Eu/Eu* Ce/Ce*

Weathered glauconite

F1

F2

Average

W1

W2

Average

14.4 24.8 3.2 11.7 2.3 0.5 1.5 0.2 1.3 0.3 0.8 0.1 0.7 0.1 61.8 0.8 0.8

15.8 25.4 3.1 10.9 1.9 0.4 1.7 0.2 1.4 0.3 0.8 0.1 0.8 0.1 62.8 0.7 0.7

15.1 25.1 3.1 11.3 2.1 0.4 1.6 0.2 1.3 0.3 0.8 0.1 0.7 0.1 62.3

34.9 76.6 10.1 41.2 8.8 2.2 8.1 1.2 6.6 1.3 3.2 0.5 2.7 0.4 197.5 0.8 0.9

28.8 66.9 6.8 25.2 4.7 1.1 3.9 0.6 3.8 0.8 2.3 0.3 2.3 0.3 147.8 0.8 1

31.8 71.8 8.4 33.2 6.8 1.6 6.0 0.9 5.2 1.0 2.8 0.4 2.5 0.4 172.7

Enrichment factor 2 3 3 3 3 4 4 4 4 4 4 4 3 3

Fig. 8. Chondrite-normalized rare earth elements patterns of the glauconite samples in comparison to that of the seawater at depth of 100 m (Elderfield and Greaves, 1982).

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Fig. 10. SEM Photomicrographs of smectite that occurs as dense, uniform, and very fine flakes (A). EDX analysis shows the composition of smectite that is dominated by Si, Al, Fe, and K (B).

Fig. 9. X-ray diffraction patterns of smectite-rich beds from the Qusseir Formation. Bulk sample (A) is composed of smectite (S) and quartz (Q) and clay fraction (B) is composed entirely of smectite.

degraded 2:1 layer silicate lattice (TOT clay) into an iron- and potassium-rich 2:1 layer silicate of the illite group. This is supposed to occur under reducing conditions. (2) The epigenetic substitution theory (Ehlmann et al., 1963) suggests that glauconite layers form through solution of preexisting minerals, by adding ions present in sea water. (3) The precipitation–dissolution–recrystallization theory (Odin, 1975; Odin and Matter, 1981; Ireland et al., 1983) involves successive processes leading to a true neoformation, and therefore implies independence between the nature of substrate and the new iron-rich clay minerals. Recent studies have confirmed the precipitation–dissolution–recrystallization theory (Chamley, 1989). The chemical evolution of green clay granules stops either after a long exposure at the sediment surface (10 5–10 6 years) or after significant burial (Chamley, 1989). The glauconite beds in the Abu Tartur Plateau are underlain by thick clays beds. Mineralogical and geochemical analyses indicated that these beds are composed mainly of smectite. These smectites are characterized by higher K2O and Fe2O3 contents (2.6% and 6.3%, respectively) compared to many smectites of which Fe2O3 ranges between 0.21 and 0.36%, while K2O varies from 0 to 0.02% (e.g., Nadeau et al., 1984; Mosser-Ruck and Cathelineau, 2004). This suggests that this smectite can be considered as a potential precursor of the overlying glauconite supporting the “layer lattice” theory in which glauconite formed through K uptake by a detrital smectite precursor. Potassium diffusion was caused by the invasion of the seawater due to the Late Cretaceous transgression at the Abu Tartur area. According

to Meunier and Albani (2007), potassium ions diffuse from the seawater sediment interface. If not interrupted, the diffusion process is active until reaction completion is reached, i.e. formation of Fe-illite or glauconite. The similarities in the major oxides trends (i.e. positive correlations between K2O and Fe2O3) in both K2O-rich smectite and glauconite samples support this interpretation. K2O-rich Smectite beds can be considered as intermediate (transitional) phase between the smectite precursor with low K2O and Fe2O3 contents and glauconite end product of glauconitization process with low K2O and Fe2O3. Smectite of relatively low K2O and Fe2O3 (Table 3 and Fig. 7) can be considered as the initial precursor for the glauconite. No intermediate mineral phases such as mixed-layer minerals were identified in the current study or previous works (e.g. Abu Zeid, 1974; Baioumy, 2004). However, formation of transitional or intermediate mineral phase(s) is not necessary in the transformation process of smectite to glauconite. Glauconite still can be formed directly from smectite if the diffusion of potassium ions to the pre-existing detrital smectite is not interrupted. According to Meunier and Albani (2007), the FeAl and Fe-rich clay minerals form two distinct solid solutions. The earliest phases to be formed are FeAl smectites or berthierine depending on the sedimentation rate. Potassium ions diffuse from the sea-water sediment interface. If not interrupted, the diffusion process is active until reaction completion is reached, i.e. formation of Fe-illite or glauconite. Mixed-layer minerals are formed when the diffusion process is interrupted because of sedimentation, compaction or cementation. Baioumy and Boulis (Submitted for publication) indicated that although glauconite occurs mainly as pellets, these pellets are composed of very fine flakes. Petrographical studies of the Qusseir Formation glauconites using optical microscope and SEM indicated that these glauconites occur as very fine flakes and not as pellets. This can be used to suggest that the morphology of the Qusseir Formation glauconite represents an intermediate stage before pelletization. Pelletization may occur in a later stage due to the circulation and/or reworking process of glauconitic mud in the depositional environments. These conditions might have not been occurred during the formation of the Qusseir Formation glauconite. Baioumy and Tada

8

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Table 3 Major oxides compositions (%) of smectite-rich beds from the Qusseir Formation. Low K2O and Fe2O3

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 L.O.I. Sum

High K2O and Fe2O3

S1

S2

S3

Average

S5

S6

S7

S8

S9

Average

68.60 1.56 17.21 2.11 0.04 0.51 0.16 0.92 0.70 0.08 7.2 99.09

69.60 1.36 18.21 1.81 0.02 0.54 0.15 0.72 0.83 0.05 6.20 99.48

68.10 1.76 17.71 1.66 0.06 0.49 0.19 0.82 0.92 0.07 7.70 99.47

68.76 1.56 17.71 1.86 0.04 0.51 0.17 0.82 0.82 0.07 7.03 99.35

65.74 1.08 14.25 6.14 0.02 1.88 0.18 0.12 2.52 0.22 7.30 99.44

68.53 1.19 14.16 5.01 0.02 1.14 0.53 0.21 2.06 0.09 6.70 99.64

56.32 0.93 13.17 7.13 0.02 2.34 0.55 0.19 2.98 0.32 15.40 99.35

56.76 0.77 12.67 7.56 0.05 2.60 0.87 0.62 3.19 0.20 14.88 100.17

55.15 0.92 14.50 5.61 0.04 1.73 0.61 0.53 2.08 0.31 17.54 99.02

60.50 0.98 13.75 6.29 0.03 1.94 0.55 0.33 2.57 0.23 12.36 99.52

(2005) suggested the pelletization of phosphatic grains, which were derived from the preexisting phosphatic mud during reworking process in a shelf environment. Formation of glauconite pellets in a similar mechanism is possible. The abovementioned observations can suggest that a possible mechanism of glauconitization process includes; 1) presence of detrital smectite or smectite-like precursor, 2) K and Fe uptake by the detrital smectite precursor in a marine environment, 3) transformation of smectite precursor to glauconite, and 4) pelletization of glauconite flakes due to reworking in the depositional environment to form the glauconite pellets. Most of the previous research work on the Qusseir Formation considered the shales of this formation as non-marine sediments (e.g., Bowman, 1926; Nakkady, 1951). However, according to Odin and Fullagar (1988), glaucony is generally assumed to occur at an average water depth between about 50 and 500 m. Chondrite-normalized REE patterns of the studied glauconites more or less match the chondritenormalized REE pattern of the seawater at 100 m depth of Elderfield and Greaves (1982), which is consistent with the conclusion of Odin and Fullagar (1988). The occurrence of glauconite in the upper part of the Qusseir Formation suggests the invasion of sea water during the deposition of the upper part of this formation. Supporting this interpretation, Faris and Hassan (1959) reported a marine fauna in the upper layers of the Qusseir shales at the Abu Tartur Plateau (Prelibycocaras fauna), which led them to suggest that occasional transgressions of the shallow water sea occurred during the deposition of these beds. 5.2. Effect of weathering on the mineralogy and chemistry of glauconite Data of the current study showed serious changes in the geochemical characteristics of glauconite due to weathering (Fig. 11) without

changes in its mineralogy. Geochemical analysis showed a significant depletion in the Fe2O3 and slight depletion in K2O contents. As a result, Al2O3 has increased in the weathered glauconite compared to the fresh glauconites. These changes in the chemistry affected the crystallinity of the glauconite as it is indicated from the XRD pattern (Figs. 5 and 6), which shows asymmetric pattern for the weathered glauconite compared to the symmetric pattern of the fresh glauconite. Although Meunier (2004) described the weathering of glauconite as a progressive transformation into illite/smectite mixed-layers and eventually into smectites, the data in the current study did not show any evidence that supports the occurrence of illite/smectite mixed-layers or smectites. Weathering effect stopped at the level of leaching of Fe2O3 and K2O with some structural deformation in the original structure and kept the original structure of the glauconite. The transformation of glauconite to illite/smectite mixed-layers or smectites probably needs more severe weathering conditions, which are not available in the studied area. Significant amount of literature exists on the behaviors of REE in clay minerals, but there are still contradictory results and interpretations among the studies published on this topic (Honty et al., 2008). Some studies (e.g. Taylor and McLennan, 1988; Condie, 1991) suggested that the REE reflect the source of the clay minerals with no changes in the REE patterns during diagenesis or by hydrothermal process. More recent studies provided evidence in favor of REE redistributions during both diagenetic (e.g. Awwiller and Mack, 1991; Milodowski and Zalasiewicz, 1991) and hydrothermal processes (e.g. Hopf, 1993; Zwingman et al., 1999; Uysal and Golding, 2003). Weathered glauconites in the study area are enriched in the REE compared to the fresh glauconites. The enrichment factors range between 2 and 4. Although both fresh and weathered glauconites show relative enrichment of LREE over HREE, the enrichment ratios are different between the fresh and weathered samples. In addition, Ce and Eu anomalies are different. These observations would support the REE redistributions during diagenesis and/or weathering. 6. Conclusion

Fig. 11. A comparison between the chemical compositions of fresh and weathered glauconites from the Qusseir Formation.

The Qusseir Formation was considered in the previous publications as non-marine green clays and is composed of smectite and kaolinite. Detailed petrographical, mineralogical, and geochemical investigations indicated that the uppermost part of this formation is composed entirely of glauconite. The underlying smectite-rich beds are characterized by higher K2O and Fe2O3 compared to typical smectite. These smectiterich beds were considered as the precursor of the overlying glauconite. The results of the current study suggest that a possible mechanism of glauconitization process requires; 1) presence of detrital smectite or smectite-like precursor, 2) K and Fe uptake by the detrital smectite precursor in a marine environment, and 3) transformation of smectite precursor to glauconite. K2O- and Fe2O3-rich smectite is considered as

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