Clay Minerals (1998) 33, 349–362

Characteristics and origin of (Meerschaum) from Central

A.SINGER,K.STAHRAND M. ZAREI*

Seagram Center for Soil and Water Sciences, The Hebrew University of Jerusalem, PO Box12, Rehovot 76100, Israel, and Institut fu¨r Bodenkunde u. Standortslehre, Universitat Hohenheim, D-70599 Stuttgart, Germany

(Received 16 December 1996; revised 3 June 1997)

ABSTRACT: Nearly pure sepiolite clay crops out in a playa-like depression near El Bur, Central Plateau region of Somalia. The deposit is associated with the Lower to Mid-Eocene Taleh Formation that includes, besides limestone, and gypsiferous marls, extensive anhydrite and various evaporites, primarily gypsum. The material was examined by XRD, DTA, IR and EM. The XRD and DTA analyses indicated that from 40 cm down to a depth of 300 cm, the material consists of well- crystallized sepiolite, accompanied in some layers by minor calcite and traces of quartz and halite. The chemical composition, determined by XRF, indicated a low-Fe mineral, with the formula: (Si11.888Al0.112)(Mg7.313Al0.154Fe0.084)O30(OH)4(OH2)4Áx8H2O. The fibres, arranged in the form of interwoven mats, are straight and have lengths varying between 2À6 mm and widths of 20À40 nm. Commonly, they are aggregated into units of two parallel-lying fibres, with a random orientation against each other, creating a dense network of pores. The high viscosity and external surface area (306À346 m2gÀ1) of the material, compared to those of the Spanish Vallecas sepiolite, suggest the high industrial suitability of this clay. The extent of the deposit is not known. Lithology and geomorphology indicate a lacustrine, closed basin evaporative environment of formation for this deposit. In contrast to the palaeolacustrine environments of formation of Spanish and Turkish sepiolite deposits, the El Bur sepiolite apparently is more recent.

The occurrence of Meerschaum (sepiolite) deposits Late Tertiary lagoonal sediments rich in in Somalia has been known for some time, and from Northern Somalia have been described by sporadically mentioned in reports (Ahrens, 1951; Hendriks et al. (1993). The origin of this mineral is, Grossher, 1978). Deposits of sepiolite in the El Bur according to the authors, from reworking of older area of Somalia were mentioned by Gala´n (1987) in marine sediments, although minor pedogenic trans- his review of world reserves of this . formation of smectite to palygorskite may have The Federal Statistical Office of Germany (1991) occurred too. Studies by Boaler & Hodge (1964) reported a yearly production of 10 t of sepiolite indicated the presence of palygorskite in soils of between the years 1985À1987, after 9 t in 1982. Northern Somalia. These studies suggest that Recent political unrest thwarted plans for expansion conditions for the formation and stability of of production and possible export. Production at palygorskite in Somalia are not unfavourable. present is for local consumption only. The first description of a Meerschaum (sepiolite) Scientific studies of palygorskite, another fibrous occurrence in the El Bur area of Central Somalia clay mineral, are available from Northern Somalia was by Stahr et al. (1990). They described a where large amounts of palygorskite characterize material extracted and worked by local inhabitants, soils of Quaternary age (Alaily et al., 1990). These that produced ceramic and popular art objects from soils occur on parent materials of different ages and the raw (unburned) clay. Access to the sites was, with a varying lithology, under a semi-arid to arid and still is, dangerous because of political unrest. climate. The provenance of this mineral is attributed Drechsel (1991) later described palygorskite and to inheritance as well as to authigenic neoformation. sepiolite occurrences in soils of the same area.

# 1998 The Mineralogical Society

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It is the objective of this paper to characterize the and Southern Somalia. Exposures of crystalline El Bur sepiolite chemically and mineralogically and basement rocks are known only from the El Bur to propose processes for its formation. region in Central Somalia, and in Northern Somalia. The El Bur area is situated in the Central Plateau that, with an area of 150,000 km2, constitutes 1/4 of GEOLOGY AND GEOMORPHOLOGY the surface of the Republic of Somalia (Fig. 1). The major portion of the Somalia peninsula This tableland is level to slightly undulating, and constitutes a wide sedimentation basin, into which includes several sub-units (Fig. 2). The Lower a sequence of marine transgressions from the NE Cretaceous Belet Uen limestone formation lies to towards the SW have deposited huge sedimentary the west, the 200À499 m thick upper Cretaceous to formations, beginning with the Jurassic and Eocene Jesomma sandstone more to the east and the continuing into the Upper Tertiary. While Auradu fossil-rich limestone from the Lower Palaeozoic rocks apparently are absent, sandstone Eocene. More to the east, the plateau is underlain from the Triassic has been found only in Northern by the Lower to Mid Eocene Taleh formation (up to

FIG. 1. El Bur location map in Central Somalia.

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FIG. 2. Schematic cross-sections of El Bur sepiolite site, seen from east (upper) and from south (lower).

270 m thick) which includes limestone and various in two short seasons. Somewhat more humid evaporites, with interlayering of marls and dolo- periods may have prevailed during the mites (Watson et al., 1982). Thus extensive gypsum Pleistocene. The vegetation of this sparsely deposits characterize this formation. Commonly, inhabited region consists of an Acacia tortilis gypsum near the surface represents a hydration savannah. Annual grasses provide some meagre product of anhydrite. The Taleh formation is pasture for the livestock of the partly nomadic followedmoretothenorthbynodularand population. massive, up to 400 m thick, limestones of the Middle to Upper Eocene Karkar formation. It is SITE DESCRIPTIONS noteworthy that the landforms underlain by the Auradu, Taleh and Karkar formations are covered El Bur is situated ~350 km NNW from Mogadishu, by a continuous layer, up to 3 m thick, of on the Central Somalia Plateau, ca. 200 m above consolidated sands, with some occasional sand sea level. The area belongs to the ‘southern dunes. The sands, of a light yellow to reddish gypsum’ division of the Taleh formation (Watson brown hue, are considered to be relatively old and, et al., 1982). at present, resistant to wind erosion. Since they lie To the west of the hamlet of El Bur, a slight unconformably on the calcareous sediments, they depression contains a saline lake, fringed by a ca. are not considered to be their weathering products, 1 km wide saline playa (Fig. 2). The sepiolite and were probably blown in from the coastal sands deposits are situated south-east of the playa, about or from the Jesomma sandstone formation to the 3 km south of the hamlet, flanking the air-strip to west. Belo Burti. The depressions in this tableland have some The excavation pits used by local inhabitants are playa-like characteristics, contain some gypsiferous 2À3 m deep. Overburden consists of a saline marls and are dominated by evaporites, particularly Aridisol and lacustrine sedimentary deposits. gypsum, in a variety of forms, including selenite Below a desert pavement is a dry, homogeneous, and transparent platy gypsum. The soils are brown, loamy sand with few roots, to a depth of moderately to highly saline and contain both ~40 cm. Below that, at 40À100 cm depth, appears a palygorskite and sepiolite in their clay fraction dry, finely laminated, grey lacustrine sediment of (Drechsel, 1991). The climate of the area is semi- hard consistency. Laminae are frequently fractured arid to arid with ~200 mm yÀ1 precipitation, falling by gypsum or halite crystallite disintegration,

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possibly as a result of dissolution. Between using a Siemens SRS 200 instrument. The viscosity 100À170 cm this changes to a dry, massively- of sepiolite suspensions was determined by the dense, white layer. At 160 cm, the lacustrine Ostwald method on a Cannon-Fenske viscosity- sediment is moist, yellowish-white, and again meter, produced by Schott. The BET surface area laminar. Below 200 cm, the white to yellowish, measurements were carried out in the Institute for moist material is of a soft consistency, with Mineralogy, University of Tubingen, on a Gemini occasional cracks. The massive layer has still 2375 instrument after outgassing the samples in N2 some coarse pores with some remains of root- at 1008C for 1 h. Both of these determinations were channels that are partly coated by Mn cutans. also carried out on a Vallecas sepiolite sample for Gypsum crystallites can be discerned visually as reference purposes. The density was determined by well as gypsum pseudomorphs. the displacement method using petrol, benzene or ethanol as immersion liquids. The chemical composition of water extracts from METHODS the sepiolite deposit, obtained by equilibrating the Untreated crumbs of the material were examined by material with water at a 1:1 ratio, was determined scanning electron microscopy (SEM), using a Jeol by conventional methods. Organic C was obtained JXA 50A instrument with EDS attachment. by the difference between total C and carbonate C. Untreated material was ground slightly and submitted to X-ray diffraction (XRD) by the RESULTS powder method, using Cu-Ka radiationona Siemens 5000-D instrument. Washed material (to Chemistry eliminate soluble salts) was examined by the combined differential thermal/gravimetric analysis Table 1 indicates that Al, Ca, Na, K, Fe and Ti method, using a Netzsch STA 409 EP instrument (expressed as oxides) are present only in very small with a 108/min heating rate. Sepiolite morphology amounts in all the layers examined. All layers was examined by electron microscopy, using a except at 100À200 cm contain some impurities. Zeiss Model EM10 transmission electron micro- Layers at 40À100 cm and 200À300 cm contain scope (TEM) and a HRSEM ABT DS 150 F some halite, as was also confirmed by XRD (Topcon Tokio) scanning electron microscope. (Table 3). Layers at 0À40 cm and 200À300 cm The chemical composition of untreated materials also contain calcite. The concentrations of calcite was obtained by X-ray fluorescence spectroscopy calculated from the CaO contents were 15.5 and

TABLE 1. Chemical composition (%) of fine earth (<2 mm) and clay (<2 mm) from a sepiolite excavation pit in El Bur, Central Somalia.

Layer 0À40 40À100 100À200 200À300 (cm) <2 mm <2 mm<2mm<2mm<2mm<2mm<2mm<2mm

SiO2 46.6 52.2 50.2 56.7 58.6 54.0 46.0 47.7 MgO 9.4 15.2 18.7 20.8 20.2 19.5 16.3 15.7 Al2O3 2.80 4.08 0.90 1.00 0.80 0.83 0.74 1.30 CaO 14.5 9.0 0.5 1.3 0.41 0.07 10.6 5.2 Na2O 0.94 0.03 3.7 0À. 0.74 0.03 3.6 0.23 K2O 1.04 1.10 0.46 0.47 0.54 0.40 0.42 0.63 TiO2 0.30 0.40 0.11 0.12 0.11 0.10 0.09 0.18 Fe2O3 1.46 2.46 0.59 0.50 0.39 0.48 0.37 0.78 MnO 0.01 0.02 0À. 0À. 0À. 0À. 0.02 0.07 P2O5 0.02 0.03 0À. 0.04 0À. 0.02 0À. 0.05 ZrO 0.03 0.02 0.01 0.02 0.02 0À. 0.01 0.01 H2O(+) 12.0 10.0 11.0 16.0 8.0 19.0 14.0 21.0 H2O(À) 10.9 5.4 13.8 3.0 10.1 5.6 7.7 7.2

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11.4% for the 0À40 cm and 200À300 cm layers, second (40À100 cm) and fourth (200À300 cm) respectively. This was less than the carbonate layers, as also shown by the XRD identification calculated from the inorganic C contents (20.25 of halite in these layers. The dominant soluble and 16.4% for the uppermost and lowermost layers, cation is Na, followed by Mg, Ca and K (Table 2). respectively), and suggests the presence of high-Mg The highest Mg/Ca ratio was found in the calcite or even traces of dolomite. While carbonate uppermost layer. Soluble K is relatively low. is disseminated throughout the matrix of the layer at Among the anions, chlorides dominate, followed 0À40 cm, in the layer at 200À300 cm it appears as by sulphates. fissure coatings. This suggests that it represents reprecipitated carbonate from leaching out of the XRD upper layer or derived from below (capillarity or surface discharge). In the clay fraction (<2 mm) Three out of the four layers examined consist of separated from the bulk material, halite is absent nearly pure sepiolite (Fig. 3). In the uppermost and also the amount of carbonate is less (Table 1). layer (0À40 cm) sepiolite is accompanied by some Using the Brauner-Preisinger model (Bailey, , distinguished by its 002 diffraction line from 1980) for ideal sepiolite: palygorskite. Calcite and minor amounts of quartz are present in this layer too. In the other layers Si Mg O (OH) (OH ) Á8H O 12 8 30 4 2 4 2 (40À300 cm), quartz is present in trace to minor the chemical composition of the clay (<2 mm) amounts (Table 3). In layers 40À100 cm and fractions of the second (40À100 cm) layer from 200À300 cm, halite is present in trace amounts. the excavation pit was used for calculating the Table 3 shows that in the three layers at following formula for the El Bur sepiolite 40À300 cm, all diffraction lines of sepiolite could (assuming the presence of 5% quartz as impurity). be identified in powder diffractograms. The sharpness and intensity of the diffraction lines are (Si Al )(Mg Al Fe )O 11.888 0.112 7.313 0.154 0.084 30 comparable to those of the sepiolite from Vallecas, (OH) (OH ) Á8H O 4 2 4 2 , suggesting the high crystallinity of the This indicates a relatively small tetrahedral Al for material. Si substitution. Octahedral cation occupancy is low Ethylene glycol solvation did not result in any (7.551) and thus the extra positive charges created expansion of the 110 reflection in the 40À100 cm by Al substitution of Mg compensate only partly for layer sepiolite. In the 200À300 cm layers, a slight the negative tetrahedral and octahedral charges. expansion from 12.26 to 12.47 A˚ was observed, This explains the relatively high cation exchange accompanied by contraction of the 130 spacing capacity of the El Bur sepiolite, ranging between from 4.52 to 4.49 A. 240À360 mmol kgÀ1. To be noted also is the very low Fe content of this sepiolite. Thermal analysis The pH of the sediment water extracts is moderately alkaline in the uppermost layer, mildly The DTA analysis of the El Bur sepiolite (layers alkaline in the lower ones (Table 2). High 40À100 cm) reveals four endothermic and one concentrations of soluble salts are present in the exothermic reactions (Fig. 4). The first endotherm,

TABLE 2. pH, CaCO3 and soluble salts in the four layers of the El Bur excavation pit.

Water soluble salts + + 2+ 2+ À À 2À Horizon pH CaCO3 Corg Na K Mg Ca Cl NO3 SO4 À1 À1 (cm) (H2O) % mg kg mmol kg

0À40 8.59 20.2* 2.2 239 6.2 91.2 32.1 214 2.7 66 40À100 7.89 0 2.2 509 12.6 210 100.1 488 3.5 116 100À200 7.86 0 2.1 105 4.7 121 69.0 182 1.0 112 200À300 8.09 16.4* 2.0 526 11.6 201 87.8 674 5.0 116

*Calculated from inorganic carbon content

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FIG. 3. Powder X-ray diffractogram (using Cu-Ka radiation) of El Bur sepiolite from layer 40À100 cm. No impurities could be identified. The sepiolite diffraction line at 3.35 A˚ possibly overlays the major quartz line at 3.34 A˚ .

FIG. 4. Combined DTA and TGA tracings of El Bur sepiolite from layer 40À100 cm.

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TABLE 3. X-ray powder data of different layers of the El Bur sepiolite compared to those of a sample from Vallecas, Spain.

Layer 40À100 cm 100À200 cm 200À300 cm Vallecas A˚ A˚ A˚ A˚ hkl dobs I/Io dobs I/Io dobs I/Io dobs I/Io

110 12.27 100 12.29 57 12.32 100 12.19 100 130 7.58 13 7.58 8 7.59 13 7.51 10 040 6.66 14 6.66 8 6.67 12 6.67 10 150 5.03 10 5.03 7 5.04 9 5.03 8 031 4.52 21 4.52 12 4.52 18 4.50 17 131 4.30 32 4.29 30 4.31 26 4.31 24 221 0ÀÀ. 3.98 6 0ÀÀ. 3.98 7 260 3.75 21 3.74 12 3.74 18 3.75 19 241 3.54 12 0ÀÀ. 0ÀÀ. 3.53 10 080 3.35 76 3.48 100 3.35 73 3.34 26 331 3.20 20 3.25 24 3.24 28 3.19 17 0ÀÀ. 3.18 15 3.18 18 0ÀÀ. 261 3.01 10 3.06 7 3.06 10 3.05 9 370 0ÀÀ. 2.91 5 0ÀÀ. 0ÀÀ. 2.83* 8 0ÀÀ. 2.83* 27 2.82 6 460 2.68 12 2.70 7 2.68 11 2.67 10 0À. 0À. 0À. 2.60 16 530 2.58 27 2.58 16 2.58 23 2.57 22 202 2.45 20 2.45 12 2.45 13 2.26 12 461 2.40 12 2.39 7 2.40 10 2.40 9 2.28 18 2.28 13 2.26 13 2.26 12 312 2.25 15 0ÀÀ. 0ÀÀ. 0ÀÀ. 640 2.13 11 2.13 10 2.13 10 2.12 6 402 2.06 10 2.06 6 2.06 8 2.06 8 1.98* 7 1.98 8 1.99* 14 0ÀÀ. 0ÀÀ. 1.92 11 0ÀÀ. 1.94 4 1.82 1.82 14 1.82 6 1.87 5 1.54 1.54 15 1.54 17 1.55 6

* halite diffraction lines

between room temperature and 172.38C, with the strong exotherm at 831.18C. A weight loss of peak at 104.98C, is associated with the elimination 2.47%, in the range between 610.58C and 875.48C of adsorbed and zeolite H2O. From the TGA curve may be related to the loss of the last portion of it can be seen that the weight loss due to this structural OH; 0.80% more weight loss was reaction was 10.92%. The second endotherm, with a recorded between 875.48C and 1096.58C. peak at 316.58C, is due mainly to the loss of From Table 4, where the thermal reactions of the coordinated water. The weight loss between El Bur sepiolite are compared to those of the 170.48C and 379.58C associated with this reaction Vallecas sepiolite analysed under the same condi- was 3.23%. The third broad endotherm, barely tions, the similarity is remarkable. These losses are visible at 508.68C, can be attributed to the also similar to the theoretical ones given by Caillere elimination of the last part of the coordinated et al. (1982), i.e. 11.1, 5.5 and 2.7% for the water and to the release of structural OH. Extending 20À2608C, 260À6208C and 620À10008C intervals, between 377.58C and 610.58C, 2.29% water was respectively. The DTA curve exhibits the effects lost in this range. The marked endotherm at which, according to Peterson & Swaffield (1987), 815.68C was followed immediately by a sharp and can be regarded as characteristic of sepiolite.

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TABLE 4. Thermal reactions of the El Bur sepiolite IR analysis (40À100 cm) compared to those of the sepiolite from Vallecas, Spain. The characteristic octahedral OH-stretching vibration is well formed, and appears in all El Bur, Somalia Vallecas, Spain spectra at 3693 cmÀ1 in the uppermost layer (0À40 cm), and at 3682 cmÀ1 in layer Reaction temp. (8C) 100À200 cm (Fig. 5). Possibly, the spread in the First endotherm 104.9 104.2 frequencies of this band is related to the number of Second 316.5 311.8 vacancies in the octahedral sites. Bands at Third 508.6 511.7 À1 À1 À1 Fourth 815.6 823.3 3620 cm , 3564 cm and 1625 cm originate Exotherm 831.1 834.9 from channel water coordinated to Mg ions at the edges of the 2:1 ribbon layers. In the lowermost Weight losses (%) (200À300 cm) layer, the 1625 cmÀ1 band is very RTÀ172.3 10.92 11.51 indistinct. The typical trioctahedral OH-deformation 172.3À379.5 3.23 3.65 doublet bands at 696 and 641 cmÀ1 could also be 379.5À611.5 2.29 2.58 611.5À875.4 2.47 2.52 observed in all samples. The position of the 875.4À1096.6 0.80 0.69 characteristic SiÀO vibration band varied between 1218 cmÀ1 in the material from layer 200À300 cm to 1204 cmÀ1 in the layer 100À200 cmÀ1. In the Vallecas sepiolite, this band appears at 1209 cmÀ1. Thermal characteristics of sepiolite from other layers are similar though not identical to those from Morphology 40À100 cm. In the 200À300 cm layer, the fourth endotherm is very small, and its peak temperature, The morphology of single fibres was examined as well as that of the exotherm, is lower. by TEM, and their structural arrangement by SEM.

FIG. 5. IR spectrum of El Bur sepiolite from layer 100À200 cm.

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The fibres have an approximate length of This indicates a very high external surface area for 1À2 mm in the uppermost layer, that increases to the El Bur sepiolite. 2À4 mm in the lowermost layer (200À300 cm). The At very low concentrations (1000 ppm), the width of the fibres is 20À40 nm (Fig. 6). When viscosities are similar, and approximately equal to dispersed, they are straight with no branching. In that of water. At higher suspension concentrations the lowermost layer (200À300 cm) features that (1310 and 1740 ppm), the viscosity of the El Bur could be interpreted as branching could sometimes sepiolite, particularly that from layer 40À100 cm, is be observed. In the uppermost layer, the fibres were distinctly higher than that of the Vallecas sepiolite. accompanied by some small, plate-like crystallites, The difference increases at higher concentrations. probably illite (as indicated by XRD). The low bulk density of the El Bur sepiolite The SEM observations revealed mats of inter- indicates the very high porosity of the material, also woven, straight fibres. Commonly, the material was observed by SEM examination. The density is aggregated into units of two, sometimes more, consistent to a depth of 200 cm, where it decreases parallel-lying and closely attached fibres, each with sharply (Table 5). a width of 40À50 nm. These units appeared to have a random orientation against each other, creating a DISCUSSION dense network of pores (Fig. 7). In the uppermost layer, these units were shorter, less straight, Results indicate that, to a depth of 3 m at least, suggesting some disintegration. massive sepiolite is present in the surface sediments The fibres are shorter than those observed for at El Bur. In the absence of detailed prospecting, sepiolite formed by epigenetic weathering of mafic the lateral extent of this deposit is not known. primary minerals nor were they arranged in thick Lithological as well as geomorphological considera- strands aligned in a parallel pattern (Singer et al., tions suggest, however, that this, or similar surface 1992). Nor does the length of the fibres and their deposits, might have a considerable extent. The arrangement resemble that of the ‘Meerschaum’ vertical extent of this deposit is not known either. sepiolite formed by replacement in At depth of 3 m there were no indications for a Central Anatolia, (Yeniyol, 1995, Ece & transition to sediments of a different nature. Coban, 1994) where up to 20 mm length fibres, Data also indicate the relatively high purity of the slightly to strongly curved and branching, and with deposit. Except for minor quantities of quartz and parallel or subparallel orientation in bundles and occasional carbonates and soluble salts, sepiolite mats or as coatings, pore-fillings and pore-bridging, clay is the sole component of all layers from 40 cm were observed. depth downward. Both external surface area and The morphology is also not similar to that viscosity of this sepiolite are higher than that from described from the Madrid Basin (Spain) where Vallecas, and thus suggest the industrial suitability the sepiolite has formed by the diagenetic alteration of this material. of smectite (Leguey et al., 1995). The idiomorphous Both the geology and geomorphology of the morphology of the El Bur sepiolite fibres suggest region indicate a lacustrine origin for the El Bur crystal growth under conditions of supersaturation. Low supersaturation may give rise to long fibres TABLE 5. Bulk density, BETsurface area and Ostwald coalesced into bundles (Gehring et al., 1995). The viscosity (1740 ppm suspension) of the El Bur shorter fibres in a completely random orientation of sepiolite compared to Vallecas sepiolite. the El Bur sepiolite are likely to have formed under higher supersaturation, associated with a stronger evaporative environment. Bulk density Surface area Viscosity gcmÀ3 m2 gÀ1 mm2/S*

Surface area, viscosity, and bulk density El Bur 0À40 cm 0.83 306 1.734 2 À1 The largest external surface area, 346 m g , was 40À100 cm 0.76 346 3.211 obtained in the 40À100 cm layer, the smallest, 100À200 cm 0.89 340 1.293 306 m2gÀ1, in the uppermost (0À40 cm) layer 200À300 cm 0.49 342 n.d. (Table 5). The Vallecas sepiolite external surface, Vallecas n.d. 224 1.171 measured on the same instrument, was 224 m2gÀ1.

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sepiolite. The physiography of the area is nearly the diagenetic replacement of magnesite pebbles level to slightly undulating, with shallow depres- with shallow burial under alkaline conditions in the sions that have playa-like characteristics and vicinity of paleoshore lines (Ece & Coban, 1994; frequently contain saline lakes of a seasonal Yeniyol, 1995). nature. Evaporites, particularly gypsum, are abun- In contrast to these palaeolacustrine environments dant in the surface sediments and occur in a variety of formation, the El Bur sepiolite is Recent or of forms. Gypsiferous marls are common. The Quaternary and appears to be forming at present. As abundance of evaporites indicate a lacustrine, such, this deposit could be likened to the closed basin evaporative environment. These sedi- occurrences described by Hay et al. (1995) in the ments are in accord with the present climate, which Amboseli Basin, East Africa, the Amargosa Desert, is semi-arid to arid and thus may have formed Nevada (Hay et al., 1986), and to those described contemporaneously. more recently by Webster & Jones (1994) from the Millot (1964) in his studies of the Tertiary basins southern High Plains, Texas. of north and west Africa, presented the classical In contrast to the Amargosa desert deposits, scheme of clay mineral sequences in closed basin gypsum is a common associate of the El Bur deposits. The species richest in Al and Fe are at the sepiolite, suggesting that chemical precipitation had periphery and are succeeded basin-ward by more occurred from water with a higher salinity. The siliceous and Mg-rich clays, with sepiolite at the absence of other authigenic Mg clays, such as those centre. Garrels & Mackenzie (1967) proposed direct described from the Amargosa desert, from the El precipitation of sepiolite from alkaline-saline lake Bur deposit, possibly indicates fewer fluctuations in water, concentrated by evaporative processes. In the water chemistry in the latter. general terms, the sequence of more magnesian and The absence of any detrital clay minerals such as siliceous clays basin-ward has been confirmed by smectite or illite indicates that no significant subsequent studies, but apparently the formation of overland run-off had taken place. The minor sepiolite as end-member is not ubiquitous (Jones & amounts of quartz may have been the result of Gala´n, 1988). Crystalline rocks undergoing weath- some short-range aeolian input. The absence of ering, or volcanic ash, are commonly given as detrital sheet minerals also supports the hypothesis sources for Si, and Mg-rich carbonates, silicates or that the sepiolite had formed by chemical precipita- residual marine salts as those for Mg. tion and not by transformation of precursor The geomorphological setting of the El Bur minerals, such as palygorskite (Bachman & sepiolite is somewhat similar to that of the Tertiary Machette, 1977). Webster & Jones (1994) deduced lacustrine basins in Spain and in Turkey. The evaporatively induced salinity shifts from brackish economically most significant of the Spanish to saline (perennial) or ephemeral-lake (playa) deposits is from the Miocene Madrid basin (Gala´n conditions from the predominance of sepiolite, & Castillo, 1984; Doval et al., 1986). In contrast interstratified Mg-smectite and palygorskite, respec- with the Millot model, most sepiolite does not tively, in Pleistocene-Holocene lacustrine sediments occur at the centre of the basin, but in marginal in the southern High Plains, Texas. positions, crystalline rocks of bordering ranges No volcanic material or crystalline rocks are being the major source for detritus and soluble Si. present in the immediate vicinity of the El Bur area Fluctuating lacustrine brackish to saline waters are that could have served as sources for the Si and Mg the principal contributors of Mg, probably derived required for sepiolite precipitation. As an alternative from the recycling of Mesozoic marine evaporite source, sustained groundwater inflow could be strata. considered. The presence of groundwater close to The Turkish Eskisehir and Central Anatolian the surface in El Bur is indicated by the moist deposits are from the Miocene and had formed by condition of the material that was sampled.

FIG. 6. Transmission electron micrographs of El Bur sepiolite (a) layer 0À40 cm; the length of the fibres is 1À2 mm; some small, plate-like crystallites probably represent illite. (b) and (c) layer 40À100 cm; length of the fibres is 2À4 mm; units of two or more, parallel-lying and closely attached fibres can be observed in (c); (d) layer 200À300 cm.

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FIG. 7. Scanning electron micrographs of El Bur sepiolite from layer 200À300 cm (a) structure of mats formed by straight interwoven fibres; units of two, sometimes more, parallel-lying, closely attached fibres, can be observed in (b).

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Sepiolitic clays at the margins of a pan in the Clay mineralogy of the Madrid basin: Comparison Kalahari desert, SW Africa (Kautz & Porada, 1976) with other lacustrine closed basins. Pp. 188À189 in: in the Amargosa desert, Nevada (Hay et al., 1986) Geochemistry of the Earth Surface and Processes of and in the Amboseli basin, Kenya (Hay & Stoessell, Mineral Formation (abst.) (R. Rodriguez-Climente & Y. Tardy, editors). 1984; Hay et al., 1995) are also proposed to have Drechsel P. (1991) Bodengesellschaften formed under the impact of Si/Mg-rich ground Centralsomalias: O¨ kologie und Genese. Bayreuther water. According to Hay et al. (1995), Si and Mg Bodenkundliche Berichte. Band 19, Bayreuth. for Mg-rich clays in the Amboseli basin were Ece O.I. & Coban F. (1994) Geology, occurrence and supplied by lake- and ground-water. Different genesis of Eskisehir , Turkey. Clays Clay mixtures of dolomite, relatively Mg-rich ground- Miner. 42,81À92. water with saline, alkaline lake water may have Federal Statistical Office (1991) Statistics of Foreign accounted for precipitation of sepiolite or other Countries, Somalia.J.Be.MetzlerandC.E. high-Mg clays at different times. The purity of the Poeschel Publ. Wiesbaden, Germany. El Bur sepiolite supports the contention of Jones & Gala´n E. (1987) Industrial applications of sepiolite from Gala´n (1988) that the purest sepiolite deposits are Vallecas-Vicalvaro, Spain: A review. Proc. Int. Clay Conf. Denver, 400À404. the result of evaporative precipitation from Gala´n E. & Castillo A. (1984) Sepiolite-palygorskite in dominantly groundwater-fed shallow water bodies Spanish Tertiary Basins: Genetical patterns in lacking reactive clay detritus. continental environments. Pp. 87À124 in: Palygorskite-Sepiolite, Occurrences, Genesis and Uses (A. Singer & E. Gala´n, editors). Dev. ACKNOWLEDGMENTS Sedimentol. 37. Elsevier, Amsterdam. The authors acknowledge the help of Mrs Harfold with Garrels R.H. & MacKenzie F.T. (1967) Origin of the the transmission electron microscopy, and that of Dr. chemical composition of some springs and lakes. Pp. Wurster with the scanning electron microscopy. Dr 222À242 in: Equilibrium Concepts in Natural Water Baumann was extremely helpful in the viscosity Systems (W. Stumm, editor). American Chemical measurements. Special thanks are also due to Prof. K. Society, Advances in Chemistry v. 67. Nickel from Tubingen University for carrying out the Gehring A.V., Keller P., Frey B. & Luster J. (1995) The surface area measurements. The opportunity to sample occurrence of spherical morphology as evidence for in the El Bur area was provided by the Special changing conditions during the genesis of a sepiolite Research Program 308 of the German Research deposit. Clay Miner. 30,83À86. Foundation (DFG). Grossher D. (1978) Somalia Hantiwadaag. Kubler, Heidelberg. Hay R.L. & Stoessell R.K. (1984) Sepiolite in the REFERENCES Amboseli Basin of Kenya: A new interpretation. Pp. Ahrens T.P. (1951) A Reconnaissance Groundwater 125À136 in: Palygorskite-Sepiolite, Occurrences, Survey of Somalia, East Africa. Comitato interminis- Genesis and Uses (A. Singer & E. Gala´n, editors). triale per la reciostruzione, Roma. Dev. Sedimentol. 37. Elsevier, Amsterdam. Alaily F., Lassonczyk B., Huth A. & Genisor B. (1990) Hay R.L., Pexton R.E., Teague T. & Kyser K. (1986) Spring-related carbonate rocks, Mg clays and Genesis of soils in the arid part of north-east associated minerals in Pliocene deposits of the Somalia. Berline Geowiss. Abh. (A) 120(2), Amargosa desert, Nevada and California. Geol. 695À704. Berlin. Soc. Am. Bull., 97, 1488À1503. Bachman G.O. & Machette M.N. (1977) Calcic soils and Hay R.L., Hughes R.E., Kyser T.K., Glass H.D. & Lin J. calcretes in the southwestern . U.S. (1995) -rich clays of the Meerschaum Geol. Survey Open-File Rept. 77À794, 163 p. mines in the Amboseli Basin, Tanzania and Kenya. Bailey S. (1980) Structures of clay minerals. Pp. 1À123 Clays Clay Miner. 43, 455À466. in: Crystal Structures of Clay Minerals and their X- Hendricks F., Behrens H., Busch W., Bussmann M., ray Identification (G. Brindley & G. Brown, editors). Gebhardt H., Gorler K., Reuleke D., Strouhal A. & Mineralogical Society, London. Uhmann A. (1993) Detrital palygorskite in lacustrine Boaler S.B. & Hodge C.A. (1964) Observations on and lagoonal clay-mineral associations of Late vegetation areas in the northern region, Somali Tertiary age from and Somalia. Zbl. Geol. Republic. J. Ecol., 52, 511À544. Palaont. Teil 1, 1992 H. 5, 415À436. Caillere S., Henin S. & Rautureau M. (1983) Jones B.F. & Gala´n E. (1988) Sepiolite and palygorskite. Mineralogie des Argiles, Vol. 2, Masson, Paris. Pp. 631À674 in: Hydrous Phyllosilicates (S.W. Doval M., Calvo J.P., Brell J.M. & Jones B.F. (1986) Bailey, editor). Reviews in Mineralogy, Vol. 19,

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