2 PROPERTIES AND APPLICATIONS Bentonite is a clay generated frequently from the alteration of volcanic ash, consisting predominantly of smectite minerals, usually montmorillonite. Other smectite group minerals include hectorite, saponite, beidelite and nontronite. Smectites are clay minerals, i.e. they consist of individual crystallites the majority of which are <2µm in largest dimension. Smectite crystallites themselves are three-layer clay minerals. Depending on the nature of their genesis, bentonites contain a variety of accessory minerals in addition to montmorillonite. These minerals may include quartz, feldspar, calcite and gypsum. The presence of these minerals can impact the industrial value of a deposit, reducing or increasing its value depending on the application. Bentonite presents strong colloidal properties and its volume increases several times when coming into contact with water, creating a gelatinous and viscous fluid. The special properties of bentonite (hydration, swelling, water absorption, viscosity, thixotropy) make it a valuable material for a wide range of uses and applications. Bentonite deposits are normally exploited by quarrying. Extracted bentonite is distinctly solid, even with a moisture content of approximately 30%. The material is initially crushed and, if necessary, activated with the addition of soda ash (Na2CO3). Bentonite is subsequently dried (air and/or forced drying) to reach a moisture content of approximately 15%. According to the final application, bentonite is either sieved (granular form) or milled (into powder and super fine powder form). For special applications, bentonite is purified by removing the associated gangue minerals, or treated with acids to produce acid-activated bentonite (bleaching earths), or treated with organics to produce organoclays. The main application of natural bentonite includes: Foundry: Bentonite is used as a bonding material in the preparation of molding sand for the production of iron, steel and non- ferrous casting. Cat Litter: Bentonite is used for cat litter, due to its advantage of absorbing refuse by forming clumps (which can be easily removed) leaving the remaining product intact for further use.
The lacustrine sequence consists of poorly compacted various coloured clays, silts, sands, tuffaceous sands and cinerites, interbedded with compact layers of limestone, sandstone and calcarious flagstone (fig 4). The sequence is capped by conglomerates. Mammalian fossils are abundant and these include australopithicenes (Lucy (Dinknesh) and family). Akilul assefa and R.F Ball traced the bentonite bed between 1.8 and 2.5 m thick. This bed is generally exposed only in a Table 2 Chemical composition of bentonites of afar and gidicho area cliff face and covered in most places by tens of meters of overburden. The reserve is estimated to be 925450m3 with average thickness of bentonite is 4.2 , surface area 647500m3 and the the volume of the overburden is found to be 925450m3. 3.3 Ledi Bentonite Bentonite sediment were noted on both sides of addis ababa assab road 20km south of desse junction with in 500m of Ledi river. The area is centered with geographic coordinates of 1109'30'' N and40041'E (fig2). The lacustrine clays underlying the plain are exposed in river gullies and outliers. In ledi area 4 bentonite occurrence were identified, Horizontally bedded grey clay with a greasy luster contains interbeds of 1- 2 cm thickness containing Iron, magnesium and gypsum is located to the south of ledi river. Extensive exposure of grey to brownish salty to sandy clay with indistinct horizontal bedding. the south of ledi river.
Figure 4 Geological map of afar area Mengesha T. et.al., 1996) On the northern bank of ledi river a grey –green bentonitic clay which is compact and massive containing minor salt and silt is exposed.in this area the betonite outcrop is without an overburden and has 8.5 thickness. 5-6km west of the gewane-mille highway grayish to brownish bentonite clay is exposed in the river valley.it has an average thickness of 7.5m . In this area the reserve was estimated to be 1784000m3 (table1) with an overburden of 1200000m3.
The oldest rock exposed in the island is the Pleistocene lacustrine sediments. These lacustrine sediments include diatomite, bentonitic clay and diatomaceous silt, which are exposed in the uplifted blocks within the island .the claybeds are mainly dominant in the lower most part of the lacustrine sequences with respect to the diatomite beds.(Million et.al., 1991) West of arura village there are small outcrops of yellowish brown, massive silty limestone which is sandwiched between the lacustrine beds. In some places within the lacustrine beds (specially Shigima area) there are gypsum anhydrite, manganese nodules and some pistolites of carbonate origin. The upper part of the diatomite layer in biaso area is partly covered by chert. The underlying beds of the lacustrine formation are not exposed. The Pleistocene lacustrine sediments are overlain by volcanic rocks. Ignimbrite which is rhyolitic, greenish grey, fine grained and vesicular overlies the lacustrine beds. The ignimbrite inturn is overlain by dark grey, vesicular basalt and the vesicles filled with calcarious material. There is a trachyte hill between shigima and arura village. Some exposure of grayish brown trachitic rock with abundant phenocrysts of feldspar are observed near the shigima village lying on top of basalt. In some places this rock is slightly weathered. Holocene lake sediments are exposed on the shore of the present lake while recent alluvium deposits are restricted at the mouth of some gullies within the island. Figure 6 Bentonite from Gewane area (geomuseum) photo. by mesfin The reserve in the area is estimated to be 3462860 m3.and the chemical composition of gidicho bentonite is almost comparable with the world bentonite producing countries (Table 4). In some areas slight variation also observed. Tests conducted on Bentonite samples from these and Gidcho deposits have shown them to be suitable for iron–ore pelletization and drilling mud after proper beneficiation (Tibebe et. al., 2002). Laboratory analysis of