The term “Bentonite” was first proposed Knight (1898) after Benton shale in the clay was believed to occur at that time. The Benton shale is named after fort Benton in Montana in the United States. According to Grim (1972) Bentonite is a rock formed of highly colloidal and plastic clays composed mainly of montmorillonite, a clay mineral of the smectite group, and is produced by in situ diversification of volcanic ash. In addition to montmorillonite, bentonite may contain feldspar, cristobalite, and crystalline quartz. The special properties of bentonite are an ability to form thixotrophic gels with water, an ability to absorb large quantities of water, and a high cation exchange capacity. The properties of bentonite are derived from the crystal structure of the smectite group, which is an octahedral alumna sheet between two tetrahedral silica sheets. Variations in interstitial water and exchange able cations in the interlayer space affect the properties of a bentonite. By extension, the term bentonite is applied commercially to any clay with similar properties. In addition to montmorillonite, bentonite may also contain feldspar, biotite, kaolinite, illite, cristobalite, pyroxene, zircon, and crystalline quartz (Parkes, 1982). Bentonite is a rock or a clay base industrial material. It is therefore a mixture of minerals. So no “molecular” formula can be given. By extension, the term bentonite is applied commercially to any plastic, colloidal, and swelling clay regardless of its geological origin. Such clays are ordinarily composed largely of minerals of the montmorillonite group. However, bentonite with this meaning is still term, consisting of more than one mineral.
Origin, exploration and mining
Bentonite is a clay-based material derived from the alteration, over geological time periods, of glassy material emitted from volcanoes - tuff and ash or from alteration of silica bearing rocks such as granite and basalt. Bentonites have formed in the course of several million years through alteration, i.e. weathering of volcano ash. As this ash only settles in valleys and shallow areas of ground, they often form multiple layers and thus small bentonite deposits develop The environmental requirements for the formation of the clay that is the main component found in Bentonite are only approximately known and different climatic and hydrological environments together with the different ages and depths of occurrence produce subtle variations in this clay. Most Bentonite deposits date from the Tertiary and Mesozoic periods i.e. up to 230 million years ago. The clay type found in Bentonite swells in water but this clay can over long periods convert to another non-swelling clay type called illite. The lack of older Bentonite deposits may be due to conversion of the clay to illite or because the conditions for initial formation of the water swelling clay were not favorable.
Explorations for bentonite involve using geological knowledge and field investigations. The initial step in exploration is examining geological environment .An understanding of the basic requirement for bentonite formations narrows the possible setting. Bentonite most commonly form through the aqueous alteration of volcanic ash. In addition the predominant environment of the past depositional alteration is marine in nature. Bentonite is largely restricted to formation of cretaceous and tertiary age. Using these factors, a typical bentonite-forming environment will be an area of the past volcanic activity that was located near a substantial body of water. The next step exploration is field investigation of a potential deposit .At this time the target area is examined for the presence of bentonite outcrop. Once the presence of bentonite is conformed the nature of the deposit is determined via surface mapping combined with drilling .The data from drill logs and maps allow a general assessment of the reserve volume and quality. After a deposit 
No comments:
Post a Comment