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编辑点评： GRE阅读考试的材料，非常强调逻辑的关系，所以逻辑要点是大家在解GRE阅读题的时候的关键，本文就为大家例举了这样一篇逻辑短文，供大家参考和练习。 随着GRE阅读考试的难度日益增加，大家需要有针对性地进行练习，本文为大家例举的就是逻辑短文的练习，文末还附有答案，供大家更好地练习。 Eight percent of the Earth s crust is aluminum, and there are hundreds of aluminum-bearing minerals and vast quantities of the rocks that contain them. The best aluminum ore is bauxite, defined as aggregates of alumi- 5 nous minerals, more or less impure, in which aluminum is present as hydrated oxides. Bauxite is the richest of all those aluminous rocks that occur in large quantities, and it yields alumina, the intermediate product required for the production of aluminum. Alumina also occurs natu- 10 rally as the mineral corundum, but corundum is not found in large deposits of high purity, and therefore it is an impractical source for making aluminum. Most of the many abundant nonbauxite aluminous minerals are silicates, and, like all silicate minerals, they are 15 refractory, resistant to analysis, and extremely difficult to process. The aluminum silicates are therefore generally unsuitable alternatives to bauxite because considerably more energy is required to extract alumina from them. 4. The author implies that a mineral must either be or readily supply which of the following in order to be classified as an aluminum ore? | 
