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In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form. Minerals are classified by key chemical constituents; the two dominant systems are the Dana classification and the Strunz classification. Silicate minerals comprise approximately 90% of the Earth's crust. Other important mineral groups include the native elements, sulfides, oxides, halides, carbonates, sulfates, and phosphates. Ores recovered by mining include metals, coal, oil shale, gemstones, limestone, chalk, dimension stone, rock salt, potash, gravel, and clay. Mining is required to obtain most materials that cannot be grown through agricultural processes, or feasibly created artificially in a laboratory or factory.
Central Africa is a subregion of the African continent comprising various countries according to different definitions. Angola, Burundi, Cameroon, the Central African Republic, Chad, the Democratic Republic of the Congo, the Republic of the Congo, Equatorial Guinea, Gabon, Rwanda, and São Tomé and Príncipe are members of the Economic Community of Central African States (ECCAS). Middle Africa is an analogous term used by the United Nations in its geoscheme for Africa. The main economic activities of Central Africa are farming, herding and fishing. Crop production based on rain is possible only in the southern belt. Slash-and-burn agriculture is a common practice. Flood recession agriculture is practiced around Lake Chad and in the riverine wetlands. Nomadic herders migrate with their animals into the grasslands of the northern part of the basin for a few weeks during each short rainy season, where they intensively graze the highly nutritious grasses. When the dry season starts they move back south, either to grazing lands around the lakes and floodplains, or to the savannas further to the south. Fisheries have traditionally been managed by a system where each village has recognized rights over a defined part of the river, wetland or lake, and fishers from elsewhere must seek permission and pay a fee to use this area. Oil is also a major export of the countries of northern and eastern Central Africa, notably making up a large proportion of the GDPs of Chad and South Sudan. Countries in Central Africa are Angola, Cameroon, Central African Republic, Chad, DR Congo, Republic of the Congo, Equatorial Guinea, Gabon, São Tomé and Príncipe, Rwanda, Burundi.