Describe chemically formed sedimentary rocks under the following headings: (a) mode of formation; (b) three examples; (c) four importance.
(a) Mode of formation of chemically formed sedimentary rocks: - major agent is water - water with existing rocks - solutions are formed -formed from the accumulation of materials derived from other rocks- mineral salts are precipitated e.g. iron stone/limonite from hydrated iron oxide - direct evaporation of water leaves solid precipitates e.g. rock salt - accumulation over a long period of time - cementation of precipitated rocks - hardening of cemented rocks - accumulation of sediments could be in layers/strata -variation in thickness of strata - variation in thickness of strata
(b) Examples of chemically formed of sedimentary rocks: rock salt (sodium chloride or common salt) - nitrates-alabasta -a form of limestone -gypsum/calcium sulphate - anhydrites- dolorite (calcium carbon- ate) - potash - borax- limorite (iron stone) - limonite - flint (silicate).
(c) Importance of chemically formed sedimentary rocks: source of minerals - construction purposes - tourist attraction provides employment soil formation- foreign exchange earning raw materials for industries - used in reducing acidity of soils - food - used in health care e.g. pop and caoline - development of mining towns.
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