Theory question 305
RocksDescribe chemically formed sedimentary rocks under the following headings: (a) mode of formation; (b) examples; (c) importance.
Show the model answer
Model answer
(a) Formed when minerals dissolved in water are precipitated or left behind as the water evaporates (e.g. in lakes or enclosed seas in hot dry areas), or by chemical reactions; the deposits harden in layers.
(b) Rock salt (halite), gypsum, potash, chalk/limestone of chemical origin, dolomite, trona, ironstone, flint, nitrates.
(c) Rock salt for cooking and the chemical industry; gypsum for cement and plaster of Paris; potash and nitrates for fertilisers; limestone for cement and lime; trona for soap and glass; export earnings and employment.
Explanation
Give at least three examples.