A story of the Carboniferous period
Limestone is a biogenic rock, but it is more compacted than chalk. It is a sedimentary rock formed from the remains of remains of billions of tiny shells and skeletons of microscopic animals or foraminifera.
Major compaction by later sediments becoming very hard. (Source: Imerys Performance Minerals)
Pure limestone is white or almost white. Because of impurities, such as clay, sand, organic remains, iron oxide and other materials, many limestones exhibit different colours, especially on weathered surfaces.
The Peak District, Derbyshire a major source of high quality limestone was formed during the Carboniferous geological period, some 340 million years ago. The Peak District limestone formed over a period of 50 million years and changes in conditions on the sea floor over that time are reflected today in variation in rock colour, strength and fossils. Other limestone producing regions in the UK include Northern Ireland and Cumbria.
Dolomite is formed similarly to limestone except that the sedimentation process occurs in the presence of magnesium. Dolomitisation results in some of the calcium ions in the crystal lattice being replaced by magnesium ions and leads to the formation of dolomite CaMg(CO3)2 – calcium magnesium carbonate.
Limestone that is partially replaced by dolomite is referred to as dolomitic limestone or magnesian limestone.
Permian dolomites are the main source of dolomite in the UK. Most significant deposits are found in a belt running north-south for some 230 km from Newcastle to Nottingham.