1.4 Classification of limestone
High calcium limestone is composed primarily of the minerals calcite or Aragonite (CaCO3) with a total oxide (CaO + MgO) content of over 95 % It can be a fine to a coarse "rained stone of varying porosity and hardness. Chalk is a soft, fine "rained, highly porous limestone. The pure, white chalks can have up to 99 %calcium carbonate whereas the grey variety can have up to 20 % impurities i.e. only 80 % CaCO3. Dolomitic and magnesian limestones, in addition to the CaCO3, contain a relatively large proportion of MgCO3. Usually limestones containing 20 % to 44 % MgCO3 are referred to as "dolomite" or as dolomitic limestone, and those containing between 5 -20 % MgCO3, as magnesian limestone. They both vary in purity, density, hardness and colour. Marble is a metamorphosed limestone. It is either high calcium or dolomitic and highly crystalline, dense and hard, and varies in purity. Oolitic limestorne is a chemically precipitated limestone of high purity. Travertine is a hard limestone formed by chemical precipitation in hot springs. Tufa has the same derivation but is softer and more porous. Hydraulic limestones, sometimes referred to as argillaceous limestones have a high proportion of clay and silicia ( 15-20 %) and can be either high calcium or magnesian. When fired a lime is produced which can set under water (hydraulic lime).
There are several other types of limestones (less common and of limited value) such as coral limestones, shell limestones, marl, cherty limestones and stalagmites and stalactites.