The composition of limestone

Ancaster Freestone seen through a microscope.

There are many ways that limestone can be classified, based on such things as:

  • the way it formed
  • the types of grains
  • the amount and kinds of fossils
  • its chemical composition

As the intention of the limestone landscapes pages is to discover how particular limestone gives rise to a distinctive landscape, we will not go into the details of classification, but concentrate on the general textures.

Sedimentary rock

Limestone is a sedimentary rock, most of which originally formed by the accumulation of sediments on the sea floor (although some formed in fresh water). These sediments, which were afterwards turned to limestone rock, are composed of over 50 per cent carbonate minerals.

There are three main components in limestone:

Calcite and aragonite

Calcite

Calcite is a form of calcium carbonate with the chemical formula CaCO3. In other words, calcite is composed of a basic structure of one atom of calcium, one atom of carbon and three atoms of oxygen. It forms as crystals of various shapes, but often forms white or colourless rhombs. Some crystals split light so that, when placed on a picture or writing, two images can be seen. It may have developed from aragonite and is an important constituent of limestone.

Aragonite

Aragonite is an unstable form of calcium carbonate with the chemical formula CaCO3. Although it has the same chemical composition as calcite, it forms a different shaped crystal. It is found in the shells of some types of animals and in some calcareous sediment, but when the shells and sediments are turned to rock, the unstable aragonite often turns into calcite.

Dolomite

Dolomite

Dolomite was named after the Dolomite mountains in southern Europe. It is a carbonate mineral that often forms from calcite.

Sometimes liquids rich in magnesium carbonate (MgCO3) (an atom of magnesium, an atom of carbon and three atoms of oxygen) pass through a limestone. When this happens, the CaCO3 of the calcite picks up the MgCO3, so forming dolomite.

Like calcite, crystals of dolomite are generally rhombs and are found in limestone and dolostone. Its chemical formula is CaMg(CO3)2.

Impurities

Impure limestone

Impurities are 'added' to the limestone in the form of sand grains, silt or mud. This material was usually eroded from nearby land and washed into the sea by rivers to be mixed with lime muds, shells, corals, etc.

Composition of limestone graph

Limestone composition

We can think of limestone as being made of varying proportions of three components:

  • calcite
  • dolomite
  • impurities

We can draw a triangular graph to show this. The axes of the graph are shown in Figure 1. As you can see, the points of the triangle represent 100 per cent of a particular component and the line opposite is 0 per cent of the component.

As an example, a rock with 50 per cent calcite, 30 per cent dolomite and 20 per cent impurities is shown on the graph.

Composition of limestone graph

All limestones have over 50 per cent calcium carbonate and true limestones have over 90 per cent calcite. Much of the Carboniferous limestones and parts of the Cretaceous chalk of England have less than one per cent impurities.

However, most sedimentary rocks (and limestone is no exception) have a mixture of impurities — these might be sand grains, silt, mud, etc.

A rock that has more than 50 per cent impurities is not a limestone (but it might be a calcareous sandstone or a calcareous mudstone). In this topic we will ignore this part of the graph (Figure 2).

The main kinds of limestone

Sometimes the calcite in limestones changes to dolomite. Although the rock is still a kind of limestone, it is given another name — dolostone.

So using the triangular graph, the main kinds of limestone can be recognised (Figure 3).