GeologyRocks

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Jon
University of Edinburgh

Introduction

Carbonates are rocks composed mainly of calcium carbonate, CaCO₃. Some examples of common carbonate rocks are limestones and chalk. Carbonates form by precipitation from water; either straight from the water, or induced by organisms, to make their shells or skeletons, and they form in many environments (Figure 1).

Figure 1: Some typical environments that carbonates can form. Redrawn from Tucker and Wright (1990).

Mineralogy

There are three main minerals that form carbonates:

• Calcite (CaCO₃), which comes in high magnesium and low magnesium forms.
• Aragonite (CaCO₃), which has a different structure to calcite.
• Dolomite (CaMg(CO₃)₂), a magnesium rich carbonate produced by diagenesis.

Only low magnesium calcite is stable at surface pressure and temperatures. It is therefore the most common mineral in ancient carbonates. However, most modern carbonates are composed of aragonite as this is the mineral that most biological organisms create to make their shells or skeletons. Examples of organisms that produce aragonite shells are bivalves (sea shells), gastropods (snails) and Halimeda (a green algae). Organisms that produce a calcite shell include brachipods (a rare type of sea shell) and ostrocods (a small crustacean).

Components

Carbonates can be made of several components. These are:

• Bioclasts.
• Ooids.
• Peloids.
• Intraclasts.
• Micrite.
• Sparite.

Bioclasts

Bioclasts are fragments of dead sea creatures. These include shells (Figure 2) and corals. The creatures precipitate the carbonate in order to produce some kind of structure.

Figure 2: A bioclastic limestone. Here the bioclasts are bivalves of Carboniferous age.

Ooids

Ooids are rounded grains formed by precipitation of calcite around a nucleus to produce concentric circles (Figure 3). They form in warm, shallow waters, with a strong tidal currents. Wave action may also contribute to their near-spherical shape.

Figure 3: The structure of an ooid. Redrawn from Tucker and Wright (1990).

Peloids

Peloids are sand sized grains (100-150 micrometers) of micro-crystalline carbonate. They are generally rounded or sub-rounded. They originate from fecal pellets, algae and mud clasts. They are sometimes found clumped together, in a formation known as a grapestone.

Intraclasts

Intraclasts are clast of other limestone that appear in younger limestones. They can be quite difficult to distinguish at times, as they may be made of a similar rock as that which encases it. For example, hardgrounds can from when sea water flows through carbonate sediment, lithifying it rapidly. Subsequently, the hardground may be broken up and incorporated into the surrounding sediment.

Micrite

Micrite is microcystalline carbonate mud, with grains less than 4 micrometers.

Sparite

Sparite is coarser than micrite, with a grain size of more than 4 micrometers and is crystalline.

Both micrite and sparite form the matrix or cement in carbonate rocks.

Classification

There are two main methods of classifying carbonate rocks; Dunham's and Folk's classification. Below is Dunham's classification which uses the texture of the rock to classify the carbonate. Dunham's classification is useful for field work.

Folk's classification is very useful for carbonates in thin section. It classifies carbonates based on their components (see above) and the matrix/cement binding the components together.

Conclusion

Carbonate rock are sedimentary rocks from from calcium carbonate, CaC0₃. Calcium carbonate is either the mineral aragonite, high magnesium carbonate or low magnesium carbonate. Only low magnesium carbonate is stable at surface pressures and temperatures, but most modern carbonate is aragonite. Carbonates are formed from various components, which include ooids, peloids, clasts and biological remnants. Carbonates are classified by either Folk's or Dunham's classification schemes.

References

M. E. Tucker and P. Wright, 1990. Carbonate Sedimentology. Buy It.