Calcium carbonate biomineralisation

Biomineralisation refers to the processes by which living things form minerals. For example, calcium carbonate biomineralisation is used extensively by marine invertebrates to build structures such as shells that give support and protection.

Over time, the accumulated shell debris from these marine invertebrates is transformed through depositioncompaction and cementation into limestone rock. In about 90% of all the limestone deposits that exist, the origin of the calcium carbonate present can be traced back to biomineralisation processes that occurred in marine invertebrates many millions of years ago.

A case study – pāua shell formation

Pāua is the Māori name for a type of large marine gastropod snail, the best known being blackfoot paua, Haliotis iris. In other countries, the term ‘abalone’ is used for this type of marine snail.

The pāua shell is an arrangement of calcium carbonate crystal forms and organic macromolecules such as proteins, lipids and polysaccharides. Mantle tissue that is found under and in contact with the shell controls and directs the laying down of the various layers that make up the pāua shell.

Calcium carbonate has several crystalline forms, and two of them, calcite and aragonite, are found in the pāua shell. It is the presence and arrangement of aragonite crystals in the inner layer of the shell (the nacre) that give its characteristic appearance when viewed in daylight. The iridescent swirl of intense green, blue, purple and sometimes pink colours make it an object of great beauty. As the pāua grows, more shell material is added to the lip of the shell and the inner portion of the shell is thickened at the same time.

Pāua shells are composed of three structurally distinct layers:

• The inner nacreous (flat pearl) layer consists of layers of aragonite crystals encased within sheaths made mainly of proteins and carbohydrates.
• The calcite-containing prismatic layer also contains proteins and carbohydrates.
• The outer periostracum is a thin organic layer that protects and decorates the shell.

Scientific interest in biomineralisation

The remarkable control that nature exerts in the growth and development of a seashell has attracted the attention of materials scientists. The way in which the calcium carbonate is laid down in a proteincarbohydrate scaffold is the focus of much attention. The high strength, resistance to fracture and aesthetic value of these natural materials have prompted scientists to try to replicate natural shell growth in the laboratory.

The long-term goal of this work is to develop materials, based on nature, that might find application for use in such areas as medical implants, structural components for buildings and vehicles and replacement for some types of plastic.

Limestone’s link to biomineralisation

When marine organisms reach the end of their life, the soft body parts decay but the hard body parts remain. These hard body parts, such as shells and skeletal parts, were formed by calcium carbonate biomineralisation.

Over time, there is an accumulation of this shell debris, and eventually, limestone rock is formed through the processes of deposition, compaction and cementation. Variations in each of these processes have led to many different types of limestone forming.

For example, the limestone deposit found near Oamaru is a 40-metre thick layer of pure bryozoan limestone. It is fine grained, strong and durable, even textured and readily cut and shaped. Some of New Zealand’s most important historic buildings have been built using Oamaru stone.

In contrast, the limestone found close to Te Kūiti is hard, dense and crystalline and is over 95% calcium carbonate. It is not suitable as a building stone, but its high calcium carbonate content makes it a valuable deposit servicing the needs of manufacturing industries.