Biomineralisation1 refers to the processes by which living things form minerals. For example, calcium carbonate biomineralisation is used extensively by marine invertebrates2 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 cementation3 into limestone4 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
Pāua5 is the Māori name for a type of large marine gastropod6 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 organic7 macromolecules such as proteins, lipids8 and polysaccharides. Mantle9 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, calcite10 and aragonite11, are found in the pāua shell. It is the presence and arrangement of aragonite crystals in the inner layer of the shell (the nacre12) 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 pearl13) 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 periostracum14 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, resistance15 to fracture and aesthetic16 value of these natural materials have prompted scientists to try to replicate17 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 plastic18.
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 deposition19, compaction20 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 bryozoan21 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.
Nature of science
Although science is often divided into member disciplines such as chemistry and biology22, nature is not. To fully understand the biomineralisation process that nature uses to make seashells requires a multidisciplinary23 approach.
- biomineralisation: The process by which living organisms produce minerals, often to harden or stiffen existing tissues. Marine invertebrates such as pāua grow a protective shell based on mineral forms of calcium carbonate.
- invertebrates: An animal without a backbone, for example butterflies, worms, snails, insects, spiders and aquatic species such as crabs and jellyfish.
- cementation: A geologic process in which dissolved minerals precipitate out of solution, gluing particles of sediment together.
- limestone: Sedimentary rocks formed mainly from the minerals calcite or dolomite. Many limestones are derived from the shells of dead marine organisms. Others are formed by chemical precipitation.
- pāua: Large edible sea snails that are found around the coastline of New Zealand. They grow large shells that, when polished, have a blue, green and purple iridescent appearance.
- gastropods: Any of a large class (Gastropoda) of molluscs such as snails and slugs. They typically have a large foot, a single coiled shell that covers the soft body, and a head with a pair of eyes and tentacles.
- organic: 1. Molecules that contain carbon and that have a biological origin. 2. Grown using natural processes with nutrients from natural sources.
- lipid: Any of a group of organic (carbon-containing) compounds, including fats, oils and waxes that are insoluble in water, oily to the touch and, together with carbohydrates and proteins, constitute the principal structural material of living cells. Lipids are characterised by their insolubility in water and solubility in organic solvents.
- mantle: 1. A layer of the inner Earth between the crust and the core. Varies in temperature from 500 °C to 900 °C. Consists of semi-fluid molten rock. 2. A layer in molluscs that covers the fleshy body. In some molluscs, it secretes a shell (for example, snails), but it doesn’t in others (for example, slugs).
- calcite: The most common and most stable mineral form of calcium carbonate (CaCO3). It is the main component of chalk, limestone and marble.
- aragonite: A mineral form of crystalline calcium carbonate similar to calcite. It is found in the shells of marine invertebrates such as pāua and in living coral reefs.
- nacre: A biomineral produced by various types of mollusc that is made predominantly from calcium carbonate and the carbohydrate chitin. Known as ‘mother of pearl’, it gives the inner shell of the mollusc its characteristic lustrous and iridescent appearance.
- pearl: A semi-precious gemstone of calcium carbonate produced in the mantle of a living shelled mollusc such as an oyster.
- periostracum: A thin mainly protein layer that is the outermost part of the shell of molluscs such as pāua and mussels.
- resistance: 1. The opposition to the flow of electric current through a circuit. 2. The ability to withstand harmful or unfavourable conditions, such as an infectious disease.
- aesthetic: Appealing to the senses.
- replicate: Make an exact copy of.
- plastic: A synthetic material made from a wide range of organic polymers (such as polyethylene, PVC and nylon) that can be moulded into shape while soft and then set into a rigid or slightly elastic form.
- deposition: A geologic process referring to the settling out or placement of sediments following transport from one place to another.
- compaction: A geologic process in which sediments are progressively buried by more sediment, causing them to press together under their own weight, reducing their thickness.
- bryozoan: A type of largely marine water-dwelling invertebrate organism with a hard calcium carbonate outer casing. They band together to form colonies with an enormous range of shapes and sizes.
- biology: The science of living things.
- multidisciplinary: Involving a combination of several disciplines.
