Biomineral

Biominerals are minerals that organisms produce. ^[1] They are true minerals in the geological sense - naturally occurring crystalline solids with definite compositions - but living organisms drive or control their formation. The “bio” prefix marks origin, not a different category of material. The outstanding example is the calcium carbonate of mollusk shells. The oyster’s shell and the pearl that may be within it are composed in large part of aragonite, identical to the inorganically formed mineral. ^[2]

Common Biogenic Minerals

Several forms of CaCO₃ (calcite, aragonite, vaterite) and monohydrocalcite (CaCO₃·H₂O) are the most common biogenic minerals (meaning “mineral formed by organisms”). ^[2] Calcite and aragonite - both CaCO₃ but with different crystal structures, making them distinct species - get secreted by marine invertebrates to build shells. Their remains are a major component of limestone layers throughout the stratigraphic record. ^[1] The volume of biologically precipitated carbonate preserved in the rock record is substantial.

Apatite [Ca₅(PO₄)₃(OH, F, Cl)] is a major structural component in vertebrate skeletons and teeth. ^[1] The apatite in a human tooth and the apatite in a phosphate rock are the same mineral - chemically and structurally identical. The human body produces essential minerals, and apatite, Ca₅(PO₄)₃(OH), is the principal constituent of bones and teeth. ^[2]

Pyrite (FeS₂) occurrences within numerous coal and shale deposits result from the metabolic activity of sulfate-reducing bacteria. ^[1] The fine-grained disseminated pyrite common in organic-rich sedimentary rocks is largely a bacterial product rather than an inorganic chemical precipitate.

Expanded List of Biogenic Species

Many other biogenic species have been recognized beyond the common carbonates and phosphates. ^[2] Biological precipitation can yield a variety of distinct mineral species, including: ^[2]

• Opal (an amorphous form of SiO₂) ^[2]
• Magnetite (Fe₃O₄) ^[2]
• Fluorite (CaF₂) ^[2]
• Several phosphates ^[2]
• Some sulfates ^[2]
• Mn-oxides ^[2]
• Pyrite (FeS₂) ^[2]
• Elemental sulfur ^[2]

This list continues to grow as researchers identify additional minerals formed through biologically induced or controlled processes.

Pathological Biomineralization

Mineral formations, commonly referred to as calculi, can also develop within the human urinary tract. ^[2] Kidney stones, bladder stones, and other calculi are unwanted but genuine examples of biomineralization. Such calculi consist predominantly of: ^[2]

• Calcium phosphates, which include whitlockite, carbonate-apatite, and hydroxylapatite ^[2]
• Calcium oxalates, a group rarely encountered in non-biological geological environments ^[2]
• Magnesium phosphates ^[2]

The calcium oxalates in kidney stones represent a mineral type extraordinarily rare in geological settings but common in biological contexts, illustrating how different the chemical environments inside organisms can be from those in typical rocks and sediments.

Significance

The list of known biologically produced minerals and mineraloids keeps growing. Numerous minerals and mineraloids are now recognized as products of biologically induced and controlled mineralization. ^[1] As scientific comprehension of biological mineralization advances, the number of recognized biogenic minerals is certain to increase. ^[1]

The chemical conditions near the Earth’s surface, along with the subsequent distribution and variety of minerals, have been heavily influenced by biological activity. ^[1] The mineralogy of soils, sediments, and shallow crustal rocks cannot be fully understood through inorganic geochemistry alone. Any definition of “mineral” that excludes biologically produced materials excludes some of the most volumetrically significant minerals at Earth’s surface.

References

1. Nesse, W. D. (2017). Introduction to Mineralogy, 3rd ed. Oxford University Press.
2. Klein, C. (2002). Manual of Mineral Science, 22nd ed. John Wiley & Sons.

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