by Paula Piilonen and Noel Alfonso

Most times, when someone uses the word species, the automatic assumption is that they are talking about a biological species, whether it is a plant, mammal, reptile, fish, algae or fungus.

Very rarely do people equate the word species with a mineral. But minerals are species too. And mineralogists use a classification or taxonomic system that is very similar to that used by biologists.

Biological Classification Mineralogical Classification
Kingdom Class
Phylum Subclass
Class Family
Order Supergroup
Family Group
Genus Series
Species Species

Humans have a natural instinct for seeing similarities and differences. Biologists use a hierarchical classification system based on similarity (shared characters) in order to group organisms. Most people are familiar with the kingdom, phylum, class, etc. categories down to the two most familiar levels, genus and species. Domain is a new level above kingdom.

Several specimens of Lake Trout (Salvelinus namaycush).
Morphological variation in the Lake Trout of Great Bear Lake, NWT, as an illustration of natural variation within a species. The role of biological taxonomists is to understand and delineate species boundaries. Image: Noel Alfonso © Canadian Museum of Nature.

Biologists struggle with the definition of a species. There are about twenty-six species concepts currently in use. The most prevalent one defines a species as groups of actually or potentially interbreeding populations. This is very difficult to test. Reproductive isolation can be demonstrated by defining a gap in a morphological or molecular character between two populations.

Defining a mineral species is more straightforward. Although minerals can be grouped in many different ways, mineralogists use a mineral classification scheme that is based on both the chemistry and atomic structure of a mineral. In order to classify a mineral, we must first know what elements it comprises.

All minerals are made of positively charged atoms (cations) and negatively charged atoms (anions). We use both these types of atoms to help us classify minerals.

To determine the class (the highest grouping in the hierarchal scheme) that a mineral falls into, we must know what anionic group acts as the main building block of the mineral. For example:

  • all silicate minerals are built from SiO44−
  • carbonates: CO32−
  • sulphides: S2−
  • oxides: O2−
  • halides: Cl or F
  • phosphates: PO43−.

We can further classify minerals based on how the cations and anions in a mineral bond together, as well as variations in chemistry between minerals.

Diamond and graphite specimens with a diagram of their atomic structure.
Diamond and graphite are both made of only carbon. It is the different ways in which the carbon is bonded in each mineral that cause their physical differences. Their respective atomic structures are represented by the diagram below each mineral. Image: Materialscientist © Materialscientist

Example 1: Both diamond and graphite comprise only one element: carbon. However, the way the carbon atoms are bonded together results in two very different mineral species.

Collage of mineral specimens: muscovite (collection #CMNMC56667) phlogopite (CMNMC30318), annite (CMNMC30318).
Muscovite, phlogopite and annite are all types of mica. They have the same atomic structure, but differences in their chemistry result in different mineral species names. Images: Michael Bainbridge © Michael Bainbridge

Example 2: There are 49 different species in the mica group. All micas have a similar atomic structure. They are sheet silicates, comprised of alternating layers of silicate tetrahedra and other cations (Mg, Fe, Al, Li, etc.). We can define each of the 49 species based on their differences in chemical composition. For example, muscovite is a K-Al mica, phlogopite is a K-Mg mica and annite (biotite) is a K-Fe mica.

Mineralogists in the museum’s Centre for Species Discovery and Change are interested in mineral species in the same way that biologists are interested in bird, plant or insect species—differences in species between geological environments can provide us with information on the evolution of that environment, such as pressure, temperature, oxygen fugacity or bulk chemical changes that may have occurred as the mineral species were crystallizing and growing.

The next time you are out for a nature walk, remember: the rocks at your feet contain important species too!