A Beginner's Guide to Mineral Chemical Properties and Classification
Part three of our Mineralogy Guide series. This guide covers the chemical properties of minerals: what they are made of, how they are classified, and what the numbers and formulae in mineral property tables actually mean. These are the more abstract properties compared to the physical and optical ones covered in guides one and two, but they are the foundation of how mineralogy works as a science and they unlock a deeper understanding of why minerals look, behave, and form the way they do.
What Is a Mineral?
Before exploring how minerals are classified it is worth establishing what a mineral actually is, because the word is used loosely in everyday language in ways that differ from its precise scientific meaning.
In mineralogy a mineral is defined as a naturally occurring, inorganic, solid substance with a definite chemical composition and an ordered internal crystal structure. Each part of this definition matters.
Naturally occurring means it must form through geological processes without human intervention. Synthetic rubies grown in a laboratory are not minerals in the strict sense, even though they are chemically and structurally identical to natural ones.
Inorganic means it is not produced by biological processes. Coal, amber, and pearl are sometimes called minerals in everyday language but are technically not minerals under the strict definition because they are of biological origin. They are sometimes referred to as mineraloids or organic gemstones.
Definite chemical composition means the chemistry of a mineral can be expressed as a chemical formula, though the formula may include ranges of substitution where one element can replace another within the structure.
Ordered internal crystal structure means the atoms are arranged in a repeating, three-dimensional pattern. Materials that lack this internal order, such as Obsidian or Amber, are amorphous and are technically mineraloids rather than true minerals.
Understanding this definition helps explain some of the distinctions made throughout the Mineral Vault guides, such as why Obsidian is described as a mineraloid rather than a mineral, and why Amber and Jet are called organic gemstones rather than minerals.
Chemical Formula
The chemical formula of a mineral is a shorthand expression of its chemical composition, showing which elements are present and in what proportions. Reading a mineral formula is straightforward once the basic conventions are understood.
Each element is represented by its chemical symbol: Si for silicon, O for oxygen, Ca for calcium, Al for aluminium, Fe for iron, K for potassium, Na for sodium, and so on. The number following each symbol indicates how many atoms of that element are present in one formula unit of the mineral.
Quartz has the formula SiO₂, meaning one silicon atom bonded to two oxygen atoms. Calcite is CaCO₃, meaning one calcium atom, one carbon atom, and three oxygen atoms. These are among the simpler mineral formulae.
Many minerals have more complex formulae that reflect the substitution of one element for another within the crystal structure. Brackets in a formula indicate a structural site that can be occupied by more than one element. The formula for Tourmaline, for example, contains multiple bracketed groups reflecting the range of elements that can occupy different sites within the structure, which is why Tourmaline occurs in such a wide range of colours and compositions.
A dot followed by H₂O in a formula, as in Gypsum's CaSO₄·2H₂O, indicates water molecules incorporated into the crystal structure rather than chemically bonded into the main framework. These are called hydrated minerals, and the water can often be driven off by heating.
The formula is important because it tells you what elements are present and in what proportions, which in turn explains many of the mineral's physical and optical properties. The copper in Malachite's formula explains its green colour. The lithium in Lepidolite's formula explains its connection to battery technology. The silicon and oxygen in Quartz's formula explain its hardness and chemical stability.
Mineral Group
The mineral group is the broadest level of chemical classification in mineralogy, grouping minerals together based on their dominant anion or anionic complex: the negatively charged ion or group of ions that defines the fundamental chemistry of the group. Think of it as the highest level of organisation in mineralogy, the point where thousands of individual mineral species are sorted into families based on what they are fundamentally made of.
Before we dive into the individual groups it is worth knowing that the group a mineral belongs to often tells you something immediately useful: where it is likely to form geologically, how it tends to behave physically, and sometimes even what colour range to expect. Carbonates, for example, are almost always formed in sedimentary or secondary environments. Sulphides tend to be dense, metallic-looking, and associated with ore deposits. Silicates dominate the Earth's crust so thoroughly that understanding them is essentially understanding the planet itself.
With that in mind, here is a top level view of the principal mineral groups and what defines each one.
Silicates are the largest and most diverse group, comprising approximately 90 percent of the Earth's crust by volume. They are defined by the presence of silicon-oxygen tetrahedra, in which one silicon atom is bonded to four oxygen atoms in a tetrahedral arrangement. The way these tetrahedra link together defines the silicate subgroups. Quartz, Feldspar, Mica, Garnet, Tourmaline, and the vast majority of rock-forming minerals are silicates.
Oxides contain oxygen bonded to one or more metal cations without the presence of other anionic groups. Hematite, Magnetite, Corundum, Rutile, and Spinel are oxide minerals.
Sulphides contain sulphur bonded to one or more metal cations. Pyrite, Galena, Chalcopyrite, Sphalerite, and Cinnabar are sulphides. Many of the most important metallic ore minerals are sulphides, and they are responsible for some of the most vivid and unusual colours in the mineral world.
Carbonates contain the carbonate ion CO₃²⁻ bonded to metal cations. Calcite, Aragonite, Dolomite, Malachite, Azurite, and Rhodochrosite are carbonates. They are abundant in sedimentary rocks and form through both inorganic and biological processes.
Sulphates contain the sulphate ion SO₄²⁻. Gypsum, Barite, Celestine, and Anglesite are sulphates. They form predominantly in evaporitic and oxidised environments.
Halides contain a halogen ion, fluorine, chlorine, bromine, or iodine, as the principal anion. Halite, Fluorite, and Sylvite are halides. Halite is common salt, sodium chloride, and Fluorite is calcium fluoride.
Phosphates contain the phosphate ion PO₄³⁻. Apatite, Turquoise, Lazulite, and Variscite are phosphates. Many vivid blue and green secondary minerals are phosphates.
Native elements are minerals consisting of a single element in its pure or nearly pure form. Gold, Silver, Copper, Diamond, Graphite, and Sulphur are native elements.
Knowing the mineral group immediately tells you something fundamental about a mineral's chemistry and often about its geological setting, physical properties, and behaviour.
Mineral Category
Within each mineral group, minerals are further organised into categories based on more specific structural or chemical characteristics. The silicate group for example is divided into subgroups based on how the silicon-oxygen tetrahedra are connected.
Nesosilicates, also called orthosilicates, have isolated silicon-oxygen tetrahedra with no shared oxygen atoms between them. Garnets, Olivine, Zircon, and Kyanite are nesosilicates. They tend to be dense, hard, and resistant to weathering.
Sorosilicates have pairs of tetrahedra sharing one oxygen atom. Epidote, Zoisite, and Tanzanite are sorosilicates.
Cyclosilicates have tetrahedra arranged in rings. Tourmaline, Beryl, and the Milarite-Osumilite group including Sugilite are cyclosilicates.
Inosilicates have tetrahedra linked in chains. Single chain inosilicates include the Pyroxene group. Double chain inosilicates include the Amphibole group.
Phyllosilicates have tetrahedra linked in sheets. All micas including Muscovite, Lepidolite, and Fuchsite are phyllosilicates, as are Talc, Chlorite, and the clay minerals. The sheet structure is directly responsible for the perfect basal cleavage and elastic tenacity characteristic of micas.
Tectosilicates have tetrahedra linked in a fully connected three-dimensional framework with every oxygen atom shared between two tetrahedra. Quartz, Feldspar, Zeolite, and Sodalite are tectosilicates. This fully connected framework produces the greatest structural stability and chemical resistance of any silicate subgroup.
Understanding the silicate subgroup of a mineral immediately tells you something about its crystal structure, its cleavage characteristics, and often its physical properties.
Specific Gravity
Specific gravity is a measure of the density of a mineral relative to the density of water. Water has a specific gravity of 1. A mineral with a specific gravity of 2.65, such as Quartz, is 2.65 times denser than water. A mineral with a specific gravity of 7.6, such as Galena, is 7.6 times denser.
Specific gravity is determined by the chemical composition of the mineral and the efficiency of atomic packing within the crystal structure. Minerals containing heavy elements such as lead, barium, or iron tend to have high specific gravities. Minerals built primarily from light elements such as silicon, oxygen, aluminium, and sodium tend to have lower specific gravities.
The practical significance of specific gravity for collectors is the heft of a specimen: a high specific gravity mineral feels noticeably heavier than a low specific gravity mineral of the same size. Picking up a piece of Galena or Barite for the first time is often surprising because the weight is so much greater than the visual impression of the size suggests. This heft test is a useful preliminary identification tool and one of the first things experienced collectors notice when handling an unfamiliar specimen.
Specific gravity can be measured precisely using a hydrostatic weighing method, weighing the specimen first in air and then suspended in water and applying Archimedes' principle to calculate the ratio. In practice, for solid specimens without significant porosity or matrix, a comparative heft assessment is often sufficient for a rough estimate.
Specific gravity values quoted in mineral tables are usually given as a range reflecting natural variation in chemistry and purity between specimens of the same species.
The Difference Between Mineral Species, Varieties, and Trade Names
Understanding the distinction between a mineral species, a variety, and a trade name helps make sense of the naming conventions used throughout the Mineral Vault guides and in mineralogy generally.
A mineral species is a formally defined mineral with a specific chemical composition and crystal structure, approved and catalogued by the International Mineralogical Association. There are currently over 5,800 approved mineral species. Quartz, Calcite, and Corundum are mineral species.
A mineral variety is a naturally occurring form of a mineral species distinguished by colour, habit, or some other characteristic that does not constitute a separate species in its own right. Amethyst, Citrine, and Smokey Quartz are all varieties of Quartz, differing only in the trace element or structural defect responsible for their colour. Ruby and Sapphire are both varieties of Corundum, the same aluminium oxide mineral coloured differently by different trace elements. Tanzanite is a variety of Zoisite. Varieties are not formally approved by the International Mineralogical Association in the same way species are, and the boundaries between them can be loosely defined, with no hard chemical or structural line separating one variety from another in the way that distinguishes separate mineral species.
Trade names sit at a further remove from formal mineralogy. They are commercial names given to minerals or varieties for marketing purposes, and they may or may not correspond to any mineralogically meaningful distinction. Caribbean Calcite is a trade name for a specific combination of blue Calcite and brown Aragonite from Pakistan. Larimar is a trade name for blue Pectolite from the Dominican Republic. Ametrine is a trade name for naturally bicoloured Amethyst and Citrine within the same crystal. Trade names are widely used in the collector and gem markets and are not inherently misleading, but understanding that they are commercial rather than scientific designations helps avoid confusion when the same material appears under different names in different contexts, or when a name implies a uniqueness that the underlying mineralogy does not quite support.
How Minerals Are Named
Mineral names follow several conventions that, once understood, make many names immediately informative rather than arbitrary.
The most common source is a person, typically the scientist who first described or discovered the mineral, or a notable figure in the relevant field. Fuchsite honours the German mineralogist Johann Nepomuk von Fuchs. Sugilite honours the Japanese petrologist Ken-ichi Sugi. This is the single most common naming convention in mineralogy and accounts for a large proportion of the more unfamiliar names encountered in collections.
Geography is another frequent source. Tanzanite takes its name from Tanzania, its sole country of origin, following the well-established convention of naming minerals after their type locality. Many minerals carry the names of the specific mines, mountains, or regions where they were first found.
Some names describe a chemical or physical property directly. Hematite derives from the Greek word for blood, a reference to its characteristic red streak rather than its often grey or metallic body colour. Magnetite is named for its magnetic properties, one of the most immediately distinctive physical characteristics in the mineral world. Malachite comes from the Greek for mallow, the plant whose soft leaf-green the mineral resembles.
Other names reference optical or structural characteristics. Mica comes from the Latin micare, meaning to glitter, a fitting description of the sparkle of its cleavage surfaces. Selenite derives from the Greek word for moon, referencing the soft, diffuse luminosity of its finest specimens. Zeolite combines the Greek words for boiling and stone, capturing the mineral's tendency to froth when heated as its structural water is rapidly expelled.
Almost all mineral names end in the suffix -ite, a convention inherited from the Greek and Latin naming traditions of early natural history. The suffix carries no specific meaning beyond signalling that the word is a mineral name, which is why it appears so consistently regardless of what the rest of the name refers to.
Summary
The chemical properties and classification of minerals; the formula, the mineral group, the mineral category, the specific gravity, and the distinction between species, varieties, and trade names are the foundation of mineralogy as a science. They explain why minerals form where they do, why they behave as they do physically and optically, and how the thousands of known mineral species relate to one another within a coherent scientific framework. Together with the physical properties covered in Guide 1 and the optical properties covered in Guide 2, these chemical properties complete the picture of what a mineral is and why understanding it matters.
As always, our inbox and DMs are open if you would like guidance or simply wish to explore further.
Love, Laura
Further Reading
- A Beginner's Guide to Mineral Physical Properties
- A Beginner's Guide to Mineral Optical Properties
- Malachite: From Ancient Egyptian Cosmetics to the Winter Palace
- Lepidolite: Same Lithium as Your Phone Battery
- Green Fuchsite: The Green Crystal That Could Have Been Red
- Tanzanite: The Gemstone Discovered in 1967 That May Run Out Within Your Lifetime
- Azurite: The Mineral That Coloured Medieval Paintings
- Understanding Pleochroism: How Crystal Structure Creates Colour Change in Gemstones
