Mineral

“Mineral” means different things depending on who you ask. ^[1] Economists use it for anything pulled from the Earth - coal, oil, natural gas, groundwater, iron ore, gold, sand, gravel, and so on. ^[1] Nutritionists apply it to compounds and elements the body needs. In everyday speech, anything that’s not animal or vegetable gets lumped in as “mineral.” ^[1] The geosciences use a stricter definition than any of these. ^[1] Each word in that definition is doing specific work - ruling out things that everyday language calls a mineral but that don’t actually qualify.

At its core: a mineral is a naturally occurring crystalline solid. ^[1] A more comprehensive definition states that a mineral must be a naturally formed solid characterized by an internally ordered atomic geometry and a specific, though potentially variable, elemental makeup-typically originating via inorganic pathways. ^[2]

Definition

Naturally occurring means the material forms without human involvement, and you need to find samples of it that formed in nature on their own. ^[1] This criterion firmly separates geological products from artificial creations. ^[2] Modern industrial and scientific facilities frequently synthesize exact structural and chemical replicas of natural minerals, creating artificial versions of precious stones like diamonds, rubies, and emeralds. ^[2]

Many crystalline solids produced in laboratories share the same chemistry and structure as their natural counterparts - but those are synthetic minerals, not true minerals. ^[1] For over a century, mineralogy has heavily depended on experimental data derived from these artificial analogs, and researchers conventionally assign these synthetic products the identical names of their natural geological equivalents. ^[2] Although this convention technically conflicts with the rigorous “naturally occurring” criterion, the mineralogical community widely accepts it. ^[2] A lab-grown ruby and a ruby from a metamorphic rock are identical in every measurable way - yet only one is a mineral. Origin matters, not just composition.

Consider a boundary case: concentric layers of CaCO₃ (calcite) occasionally precipitate within municipal water pipes-a natural chemical process occurring inside an artificial environment. ^[2] Since human involvement in its precipitation wasn’t intentional, the majority of mineralogists still classify this material under its proper mineral name, calcite. ^[2] If the process is natural even though the setting is not, the distinction is rarely enforced.

Solid is a straightforward requirement but has implications that aren’t always obvious. ^[2] A glacier’s solid H₂O (ice) qualifies as a mineral, whereas its liquid counterpart does not. ^[2] Similarly, despite existing in geological deposits, native liquid mercury fails a rigorous application of the “solid” criterion and thus is excluded. ^[2] To accommodate these edge cases, naturally occurring materials that mimic mineral chemistry and occurrence but lack a solid or crystalline state are classified as mineraloids, keeping them within the purview of mineralogy. ^[2] The line is formal, not practical - mineralogists study these materials whether they meet the definition or not.

Crystalline means the atoms are locked into a regular, repeating pattern that extends over long distances. ^[1] Having a “highly ordered atomic arrangement” means that constituent atoms or ions are locked into a predictable, repeating geometric framework. ^[2] Since this internal structural regularity defines a crystalline solid, all true minerals must inherently be crystalline. ^[2]

You can see this order at two scales:

- At the hand-specimen scale - symmetrically arranged crystal faces and cleavage planes are the most visible signs.

• At the atomic scale - the ability to diffract X-rays. Only a regular periodic lattice can do this coherently. ^[1]

The bonding that locks atoms into that regular pattern also forces the material to be solid. Under the right temperature and pressure conditions, crystalline solids can still deform plastically - but they remain solid. ^[1]

Solid matter doesn’t guarantee crystallinity. Materials completely devoid of long-range internal atomic order-such as glass-are termed amorphous. ^[2] Glass is rigid but its atoms lack long-range order - that makes it amorphous, not crystalline. ^[1] Examples of naturally occurring amorphous solids include volcanic glass (excluded from mineral standing due to its fluctuating chemistry and structural disorder), limonite (hydrous iron oxide), and allophane (hydrous aluminum silicate). ^[2]

Metamict minerals, including species like microlite, gadolinite, and allanite, represent another non-crystalline category. Their initially ordered lattice has been progressively dismantled over time by ionizing radiation emitted from decaying isotopes incorporated during their formation. ^[2] Alongside structurally disordered liquids like water and native mercury, these radiation-damaged solids are collectively grouped as mineraloids. ^[2]

The line between crystalline and amorphous is usually clear, but not always. ^[1] During biomineralization, nanoparticles (~1 to 100 nm) with only partial atomic order can be the first thing to precipitate - sitting in a gray zone between the two. ^[1] Even in clearly crystalline materials, the order is never flawless - imperfections and disorder are always present. ^[1] There’s no agreed-upon threshold for how much disorder tips a material from crystalline to amorphous, so this gray zone remains genuinely unresolved. ^[1]

Definite chemical composition follows directly from being crystalline. When atoms lock into a repeating lattice, the ratios between elements get constrained. That’s why minerals have a definite - but not necessarily fixed - composition. ^[1] Saying a mineral possesses a definite chemical composition means its elemental makeup can be accurately captured within a specific chemical formula. ^[2] You can write a formula for any mineral. ^[1]

Quartz (SiO₂) is the simplest case - every quartz sample anywhere has silicon and oxygen in a 1:2 ratio, with no exceptions. ^[1] Quartz’s chemical makeup is universally denoted as SiO₂. Given the absolute exclusion of any elements beyond silicon and oxygen in its lattice, this composition is rigidly defined. ^[2] Consequently, mineralogists frequently cite quartz as an example of a pure substance. ^[2]

“Definite” does not mean locked to a single composition, though. Most minerals do not have such well-defined compositions. ^[2] Many species vary in composition within limits. ^[1] Olivine is the standard example - it ranges from iron-rich (Fe₂SiO₄) to magnesium-rich (Mg₂SiO₄), with every intermediate composition in between. ^[1] What varies is only the iron-to-magnesium split. The combined total - iron plus magnesium relative to silicon and oxygen - never changes. The ratio (Fe + Mg):Si:O always works out to 2:1:4, no matter how much iron or magnesium is present. ^[1]

Unlike quartz, dolomite [CaMg(CO₃)₂] rarely exists as a perfectly pure calcium-magnesium carbonate system. Significant quantities of iron and manganese frequently substitute for magnesium within its lattice. ^[2] Given this fluctuating elemental substitution, dolomite’s bulk chemistry spans a defined compositional spectrum rather than holding to one fixed ratio. ^[2] Standard geological notation assigns the pure formula CaMg(CO₃)₂ to represent the theoretical extreme, which mineralogists call an end-member composition. ^[2] To represent a natural dolomite containing mixed divalent cations, the formula expands to Ca(Mg,Fe,Mn)(CO₃)₂, deliberately omitting exact subscripts for the variably mixed Mg, Fe, and Mn sites. ^[2]

Both the idealized end-member and the chemically complex variants perfectly share their fundamental stoichiometric ratios. The pristine formula forces a Ca:Mg:CO₃ ratio of 1:1:2, while the substituted variety simply expands the middle term to maintain an identical 1:(Mg+Fe+Mn):2 structural ratio. ^[2] Ultimately, the underlying crystallographic architecture locks the total atomic ratios firmly in place (they are fixed), despite the fact that the specific elemental occupants occupying those ratio slots can freely alternate (its chemistry remains variable). ^[2]

Distinct specimens belonging to a single mineral species often exhibit differing elemental chemistries-yet structural geometry strictly bounds this variability. ^[1] That bounded variability is what qualifies something as a mineral rather than a random mixture. And because minerals are crystalline with a definite composition, their physical properties - hardness, density, cleavage, color - follow predictably from that composition. ^[1] When composition varies within limits, those properties vary too - within limits, since composition controls them. ^[1] That’s why field identification by physical properties works reliably.

The Inorganic Requirement

Some definitions add a rule that minerals must form by inorganic processes - a restriction rooted in an older philosophical division of matter into animal, vegetable, and mineral categories. ^[1] According to the traditional definition, a mineral is formed by inorganic processes. Historically, strict definitions mandated that an authentic mineral must form exclusively through inorganic mechanisms. Modern definitions soften this by adding the caveat “usually,” thereby granting mineralogical status to biogenic crystalline compounds that successfully meet all structural and chemical criteria. ^[2]

That restriction should have been dropped long ago. Minerals play central roles in biological structures and processes. ^[1] Excluding biologically produced minerals means rejecting large volumes of naturally occurring crystalline solids simply because an organism made them rather than an inorganic fluid.

The evidence is straightforward:

• Calcite and aragonite - both CaCO₃ but with different crystal structures, making them distinct species - get secreted by marine invertebrates to build shells. Their remains form major limestone units throughout the stratigraphic record. ^[1] Vertebrate skeletal systems and teeth owe a massive portion of their structural integrity to the mineral apatite [Ca₅(PO₄)₃(OH, F, Cl)]. ^[1]

Molluscan calcium carbonate precipitation offers perhaps the most famous illustration. Both the protective shell of an oyster and any pearls forming inside consist predominantly of aragonite-crystallographically indistinguishable from aragonite precipitating in purely geological environments. ^[2]

Although several forms of CaCO₃ (calcite, aragonite, vaterite) and monohydrocalcite (CaCO₃·H₂O) are the most common biogenic minerals (meaning “mineral formed by organisms”), many other biogenic species have been recognized. ^[2] Opal (an amorphous form of SiO₂), magnetite (Fe₃O₄), fluorite (CaF₂), several phosphates, some sulfates, Mn-oxides, and pyrite (FeS₂), as well as elemental sulfur, are all examples of minerals that can be precipitated by organisms. ^[2]

Human physiology also relies on intrinsic mineral manufacturing. Hydroxylapatite, Ca₅(PO₄)₃(OH), serves as the primary building block for our skeletal and dental frameworks. ^[2] Pathological mineral precipitation can also occur, yielding solid concretions (calculi) within urinary tracts. These intrusive masses are dominated by biogenic calcium phosphates (including whitlockite and carbonate-apatite), magnesium phosphates, and calcium oxalates-the latter being exceptionally rare outside of biological systems. ^[2]

These are naturally occurring crystalline solids with definite compositions - they are minerals. If you want to flag their origin, call them biominerals. ^[1]

Fossil fuels like coal and petroleum fail the test entirely, despite colloquial labels as “mineral fuels.” While forged by natural planetary processes, their internal structures lack long-range geometric order, and their hydrocarbon chemistries defy precise stoichiometric formulas. ^[2] A geological loophole occurs if extreme thermal metamorphism bakes a coal bed. The intense heat drives away volatile organic fractions and forces the residual carbon to lock into a crystalline lattice, yielding genuine flakes of the mineral graphite. ^[2]

Mineral Nomenclature

A mineral species has a unique combination of chemical composition and crystal structure - both criteria matter, and neither alone is sufficient. ^[1] Calcite and aragonite are the clearest example: same formula (CaCO₃), different structures, different minerals. Over 4,900 species have been identified and named, though fewer than a hundred are at all common in the field. ^[1] New species get discovered regularly, and each must pass criteria set by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association before it gets official status. ^[1]

A mineral variety is a subdivision within a species - usually defined by differences in color, shape, or other visible properties. ^[1] Corundum (Al₂O₃) is normally dull and gray. Its varieties ruby and sapphire look nothing like it - but they are chemically and structurally the same mineral. ^[1] Ruby is red from trace Cr; sapphire is blue from trace Ti + Fe. Both are still corundum - varieties of one species, not separate species. ^[1] Variety naming mostly follows historical convention rather than any formal scheme. ^[1]

A mineral series links two or more minerals across a continuous compositional range. ^[1] The plagioclase feldspar series is the most familiar example. Its end members are albite (NaAlSi₃O₈) and anorthite (CaAl₂Si₂O₈), and most natural plagioclase sits somewhere between the two. ^[1] Sodium and calcium swap places in the plagioclase structure across an unbroken range - a direct example of definite-but-variable composition.

A mineral group shares one crystal structure across members with different compositions. ^[1] The calcite group shows this clearly. Its general formula is XCO₃, where different metals occupy position X - giving you calcite (CaCO₃), magnesite (MgCO₃), rhodochrosite (MnCO₃), siderite (FeCO₃), and smithsonite (ZnCO₃), all sharing the same atomic arrangement. ^[1] Group and series aren’t mutually exclusive. Magnesite and siderite also form a series, because intermediate compositions between them are common in nature. ^[1]

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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