The term "mineral" is precise. To be classified as a mineral, a substance must meet five specific criteria.
Definition of a Mineral: A mineral is a...
- Naturally Occurring: It must be formed by natural geological processes, not man-made (e.g., synthetic diamond is not a mineral).
- Inorganic: It must not be formed from living organisms or their remains (e.g., coal, which comes from plants, is not a mineral).
- Solid: It must be a solid substance at standard Earth surface temperatures (e.g., liquid water is not a mineral, but ice is).
- Definite Chemical Composition: Its chemical makeup is fixed or varies within a specific, limited range (e.g., Quartz is always SiO₂, Halite is always NaCl).
- Ordered Internal Structure: The atoms within it are arranged in a repeating, orderly, three-dimensional pattern. This is what makes it crystalline.
Substances that have a chemical composition but lack an ordered internal structure (like volcanic glass) are called mineraloids.
Minerals can be broadly grouped based on their abundance and economic value.
These are the minerals that make up the vast majority (over 90%) of the Earth's crust. They are the common "building blocks" of most rocks.
These are minerals from which a metal or other valuable element can be extracted profitably. The key distinction is economic viability.
| Ore Mineral | Chemical Formula | Element Extracted |
|---|---|---|
| Hematite | Fe₂O₃ | Iron (Fe) |
| Galena | PbS | Lead (Pb) |
| Bauxite | (An aggregate, primarily Al oxides) | Aluminum (Al) |
| Chalcopyrite | CuFeS₂ | Copper (Cu) |
| Sphalerite | ZnS | Zinc (Zn) |
Minerals are classified into groups based on their dominant anion (negatively charged ion) or anionic group (a complex ion like (CO₃)²⁻). This is the most widely used chemical classification system.
| Mineral Class | Dominant Anion / Group | Examples |
|---|---|---|
| Native Elements | Single elements (e.g., C, S, Au) | Gold (Au), Silver (Ag), Copper (Cu), Sulfur (S), Graphite (C), Diamond (C) |
| Sulfides | Sulfide ion (S²⁻) | Galena (PbS), Pyrite (FeS₂), Sphalerite (ZnS) |
| Oxides | Oxygen ion (O²⁻) | Hematite (Fe₂O₃), Magnetite (Fe₃O₄), Corundum (Al₂O₃) |
| Hydroxides | Hydroxyl ion (OH⁻) | Goethite (FeO(OH)), Bauxite (Al-hydroxides) |
| Halides | Halogen ions (Cl⁻, F⁻, Br⁻) | Halite (NaCl), Fluorite (CaF₂) |
| Carbonates | Carbonate group (CO₃)²⁻ | Calcite (CaCO₃), Dolomite (CaMg(CO₃)₂), Malachite (Cu₂(CO₃)(OH)₂) |
| Sulfates | Sulfate group (SO₄)²⁻ | Gypsum (CaSO₄·2H₂O), Barite (BaSO₄) |
| Phosphates | Phosphate group (PO₄)³⁻ | Apatite (Ca₅(PO₄)₃(F,Cl,OH)) |
| Silicates | Silicate group (SiO₄)⁴⁻ | Quartz, Feldspar, Mica, Olivine (The largest and most important class) |
Physical properties are the primary tools used to identify minerals in the field (hand-specimen identification). These properties are a direct expression of the mineral's chemical composition and internal crystalline structure.
The first thing you notice, but often the least reliable property.
The color of a mineral's powder when scraped against an unglazed porcelain plate (a "streak plate").
The way a mineral's surface reflects light. This is independent of color.
A mineral's resistance to being scratched. This is measured using Mohs' Scale (see next section).
The characteristic external shape a mineral takes if it grows unimpeded. This is the outward expression of its internal atomic arrangement.
The tendency of a mineral to break along flat, parallel planes of weakness in its crystal structure.
How a mineral breaks when it does not break along cleavage planes.
The ratio of a mineral's density to the density of water (i.e., how "heavy" it feels for its size).
Developed by Friedrich Mohs, this is a relative scale of hardness from 1 (softest) to 10 (hardest). It's not a linear scale; the difference in hardness between 9 (Corundum) and 10 (Diamond) is greater than the difference between 1 and 9.
The test is simple: a harder mineral will scratch a softer one.
| Hardness (H) | Mineral | Common Field Test |
|---|---|---|
| 1 | Talc | Can be scratched easily by a fingernail (H ≈ 2.5) |
| 2 | Gypsum | |
| 3 | Calcite | A copper penny (H ≈ 3.5) will scratch it. |
| 4 | Fluorite | |
| 5 | Apatite | A steel knife or glass plate (H ≈ 5.5) will scratch it. |
| 6 | Orthoclase (Feldspar) | |
| 7 | Quartz | Will scratch glass and steel. (H ≈ 7+) |
| 8 | Topaz | |
| 9 | Corundum | |
| 10 | Diamond | Scratches all other substances. |
Mnemonic for Mohs' Scale:
"Tall Geologists Can Find All Old Quartz Tombs, Caves, Diamonds."
(Talc, Gypsum, Calcite, Fluorite, Apatite, Orthoclase, Quartz, Topaz, Corundum, Diamond)
Silicate minerals are the most important and abundant group of minerals, making up ~95% of the Earth's crust.
Their fundamental building block is the Silicon-Oxygen Tetrahedron (also called the silica tetrahedron). This structure consists of one small central Silicon atom (Si⁴⁺) bonded to four larger Oxygen atoms (O²⁻), forming a pyramid shape.
Formula: (SiO₄)⁴⁻
This basic unit has a net charge of -4, which must be balanced by bonding with positive ions (cations) like Mg²⁺, Fe²⁺, K⁺, etc., or by sharing oxygen atoms with other tetrahedra.
The classification of silicates is based on how these tetrahedra are linked together, a process called polymerization. This linkage involves sharing one or more oxygen atoms between adjacent tetrahedra.
| Class | Structure (Polymerization) | Si:O Ratio | Examples |
|---|---|---|---|
| Nesosilicates (Isolated Tetrahedra) |
Independent (SiO₄)⁴⁻ tetrahedra are bonded only by cations. No shared oxygens. | 1:4 | Olivine, Garnet |
| Sorosilicates (Double Tetrahedra) |
Two tetrahedra share one oxygen atom. Forms (Si₂O₇)⁶⁻ group. | 2:7 | Epidote |
| Cyclosilicates (Ring Structures) |
Tetrahedra share two oxygens each, forming closed rings (e.g., (Si₆O₁₈)¹²⁻). | 1:3 | Beryl, Tourmaline |
| Inosilicates (Chain Structures) |
Single Chains: Share two oxygens. (SiO₃)²⁻ Double Chains: Share 2 and 3 oxygens alternately. (Si₄O₁₁)⁶⁻ |
1:3 4:11 |
Pyroxene Group (e.g., Augite) Amphibole Group (e.g., Hornblende) |
| Phyllosilicates (Sheet Structures) |
Tetrahedra share three oxygen atoms, forming continuous 2D sheets. (Si₂O₅)²⁻ | 2:5 | Mica Group (Muscovite, Biotite), Clay Minerals (Kaolinite) |
| Tectosilicates (Framework Structures) |
All four oxygen atoms are shared, forming a continuous 3D framework. | 1:2 | Quartz (SiO₂), Feldspar Group (Orthoclase, Plagioclase) |
Iso = "same", Morph = "form"
Isomorphism describes a relationship where two or more different minerals have different chemical compositions but the same crystal structure.
Poly = "many", Morph = "form"
Polymorphism is when a single chemical substance (same composition) can exist in two or more different crystal structures. These different structures are called polymorphs.
Pseudo = "false", Morph = "form"
A pseudomorph is a mineral that has the outward crystal form (shape) of one mineral, but is actually composed of a different mineral that has replaced it. It's a "false form."
A solid solution refers to a range of chemical compositions in a mineral, resulting from the substitutional replacement of one type of ion for another in the crystal lattice.
Instead of a fixed formula (like Quartz, SiO₂), minerals in a solid solution series have a variable formula written with parentheses.
Here, a coupled substitution of (Na⁺ + Si⁴⁺) is replaced by (Ca²⁺ + Al³⁺). The size is similar, and the total charge (+5) is balanced in both cases.
Exsolution is the "un-mixing" of a solid solution that was stable at high temperatures, but becomes unstable as it cools down. The single mineral separates into two or more distinct mineral phases.