Mineral facts for kids
A mineral is a naturally occurring solid substance with a specific chemical recipe and an orderly arrangement of its atoms. Think of minerals as the fundamental building blocks of our planet!
Minerals are different from rocks. A rock is usually a mixture of one or more minerals, or even other materials. For example, granite is a rock made of several different minerals like quartz and feldspar. But a mineral, like quartz, is a single, pure substance.
Some natural solids, like opal or obsidian, don't have that perfect, orderly atomic structure. These are called mineraloids, not true minerals. If a chemical compound can form with different crystal structures, each unique structure is considered a different mineral. For instance, quartz and stishovite are both made of silicon dioxide, but they are different minerals because their atoms are arranged differently.
The International Mineralogical Association (IMA) is the official group that names and defines minerals. As of June 2026, they recognize over 6,145 different mineral species!
A mineral's chemical makeup can vary slightly due to tiny amounts of other elements, called impurities. For example, amethyst is a purple type of the mineral quartz. Some minerals can even have different amounts of certain elements that swap places in their structure, like the olivine group, which can be rich in either magnesium or iron.
Scientists describe minerals using many features. These include their shape (called habit), how hard they are (their hardness), how shiny they look (their lustre), how much light passes through them (their diaphaneity), their colour, the colour of their powder (their streak), how they break (their cleavage or fracture), and how heavy they feel for their size (their specific gravity). Some minerals even have special traits like magnetism, fluorescence, or radioactivity.
Minerals are sorted into groups based on their main chemical ingredients. The most common minerals in the Earth's crust are silicate minerals, making up about 90% of it! Other important groups include native elements (like pure gold), sulfides (like galena), oxides (like quartz), halides (like rock salt), carbonates (like calcite), sulfates (like gypsum), and phosphates.
Contents
What Makes a Mineral?
The International Mineralogical Association (IMA) has clear rules for what counts as a mineral.
Natural and Solid
A substance must be found in nature, formed by geological processes on Earth or even other planets. This means things made only by humans or living creatures (like seashells) are usually not minerals. However, if geological processes later change these substances, they might become minerals.
Minerals must also be solid in their natural state. A famous exception is native mercury, which is liquid at room temperature but was classified as a mineral before the rules were set. Water ice is considered a mineral because it's a solid with a crystal structure.
Orderly Structure and Chemistry
Minerals must have a clear, ordered arrangement of their atoms, like a repeating pattern. This gives them properties like a specific shape and hardness. Materials without this order, like obsidian (volcanic glass), are called mineraloids.
Minerals also need a fairly defined chemical recipe. Some minerals can have a range of compositions, where one element can swap places with another. For example, mackinawite can have varying amounts of iron and nickel. Sometimes, minerals with a continuous range of compositions are divided into separate species, like the olivine group, which includes both magnesium-rich and iron-rich types.
Minerals from Living Things
Some scientists believe that minerals formed by living organisms, called biominerals, should also be included. Organisms can create many different minerals, some of which don't form in other ways. For example, tiny microorganisms can help create minerals in rocks and soil. Even though the IMA has strict rules, the study of how life and minerals interact is a growing field!
Rocks, Ores, and Gems: Mineral Connections
A rock is a natural solid material made of one or more minerals or mineraloids. Some rocks, like limestone, are mostly one mineral (calcite). Others, like granite, are a mix of several minerals. Minerals that make up most rocks are called rock-forming minerals, such as quartz, feldspars, and micas.
Industrial minerals are valuable minerals and rocks used in industries, but they are not gemstones or metal ores. For instance, muscovite (a type of mica) can be used in windows or as an insulator.
Ores are minerals that contain a high amount of a specific element, usually a metal, making them valuable to mine. Examples include cinnabar (for mercury) and sphalerite (for zinc).
Gems are minerals prized for their beauty, durability, and rarity. There are about 20 main types of gem minerals, but they come in many varieties. For example, both ruby and sapphire are types of the mineral corundum.
Where Did the Word "Mineral" Come From?
The word "mineral" first appeared in the English language around the 15th century. It comes from a medieval Latin word, minerale, which means "mine" or "ore." The word "species" comes from the Latin word species, meaning "a particular sort or kind."
Mineral Chemistry: Earth's Building Blocks
The types and amounts of minerals we see depend on the chemical elements available in the Earth. Most minerals come from the Earth's crust. Eight elements make up over 98% of the crust by weight: oxygen, silicon, aluminium, iron, magnesium, calcium, sodium, and potassium. Oxygen and silicon are the most important, making up 47% and 28% of the crust, respectively.
Minerals form when elements combine in ways that are most stable at certain temperatures and pressures. For example, in many volcanic rocks, aluminum and alkali metals (sodium and potassium) combine with oxygen, silicon, and calcium to form feldspar minerals. If there's too much of one element, different minerals might form.
Minerals can also have slightly different chemical recipes within the same type. For instance, plagioclase feldspars range from sodium-rich albite to calcium-rich anorthite. This happens because similar-sized atoms can sometimes swap places in the mineral's structure.
How to Identify Minerals: Physical Properties
Identifying minerals can be easy or challenging. Sometimes, a few simple tests are enough. Other times, scientists need special equipment like X-rays. Here are some common physical properties used to identify minerals:
Crystal Structure and Shape
A mineral's crystal structure is the orderly, repeating arrangement of its atoms. This internal structure often shows up in the mineral's external shape, called its crystal habit. Even if the crystals are tiny, their internal structure is always regular.
Minerals are grouped into six main crystal families based on their symmetry:
| Crystal family | Example Minerals |
|---|---|
| Isometric | Garnet, halite, pyrite |
| Tetragonal | Rutile, zircon, andalusite |
| Orthorhombic | Olivine, aragonite, orthopyroxenes |
| Hexagonal | Quartz, calcite, tourmaline |
| Monoclinic | Clinopyroxenes, orthoclase, gypsum |
| Triclinic | Anorthite, albite, kyanite |
Minerals with the same chemical formula but different crystal structures are called polymorphs. For example, diamond and graphite are both made of pure carbon, but their atoms are arranged very differently, giving them vastly different properties. Diamond is super hard and shiny, while graphite is soft and greasy.
Sometimes, two or more crystals of the same mineral can grow together in a special way called twinning. This creates interesting patterns, like the cross-shaped crystals of staurolite.
Crystal habit describes the overall shape of a mineral's crystals. Some are needle-like (acicular), some look like tree branches (dendritic), some are blocky (equant), and some are flat (tabular).
Hardness: How Scratch-Resistant is it?
A mineral's hardness tells us how well it resists scratching. This property depends on its chemical makeup and crystal structure. The most common way to measure hardness is using the Mohs scale, which has ten indicator minerals. A mineral with a higher number can scratch any mineral with a lower number.
Here is the Mohs hardness scale:
| Mohs hardness | Mineral | Chemical formulae |
|---|---|---|
| 1 | Talc | Mg3Si4O10(OH)2 |
| 2 | Gypsum | CaSO4·2H2O |
| 3 | Calcite | CaCO3 |
| 4 | Fluorite | CaF2 |
| 5 | Apatite | Ca5(PO4)3(OH,Cl,F) |
| 6 | Orthoclase | KAlSi3O8 |
| 7 | Quartz | SiO2 |
| 8 | Topaz | Al2SiO4(OH,F)2 |
| 9 | Corundum | Al2O3 |
| 10 | Diamond | C |
A mineral's hardness isn't always the same in every direction. For example, kyanite is softer when scratched in one direction than in another.
Lustre and Diaphaneity: How Light Interacts
Lustre describes how light reflects off a mineral's surface. Minerals can have a shiny metallic lustre, like galena or pyrite. Or they can have non-metallic lustres, such as:
- Adamantine (like a diamond)
- Vitreous (glassy, common in many silicates)
- Pearly (like talc)
- Resinous (like some garnets)
- Silky (common in fibrous minerals)
The diaphaneity of a mineral describes how much light can pass through it.
- Transparent minerals let light pass through clearly, like muscovite mica, which was once used for windows.
- Translucent minerals let some light pass, but you can't see clearly through them, like jade.
- Opaque minerals don't let any light pass through, like hematite or pyrite.
Colour and Streak: Visual Clues
A mineral's colour is often the first thing you notice, but it can be misleading. Some minerals always have the same colour, like green malachite. Others, like corundum, can come in many colours (red ruby and blue sapphire are both corundum!). These colour differences are often due to tiny impurities.
Minerals can also show special colour effects, like a "play of colours" in opal, or a "cat's eye" effect (chatoyancy) in some gems.
The streak of a mineral is the colour of its powder. You can test this by rubbing the mineral on a rough, unglazed porcelain plate (a streak plate). The streak colour is often more reliable than the mineral's body colour because it's not affected by impurities or weathering. For example, hematite can look black or silver, but its streak is always reddish-brown. This test works best for minerals softer than the streak plate (Mohs hardness 7).
Cleavage, Fracture, and Tenacity: How Minerals Break
Minerals have planes of weakness in their crystal structure. When a mineral breaks along these smooth, flat planes, it's called cleavage. The quality of cleavage can be "perfect," "good," "distinct," or "poor." Some minerals, like quartz, have no cleavage because their atomic bonds are equally strong in all directions. Micas, however, have perfect cleavage, meaning they easily split into thin sheets.
Cleavage can occur in one, two, three, four, or six directions. For example, halite (table salt) has cubic cleavage, breaking into perfect cubes.
Parting looks like cleavage but is caused by stress or defects in the crystal, not a fundamental weakness in its atomic structure. It's not as consistent as true cleavage.
When a mineral breaks in a direction that is not a cleavage plane, it's called fracture.
- Conchoidal fracture creates smooth, curved surfaces, like broken glass (common in quartz).
- Other types include fibrous, splintery, or hackly (jagged, like broken metal).
Tenacity describes how a mineral resists breaking or changing shape. Minerals can be brittle (easily broken), ductile (can be drawn into a wire), malleable (can be hammered into sheets), sectile (can be cut with a knife), flexible (bends but doesn't return to original shape), or elastic (bends and returns to original shape).
Specific Gravity: How Heavy is it?
Specific gravity tells us how dense a mineral is compared to water. Most common rock-forming minerals have a specific gravity between 2.5 and 3.5.
Minerals with a high specific gravity feel heavy for their size. This is often a good clue for identification. Minerals containing heavy elements like lead or iron tend to have higher specific gravities. For example, galena (lead sulfide) has a specific gravity of 7.2–7.6, while pure gold can be as high as 19.3!
Other Special Properties
Some minerals have unique properties that help identify them:
- Reaction to Acid: Carbonate minerals (like calcite) will fizz and release carbon dioxide gas when a drop of dilute acid is placed on them. This is a common test for geologists.
- Magnetism: A few minerals, like magnetite, are strongly magnetic and will attract a magnet.
- Taste or Smell: Halite (table salt) tastes salty. Some sulfide minerals can have a sulfurous smell, especially when broken.
- Radioactivity: Minerals containing elements like uranium or thorium, such as uraninite or carnotite, are radioactive.
Classifying Minerals
Scientists classify minerals to organize and understand them better.
Early Ideas
Ancient thinkers like Theophrastus (around 315 BCE) grouped minerals simply as stones, earths, or metals. Later, in 1546, Georgius Agricola created a more detailed system.
Modern Classification Systems
Today, minerals are classified based on their chemistry and crystal structure. The two main systems are the Dana classification and the Strunz classification.
Minerals are grouped into:
- Varieties: Specific types of a mineral species (e.g., amethyst is a variety of quartz).
- Species: Each unique mineral (e.g., quartz).
- Series: A range of compositions between two mineral species (e.g., the olivine series).
- Groups: Mineral species with similar chemistry and crystal structure (e.g., the pyroxene group).
The largest group of minerals by far are the silicates, which make up over 90% of the Earth's crust. They are built from silicon and oxygen, the two most abundant elements. Other important groups, called non-silicates, include:
- Native Elements: Minerals made of a single pure element, like gold, silver, or diamond (pure carbon).
- Sulfides: Minerals where metals are combined with sulfur, like galena (lead sulfide) or pyrite (iron sulfide). Many are important metal ores.
- Oxides: Minerals where metals are combined with oxygen, like hematite (iron oxide) or corundum (aluminum oxide).
- Halides: Minerals where metals are combined with halogens like chlorine or fluorine, such as halite (table salt).
- Carbonates: Minerals containing the carbonate group, like calcite (calcium carbonate), which forms limestone.
- Sulfates: Minerals containing the sulfate group, like gypsum.
- Phosphates: Minerals containing the phosphate group, like those in the apatite group, which are found in bones and teeth.
- Organic Minerals: A very rare group of minerals that contain organic carbon, but are formed by geological processes.
Silicate Minerals: Earth's Most Common
The basic unit of a silicate mineral is a silicon atom surrounded by four oxygen atoms, forming a tetrahedron. These tetrahedra can link together in different ways to create various structures:
- Tectosilicates (Framework Silicates): These have the most linked tetrahedra, forming strong, stable structures. Examples include quartz and the feldspars, which are the most abundant minerals in the Earth's crust.
- Phyllosilicates (Sheet Silicates): These form flat sheets of tetrahedra. They often have perfect cleavage, allowing them to split into thin flakes, like the micas (e.g., muscovite). Some serpentine minerals, like chrysotile, were used as asbestos in building materials because of their strong fibers, but their use is now limited due to health concerns.
- Inosilicates (Chain Silicates): These form long chains of tetrahedra. The pyroxenes are single-chain silicates, and the amphiboles are double-chain silicates. Some amphibole minerals also formed asbestos fibers.
- Cyclosilicates (Ring Silicates): These form rings of tetrahedra. Examples include the colourful tourmaline group and beryl (which includes gemstones like emerald and aquamarine).
- Sorosilicates (Double Tetrahedra): These have two tetrahedra linked together. The epidote group is a common example.
- Orthosilicates (Isolated Tetrahedra): These have single, unlinked tetrahedra. Important examples include the olivine group and the garnet group, which often form blocky, hard crystals.
Astrobiology: Minerals in Space
Scientists are studying minerals to help search for life beyond Earth. Minerals formed by living things (biominerals) could be important clues for finding past or present life on other planets, like Mars.
In January 2014, NASA's Mars rovers, Curiosity and Opportunity, began looking for signs of ancient life and water on Mars. The search for habitable environments and organic carbon (a key ingredient for life) on Mars is a major goal for NASA.
Images for kids
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Mohs Scale versus Absolute Hardness
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Contact twins, as seen in spinel
See also
In Spanish: Mineral para niños
