Characteristics of minerals
A mineral is a substance that usually
- is an inorganic solid. (elemental mercury is an exception)p184
- has a definite chemical make-up
- usually has a crystal structure; some do not
- is formed naturally by geological processes
One recent definition is:
- "A mineral is a homogeneous naturally occurring substance with a definite but not necessarily fixed chemical composition. Most minerals are solids with an ordered atomic arrangement, and most are inorganic in the chemical sense of that word".
Alternatively, a mineral is one listed as such by the International Mineralogical Association.
Minerals and rocks
A rock has only two of the characteristics minerals have–it is a solid and it forms naturally. A rock usually contains two or more types of minerals. Two samples of the same type of rock may have different kinds of minerals in them. Minerals are always made up of the same materials in nearly the same proportions. A ruby is a mineral. Therefore, a ruby found in India has similar makeup as a ruby found in Australia.
Formed in nature
Minerals are formed by natural processes. A few substances with the same chemical composition as minerals can be produced by living creatures as part of their shells or bones. The shells of molluscs are composed of either calcite or aragonite, or both.
Traditionally, chemicals produced by living things are not considered minerals. However, it is difficult to see why an organic substance should not be called a mineral if its chemical nature and its crystalline structure is identical with its inorganic twin. This issue is now under debate: see Railsback part II.
Minerals form in many ways. The mineral halite, which is used as table salt, forms when water evaporates in a hot, shallow part of the ocean, leaving behind the salt it contained. Many types of minerals are made when molten rock, or magma cools and turns into a solid. Talc, a mineral that can be used to make baby powder, forms deep in Earth as high pressure and temperature causes changes in solid rock.
The extraordinary thing is, that most minerals owe their formation to life, or at least to the Great Oxygenation Event. "Sturdy minerals rather than fragile organic remains may provide the most robust and lasting signs of biology".
A mineral is a solid—that is, it has a definite volume and a rough shape. Volume refers to the amount of space an object takes up. For example a golf ball has a smaller volume than a baseball, and a baseball has a smaller volume than a basketball.
A substance that is a liquid or a gas is not a mineral. However, in some cases its solid form is a mineral. For instance, liquid water is not a mineral, but ice is.
These properties are used most often:
- Crystal structure: see below
- Hardness: on the Mohs scale, a ten-point scale running from the softest, talc to the hardest, diamond.
- Lustre: appearance in light
- Streak: colour of a mineral when it has been ground to a fine powder. Often tested by rubbing the specimen on an unglazed plate.
- Cleavage: how mineral splits along various planes
- Fracture: how it breaks against its natural cleavage planes
- Specific gravity: density compared with water
- Any other properties
The particles of ice that make up frost have smooth, flat surfaces. These flat surfaces form because of the arrangement of atoms in the ice, which is a mineral. Such an internal arrangement is a characteristic of minerals. It is the structure of a crystal, a solid in which the atoms are arranged in an orderly, repeating three-dimensional pattern.
Each mineral has its own type of crystal structure. In some cases, two minerals have the same chemical composition but different crystal structures. For example, both diamond and graphite consist of just one element—carbon. But the arrangements of the carbon atoms in these two minerals are not the same, so they have different crystal structures and very different properties. Diamonds are extremely hard and have a brilliant sparkle. Graphite is soft, gray, and dull.
In nature, a perfect crystal is rare. Most crystals have imperfect shapes because their growth was limited by other crystals forming next to them.
Definite chemical makeup
Each mineral has a definite chemical makeup: it consists of a specific combination of atoms of certain elements. An element is a substance that contains only one type of atom.
Scientists can classify minerals into groups on the basis of their chemical makeup. Though there are thousands of different minerals, only about 30 are common in Earth's crust. These 30 minerals make up most rocks in the crust. For that reason, they are called rock-forming minerals.
- Silicates are most common group. All the minerals in this group contain oxygen and silicon—the two most common elements in Earth's crust—joined together. Silicates may include other elements such as aluminium, magnesium, iron and calcium. Quartz, feldspar, and mica are common silicates.
- Carbonates are the second most common group of rock-forming minerals is the carbonates. All the minerals in this group contain carbon and oxygen joined together. Calcite, which is common in seashells, is a carbonate mineral.
- Oxides include the minerals from which most metals, such as tin and copper, are refined. An oxide consists of an element, usually a metal, joined to oxygen. This group includes haematite, a source of iron.
- Sulphates contain the sulphate group SO4. Sulphates commonly form in evaporites where highly salty waters slowly evaporate, allowing sulfates and halides to precipitate where the water evaporates. Sulphates also occur where hot waters are forced through the rock, as with geysers.
There are many other mineral groups.
Some uses of minerals
People use minerals for many everyday purposes. Every time people turn on a microwave oven or a TV, minerals are being used. The copper in the wires that carry electricity to the machine is made from a mineral. Table salt or halite, is another mineral that people use in their everyday life.
- Graphite is used to make pencils
- Rock salt is used in cooking
- Mineral ores are the source of metals.
When minerals react, the products will sometimes assume the shape of the reagent; the product mineral is termed to be a pseudomorph of (or after) the reagent. Illustrated here is a pseudomorph of kaolinite after orthoclase. Here, the pseudomorph preserved the Carlsbad twinning common in orthoclase.
Topaz has a characteristic orthorhombic elongated crystal shape.
Pyrite has a metallic lustre.
Mineral for Kids. Kiddle Encyclopedia.