Minerals

Components of the Geosphere

We define geosphere as the solid part of the planet. Nearly the whole solid components of the planet are minerals or rocks. Although there are some exceptions, such as the soil or the living beings, that are solid substances but are not minerals or rocks. The organic elements related to living things are a part of the Earth’s component called Biosphere.

Minerals and rocks are very important for human beings, not only because they are the main component of the planet, but also because they are the main source for raw materials. Lots of materials, such as the concrete used to build our house, the iron used to make a knife or the silicon used to make a computer come from different minerals or rocks.

Minerals

Definition

Minerals are solid, inorganic, naturally occurring but abiogenic, crystalline substances. They have to be homogeneous and have a representative, constant and fixed chemical composition.

The properties of minerals are inherent to this definition. As a result, all minerals have to satisfy all the criteria of the definition, and no substance can be named a mineral if it does not satisfy any point of the definition.

Let's analyse all the items of the definition.

• Minerals have to be solid substances. So they can't be liquid or gases.
• Minerals have to be inorganic. So they are always simple substances and are never made up of carbon chains.
• Minerals are naturally occurring. This fact means that they can't be produced by human beings. We can produce, for instance, artificial diamonds in a factory using carbon and high
  
  pressure machines, but they are not real minerals.
• Minerals are abiogenic. This means that they can't be made by living beings. Some inorganic substances produced by living things, such as the shells of some animals or  the hydroxyapatite in the bones of vertebrates are very similar to minerals, but they are not real minerals due to their origin.
• Minerals are crystalline. Their atoms must have an orderly repeated pattern. The atoms are, as a result, extremely ordered in tridimensional structures. This extremely ordered structures sometimes leads to the external shape of the mineral, so some minerals have a geometric shape. Although this is not a general rule and many minerals don't have a definite external shape and are amorphous.
• Minerals are homogeneous. This means that a mineral has the same properties and chemical composition in all its points.
• Minerals have definite chemical composition. A mineral must have the same chemical composition, wherever it is originated.

Characteristics

Minerals can be classified according to different characteristics. These characteristics can be divided into two groups: chemical and physical characteristics.

• Chemical characteristics.
• Chemical composition.
• Crystallisation (crystalline structure).
• Physical characteristics.
• Colour and brightness.
• Streak.
• Shape and cleavage.
• Hardness.
• Density.

Chemical composition

The chemical composition of a mineral is related to the chemical components of the mineral and the formulation. Minerals have, by definition, a definite chemical composition. They are always inorganic substances, and usually have a simple formula.

Two minerals can be different and have the same chemical composition. Diamond and graphite, for instance, have the composition: carbon. The final properties of a mineral depend not on their chemical composition, but also on other characteristics, such as the crystalline structure.

Some minerals can also have impurities. Impurities are little amounts of elements or chemical components that are not a part of the regular formula. Impurities can change some properties of the mineral, such as its colour or brightness.

Mineral can be classified according to their chemical composition. There are two big groups, named silicates and non-silicates.

Silicates are minerals with silicon (Si) in their composition. Silicon is, apart from oxygen the most abundant chemical element of the Earth's crust. Due to this, silicates are very abundant and there are many types of silicates. Quartz, muscovite, biotite or olivine are some examples.

Quartz

Olivine

Non-silicates are minerals without silicon in their formula. There are seven groups of non-silicate minerals.

• Native minerals: these minerals have only one chemical element in their composition. There are many examples, such as sulphur, gold or silver.
• Sulphides: these minerals have sulphur linked to metallic or semi-metallic elements. There are many examples, such as pyrite (FeS2), galena (PbS) or cinnabar (HgS).

Sulphur

Pyrite

• Oxides: minerals with oxygen in their composition. Corundum (Al2O3), hematite (Fe2O3) or cuprite (Cu2O) are some examples.
• Halides: minerals with halogen elements (fluorine, chlorine, bromine or iodine) in their composition. Fluorite (CaF2), halite (NaCl) and sylvite are three typical examples.

Corundum

Fluorite

• Carbonates: minerals with CO3 in their formula. The most typical ones are the calcite (CaCO3) and aragonite (also CaCO3).
• Sulphates: minerals with SO4 in their composition. Gypsum (CaSO4), barite (BaSO4) or celestine (SrSO4) are some examples.
• Phosphates: minerals with PO4 in their composition. Fluoroapatite (Ca5(PO4)3F) or hydroxyapatite (Ca5(PO4)3OH) are the most typical examples.

Calcite

Gypsum

Hydroxiapatite

Crystallisation

The crystallisation is the tridimensional distribution of the atoms. The atoms of the minerals are arranged in extremely ordered microscopic structures. The repeated geometric figures that the atoms form and their axis and mirror planes define crystalline structure of the mineral.

According to these geometric characteristics there are seven different crystalline groups: monoclinic, triclinic, orthorhombic, tetragonal, trigonal, hexagonal and cubic.

The crystalline structure is extremely important. Two minerals with the same chemical composition can have different properties because of their crystalline structure. Diamond and graphite, for instance, have the same chemical composition: carbon (they are native minerals). Their properties are, however, different: diamond is the hardest mineral, graphite is so soft that it can be scratched with our fingernail. The main difference between these two minerals is their crystalline structure: diamond 

Colour

The colour of a mineral depends on how it the absorbs or reflects light. Some minerals have only one colour, so we it can be said that the colour is characteristic for that mineral. Most of them, however, can have different colours.

This variability depends on several factors, such as the origin and formation of the mineral (temperature and pressure during the formation of the mineral) or the presence of impurities. We have studied that impurities are chemical components that contaminate the mineral, but are not a part of its regular composition. Some of these chemical components can cause changes in colour even in very low quantities.

Brightness and lustre

These characteristics refer to the quality and quantity of light reflected by the minerals. Minerals can be classified according to their brightness. The most typical types of brightness or lustre are:

• Metallic: typical in mineral with metallic elements.
• Vitreous: similar to the brightness of glass.
• Greasy: like being covered by a film of grease.
• Silky: similar to the brightness of silk.
• Earthy: like being made of sand or soil.
• Adamantine: similar to the brightness of a diamond. 

Silver: metallic brightness

Fluorite: vitreous brightness

Calcite: greasy brightness

Talc: silky brightness

Limonite: earthy brightness

Diamond: adamantine brightness

Streak

It is also called powder colour. It is the colour of the powder when the mineral is dragged across a hard surface, called streak plate. Streak plates are usually made of porcelain. This characteristic is more consistent than colour: although the colour of a mineral can change, its streak is nearly constant. 

The colour of the powder can be different to the colour of the mineral. Nearly all the metallic minerals, for instance, have black streak, although their colour is very variable.

Streak (hematite) by KarlaPanchuk

Shape and cleavage

Cleavage is the tendency of a mineral to split along definite planes. The shape of the mineral depends on these planes and how the mineral split.

The planes and the shape are related to how the atoms of the mineral are ordered. According to their shape, we can classify minerals as basal, cubic, octahedral, prismatic, dodecahedral or rhombohedral.

Hardness

Hardness is defined as the resistance of a mineral to scratching. The reference to measure the hardness of a mineral is a group or list of ten minerals called Mohs scale of mineral hardness.

Mohs scale

The ten minerals that form this scale are ordered and numbered from softest to hardest.

The hardness of a mineral depends on which minerals of the scale can be scratched with this mineral. A mineral can be scratched by a mineral with its same hardness value or higher.

According to this, a mineral with a hardness of 6, for instance, could be scratched by orthoclase (or harder minerals), but it couldn't be scratched by apatite (or softer minerals).

The hardness of a mineral can also be approximately measured using ordinary objects. Minerals with a hardness of 2 or lower can be scratched by our fingernails. Minerals with a hardness of 3 or 4 can’t be scratched by our fingernail and they are hard enough to scratch copper (a coin, for instance). But they can't scratch iron or steel.

Minerals with hardness bigger than 4 can scratch iron and they can scratch steel if their hardness is 6 or bigger (they can scratch the blade of a knife, for instance). Minerals with a hardness lower than 6 can’t scratch a piece of glass. Minerals with hardness bigger than 7 can scratch a piece of porcelain.

Density

Density is defined as the mass (or weight) of a substance per unit of volume.

The density of a mineral can easily be measured weighing the mineral to know its mass and, after that, immersing the mineral in a graduated cylinder with water in order to measure its volume (this shouldn’t be done with soluble minerals).

The density of a mineral is nearly constant, so it tis a good property to distinguish minerals that have other common physical or appearance. 

Classification of minerals

Minerals are usually classified according to their chemical composition. We have already studied the chemical groups of minerals.

They can also be classified according to their crystalline system.

Finally, minerals can also be classified according to their origin (although this classification is more typical for rocks):

• Igneous minerals: they come from molten minerals that solidify.
• Metamorphic minerals: they come from other minerals that change their chemical composition or crystallisation due to high temperature or pressure.
• Sedimentary minerals: they come from other minerals after processes of weathering, erosion and lithification.