Crystals, Minerals and Rocks: Key Geology Concepts Explained

Crystals and Their Internal Order

Crystals are solids whose components have an ordered internal structure. The first step in their formation is an external, often irregular, polyhedral form, such as crystalline marble.

Amorphous Materials

Amorphous materials lack an ordered internal structure. Their components do not follow a fixed internal arrangement, and no particular external organization or internal regularity exists. An example is glass.

Glass

Glass is a polyhedral material that, unlike natural crystalline forms, has a non-crystalline matrix.

Isotropic and Anisotropic Properties

• Isotropic: A mineral whose physical properties, such as hardness or refraction, are equal in all directions.
• Anisotropic: A mineral whose physical properties vary with direction within the material.

Minerals

A mineral is a natural, inorganic solid with a fixed or variable chemical composition within defined limits and a three-dimensional atomic order. Its chemical composition and physical properties are uniform, and they show variations only in some cases. To be considered a mineral, a substance must be a natural solid, must not be produced exclusively by processes linked to its origin, and must be inorganic. It must also have a homogeneous composition and must not be separable into different substances by mechanical means. Its physicochemical properties and fixed chemical composition are defined by its internal crystalline structure.

Pseudomorphism

Pseudomorphism refers to products that retain the external form of a mineral or glass while their internal structure has been altered.

Crystallization and Solidification

Crystallization is the solidification process that results from a series of physicochemical transformations. In these transformations, atoms form lasting bonds that give the solid a rigid form.

Hydrothermal Deposits

Hydrothermal deposits form from the activity of aqueous solutions at temperatures between 100 and 400 degrees Celsius. They are highly variable and can be found under pressure, descending to magmatic chambers at similar temperatures.

Mineral Deposits

A mineral deposit is an aggregate of substances of mineral origin found in the Earth's crust that can be economically exploited.

Rocks

A rock is a natural aggregate of minerals formed under the same process.

Detrital Rocks

Detrital rocks are formed from fragments of other rocks. They are classified as conglomerates, sandstones and clays.

Chemical Rocks

Chemical rocks form through precipitation caused by chemical reactions. Examples include limestone and evaporites.

Organic Rocks

Organic rocks form from the accumulation of remains of living organisms.

Metamorphic Rocks

Metamorphic rocks result from the change of an existing rock into a different form through modification of its structure and original characteristics. This transformation takes place below the Earth's crust.

Plutonic Rocks

Plutonic rocks form when magma crystallizes slowly at depth, as in the case of granite.

Volcanic Rocks

Volcanic rocks form through rapid solidification at the surface of the terrestrial crust, as in the case of basalt.

Filonian Rocks

Filonian rocks form when magma solidifies inside cracks and fractures at intermediate speed, as in the case of porphyry.

Sedimentary Rocks

Sedimentary rocks form through chemical precipitation, evaporation, or the accumulation of organic remains or minerals. Some are associated with carbonatites, alkaline processes with low impregnations, and strongly sulfurous sulfides. A notable example is sulfur, which has long been used in the manufacture of gunpowder.

Ornamental Rocks

Ornamental rocks are used in home decoration and building construction, as well as in other practical works. The existence of certain rocks in a given place offers an opportunity to use them locally. For example, granite is used in Galicia, and slate is abundant in areas such as the Pyrenees and the Alps, and in many traditional constructions.

In the Canary Islands, basalt is widely used thanks to its abundant structural varieties, regardless of its mineralogy, as in the case of Molinillo basalt. Pyroclasts are also important, and deposits of them have been used as cement to make mortar. White fields are characterized by the presence of pumice stone. Rocks from the area are studied for use in ignimbrite, and the blue stone of Yaiza and Arucas is used for construction thanks to its strength.