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Igneous rock: Pumice
How do minerals form?
Minerals occur when particles of the same substance are joined together. Often, this takes place in a dissolved state, or when molten rock (e.g. magma) is cooling from a high temperature. Often a regular pattern is formed, a Crystal structure' target='_parent'>crystal structure, and we speak of crystallization. However, some minerals can also occur amorphous.
The crystallization of minerals can occur from molten magma. If the temperature falls in magma then there will be minerals formed in the liquid magma. With a slow decrease in temperature, molecules of minerals can join to the Crystal structure' target='_parent'>crystal structure of already existing crystals. The slower the decrease in temperature, the larger the crystals can grow. For example, if you find a granite with large crystals of Quartz and Feldspar, then this cooled slower than a fine granite. When molten rock cools quickly, a homogeneous mass is created.
Minerals can also be formed by precipitation of ions, which are dissolved in water. This can happen, for example, between the grains of a sediment so that it hardens. Cavities can be filled by precipitation of minerals. If such a cavity partially filled, a nice crystal geode can be formed in which nice crystals can be seen.
If a mineral grows, crystal forms occur. Crystal forms give information about the way in which the atoms and molecules are arranged in the mineral. The structure of each crystal is characteristic of a particular mineral . Most of the crystals grow against each other because there is no room for free growth. Only if minerals have the space (e.g. in cavities), they can form beautiful exposed crystals. In that case, the shape of the crystal reflects the internal chemical and molecular composition of the mineral. The crystal is surrounded by symmetrically shaped crystal faces. By measuring the angle between the different planes of the crystals, information about the internal structure of the mineral Crystal structure' target='_parent'>crystal structure can be found.
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