Minerals can be formed in various ways. The same mineral species can form under completely different conditions. Most minerals require thousands of years to crystallize, others, on the other hand, need only a few years or even a few hours.
The genesis of minerals takes place either in the zone of molten rocks, i.e. in the magma, or at or near the surface of the earth’s crust, or at depth, under the influence of transforming forces, described as metamorphic. The specialist speaks, according to the case, of magmatic, sedimentary or metamorphic genesis.
Many minerals are formed directly from magma. Feldspar, mica and quartz, for example, crystallize during the cooling of magma deep in the earth’s crust, at temperatures of 1100-550°C.
Other minerals are formed from gases exhaled from the magma. These gases, as they cool and react with the surrounding rock, produce minerals containing chlorides, fluorides and sulfates, but also gold and silver.
As the magma continues to cool below 400°C, the formation of minerals occurs from the precipitation of substances and inputs from the surrounding rock. This is how the minerals of the alpine cracks were formed.
Some minerals originate at or near the surface of the earth during the alteration or formation of rocks. The main actors here are water, carbon dioxide and atmospheric oxygen.
These elements dissolve substances in the upper layers of the soil, which then seep into the ground and finally, through the action of groundwater, lead to the formation of minerals in enrichment areas, such as silver and copper deposits.
In both dry and warm regions, due to a high evaporation rate, evaporites are formed by chemical precipitation in lakes and salt marshes or isolated inlets.
Many organisms also take part, directly or indirectly, in the formation of minerals, for example by oxygen supply or carbonic acid removal, by putrefaction processes, or by the production of calcareous shells and siliceous skeletons secreted from elements in solution.
When rocks are buried at great depths in the earth’s crust as a result of progressive burial or orogenic processes, new minerals are formed by transformation of pre-existing minerals under the effect of high temperatures associated with high pressures.
A comparable metamorphic transformation, although much more limited in space, occurs when liquid and incandescent magma, penetrating in chimneys or along cracks, comes into contact with the surrounding rock.
Due to similar or very similar genetic processes, many minerals are found grouped in characteristic associations called paragenesis. On the other hand, some minerals are mutually exclusive. The knowledge of paragenesis is a precious help for the research or the identification of minerals, especially in geology.
For example, barite, fluorite and galena are always found together in some rocks. And conversely, feldspars and rock salt will never be found on the same group of crystals.