What is an inclusion in geology ?

Inclusion : definition

In mineralogy / gemology, an inclusion is a solid, a liquid, or a gas, trapped inside a mineral. These inclusions can give very interesting information on the origin or the conditions of formation of a mineral or a gem. When the trapped element is a fluid and / or a gas, one speaks of “fluid inclusions”. When the inclusion has several phases (a liquid and a gas, or a solid, a liquid and a gas) we speak of polyphase inclusions. If the inclusion has 2 distinct phases, we speak of two-phase inclusion, if it has 3, we speak of three-phase inclusion.

A fluid inclusion is a cavity of often microscopic size (1 to 100 µm on average) within a mineral, but which can be exceptionally pluricentimetric, as in the halite crystal presented below or in the so-called quartz and amethysts "enhydro" or "bubble". These cavities can have the external shape of the host mineral, this is called a negative crystal. Fluid phases are usually salt water with CO2, sometimes H2S) sometimes accompanied by one or more solid phases. There are primary and secondary fluid inclusions, the former being contemporaneous with the crystallization of the host mineral while the latter are formed by healing of late fractures (of tectonic or other origin).
2 phase-inclusion in an halite
(CO2 + liquid)
3 phase-inclusion in a colombian emerald
(halite + liquid + CO2)
2 phase negative crystal inclusion in quartz
(liquid + CO2)