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Classification of inclusions by phase filling
Classification by phase filling is based on the quantity, physical state (solid, liquid or gaseous) and combinations of different phases observed within the inclusion. The term “phase” in natural sciences is used to describe a portion of substance with identical characteristics of physical state, chemical composition and physical properties. This way, for example, two immiscible liquids inside the inclusion are described as two different phases, despite being both presented in the liquid state.
There are inclusions consisting of only one phase and others having more than one phase inside the same cavity. It’s important to emphasize that inclusions are classified on a basis of the phases observed under standard conditions. The vast majority of inclusions presenting different phases at room temperature were trapped as homogeneous fluid at higher temperatures corresponding to the host mineral formation, and later separated in different phases due to cooling of the sample to room temperature. Such inclusions in general are also called “fluid inclusions”. If the sample is cooled or heated the phase filling of fluid inclusions will change, as shown in the following sections.
The same fluid inclusion in emerald from the Ural Mountains, Russia, observed at different temperatures. Note the variations of phase assemblages depending on temperature changes. Unfortunately, this particular inclusion decrepitated at around 250ºC, so the final single phase homogeneous fluid picture is missing in this sequence.
The types of inclusions that we can find in a certain mineral are closely related to its conditions of formation – type of deposit, temperature and pressure of the growth environment. For example, in magmatic deposits and synthetic gems grown from a melt, we can frequently find solid and gaseous inclusions but not cavities containing liquid water phase, typical for hydrothermal and metamorphic deposits. Also, more than one type of inclusions can be frequently found within the same crystal. A very typical combination can be solid inclusions of other minerals together with cavities filled with liquid fluid with gaseous bubble.
Different cases and combinations of phases as inclusions are considered in the following sections of our site, with typical examples of gem and mineral world.
Rutile needle, two phase inclusions and elongated rhombohedron of calcite (lower left corner) as inclusions in quartz.
Field of view 6 mm.