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Two-phase solid-gaseous inclusions
In this case, when observed at room temperature conditions, we actually see solidified glass with gas bubble; it is not fluid inclusion anymore, but it was in the moment of entrapment.
Such inclusions are typical for synthetic materials crystallized by flux method (emerald, ruby, sapphire, alexandrite, spinel). In this process, a molten substance (flux) is used as a solvent for the components of a mineral to crystallize and as a medium for crystallization. When portions of flux are trapped by the growing crystal, they are liquid, but subsequent cooling to the room temperature makes them solidify and contract, forming a tiny vacuum bubble within each cavity. As a result we observe inclusions that can be confused with two-phase liquid-vapor inclusions typical for natural gems.
Two-phase solid-gaseous inclusion in Russian synthetic flux emerald. Field of view 0.15 mm.
The same process takes place in some treated gemstones. For example, in heat treated corundums (rubies and sapphires), when heating is performed in presence of borax or other substances used as flux with the aim to “heal” the fractures present in natural crystals. These substances melt, enter in open fractures and act as a solvent to the host crystal causing its recrystallization and “healing” of fissures.
Of course, part of flux material is trapped within the fissures too and, when cooled, form solid inclusions with bubbles, the same way as flux synthetics described before. Actually, the appearance of the veils of inclusions in heat treated corundums can be very similar to those in synthetic flux-grown crystals.
Two phase solid-gaseous inclusions in heat-treated green sapphire. Field of view 2 mm.
In emeralds treated to fill their fractures and cavities with artificial resins, in some cases trapped gas bubble can also be seen, like in the image below, where solidified resin fills up a large cavity, probably formed by dissolution of a mineral inclusion (pyrite or calcite), and a large bubble can be clearly seen in the filling material.
Artificial resin with gas bubble filling very large cavity in natural emerald from Colombia, as a result of treatment to improve the clarity of emerald. Field of view 5 mm. Photo Juan Cozar.
Finally, two phase solid-gaseous inclusions can also be formed as a result of heat treatment and subsequent melting of solid inclusions present in the gem. Rubies and sapphires can stand heat treatment at very high temperatures. The melting point of corundum is 2044ºC and heat treatment can be conducted at up to 1800ºC. When the melting point of minerals included in heat-treated stones is lower than the heating temperature, they will melt and in some cases decrepitate forming circular halos filled with solidified glass. In other cases, melted and solidified material will stay within the same cavity of the initial solid inclusion. Anyway, as a result, two-phase solid-vapor inclusions of solidified glass with bubbles can be formed.
Molten solid inclusion in natural heat-treated orange sapphire, resulting in glass inclusions with gas bubbles filling cavities of initial mineral inclusions. Field of view 5 mm.