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Two-phase liquid-liquid inclusions
In this case, the inclusion cavity is filled by two liquids, immiscible at room temperature conditions. As for liquid-vapor inclusions, if the sample is heated, homogenization of both liquids in one fluid will occur, corresponding to the primary fluid present during crystallization. But cooling to room temperature makes two liquids separate, forming two different phases at standard observing conditions. The most common combinations of two immiscible liquid phases are water and CO2 or water and liquid hydrocarbons (petroleum).
True primary liquid-liquid inclusions are rare because normally a bubble of gaseous phase is also present within the same cavity (see for example of water-CO2liq-vapor inclusion in Russian emerald, described in the corresponding section).
However, liquid-liquid inclusions can also be formed as a result of “necking down” of larger liquid-liquid-vapor inclusions, when a primary large cavity is separated in two or more different cavities and the bubble of vapor phase is separated to one of them, leaving only two immiscible liquids in other cavities. Necking down is a common phenomenon for fluid inclusions of all types; it produces due to modification of the borders of a primary cavity, with dissolution of the host crystal in some areas and reprecipitation of dissolved material in others. It tends to form more energy-effective negative-crystal shaped cavities with borders corresponding to the faces of the host crystal.
This process, responsible for formation of cavities in the form of negative crystals, can also lead to total separation of one part of a large inclusion from the other by a barrier of reprecipitated material.
Image: Two phase liquid-petroleum inclusion in Pakistani quartz, result of separation of a larger inclusion in smaller parts due to necking down process. Vapor bubble has been isolated in one of the resulting parts of previously united large inclusion.