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Three-phase inclusions are normally formed when homogeneous fluid trapped into a cavity separates in three different phases due to cooling to the room temperature. Most common examples are liquid-solid-vapor inclusions and liquid-liquid-vapor inclusions containing two immiscible liquids. Of course, captured phases may also be present in three-phase inclusions, as discussed in the “Two phase liquid-solid inclusions” section.
Three-phase liquid-solid-vapor inclusions are very familiar to gemologists as typical for Colombian emeralds. In this case, primary fluid is saturated in sodium chloride, so when emerald crystal is cooled down from the temperature of its formation (290-360ºC, according to Cheilletz et al., 1994) to the room temperature, a cubic crystal of NaCl is formed within the inclusion cavity, a so called “daughter mineral”, precipitated from the solution trapped as fluid inclusion. Aqueous fluid also separates in two phases due to cooling – liquid and vapor phases, according to the process described in the two-phase liquid-vapor inclusions section. When such inclusions are heated, complete homogenization of all three phases can be observed as a result of dissolution of the salt crystal and disappearance of the vapor bubble.
Three phase liquid-solid-vapor inclusion, typical for Colombian emeralds. Field of view 0.2 mm.
There is also another very common case of three-phase inclusions, those containing two immiscible liquid phases and a vapor bubble. Two immiscible liquids most frequently found in inclusions are water and CO2, but inclusions with water and liquid carbon hydrates (petroleum) are also quite common. Same as for other types of fluid inclusions, in the moment of entrapment the fluid is homogeneous, but it separates in three different phases due to cooling to room temperature. Heating up to the homogenization temperature will again produce single phase fluid corresponding to that trapped during crystal growth.
Three phase inclusion with water, liquid CO2 and vapor phases in emerald from the Urals, Russia. Data of typical volume percentages of each phase, microthermometry data, P-T conditions and fluid salinity are also shown in the figure.
Field of view 0.15 mm.
Liquid petroleum contained in fluid inclusions can be easily detected using UV light. In contrast, liquid CO2 present in many inclusions sometimes is more difficult to see. The density of liquid CO2 is less than that of water so it is always concentrated close to the gas bubble, forming an additional phase between water and vapor. If the quantity of liquid CO2 is significant, one can easily see a double rim around the gas bubble, sometimes in the form of a crescent moon. In contrast, a small amount of liquid CO2 can be difficult to observe; sometimes it only makes the rim between water and vapor look thicker and better marked. Cooling of such inclusions will increase the amount of visible liquid CO2, while slight heating to more than 31.1Cº will always lead to homogenization of liquid and vapor CO2, transforming the inclusion to two-phase liquid-vapor type.
Video footage of microthermometric study of fluid inclusion in emerald from the Urals, Russia.