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Two-phase liquid-vapor inclusions
This is for sure the most common type of fluid inclusions, widely represented in minerals and gems. Many crystals are formed from aqueous fluids, at temperatures much higher than our observation conditions. If the sample is heated to what is called homogenization temperature, we can observe again only one single phase, corresponding to the primary fluid. The temperature of homogenization corresponds to the minimum temperature of crystal formation, representing valuable data for mineralogical studies.
Groups of two-phase liquid-vapor inclusions in fluorite from China. Field of view 3 mm.
Not all two phase inclusions are initially trapped as homogeneous fluid. One particular case when liquid and vapor phases may coexist in the primary fluid is when boiling naturally occurs during mineralization. One common scenario to cause boiling is an opening of closed fractures and consequent lowering of hydrostatic pressure in the system, leading to boiling with no need of additional heating. Moreover, lowering of pressure can cause faster crystal growth (and entrapment of numerous inclusions) because the solubility of many minerals increases with pressure.
Two phase water-vapor inclusions of this type can be large and sometimes even observed by the naked eye. Such samples are rare and valuable collectibles, commercially called “enhydro”. Many samples of rock crystal and amethyst with large aqueous inclusions and moving bubble come from Brandberg, Namibia, but also can be found in other deposits.
Large two-phase liquid-vapor inclusion with moving bubble from Brandberg, Namibia. Field of view 10 mm.
The liquid phase in this type inclusion is not always represented by water. Two phase liquid-vapor CO2 inclusions are common in Sri Lankan sapphires, sometimes also with some additional solid phases within the same cavities. Gentle heating of such inclusions to only 31.1ºC is enough to lead to their homogenization to liquid phase (see Photoatlas of inclusions in gemstones, E. J. Gubelin and J. I. Koivula, Volume 1, pp. 78-79.)