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Growing crystals are usually not perfect at microscopic scale. Very frequently they entrap tiny cavities with portions of fluid from the crystallization environment, producing primary fluid inclusions. Such inclusions can be formed, for example, upon solid particles of other minerals deposited on the growing crystal surface, when it closes the irregularity caused by solid inclusion.
Areas of appearance of inclusions can also be related to crystal structure defects of the host mineral. In many cases, massive entrapment of primary fluid inclusions will correspond to periods of faster growth of the crystal, produced by certain changes in growth conditions (fast cooling, lowering of pressure or more dissolved components in the fluid).
Concentration of primary fluid inclusions in certain growth zones observed perpendicular to the C axis of faceted Brazilian emerald. Note hexagonal color zoning and concentration of fluid inclusions (darker zones) in certain growth zones.
Immersion, field of view 5 mm.
Two primary two-phase fluid inclusions starting on a growth zone parallel to marked color zoning. Fluorite from China,
field of view 3 mm.
Primary inclusions are very important in geological studies because they contain well preserved samples of fluid responsible for the mineralization. Microthermometric and spectroscopic studies of these inclusions help to reveal chemical composition of mineralization fluids and also pressure and temperature conditions of mineral deposits’ formation.
Two different types of inclusions in natural emerald from the Ural mountains – solid protogenetic inclusions of phlogopite mica and primary two-phase fluid inclusion formed between them. Field of view 0.15 mm.
Another example from the same type of gemstone, again with primary two-phase inclusion associated to a trapped protogenetic mica particle on the right. Note also a solid captured particle inside inclusion cavity, probably mica too (not daughter mineral, but captured crystal). Field of view 0.15 mm.