Fluid inclusion

A fluid inclusion is a microscopic bubble of liquid and/or gas that is trapped within a crystal. As minerals often form from a liquid or aqueous medium, tiny bubbles of that liquid can become trapped within the crystal, or along healed crystal fractures. These small inclusions range in size from 0.01 mm to 1 mm and are usually only visible in detail by microscopic study.

Trapped in a time capsule the same size as the diameter of a human hair, the ore-forming liquid in this inclusion was so hot and contained so much dissolved solids that when it cooled, crystals of halite, sylvite, gypsum, and hematite formed. As the samples cooled, the fluid shrank more than the surrounding mineral, and created a vapor bubble. Source: USGS

These inclusions occur in a wide variety of environments. For example, they are found within cementing minerals of sedimentary rocks, in gangue minerals such as quartz or calcite in hydrothermal circulation deposits, in fossil amber, and in deep ice cores from the Greenland and Antarctic ice caps.[1] The inclusions can provide information about the conditions existing during the formation of the enclosing mineral. Fourier transform infrared spectroscopy and Raman spectroscopy can be used to determine the composition of fluid inclusions.