Inclusions

in minerals, foreign solid, liquid, and gaseous substances, enclosed by minerals during the growing process and trapped inside their crystals. The distribution of inclusions can be random or it can be distinguished by a regular orientation, determined by the crystalline properties of an enclosing mineral. Various minerals have solid inclusions— rutile, mica, ilmenite, and others. These inclusions sometimes endow certain qualities to minerals, which allow them to be used as decorative gems (for example, aventurine is a variety of quartz with inclusions of the tiniest bits of mica that give it a special sparkling shine). The most wide-spread inclusions are those of liquids and gases, which occur in various proportions in the voids of the inclusion and are called gaseous-liquid inclusions. Their liquid phase is a salt solution, usually containing sodium, potassium, chlorine, bicarbonate, and other ions and often containing liquid carbon dioxide. The gaseous phase of inclusions may contain hydrogen, oxygen, nitrogen, hydrogen sulfide, sulfur dioxide, carbon dioxide, and other gases. The size of inclusions varies from microscopic to those visible by an unaided eye. The number of inclusions varies and may reach tens of thousands in 1 mm³.

The study of inclusions helps determine the physical and chemical conditions of processes of mineral formation: temperature (geological thermometry), pressure, the chemical composition of mineral-forming solutions, and other properties. Thus, for example, inclusions of glass show that the mineral was formed from melted magma; the presence of liquid carbon dioxide proves that the mineral grew under high pressure. It is possible to deduce the temperature of formation of inclusions from the temperature to which the mineral must be heated to make the liquid-gaseous inclusions observed under a microscope become homogenous again (homogenization method). This is based on the hypothesis that at the time of the trapping of the inclusion in the body of the mineral, the trapping medium in which the mineral grew was homogenous, but on cooling of the mineral heterogeneity of the trapped inclusion occurred. The increase in pressure within inclusions during heating leads to their cracking (decrepitation). The decrepitation temperature may be registered by various methods, and the temperature at which inclusions form (splitting method or decrepitation) can be determined from it. At the present time methods are being worked out for seeking out hidden ores using inclusions (decreptophonic and dehydration methods).