Sulfur and hematite crystals formed around fumaroles during the final stages of volcanic activity, phlogopite and hematite occurring in small druses in igneous rocks, and large and highly perfect single crystals of quartz, beryl, topaz, tourmaline, and other minerals occurring in pegmatite are all formed in supercritical vapor phases concentrated in the final stage of magma solidification. These crystals are all grown in vapor phases in which a chemical reaction (such as oxidation) takes place, or in hydrothermal solution at elevated temperatures. How these crystals grow and how their perfection and homogeneity fluctuate in single crystals will be analyzed using beryl, hematite, and phlogopite as representative examples. The analysis will be made in relation to the size of free space in which the respective crystals grow.

Crystal growth in pegmatite

Since pegmatite in granite produces large and highly perfect idiomorphic crystals, it is a treasure box of colored gemstones suitable for facet cutting, and also of rare mineral specimens containing rare earth elements. Free space on a large scale occurs due to the concentration of volatile components, and crystals can grow freely in such spaces. Since crystal growth proceeds either in a void in a solidifying magma or in a crack in the surrounding strata, at depths < 10km, pressures < 30kbar, and temperatures from 700°C and lower, the conditions range from the supercritical vapor phase to hydrothermal solutions.

The morphology of single crystals, the surface microtopographs of crystal faces, and the inhomogeneities and imperfections in crystals have been investigated in detail for quartz, topaz, and beryl formed in pegmatite.