Final answer:
Gases and volatiles like water vapor and various hydrocarbons significantly affect the crystallization of pegmatites by lowering the melting points, enhancing ion mobility, and increasing mineral solubility, which promotes the growth of large crystals. Feldspar crystallizes early with straight edges, and quartz fills the spaces later on in the process.
Step-by-step explanation:
The crystallization of pegmatites, which are a type of intrusive igneous rock with very large crystals, is greatly influenced by the presence of gases and volatiles. During the crystallization process, these elements intrude into other rocks, allowing individual crystals to grow large due to the slow cooling that occurs deep below the earth's surface. Gases like water vapor and volatile components such as hydrogen sulfide, ammonia, methane, and other hydrocarbons play a critical role in this process. Volatiles, particularly water, decrease the melting point of rocks, enhance the mobility of ions, and increase the solubility of minerals, allowing for the growth of the unusually large crystals that characterize pegmatites.
Feldspar, for instance, which crystallizes at high temperatures, can achieve straight crystal edges due to the high temperature and presence of volatiles early in the cooling process, while quartz tends to crystallize later and fill the remaining spaces. The pegmatite's overall mineral composition and the sequence in which these minerals crystallize can also be affected by the specific volatiles present during the formation.
Overall, without the presence of gases and volatiles, the unique features of pegmatites, such as their large crystal sizes and the variety of minerals present, would not be possible.