5.5 Formation of Minerals

In order for a mineral crystal to grow the following are required:

  • The elements needed to make it must be present in the appropriate proportions.
  • The physical and chemical conditions must be favourable.
  • There must be sufficient time for the atoms to become arranged.

Physical and chemical conditions include factors such as temperature, pressure, amount of oxygen available, pH, and the presence of water. The presence of water makes it easier for ions to move to where there are needed, and can lead to the formation of larger crystals over shorter time periods. Time is one of the most important factors because it takes time for atoms to line themselves up into an orderly structure. If time is limited, the mineral grains will remain very small.

Most of the minerals that make up the rocks in the crust and mantle formed through the cooling of molten rock, known as magma. At the high temperatures that exist deep within Earth, some geological materials are liquid. As magma rises up through the crust, either by volcanic eruption or by more gradual processes, it cools and minerals crystallize. If the cooling process is rapid (minutes, hours, days, or years), the components of the minerals will not have time to become ordered and only small crystals can form before the rock becomes solid. The resulting rock will be fine-grained (i.e., crystals less than 1 mm). If the cooling is slow (from decades to millions of years), the degree of ordering will be higher and relatively large crystals will form. In some cases, the cooling will be so fast (seconds) that the texture will be glassy, which means that no crystals at all form. Volcanic glass is not composed of minerals because the magma has cooled too rapidly for crystals to grow, although over time (millions of years) the volcanic glass may crystallize into various silicate minerals.

Minerals can also form in several other ways:

  • Precipitation from aqueous solution (e.g., from hot water flowing underground, from evaporation of a lake or inland sea, or in some cases, directly from seawater)
  • Precipitation from gas (e.g., from vents releasing volcanic gases)
  • Metamorphism — formation of new minerals directly from the elements within existing minerals under conditions of elevated temperature and pressure
  • Weathering — during which minerals unstable at Earth’s surface may be altered to other minerals
  • Organic formation — formation of minerals within shells (primarily calcite) and teeth and bones (primarily apatite) by organisms (these organically formed minerals are still called minerals because they can also form inorganically)

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