Factors for Large Crystal Formation in Rocks

Temperature

• Some types of rock cool slowly during formation, and this allows large crystals to develop. In such cases, the individual mineral grains can be seen with the naked eye. Intrusive or "plutonic" igneous rock, one of the coarse-grained rock types, is characterized by these large, slow-cooled crystals, as described by California State Polytechnic University.

Solidification Depth

• Phaneritic rock is another example of an igneous rock type that exhibits large crystals, because it solidifies from its molten state far below the surface of the Earth. Although this is related to slow cooling, the reason is different -- it's the location of the molten rock at the time of formation that leads to giant crystal sizes. Phaneritic rock is one of the so-called "intrusive" rock types.

Length of Time

• Rocks that form over long periods of time deep in the Earth's crust are called "porphyritic" rocks, a subset of the igneous variety. These are characterized by the close proximity of large and small crystals, which relates to different melting temperatures and growth rates. Although the large crystals have formed, some surrounding material may be slower to solidify, which results in "phenocrysts", the large crystals inside porphyritic rocks.

Insulation

• For the crystals within rocks to form into large structures, there needs to be a certain level of insulation around them. This relates to the depth at which the molten magma solidifies underneath the Earth's surface crust, but represents a slightly different factor in their formation. The insulation layer is provided by other rocks and finer sediments, and is more about how tightly packed the material is than how deep it is in the ground.

Pressure

• The pressure must be low enough for the molten, liquid version of the rock to be able to expand at a rate that allows large crystal formation. Pressure often works alongside temperature. As the temperature increases and the rock melts, the pressure reduces, and the expansion begins. As the temperature reduces during the cooling period, the pressure remains low for a short time (the rate of change is less for pressure). As the rocks harden, the coarse grains develop and set.