Grain

 

Grains are little crystals.

 

 Large crystals are not easy to grow and that’s why some stones are so expensive beyond a certain size. Diamonds can be manufactured for industrial purpose, but they are only small.They are not as expensive as the one sused in jewellery. A crystal takes millions of years to grow up to a significant size. Apart from that, it needs steady conditions of extreme heat and pressure ( like inside the earth). If these requirements are not met, only small crystals will form : grains.

 

That’s why extrusive igneous rocks have fine grains: the rapid cooling doesn’t allow for the growing of the crystals.

 

A large crystal has better mechanical properties than a group of small crystals (of the same mineral) cemented together.

 

 

Grains can often be seen with the naked eye, but a microscope is usually required to analyze a piece of rock.  Grain size is an important criterion  for the identification of stones. Different grain sizes are characteristic of different stones.

The size of the grains will also tell us a lot about the previous history of that stone.

 

 

Grains and metamorphism

 

When a material is subjected to deformation, it becomes strained. The material wants to relieve that strain. If the deformation is caused by other stones, which exert  compression forces, it will be difficult for this material to restore the initial shape, in order to release this strain . There is an analogy with a spring (on Petrograph from Philpots), where the compressed spring stores energy which is released (elastic potential energy, Hooke’s law)  when the spring  restores its initial  position. Our crystal here cannot restore its initial position, so the only way to relieve that energy is to rearrange its atoms , by  forming new chemical bonds (like graphite turning into diamond, at sufficient high pressures and temperatures).This process is called recrystallization. The newly produced unstrained grains will grow and the driving force of this process will be the strain energy that will be liberated.

 

This process is too slow to be studied in stones but it has been  extensively studied in metallurgy.