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What are glassy, crystalline and holocrystalline?

When the high temperature magma cools and forms solid rock, if the cooling time is very short and allows no time at all for the atoms to form orderly patterns (i.e. according to the crystalline structure), micronized particles will aggregate irregularly and form a glass-like texture, namely “glassy”. When it happens, it is impossible to identify any mineral using microscope or any other instrument.

If the cooling period is relatively longer, which allows atoms to form orderly patterns but not to complete the formation of crystal, the rock is called “crystalline”. When it happens, the minerals thereof cannot be identified using eyes or low-magnification microscope, but can be seen using high-precision high-magnification microscope or X-ray diffractometer.

If the cooling period is long enough to let the atoms form orderly patterns and mineral crystals fully grow, the rock then has minerals with large particle size and crystal shape, which can be seen by eyes or low-magnification microscope. In this case, the rock is known as “holocrystalline”.

With respect to the said three different types of structure, below are some rock examples:

Glassy: It is often seen in ordinary ocean floor basalt, where the magma quickly cooled down due to its contact with seawater.

Crystalline: Penghu basalt, where the magma exploded out of the ground. Although it took slightly longer time to cool down, it was still considered as a quick cooling process. Therefore, the minerals thereof have small particles.

Holocrystalline: Kinmen granite, where the particles of mineral are relatively coarser. The reason is that the magma slowly cooled down under the ground, offering minerals sufficient time to grow.

Note: These three rocks are classic examples. However, some igneous rocks can have a mixed structure – it totally depends on the temperature of where the magma existed.

For more information, please go to the “Taiwan Geoscience Portal” website.