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TEXTURES OF IGNEOUS ROCKS

Description of textures:

1. Physical appearance:
• - Holocrystalline - every components are crystallized
• - Hipocrystalline – part of the components are “glass-like”, part are crystallized
• - Hyaline texture – it is “glass–like” texture; there are no crystals

2. Grain size:
• - Phaneritic:
• large crystals – 5-30mm
• medium crystals – 5-1mm
• small crystals – 1-0,1mm
• - Aphanitic:
• micro-crystalline – 0,1-0,01mm
• crypto-crystalline – smaller than 0,01 mm

3. The proportion of the crystals sizes:
• - equally-crystalline (f.e. phaneritic)
• - irregularly-crystalline (f.e. porphyritic)

4. The degree of crystallizing the right crystallographic shape:
• - euhedral - well-formed crystals
• - subhedral – some faces are well-formed, some do not become their right crystallographic shape
• - anhedral – no well-formed crystals

5. Spatial arrangement of the components
• - disorderly texture – the components are not regularly arranged
• - direction texture – the components are arranged in a preferred direction:
• 5.1. center texture – the components are arranging around one center
• 5.2. ruler texture – the long components are arranged in one direction
• 5.3. stratum texture – the flat components are arranged in stratums

6. The fulfillment the rock space with the components:
• - compact texture
• - porous texture – there are spaces in the rock with gas

There are four main types of textures of igneous rocks:

• Phaneritic – this texture is found generally in plutonic rocks like gabbro, diorite, granite. The rocks are coarse-grained and crystals can be seen by the naked eye. Magma cools so slowly below the Earth`s surface that the crystals have time to form their right crystallographic shapes. Generally crystals are holocrystalline and euhedral

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• Aphanitic – it is characteristic for volcanic rocks like andesite, basalt, rhyolite. The crystals do not have time to grow because the magma cools suddenly when contacts with the atmosphere. The components are euhedral or subhedral.
• Porphyritic – they are typical for volcanic rocks. In aphanitic groundmass are found some fine-crystalline grains which are called phenocrysts. These grains crystallized before the remainder. Phenocrysts crystallized below the Earth`s surface in constant conditions and the remainder because of sudden cooling the magma do not become the right crystallographic shape. Phenocrysts are commonly euhedral.

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• Glassy – this texture appears when the magma is cooling down immediately. There are no fine-shaped crystals, the groundmass develop as volcanic glass (obsidian, pumice). The volcanic rocks are holocrystalline, hipocrystalline or glassy.