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basalt

basalt

SRI Petrography ID: 25

Names

[de] Basalt
[en] basalt

Description

Mostly fine grained dark volcanic rock, can contain coarser crystals. Formed from a low-silica (around 50 wt.% silica) lava that is rich in magnesium and iron.
Basalt is a hard, often dense, volcanic rock composed of plagioclase feldspar, pyroxene, biotite and olivine (as well as apatite and magnetite), often having a glassy appearance, contains no quartz.
Basalt can also be vesicular (porous) due to gas bubbles in the lava. These vesicles can be filled with secondary minerals like Quartz, Calcite or some rare Silicates (amygdaloidal basalt).
Molten basalt spreads very rapid and forms big lava fields. Basalt is the principle rock of the ocean floor.
Some basalts are porphyritic, they contain large crystals (“phenocrysts”) embedded in a matrix (groundmass). Phenocrysts (often olivine and pyroxene) crystallize in the magma chamber.

Color

Dark coloured, mostly black or brown

Grain Size

Fine grained, sometimes glassy, sometimes containing phenocrysts

Texture

no texture

Minerals

plagioclase, pyroxene, olivine, biotite, apatite, magnetite;
matrix homogeneous, sometimes phenocrysts (olivine, pyroxene)

Occurences

Iceland, Czech Republic, Western Germany, Italy, Turkey, Eastern Austria

Comments

Gabbros are similar in chemical composition to basalt, but they are well crystallized, coarse-grained rocks.

Classification

Class

IGNEOUS ROCKS

Broader Terms

fine-grained crystalline rocks

Narrower Terms

Austria-basalt

Finland-basalt

France-basalt

Germany-basalt

Italy-basalt