Topic summary
Picritic rock
Extracted from the Wikipedia article Picrite basalt.
More generally the classification of fine grained rocks recognizes a group known as 'picritic rocks' that are characterised by high magnesium content and low SiO2 content. They fit in the TAS classification system only at the lowest level of SiO2 (41 to 43% by weight) and Na2O + K2O (up to 3% by weight). They include picrite, komatiite and meimechite.