Picrite

Picrite is a mafic (dark) and heavy volcanic rock. Picritic lavas are derived from magma that was sourced from the bottom of magma-chamber where heavier crystals accumulated. With sufficient time, the magma in the magma-chamber may separate and become compositionally zoned. Heavier minerals rich in magnesium and iron, like olivine and pyroxene (augite), settle in its lower part. These minerals then give the characteristic black to black-green colour and high specific weight to picrite.

The ascent to the surface is an uneasy task for a heavy picritic magma that is full of crystals. To move the picritic magma towards the surface, the zoned magma chamber must be completely exhausted, and the lazy crystal-rich magma on its bottom must be pushed away by a new portion of fresh magma. As they are often the last to erupt, picrites usually cap lava sequences, like on the Pustý zámek Hill. Other outcrops of picrite in Doupovské hory Mts. are known from Bošov, Tureč, Žďár, Velichov and Pulovice.

The olivines are prone to rapid weathering in the surface conditions, and their colour turns from olive grey to yellow to ginger, as can be seen on the presented sample.

Sample donated by Hradiště military training area.