Picrite

picrite, intrusive igneous rock of ultramafic (very silica-poor) composition that is composed largely of olivine and augite and is somewhat similar to peridotite. Picrites are dark, heavy rocks and contain a small but variable amount of plagioclase feldspar; hornblende and biotite may also be present. Picrites usually occur in sills (tabular bodies inserted while molten between other rocks), but, unlike peridotites, they seldom are found in large plutonic masses. Varieties include augite-, enstatite-, and hornblende-picrite. The term picrite-basalt is reserved for feldspar-poor basalts rich in olivine.

The minerals in picrite are, in many cases, altered. Serpentine partially or wholly replaces olivine, and hornblende, talc, and chlorite appear as secondary products after the mineral. Augite passes into hornblende or chlorite, and the essential feldspar is often represented by epidote, prehnite, and white mica. In some picrites, as in the peridotites, a lustre mottling is produced by the inclusion of unoriented grains of olivine within large crystals of augite or hornblende.

The augite picrites of Scotland and the Czech Republic contain interstitial analcime and are closely related to the teschenites with which they are associated. Other picrites are more clearly calc-alkaline and are associated with diorites or diabases; the hornblende picrites of Gwynedd and Anglesey, Wales, are of this type. Picrites accompany diabases in the Devonian rocks of the Fichtelgebirge (in Germany) and Nassau, as well as Cornwall and Devon, Eng.