shonkinite

Shonkinite is a rare igneous rock. It is a type of ultramafic to mafic shoshonite, a potassium-rich volcanic rock. Shonkinite is characterized by a high content of potassium feldspar, typically anorthoclase, and a mafic mineralogy that includes augite, biotite, and sometimes olivine. It is also notable for its relatively coarse grain size. This rock is usually dark in color, ranging from black to dark green.

The formation of shonkinite is linked to specific geological environments, often associated with continental rift zones or intraplate volcanic settings. These environments provide the conditions for the partial melting of the mantle, leading to magmas that are enriched in potassium and other incompatible elements. The slow cooling of these magmas at or near the Earth's surface allows for the growth of larger mineral crystals, resulting in the characteristic texture of shonkinite.

Shonkinite is found in limited locations worldwide. Notable occurrences include the Highwood Mountains in Montana, USA, and the Ustedalr region of Norway. The discovery and description of shonkinite are attributed to American geologists. Its rarity and specific mineral composition make it of interest to petrologists studying magmatic processes and the composition of the Earth's crust and upper mantle. Analysis of shonkinite can provide insights into the geochemical evolution of volcanic provinces.