Fenitized Gneiss
Photographed by Michael P. Klimetz

A fenite is a quartzofeldspathic rock that has been altered by alkali metasomatism at the contact of a carbonatite intrusive complex. The process is called fenitization. Fenite is comprised mostly of alkalic feldspar, with some aegirine, subordinate alkali-hornblende, and accessory sphene and apatite. Chemically, fenites are Na- and K-rich silicate rocks which develop at the contact between alkaline (carbonatite) igneous intrusions and their surrounding country rocks. Fenites are not necessarily confined to the intrusive contact and may develop at a significant distance from the intrusion through interaction of the country rock with percolating fluids, or inside the intrusion through reaction of xenoliths with their entraining magma. Fenites typically comprise potassium feldspar, albite, aegirine, various sodic amphiboles and, in some cases, nepheline.