Xenolith – Piece of rock having a different origin from the igneous rock in which it is found.

X-Ray Diffraction – Process in mineralogy of identifying mineral structures by exposing crystals to X-rays and studying the resulting diffraction pattern.

Yellowcake – Partially refined uranium ore typically containing 80-90% U3O8.

Younging Direction – Term used to identify the top of a sedimentary sequence because when moving towards the top of a sedimentary sequence the material becomes younger or more recently deposited.

Youth (geomorphology) – Stage in the geomorphic cycle in, or the science of geomorphology used to describe a landscape which has just been uplifted and is beginning to be dissected by canyons cut by young streams.

Z/A Effect – Ratio of the atomic number ‘Z’ to the atomic weight ‘A’ which affects the relation between the response of gamma-gamma logs and bulk density used in the oil and gas industry.

Zeolite – Microporous alum inosilicate minerals commonly used as commercial adsorbents. Uniquely, zeolites have the ability to selectively sort molecules due to a regular pore structure of molecular dimensions.

Zoned Crystal – Single crystal of one mineral that has a different chemical composition in its inner and outer parts. Formed from minerals belonging to a solid-solution series and caused by the changing concentration of elements in a cooling magma that results from crystals settling out.