Greensand

a natural mineral, primarily composed of complex silicates, which possess ion exchange properties.

Definitioner

Mineral: a term applied to inorganic substances, such as rocks and similar matter found in the earths strata, Minerals normally have definite chemical composition and crystal stucture. The term also applied to matter derived from minerals, such as inorganic ions found in water.
Ion Exchange: a reversible process in which ions are released from an insoluble permanent material in exchange for other ions in a surrounding solution; the direction of the exchange depends depends on the affinities of the ion exchanger for the ions present, and the concentrations of the ions in the solution.
Ion: an atom, or group of atoms which function as a unit, and has a positive or negative electrical charge, due to the gain or loss of of one or more electrons.