In ion-adsorption type deposits, rare earth elements (REE) are adsorbed electrostatically on the surface of clay minerals as outer-sphere complex and they are extractable with an electrolyte (e.g., ammonium sulfate) solution. This is the greatest characteristic of this deposit type and is different from the other deposits. In residual deposits containing REE minerals which are resistant to chemical weathering REE are incorporated in the mineral structure or are strongly adsorbed on the surface of minerals as inner sphere complex. The residual ores have to be treated with strong acid or alkali solutions and high impurities are found in leachate. Concentrations of ion-exchangeable REE need to be reported when an ion-adsorption type deposit is evaluated in order to distinguish from a residual deposit.
