Assimilation of Trace Elements in Crystals

The presence of trace elements in minerals can be ascribed to at least four main phenomena (Mclntire, 1963)  

Surface adsorption. Foreign ions are kept in a diffuse sheet at the surface of the crystal, as a result of electrostatic interaction with surface atoms whose bonds are not completely saturated. 

Occlusion. Adsorbed impurities on the surface of the crystal are trapped by subsequent strata during crystal accretion. 

Substitutional solid solution with a major component. The trace element substitutes for a major element in a regular position of the crystal lattice. interstitial solid solution. Similar to the preceding phenomenon, but here the trace element occupies an interstitial position in the crystal lattice. 

The first two processes may be operative at ultratrace concentration levels. The first one is relevant whenever the mineral has a high surface-to-mass ratio, as in the case of colloids. The last two processes are by far the most important in geochemistry and can be appropriately described in thermodynamic terms. 

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