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Understanding Our Planet Through Chemistry

Mineral Scavengers Provide a Clue

There is another way of detecting the trace elements carried from a deposit by ground water. Ground water is drawn upward by evaporation at the surface. During this upward migration, trace elements in the water are affixed to minerals in the overburden. The affixation, or bonding, may range from weak to very strong. The strength of this bonding depends on the chemical nature of both the trace element and the host mineral. The differences in bond strength is comparable to the difference between the weak electrostatic attraction that holds an inflated balloon to a wall and a nail driven into a stud.

Minerals that are capable of scavenging trace elements from ground water with increasing bond strength include hydrated aluminum silicates (clays), secondary carbonates, amorphous (noncrystalline) oxides of manganese, and the amorphous and crystalline oxides of iron. Trace elements scavenged by these minerals are removed by treating samples of overburden with chemicals that react selectively with each mineral phase. Sequential selective extractions are used to release trace elements from the host minerals in the order of increasing bond strength such as clays first and crystalline iron oxides last.

The principal advantage of selective extractions is that they facilitate the distinction of elements that have migrated from other sources from those normally present in the overburden. Thus the presence of a gold deposit in Nevada may well be indicated by the occurrence of gold, or its associated elements, arsenic and antimony, in a specific mineral phase in the overburden.

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