Metals, their Biological Functions and Harmful Impacts - Mining and the Environment: From Ore to Metal - page 477

Of the economically important elements only four elements (aluminium, iron, potas-
sium and magnesium) exceed the 1% mark. In any given rock all remaining economically
important elements combined such as gold, silver, copper, lead, zinc, tin, nickel, or plati-
num typically account for less than half a percent of its mass.

11.1 PERSISTENCE, BIOACCUMULATION AND
TOXICITY OF METALS

Life of course has evolved in the presence of metals, some of which - the essential metals -
have become incorporated into metabolic processes crucial to the survival, growth
and reproduction of organisms, including humans. Organisms have developed several
mechanisms - with varying efi ciencies - for the uptake and excretion, regulation and
detoxii cation of both essential and non-essential metals.
Several metals (e.g. sodium, potassium, magnesium and calcium) occur in large concen-
trations in organisms. Other metals, termed trace metals, occur at much lower concentra-
tions (normally

0.01%) in organisms. A number of metals, such as Fe, Mn, Zn, Cu, Co
and Mo, have been identii ed as essential for all living organisms, while the necessity of
other metals, such as Ni, V, I, Si and B, has only been established for a limited number of
species. Other metals such as As, Pb, Cd, Hg, Al, Li and Sn have no known useful functions

FIGURE 11.2
Composition of Earth's Crust in
Percentage by Weight

The two most common elements in
the crust, silicon and oxygen, consti-
tute about two-thirds of the crust's
weight. Only eight elements make up
more than 98% of the Earth's crust
by weight.