Steven Dutch, Natural and Applied Sciences, University
of Wisconsin - Green Bay
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We can recognize a number of groups of elements that tend to occur together in rocks
and minerals, because of chemical similarities, ionic charge, or ionic radius.
- Lithophile ("rock loving") elements occupy the left side and
right top of the periodic table and tend most often to occur as silicate minerals.
- Siderophile ("iron loving") elements occupy the left center
of the periodic table. They tend to occur as oxides, are abundant in iron meteorites, and
tend to be concentrated in the Earth's mantle and core.
- Chalcophile ("brass loving") elements occupy the right center
of the periodic table. They tend to form sulfides and are are also enriched in the Earth's
- Platinum Group elements are usually regarded as part of the siderophile
group, but they are shown here separately because they are the elements most likely to
occur in the metallic form in nature, usually as natural alloys rather than pure elements.
- Noble Gases have complete outer octet shells. They can be made to
combine with other elements in the laboratory but have not been found in compounds in
- Principal Anions include oxygen, far and away the most important anion
in the Earth, followed by sulfur, with chlorine and fluorine trailing distantly.
- Minor Anions include the other two halogens, bromine and iodine, plus
some of the semi-metals. They are uncommon and the semi-metals can also occur as cations.
- Radical-formers include a cluster at the top right of the periodic
table (easily the most important group) plus another cluster on the left side of the
- Small Cations include Li+1, Be+2, B+3, C+4, N+5, Al+3, Si+4, P+5, Ga+3,
and Ge+4, with ionic radii smaller than 0.5 Angstrom units. Their small sizes enable them
to substitute for Si or Al, or cause them to form compounds with different packings than
compounds of elements further down the columns. The wide range of ionic charges means they
don't always substitute for one another: Li can sometimes substitute for Al but rarely for
Si, for example.
- Large Cations include Na, Mg, and the elements beneath them, plus a few
others with +1 or +2 charge, notably Sn+2 and Eu+2. These are typically about 1 Angstrom
Unit in diameter. Europium is a particularly interesting case. The rare earths are so
similar that they tend to occur interchangeably in minerals, but because Eu is a close
twin in size and charge to Ca and Sr, it tends to substitute for those elements. Thus, if
Eu is conspicuously enriched or depleted compared to other rare earths, it signals major
movements of Ca or Sr, such as removal or accumulation of feldspar in a magma. Note also
that Li and Be are not part of this group. The charges and chemical affinities are right,
but they are simply too small to occupy the sites that large cations do. Li would simply
slip through the holes in a feldspar structure, for example.
- Large Cations of High Charge include the block with corners at Sc and W
plus a few others like Pb+4.
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Created 18 September 1998, Last Update 18 September 1998
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