Introduction and historical review

The name “geochemistry” was first introduced by Schonbein since more than 150 years.

Clark, who was a chief chemist of the US. Geological Survey from 1884 to 1925, has contributed very much to the science "geochemistry".  The modern science of geochemistry can be dated back to Clark who published a very large number of chemical analyses of the various rocks in the earth's crust.  He computed averages for each common rock type and studied the mutual abundance and distribution of many major and minor elements.

Goldschmidt (1888 - 1947) contributed significantly to the roles of ionic size, coordination and atomic substitution in crystal lattices. He gave a practical definition for the science geochemistry, as it deals with:

1) the abundance of elements in rock, mineral or crystal, 

2) the distribution of the elements, and

3) lows governing the abundance and distribution of elements in rock, mineral or crystal.  

The contributions of the USSR geochemists are remarkable, especially after the improvement of the analytical techniques at the early decades of the present century.

There are several trials to classify elements on geochemical basis. Names such as siderophile, chalcophile, lithophile, hydrophile, thalassophile, atmophile are commonly used to denote particular geochemical affinity of elements.

Modern advances in geochemistry are enormous in different academic and applied disciplines. The revolution in the analytical techniques, especially the coupled inductively plasma-mass spectrometry facilitated data with very high precision on all components of the earth, such as water, soil, sediments, air, rock, crystals and minerals.

Recently, huge data are accumulating on the geochemistry of the REE, PGM and the inert gases. 

What makes an element siderophile or lithophile?  Notably, the Goldschmidt categories are well-grouped in the periodic table of the elements:

Properties derived from outer electrons

Ionization potential à energy required to remove the least tightly bound electron

Electron affinity à energy given up as an electron is added to an element

Electronegativity à quantifies the tendency of an element to attract a shared electron when bonded to another element.

Electronegativity of elements

Shielding

Nucleus of an atom contains protons(+) and neutrons à positively charged center of the atomLargest volume of the atom contains the electrons, zipping around, held in place by the electrostatic attraction to the nucleusOutermost electrons determine bonding, more electrons in between the outermost e- and nucleus, the less net charge and attraction they ‘feel’.In general, first ionization potential, electron affinity, and electronegativities increase from left to right across the periodic table, and to a lesser degree from bottom to top.

Atomic Radius

    Another function of shielding, size is critical in substitution of ions, diffusion, and in coordination numbers