Trace Elements

Other than Silicon (Si), Oxygen (O), Iron (Fe), Aluminium (Al), Sodium (Na), Calcium (Ca), Magnesium (Mg), and Phosphorus (K), the term trace element is applicable for elements that occur in minuscule concentrations of lower than 0.1% by weight. Parts per million (ppm) is the general unit for expressing the number of trace elements present. 

Trace elements are minerals that exist in living tissues in small quantities. Trace elements are primarily products of the petrogenesis elucidation of igneous rocks. 

The deficiency of trace elements may cause diseases. Similarly, excess of these trace elements is harmful to the body (in some cases, poisonous).

Goldschmidt Classification of Trace Elements

There are multiple ways to classify trace elements, for example:

• Siderophile elements

Metallic iron contains a high concentration of these elements. Trace element examples of this category include Cobalt (Co), Iridium (Ir), Osmium (Os), and Nickel (Ni).

• Chalcophile elements

Chalcophile elements are elements that condense as sulphides. Trace element examples of such categories include Arsenic (As), Selenium (Se), Cadmium (Cd), and Zinc (Zn).

• Lithophile elements

Lithophile elements exist mainly (concentrated) in silicate phases. Trace element examples of this category include Niobium (Nb), Strontium (Sr), Rubidium (Rb), Barium (Ba), Uranium (U), Tantalum (Ta), and Thorium (Th).

• Atmophile elements

Atmophile elements occur naturally in the atmosphere in the form of gases. Examples include some rare gases and Nitrogen.

Compatible and Incompatible Trace Elements

When minerals and silicate melt coexist in equilibrium, by preference, compatible elements segregate into the solid phases, and incompatible elements separate into the melted part. 

When mantle rocks melt into basal magma, this concept helps understand the process. Olivine, pyroxenes and trace elements such as Scandium, Chromium (Cr), Cobalt, and Nickel enrich the upper mantle rocks. Compared to silicate, they are more compatible with these phases. 

Since Scandium has compatibility in Clinopyroxene but not with Olivine, defining which mineral phases are being considered becomes necessary. Zirconium (Zr) has compatibility in Zircon. Phosphorus has compatibility in Apatite. But none of these is compatible with Pyroxene or Olivine.

We can further categorise incompatible elements based on  charge(Z)ionic radius(r).

The charge and radius of trace element ions define a trace element’s embodiment in the crystal structure. Electronegativity of these elements & crystal field effects (CFE) plays the deciding part in the embodiment of trace elements. 

Consequently, a trace element will either exchange for a major element in the framework of a shaping mineral or persist in the liquid.

Large Ion Lithophile Elements (LILE)

The terms large-ion lithophile element (or LILE) and incompatible trace elements are often interchangeable. Scholars sometimes use LILE to refer to a particular subdivision of incompatible trace elements. We can identify such elements by a large ratio of radius to charge of ions.

Geochemist Gast was the first to use this term in 1972. He did this to incorporate the cations of Cesium, Barium, Potassium, Rubidium, Strontium, rare earth elements, Uranium and Thorium. He included Lithium as a large-ion lithophile element (or LILE) because it has a large ratio of radius to charge, although it is a considerably small element.

There exists uncertainty regarding the term’s use. Therefore, researchers advise restricting usage of LILE to lithophile trace elements with a large ionic radius to charge ratio. 

Such elements should have ionic radii greater than Ca2+ and Na+. Calcium and sodium ions are the largest cations generally participating in the rock formation of minerals. Examples of large-ion lithophile trace elements include Sr, K,  Cesium (Cs), Rb, Ba, Europium (Eu2+) and Lead (Pb).

High Field Strength Elements (HFSE)

These elements are generally incompatible, even though they do not have a significant infrared (IR)value. It is because they cannot attain charge balance easily. The greater the charge, the lesser the stability.  

Generally, we can define Ta, Hafnium (Hf), Titanium (Ti), Zr, Nb, and all other elements with Zr>2 as High Field Strength Elements (HFSE). Nonetheless, these high field strength elements (HFSE) do not always separate analogously. 

High Field Strength Elements (HFSE) have a greater cationic size and larger charges. These trace elements are kept out from phases of the mantle. These trace elements are highly accumulated in the melted liquid part of the residue and thus called incompatible elements. 

HFSE trace element examples include Nb, Ta, Zr, Hf, U, and Th.

Rare Earth Elements (REE)

This category includes elements from atomic numbers 57 to 72. We can distinguish these elements by valencies +2 or +3 and relatively greater ionic radii. 

Petrogenetic interpretations are the most important and common application of REE. Since these elements have deficient concentrations in the igneous rocks, they can not undergo analysis easily.

Conclusion

Trace elements are elements that exist in a mineral in tiny amounts such that the chemical formula of the mineral does not include them. Trace elements examples include Ni, Copper (Cu), Yttrium (Y), Co, Sr, Cr, Rb, Zr, Zn, Nb, Sc, Gallium (Ga), Th, Ba, Lithium (Li), Ce, Vanadium (V), Neodymium (Nd), Pb, Praseodymium (Pr), Lanthanum (La), U, Eu, Erbium (Er), Holmium (Ho), Ytterbium (Yb), Gadolinium (Gd), Hf, Dysprosium (Dy), Lutetium (Lu), Thulium (Tm), and Terbium (Tb). 

Researchers use these trace elements according to the nature, objectives and interest of the research project. Apart from research, we can utilise trace elements in testing models of magma differentiation or for discovering the primary magma’s generation depth.