Trace Elements | Geology Optional Notes for UPSC PDF Download

Trace Elements Overview

Definition: Trace elements, also known as microminerals, are elements with concentrations lower than one thousand parts per million or 0.1% of the original rock composition. These elements are present in tiny amounts in living tissues and are primarily derived from the formation of igneous rocks. Examples include Silicon, Oxygen, Iron, Aluminium, Sodium, Calcium, Magnesium, and Phosphorus.
Deficiencies or excesses of trace elements can lead to health issues, and some can become toxic in high amounts.

Goldschmidt Classification of Trace Elements

Siderophile Elements

• Concentrated in metallic iron.
• Examples: Cobalt, Iridium, Osmium, Nickel.

Chalcophile Elements

• Form sulphides.
• Examples: Arsenic, Selenium, Cadmium, Zinc.

Lithophile Elements

• Found in silicate phases.
• Examples: Niobium, Strontium, Rubidium, Barium, Uranium, Tantalum, Thorium.

Atmophile Elements

• Naturally occur as gases in the atmosphere.
• Examples: Rare gases, Nitrogen.

Understanding Trace Element Behavior

• When minerals and silicate melt coexist, compatible elements preferentially move into solid phases, while incompatible elements remain in the melted portion.

Examples of Compatibility

• Scandium, Chromium, Cobalt, and Nickel concentrate in upper mantle rocks due to their compatibility.
• Scandium is compatible with Clinopyroxene but not with Olivine.
• Zirconium (Zr) is compatible with Zircon, while Phosphorus is compatible with Apatite.

Factors Influencing Behavior

• Charge (Z) and ionic radius (r) of trace element ions dictate their incorporation into crystal structures.
• Electronegativity and crystal field effects play critical roles in determining the residence of trace elements.
• Incompatible elements either substitute for major elements or remain in the liquid phase.

Key Classifications of Trace Elements

Large Ion Lithophile Elements (LILE)

• Definition: Trace elements with a high ratio of ion radius to charge.

Characteristics:

• Include elements with ionic radii exceeding those of cations like Ca²+ and Na+.
• Despite its small size, Lithium (Li) is included due to its distinctive radius-to-charge ratio.

• Examples: Strontium (Sr), Potassium (K), Cesium (Cs), Rubidium (Rb), Barium (Ba), Europium (Eu²+), Lead (Pb).

High Field Strength Elements (HFSE)

• Definition: Elements that are incompatible due to difficulty achieving charge balance.

Characteristics:

• High charges and larger cationic sizes.
• Typically excluded from mantle phases, accumulating in the molten liquid portion.

• Examples: Niobium (Nb), Tantalum (Ta), Zirconium (Zr), Hafnium (Hf), Uranium (U), Thorium (Th).

Rare Earth Elements (REE)

• Definition: Elements with atomic numbers 57 to 72, characterized by valencies of 2 or 3 and larger ionic radii.
• Applications: Used in petrogenetic interpretations to understand the origin and evolution of rocks.
• Challenges: Low abundance requires specialized techniques for analysis.

Conclusion

Trace elements are elements present in minerals in very small quantities, not included in the mineral's standard chemical formula. Examples include Nickel (Ni), Copper (Cu), Yttrium (Y), Cobalt (Co), Strontium (Sr), Chromium (Cr), Rubidium (Rb), Zirconium (Zr), and many more.

Applications:

• Testing models of magma differentiation.
• Determining the depth of primary magma generation.