Trace Elements | Geology Optional Notes for UPSC PDF Download

• Trace Elements Overview:
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  • 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.
  • They include elements like Silicon, Oxygen, Iron, Aluminium, Sodium, Calcium, Magnesium, and Phosphorus.
  • These elements are present in tiny amounts in living tissues and are primarily derived from the formation of igneous rocks.
  • Deficiencies or excesses of trace elements can lead to health issues, with some becoming toxic in high amounts.
• Goldschmidt Classification of Trace Elements:
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  • Siderophile Elements:
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    • Siderophile elements are concentrated in metallic iron and include Cobalt, Iridium, Osmium, and Nickel.
  • Chalcophile Elements:
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    • Chalcophile elements are those that form sulphides and include Arsenic, Selenium, Cadmium, and Zinc.
  • Lithophile Elements:
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    • Lithophile elements are mainly found in silicate phases and include Niobium, Strontium, Rubidium, Barium, Uranium, Tantalum, and Thorium.
  • Atmophile Elements:
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    • Atmophile elements naturally occur in the atmosphere as gases, such as rare gases and 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.
• This phenomenon is crucial in comprehending how mantle rocks transform into basal magma. For instance, Olivine, pyroxenes, and trace elements like Scandium, Chromium (Cr), Cobalt, and Nickel tend to concentrate in upper mantle rocks due to their compatibility with these phases.
• It is essential to determine the specific mineral phases under consideration. For example, Scandium exhibits compatibility with Clinopyroxene but not with Olivine, emphasizing the importance of understanding the relevant mineralogy.
• Elements such as Zirconium (Zr) are compatible with Zircon, while Phosphorus shows compatibility with Apatite. These elements, however, are incompatible with Pyroxene or Olivine.
• Incompatible elements can be further classified based on their charge (Z) and ionic radius (r).
• The charge and radius of trace element ions dictate their incorporation into the crystal structure. Factors like electronegativity and crystal field effects play a significant role in determining where trace elements will reside.
• Consequently, a trace element may either substitute for a major element within a forming mineral's framework or remain in the liquid phase.

Large Ion Lithophile Elements (LILE)

• The concept of large-ion lithophile elements (LILE) is closely related to incompatible trace elements, often used interchangeably in scholarly discussions.
• Initially introduced by geochemist Gast in 1972, LILE encompasses certain elements characterized by a significant ratio of ion radius to charge.
• Lithium, despite its small size, is classified as a large-ion lithophile element due to its distinctive ratio of radius to charge.
• There is some ambiguity surrounding the term LILE, leading researchers to suggest a more specific application, focusing on lithophile trace elements with a substantial ionic radius to charge ratio.
• For an element to be considered a large-ion lithophile trace element, it should possess ionic radii exceeding those of commonly involved cations like Ca2+ and Na+.
• Examples of large-ion lithophile trace elements include Strontium (Sr), Potassium (K), Cesium (Cs), Rubidium (Rb), Barium (Ba), Europium (Eu2+), and Lead (Pb).

High Field Strength Elements (HFSE)

• Definition: These elements, known as High Field Strength Elements (HFSE), are characterized by being generally incompatible due to their inability to achieve charge balance easily, despite not having a significant infrared (IR) value. The stability of these elements decreases as their charge increases.
• Identification of HFSE: Elements such as Ta, Hafnium (Hf), Titanium (Ti), Zirconium (Zr), and others where Zr>2 are classified as High Field Strength Elements (HFSE). However, it is important to note that the behavior of these high field strength elements may not always follow a similar pattern.
• Characteristics: HFSE exhibit larger cationic sizes and possess higher charges. They are typically excluded from mantle phases and tend to accumulate significantly in the molten liquid portion of the residue, earning them the label of incompatible elements.
• Examples of HFSE Trace Elements: Some examples of HFSE trace elements include Niobium (Nb), Tantalum (Ta), Zirconium (Zr), Hafnium (Hf), Uranium (U), and Thorium (Th).

Rare Earth Elements (REE)

• Definition: Rare Earth Elements (REE) refer to a group of elements with atomic numbers ranging from 57 to 72. These elements are characterized by having valencies of 2 or 3 and relatively larger ionic radii.
• Application in Petrogenetic Interpretations: One of the primary uses of Rare Earth Elements is in petrogenetic interpretations. Due to their low concentrations in igneous rocks, they play a crucial role in understanding the origin and evolution of rocks, especially in the field of geology.
• Challenges in Analysis: Analyzing Rare Earth Elements can be challenging due to their scarcity in rocks. Their low abundance makes it difficult to conduct detailed analyses, necessitating specialized techniques and equipment.

Conclusion

• Trace elements refer to elements present in minerals in very small quantities, not included in the mineral's standard chemical formula.
• Examples of trace elements are Nickel (Ni), Copper (Cu), Yttrium (Y), Cobalt (Co), Strontium (Sr), Chromium (Cr), Rubidium (Rb), Zirconium (Zr), Zinc (Zn), Niobium (Nb), Scandium (Sc), Gallium (Ga), Thorium (Th), Barium (Ba), Lithium (Li), Cerium (Ce), Vanadium (V), Neodymium (Nd), Lead (Pb), Praseodymium (Pr), Lanthanum (La), Uranium (U), Europium (Eu), Erbium (Er), Holmium (Ho), Ytterbium (Yb), Gadolinium (Gd), Hafnium (Hf), Dysprosium (Dy), Lutetium (Lu), Thulium (Tm), and Terbium (Tb).
• Researchers utilize trace elements based on the specific nature, objectives, and interests of their research projects.
• Besides research purposes, trace elements can be used in testing models of magma differentiation and determining the depth of primary magma generation.

Frequently Asked Questions

• Find answers to common queries regarding UPSC Examination Preparation.

What are Trace Elements?

• Trace elements are elements found in minerals in minimal quantities not accounted for in the minerals' chemical formulas.
• For instance, Copper (Cu) and Zinc (Zn) are trace elements present in certain minerals.

What are Atmophile Elements?

• Atmophile elements are elements that have a high affinity for combining with gases.
• One example is Oxygen (O), which readily combines with other elements in the atmosphere.

What are HFSEs?

• HFSEs stand for Highly Siderophile Elements, which have a strong affinity for iron and tend to be found in metal-rich minerals.
• An example of an HFSE is Platinum (Pt), which is often found in association with iron-rich ores.

What are REEs?

• REEs are Rare Earth Elements, a group of elements that exhibit similar chemical properties and are often found together in minerals.
• One example of a REE is Neodymium (Nd), commonly used in magnets due to its magnetic properties.

Understanding LILEs

• Trace elements refer to those elements found in very small amounts, less than 0.1% by weight, excluding common elements like silicon, oxygen, iron, etc. These are often expressed in parts per million.
• Atmophile elements are gases occurring naturally in the atmosphere. Nitrogen and rare gases are examples of trace elements in this category.
• High Field Strength Elements (HFSE) include elements like Ta, Hf, Ti, Zr, and Nb, characterized by their atomic number being greater than 2.
• Rare Earth Elements (REE) are elements with valencies of 2 or 3 and larger ionic radii. They span from La (57) to Lu (72) in the periodic table.
• LILEs, or large-ion lithophile elements, are trace elements with a high ionic radius to charge ratio, making them lithophilic.

Examples:

• Trace Elements: Think of how a dash of salt can significantly alter the taste of a large pot of soup.
• Atmophile Elements: Consider how oxygen and nitrogen, vital gases in our atmosphere, interact with other elements.
• HFSE: Imagine the strength and stability that titanium provides in aircraft construction.
• REE: Picture the unique properties of rare earth magnets used in various technologies.
• LILEs: Visualize how elements like cesium or rubidium interact with minerals in the Earth's crust due to their large ionic radii.