Geochemistry for CSIR NET Earth Science: Element Classification, Trace Elements and PYQ Strategy

Geochemistry explains how elements are distributed in the Earth and why they behave the way they do in magmas, minerals, water and the atmosphere. CSIR NET Earth Science questions usually test a few core ideas, such as element classification, partition coefficients and trace-element patterns, so a clear understanding pays off quickly.

For radiogenic and stable isotopes, see our separate guide on isotope geochemistry for CSIR NET Earth Science.

What the syllabus covers

• Cosmic and Earth abundances: solar system abundances, chondrites as a reference, composition of the core, mantle and crust.
• Goldschmidt’s classification: lithophile, siderophile, chalcophile and atmophile elements.
• Crystal chemistry: ionic radius, charge, Goldschmidt’s rules of substitution, coordination.
• Trace-element geochemistry: partition coefficients, compatible and incompatible elements, rare earth elements (REE), spider diagrams.
• Magmatic processes: partial melting and fractional crystallisation, and how trace elements record them.
• Low-temperature geochemistry: weathering, solubility, Eh–pH diagrams, the carbonate system.

High-yield topics

1. Goldschmidt’s classification with examples: lithophile (Si, Al, Na, K, Ca, REE), siderophile (Fe, Ni, Co, Pt-group elements), chalcophile (Cu, Zn, Pb, S, As), atmophile (N, noble gases).
2. Abundance of crust elements: O, Si, Al, Fe, Ca, Na, K and Mg make up about 99% of the continental crust by mass.
3. Partition coefficient: D = C_mineral/C_melt. D greater than 1 means the element is compatible and stays in the solid; D less than 1 means it is incompatible and goes into the melt.
4. REE patterns: chondrite-normalised plots, LREE enrichment in continental crust, and the europium anomaly caused by plagioclase.
5. Eh–pH diagrams: reading which iron species (Fe²⁺, Fe³⁺, hematite, pyrite) is stable under given conditions.

Concepts examiners love

• Why the core is iron-rich: siderophile elements followed metallic iron during core formation, which depleted them in the mantle.
• Incompatible elements in the crust: large-ion lithophile elements (K, Rb, Ba) and high field strength elements (Zr, Nb, Hf) become enriched in melts and therefore in continental crust.
• Negative Eu anomaly: Eu²⁺ substitutes for Ca²⁺ in plagioclase, so removing plagioclase from a magma leaves the melt depleted in europium.
• Ni and Cr as compatible elements: they go into olivine and pyroxene, so primitive mantle-derived basalts are rich in them.

How to revise in 10 days

• Days 1–2: cosmic and Earth abundances, and Goldschmidt’s classification.
• Days 3–4: crystal chemistry and substitution rules.
• Days 5–7: trace elements, partition coefficients, REE and spider diagrams.
• Days 8–9: low-temperature geochemistry and Eh–pH diagrams.
• Day 10: a timed set of previous year questions.

Common mistakes

• Classifying iron only as siderophile. It is also lithophile and chalcophile, depending on conditions.
• Mixing up compatible (D > 1) and incompatible (D < 1) elements.
• Reading spider diagrams without first checking which reference they are normalised to.