3.4 Geochemistry (elements, isotopes, weathering, aqueous)

Elemental Geochemistry

Geochemistry studies the distribution and behavior of the chemical elements in Earth materials. The continental crust is dominated by eight elements: oxygen (about 46% by weight), silicon (about 28%), aluminum, iron, calcium, sodium, potassium, and magnesium; together these exceed 98% of the crust. Oxygen and silicon alone make up over 74%, which is precisely why silicate minerals dominate the crust.

Major, Minor, and Trace Elements

• Major elements (greater than 1 wt%, reported as oxides such as SiO2, Al2O3, FeO, MgO, CaO, Na2O, and K2O) define rock classification.
• Minor elements (0.1 to 1%) and trace elements (less than 0.1%, reported in parts per million) — for example Rb, Sr, Zr, and the rare earth elements (REE) — track petrogenetic processes.
• Compatible elements (such as Ni and Cr) preferentially enter early-crystallizing minerals, whereas incompatible elements (such as K, Rb, REE, and U) stay in the melt and concentrate in late, felsic differentiates. Whether an element enters a given mineral is governed by ionic radius and charge (Goldschmidt's substitution rules): ions of similar size and charge readily substitute, which is why particular trace elements track specific host minerals.

Goldschmidt Classification

V. M. Goldschmidt grouped elements by their geochemical affinity, meaning where they concentrate:

• Lithophile ("rock-loving") — bond with oxygen and concentrate in the silicate crust and mantle (Si, Al, Na, K, Ca, Mg, O, and the REE).
• Siderophile ("iron-loving") — have metallic affinity and concentrate in the metallic core (Fe, Ni, Co, Au, Pt).
• Chalcophile ("sulfur-loving") — bond with sulfur to form sulfide ore minerals (Cu, Pb, Zn, Ag, Hg, S).
• Atmophile — volatile elements that concentrate in the atmosphere (H, N, and the noble gases).

Isotopes

Isotopes are atoms of an element with the same number of protons but different numbers of neutrons. Radioactive (radiogenic) isotopes decay at a constant rate (a fixed half-life) and drive radiometric dating: uranium-lead (in zircon, for the oldest rocks), potassium-argon and argon-argon, rubidium-strontium, and carbon-14 (organic material younger than about 50,000 years). Ages are calculated from measured parent-to-daughter ratios. Each system suits a different age range and material: carbon-14 (half-life about 5,730 years) dates young organic matter, potassium-argon and argon-argon date volcanic minerals, and uranium-lead in zircon dates the oldest terrestrial and lunar rocks and anchors the roughly 4.54-billion-year age of the Earth. Reliable ages require a closed system — no gain or loss of parent or daughter since the mineral cooled below its closure temperature. Stable isotopes (such as the oxygen-18 to oxygen-16 ratio, and carbon-13, deuterium, and sulfur-34 ratios) do not decay; their ratios fractionate with temperature and process, serving as paleoclimate and fluid-source tracers. For example, oxygen-18 ratios in ice cores and carbonates record paleotemperature.

Chemical Weathering

Chemical weathering decomposes minerals through reactions with water, oxygen, and acids:

• Dissolution — soluble minerals dissolve directly; calcite dissolves in carbonic acid, CaCO3 + H2CO3 giving Ca(2+) plus 2 HCO3(-), the reaction that drives karst.
• Hydrolysis — the single most important reaction for silicates; feldspar reacts with weakly acidic water to form clay minerals plus dissolved cations and silica (for example, potassium feldspar alters to kaolinite plus potassium ions, bicarbonate, and dissolved silica).
• Oxidation — iron-bearing minerals react with oxygen to form iron oxides (rust, hematite, limonite); olivine and pyroxene are readily oxidized.
• Hydration — water is incorporated into a mineral structure (anhydrite converts to gypsum).

The Goldich stability series states that minerals crystallizing first in Bowen's series (olivine and calcium plagioclase) weather most readily, while quartz, the last to crystallize, is the most stable. As a result, residual soils become enriched in quartz and clay minerals. The intensity and products of chemical weathering depend strongly on climate: warm, humid settings drive intense hydrolysis that yields thick, clay- and iron-oxide-rich residual soils (laterites), whereas cold or arid climates favor physical weathering. Common clay products include kaolinite, smectite, and illite, and which forms reflects the parent mineral and the leaching intensity.

Aqueous Geochemistry: pH and Eh

Natural water chemistry is governed by two master variables:

• pH — acidity or alkalinity, the negative log of hydrogen-ion activity; it controls mineral solubility and which dissolved species dominate. Carbonate buffering keeps most natural waters near pH 6 to 9, whereas acid mine drainage (pyrite oxidation producing sulfuric acid) drives pH very low.
• Eh — the oxidation-reduction (redox) potential in volts; positive Eh is oxidizing (near-surface, oxygen-rich), and negative Eh is reducing (deep groundwater, organic-rich).

Water quality is also described by total dissolved solids (TDS), hardness (dissolved Ca and Mg), and ionic strength. Redox reactions govern the fate of many contaminants: as groundwater becomes progressively more reducing, dissolved oxygen is consumed first, followed by nitrate, manganese, iron, and finally sulfate reduction in a predictable sequence that controls the mobility of metals and nutrients.

Eh-pH (Pourbaix) diagrams map the stability fields of dissolved species and solid minerals. Iron, for instance, is soluble as the ferrous ion (Fe(2+)) under low-pH, reducing conditions but precipitates as ferric oxides and hydroxides (hematite, ferrihydrite) under oxidizing conditions. These same controls govern contaminant mobility, ore deposition, and groundwater quality, making them core applied-geochemistry topics on the PG exam.