Weathering and Aqueous Geochemistry Questions and Answers

Answer: B

The correct match is a-2, b-4, c-3, d-1.

• Carbonation is the reaction of minerals with carbon dioxide dissolved in water as carbonic acid, as when limestone dissolves (a-2).
• Hydrolysis is the decomposition of a mineral by reaction with water, its H\(^{+}\) and OH\(^{-}\) ions replacing cations, as when feldspar turns to clay (b-4).
• Chelation is the binding of metal cations from a mineral into organic compounds, the chelating agents released by organisms (c-3).
• Hydration is the addition of water to the molecular structure of a mineral, as anhydrite becomes gypsum (d-1).

Each other code gets at least two of these pairs wrong.

Answer: D

The full statement is that pyrite is deposited in a reducing environment under both acid and alkaline conditions.

Pyrite (FeS₂) needs sulphide, which is produced only where oxygen is absent, typically by bacteria reducing sulphate in organic-rich mud. On an Eh-pH diagram, such as the fence diagram of Krumbein and Garrels, its field lies at low Eh and extends across the neutral line, so it forms on both the acid and the alkaline side.

Option A is true as far as it goes but leaves out the pH range that the paper is testing, so the complete statement D is preferred. Option B says nothing about the redox condition, which is the decisive control. Option C is wrong: in an oxidising environment iron sulphide is not deposited but destroyed, its iron going to oxides and hydroxides.

Answer: A

The true statement is Oxidation of Mn (II) to Mn (IV) is required for manganese ore precipitation.

Manganese is soluble as Mn²⁺ in reduced, oxygen-poor waters, and is carried in solution in that form. When those waters meet oxygenated conditions, Mn²⁺ is oxidised to Mn⁴⁺, which forms insoluble oxides such as pyrolusite and the oxides of manganese nodules and crusts. Most manganese ores are deposited by this oxidation.

Reduction does the reverse: it dissolves manganese oxides and mobilises the metal, so it cannot precipitate ore. Precipitation is therefore strongly redox-dependent, and both B and D are wrong.

Answer: B

The change is an example of hydration.

In hydration, water molecules are taken into a mineral's structure without breaking it down chemically. Haematite, Fe₂O₃, adds water to become limonite, Fe₂O₃.3H₂O (more strictly a mix of hydrated iron oxides). The mineral swells and softens, which helps weathering.

Hydrolysis is different: water's H⁺ and OH^- ions react with the mineral and replace its cations, as when feldspar becomes clay. Carbonation is reaction with carbonic acid, as in limestone solution. Chelation is the binding of metal ions by organic compounds.