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Difficulty: HardRocks, Minerals, and Soil

A major river system erodes an upland terrain containing both mafic bedrock (rich in olivine, pyroxene, and calcium-rich plagioclase) and felsic bedrock (rich in quartz and potassium feldspar). Over hundreds of miles of transport through a warm, humid climate, the sediment undergoes intense chemical and physical weathering before reaching a coastal delta. Which mineral overwhelmingly dominates the resulting mature beach sand, and what explains its persistence?

  1. A
    Olivine, because its early crystallization from high-temperature magma provides the strongest chemical resistance under Earth surface conditions.
  2. Quartz, because its framework silicate structure and low formation temperature make it exceptionally resistant to chemical weathering.Answer
  3. C
    Calcium-rich plagioclase, because it forms dense interlocking lattices that resist dissolution and hydrolysis during long-distance water transport.
  4. D
    Pyroxene, because its single-chain silicate bonds prevent chemical alteration and hydration when exposed to weathering agents.

Answer

Quartz, because its framework silicate structure and low formation temperature make it exceptionally resistant to chemical weathering.
Quartz is a framework silicate composed purely of silicon-oxygen tetrahedra (SiO2SiO_2) and crystallizes at the lowest temperatures of Bowen's reaction series. Because its crystallization environment is closest to surface conditions, it exhibits supreme chemical stability and high physical durability (Mohs hardness 7, no cleavage), enabling it to survive extensive weathering and dominate mature sands.

Step-by-Step Solution

1
Relate mineral crystallization conditions to surface chemical stability using Bowen's Reaction Series and Goldich's Dissolution Series.
Minerals formed at highest temperatures and pressures (such as olivine and calcium plagioclase) are farthest from equilibrium with surface conditions and weather most rapidly, whereas minerals formed at lowest temperatures (such as quartz) are most stable at Earth's surface.
Chemical weathering susceptibility is directly related to the discrepancy between a mineral's formation environment and surface ambient conditions.
2
Evaluate the chemical and structural resistance of the candidate minerals during prolonged humid transport.
Olivine, pyroxene, and calcic feldspar break down rapidly via hydrolysis and oxidation into clays and soluble ions. Quartz possesses a strong three-dimensional framework of silicon-oxygen tetrahedra (SiO2SiO_2) with no easily weathered metal cations.
Quartz lacks cleavage planes, has a Mohs hardness of 77, and does not readily react with weak surface carbonic acids.
3
Determine the dominant mineral constituent in the mature sedimentary deposit.
After long-distance fluvial transit and chemical decomposition of all unstable minerals, durable quartz grains persist to form mature sand.
Sedimentary maturity reflects the progressive elimination of unstable mineral species over distance and time.

Key Concept

Mineral stability and chemical weathering resistance in sedimentary processes (Goldich Dissolution Series / Bowen's Reaction Series)
Estimated Time:1m 30s
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Topics

Rocks, Minerals, and Soil

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