Quartz in Sandstones

Introduction

Quartz (SiO₂) is the dominant mineral in most sandstones, making up on average about 50-60 percent of the framework fraction. It is comparatively easy to identify both in hand specimens and by petrographic thin-section examination, although it can be confused with feldspars. Because of its superior hardness and chemical stability, quartz can survive multiple recycling episodes through erosion, transport, and redeposition. ^[1]

The dominance of quartz in most sandstones is not accidental. It is the end product of prolonged chemical and mechanical filtering. Feldspars, pyroxenes, amphiboles, and most other silicate minerals are consumed by weathering or shattered during transport. Quartz, being resistant to both, accumulates in the residue. A sandstone very rich in quartz - a quartz arenite - therefore records either a long and vigorous weathering history, multiple cycles of recycling, or both.

Grain Types: Monocrystalline and Polycrystalline

Quartz can occur as single (monocrystalline) grains or as composite (polycrystalline) grains. ^[1]

When examined under crossed polarising prisms with a petrographic microscope, many quartz grains display sweeping patterns of extinction as the stage is rotated. This property is called undulatory extinction. Some authors suggest that polycrystallinity and undulatory extinction can be used to distinguish quartz derived from different source types. ^[1]

Monocrystalline quartz gives uniform extinction across the entire grain. Polycrystalline quartz is made up of multiple sub-grains, each extinguishing at a slightly different stage rotation. Undulatory extinction is a deformation feature: the crystal lattice has been bent by stress, so different parts of the grain extinguish at different orientations. In principle, quartz from a highly deformed metamorphic rock is more likely to show undulatory extinction and polycrystallinity than quartz from an undeformed granite. Using these criteria for provenance is possible but requires careful calibration, since diagenesis and transport can also affect the textural state of grains.

Source Rocks

Quartz is derived from plutonic rock - particularly felsic plutonic rocks such as granites - from metamorphic rocks, and from older sandstones. Very little sand-sized quartz is derived from volcanic rocks. ^[1]

The near-absence of volcanic-derived sand-sized quartz reflects how quartz crystallises in volcanic systems. In felsic volcanic rocks, quartz phenocrysts can form, but volcanic eruption and rapid cooling produce fine-grained or glassy groundmasses in which quartz is too fine to contribute sand-sized grains. In contrast, granites and gneisses contain coarse quartz crystals that break down directly to sand-sized fragments. Older sandstones contribute recycled quartz grains that may already be well rounded from a previous transport cycle, further enriching the quartz content of new sediments.

Transport Rounding

Quartz grains in many sandstones display some degree of rounding acquired by abrasion during one or more episodes of transport, particularly transport by wind. ^[1]

Wind is by far the most effective agent for rounding quartz grains. In water, grains are cushioned by a fluid film during impacts; in air, the impacts are direct and the abrasion rate is orders of magnitude higher. Quartz grains that have undergone significant eolian transport therefore show a high degree of rounding and a frosted surface texture, whereas water-transported grains are often less rounded and their surfaces smoother. These textural contrasts allow the depositional environment to be partially inferred from grain morphology.

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