Question

Describe the textural change that occurs to sand as it becomes quartzite. Indicate each intermediate state.

Short answer

Sand becomes quartzite through compaction into sandstone and metamorphism.

Step-by-step solution

Identifying the Origin

Sand originates from various mineral fragments, particularly quartz, and is initially loose and granular. It forms from the weathering and erosion of rocks, especially granite.

Lithification into Sandstone

Over time, layers of sand accumulate and are compacted by overlying materials. The pressure and mineral-rich water help cement the grains together, turning sand into sandstone.

Metamorphism to Quartzite

Sandstone undergoes metamorphism due to high temperature and pressure, often from tectonic forces. During this process, quartz grains recrystallize and interlock, enhancing the rock's hardness and density, forming quartzite.

Key concepts

Sandstone

Sandstone is a type of sedimentary rock that primarily consists of sand-sized mineral particles or rock fragments. The most common minerals found in sandstone are quartz and feldspar, given their abundant nature in the Earth's crust. The formation of sandstone begins with the weathering and erosion of pre-existing rocks, which break down to create loose sand grains. These grains are then transported by wind, water, or ice and often settle in layers to form sedimentary environments like beaches, riverbeds, or dunes.

Over time, these layers of sand are buried by additional sediment, and the process of lithification ensues. Lithification involves the compaction of sand grains due to the pressure from overlying materials, and cementation, where mineral-rich groundwater permeates the spaces between grains and precipitates minerals that bind them together. The resulting rock is a coherent, solid mass known as sandstone, recognized for its layered structure and often reddish or yellowish hues, depending on the mineral cements used.

Metamorphism

Metamorphism refers to the transformation of existing rock types, such as sandstone, into new forms due to intense heat and pressure conditions. This process generally occurs below the Earth's surface and is driven by tectonic movements that create significant stress and temperature changes. These conditions cause structural and chemical changes in the rocks without melting them.

Commonly, during metamorphism, minerals within the rock undergo recrystallization, but the rock remains solid throughout. The key to understanding metamorphism is recognizing the role of

• heat – which can facilitate chemical reactions and recrystallization.
• pressure – that compresses and rearranges the minerals.
• chemically active fluids – which can promote transformations by introducing or removing elements.

The result is often a more densely packed, stronger rock with new mineral assemblages, typically leading from sandstone to quartzite.

Recrystallization

Recrystallization is a fundamental process during metamorphism where the texture and mineral composition of a rock change without altering its chemical structure. This process occurs when rock is subjected to conditions of elevated temperature and pressure, leading to the growth and interlocking of new mineral crystals.

During recrystallization, the original quartz grains in sandstone begin to grow and merge at their boundaries, eliminating any gaps or pore spaces. This development increases the overall hardness and density of the resulting rock, which is quartzite. The new crystal structure also provides structural integrity, making quartzite much more resistant to weathering and erosion than its precursor, sandstone. The process does not involve melting but instead a solid-state restructuring that enhances the rock's toughness.

Sedimentary Processes

Sedimentary processes are crucial in understanding the formation of rocks like sandstone, which later metamorphose into quartzite. These processes include various stages starting from weathering, erosion, transportation, deposition, to lithification.

Initially, weathering breaks down rocks into smaller particles, typically sand-sized. These particles are then carried away by agents like wind, rivers, glaciers, or ocean currents. The transportation phase allows them to settle in new locations, often layering over bedrock or older sedimentary deposits.

• Deposition occurs as the velocity of the transporting medium decreases, causing particles to settle out.
• Over time, accumulated layers compact and often become cemented by minerals such as silica or calcium carbonate that crystallize from water percolating through the sediments.
• This combined process of compaction and cementation solidifies sediments into rock, known in this example as sandstone.

Understanding these processes helps in comprehending how surface processes contribute to geological formations that are later transformed into more robust metamorphic rocks like quartzite.