Quartz sandstone is a clastic sedimentary rock. Clastic rocks are distinguished by their formation from fragments of pre-existing rocks and minerals. Sandstones are common, comprising approximately 20–25% of all sedimentary rocks, and are defined by their sand-sized grains. Quartz sandstone specifically represents a highly purified and stable end-member of the sandstone family.
Defining Clastic Sedimentary Rocks
Clastic sedimentary rocks are formed from the accumulation and cementation of pieces of older rock. The process begins with the physical and chemical breakdown of source rock through weathering and erosion.
These fragments are then transported by agents like water, wind, or ice away from their original location. As the energy of the transporting medium decreases, the sediments settle out in a process known as deposition. Over time, the weight of overlying sediments compacts the deposited material.
The final step in forming a solid clastic rock is lithification, which involves cementation. Dissolved minerals precipitate into the spaces between the clasts, effectively gluing the loose sediment together. Common cementing agents include silica, calcite, and iron oxides, which bind the sand grains into a coherent rock mass.
The Specific Formation of Quartz Sandstone
Quartz sandstone is the result of prolonged and intense clastic processes. It is characterized by high purity, typically containing more than 90% quartz grains. This dominance is a direct consequence of quartz’s exceptional stability and resistance to both chemical and physical weathering.
As sediments undergo repeated cycles of weathering and transportation, less stable minerals, such as feldspar and mica, are chemically altered and dissolved. Quartz is highly durable and survives these harsh conditions, resulting in a residue of nearly pure quartz sand. This extensive reworking and removal of unstable components defines quartz sandstone as a compositionally mature sedimentary rock.
The sand grains in a mature quartz sandstone are often well-rounded. This indicates they have been transported over long distances or subjected to high-energy environments, such as beaches or dune fields. The resulting rock is extremely hard and weather-resistant.
Classifying Sandstone Based on Mineral Content
While all sandstones are clastic rocks, geologists classify them further based on the specific composition of their sand-sized grains. This classification helps determine the rock’s history, including the source rocks and the intensity of weathering and transport. The relative percentages of quartz, feldspar, and lithic fragments are the primary factors used.
Quartz sandstone, with its high purity, represents the most mature composition. In contrast, Arkose is a less mature sandstone containing a substantial amount of feldspar, typically more than 25%. The presence of feldspar indicates less chemical weathering and shorter transport distances from the source rock.
Another type is Graywacke, a poorly sorted sandstone containing a significant amount of fine-grained matrix material and lithic fragments. The unstable fragments and muddy matrix suggest rapid deposition near the source area with minimal weathering or sorting.