Sand is defined by the size of its individual particles rather than its specific composition. Geologically, sand grains typically range in diameter from 0.0625 millimeters to 2 millimeters. These particles are larger than silt but smaller than gravel, and they originate from the breakdown of various rocks and minerals.
The Dominant Mineral
Quartz, a form of silicon dioxide (SiO2), is the most common mineral found in sand, especially in inland continental and non-tropical coastal settings. Its prevalence stems from its durability and resistance to both physical and chemical weathering. Quartz ranks high on the Mohs hardness scale at 7, resisting mechanical breakdown. It is also chemically inert, not reacting with most substances, such as weak acids in rainwater. These properties allow quartz grains to persist and accumulate after extensive transport.
Beyond the Primary Component
While quartz is dominant, sand often contains other mineral and organic components, depending on its source and environment. Feldspar, a mineral group that constitutes over half of Earth’s crust, is commonly present, particularly in less weathered sands. Other minerals include mica, appearing as shiny flakes, and magnetite, an iron-rich mineral that can color sand dark or black. These accessory minerals often provide clues about the sand’s origin.
Sand can also incorporate biological components, especially in marine and tropical settings. Shell fragments, coral, and other organic debris, composed of calcium carbonate, are common constituents in such sands. For example, the white sands of many tropical beaches form from eroded limestone, coral, and shell material. These diverse materials contribute to the unique appearance and characteristics of sand from different locations.
How Sand Forms and Varies
Sand forms through the weathering and erosion of pre-existing rocks over time. Physical weathering, like wind, water, and temperature changes, breaks rocks into smaller fragments, while chemical weathering alters mineral compositions. These processes liberate sand-sized particles from parent rocks such as granite, basalt, or limestone. Once formed, these grains are transported by natural agents like rivers, wind, ocean currents, or glaciers.
The journey and source material influence the sand’s final characteristics, including its shape, size distribution, and color. Long-distance transportation tends to round the grains and sort them by size, creating more uniform sand. Sand color reflects its mineral composition; white sands are rich in quartz or calcium carbonate from shells and coral. Black sands originate from volcanic rocks like basalt and contain dark minerals such as magnetite or olivine. Red or orange sands derive their hue from iron oxide coatings on the grains, commonly hematite.