While most people associate sand with the light-colored grains found on beaches and in deserts, the answer to whether all sand contains silica is definitively no. The vast majority of sand across Earth’s continental landscapes is made of the mineral quartz, which is the crystalline form of silicon dioxide (\(\text{SiO}_2\)). However, sand is fundamentally a material defined by the size of its individual grains, not by its chemical composition. This means a wide variety of minerals and organic materials can form sand, particularly in specific geological environments.
Defining Sand Grain Size and Composition
Sand is classified in geology based on the physical size of its particles, a standard set by the Wentworth scale. Sand grains range in diameter from 0.0625 millimeters up to 2 millimeters. Anything smaller is considered silt or clay, while larger particles are classified as granules or pebbles. The definition is purely physical, meaning any material that breaks down into this size range, from volcanic rock to seashells, can technically be called sand.
The mineral responsible for the familiar appearance of most sand is quartz. Quartz is the most common crystalline structure of silicon dioxide, or silica. Quartz sand is often referred to as silica sand because of its high purity, with industrial-grade deposits containing over 95% silicon dioxide. This widespread abundance creates the common misconception about sand’s universal chemical makeup.
Why Quartz Sand Dominates Global Landscapes
The prevalence of quartz in sand is a result of its unique durability. Quartz is one of the most abundant minerals in the Earth’s crust, forming a significant component of common source rocks such as granite. When these rocks are exposed to the elements, they undergo a process called weathering, which breaks them down into smaller fragments.
During both mechanical and chemical weathering, quartz is exceptionally resistant compared to other rock-forming minerals like feldspar and mica. Most other silicate minerals chemically break down into clay minerals or dissolve completely in water, especially in the presence of slightly acidic rainwater. Quartz, however, maintains its chemical structure and hardness, allowing it to survive long periods of transport and erosion.
This resistance means that over time, the weaker minerals are eliminated, leaving behind an increasing concentration of quartz grains. The final product of this prolonged weathering is known as “mature sand,” which is nearly 100% quartz and often characterized by well-rounded grains. Quartz’s persistence makes it the default mineral for sand in most of the world’s ancient and well-traveled sedimentary systems.
Sand Types Lacking Silicon Dioxide
While quartz dominates in continental and temperate environments, specific geological settings produce sands where silicon dioxide is absent or only a minor component. These exceptions highlight the physical definition of sand and demonstrate a diverse chemical makeup.
Calcareous Sand
One of the most common non-silica sands is biogenic, or calcareous, sand, which is primarily composed of calcium carbonate (\(\text{CaCO}_3\)). This light-colored sand forms in tropical and subtropical regions where marine life thrives, such as near coral reefs. The grains are fragments of the hard skeletal remains of organisms, including corals, sea urchins, foraminifera, and mollusks. When these organisms die, the fragments break down into sand-sized particles. Such beaches are chemically distinct from quartz sands and will react with weak acids due to the carbonate content.
Black Sand
Another major category of non-silica sand is black sand, which originates from volcanic activity. These sands are typically found on the shores of volcanically active islands, like Hawaii and Iceland. They are composed of dark-colored minerals like basalt, olivine, and magnetite. Basalt is a mafic rock, rich in iron and magnesium silicates, which gives the sand its characteristic dark color. Lava flows rapidly cool and shatter upon contact with water, creating sand-sized fragments of volcanic rock and minerals that are low in free silica.
Gypsum Sand
A geographically rare but striking example is the sand found at White Sands National Park in New Mexico, which is composed of gypsum (\(\text{CaSO}_4 \cdot 2\text{H}_2\text{O}\)). Gypsum is a soft mineral made of hydrated calcium sulfate that rarely forms sand because it is highly water-soluble. The unique conditions of the Tularosa Basin, which lacks an outlet and allows mineral-rich water to evaporate, resulted in massive deposits of gypsum crystals. Wind then breaks these crystals down into the sand-sized grains that form the world’s largest gypsum dune field, creating brilliant white dunes that contain no silicon dioxide.