Why Doesn't Sand Dissolve in Water?
Silicon dioxide, the primary component in sand, forms in large crystalline structures, which are held together by covalent bonds. These bonds require more energy to break than the polarity of water is able to supply. While water is able to dissolve a minute amount of silicon dioxide, natural water supplies are at the saturation point and unable to dissolve any more sand.
Silicon dioxide is the most abundant chemical compound in the earth's crust. It is the primary component of quartz, sandstone, opal, granite, clay and many rocks. It forms the skeletal parts of diatoms and sponges. In higher plants, the chemical forms stems and other tissues. It is relatively unreactive with most chemicals, including strong acids, with the exception of hydrofluoric acid, making silica glass a good choice for laboratory glassware.
According to About.com, sand is technically just a size category. It is a particulate matter larger than slit, yet smaller than gravel. Many scientists and geologists have different definitions for what sand is.
Dirty sand is quartz that mixes with other mineral grains of similar size. In certain locations, basalt lava weathers to create black sand. In others, olivine forms green sandy beaches. While these other types of sands include chemicals other than silicon dioxide, they are also relatively insoluble in water.