Sand that remembers the rock it once was

Sand is usually composed of mineral grains that come from disintegrated rocks. Rocks are mineral aggregates, they generally contain several distinct minerals. If we have a sand that is compositionally close to the rocks it comes from, then we call it immature. The opposite situation is a sand that is almost exclusively composed of quartz and other ultraresistant minerals. Such sand is mature or evolved because it has lost significant part of its former components.

Sand sample collected near Bridalveil Fall, Yosemite National Park, California. This is river sand. Its source rocks are very near. Sand is composed of plagioclase (white), quartz (transparent), K-feldspar (yellow, reddish), and biotite (black, some are brown and green due to weathering). These minerals put together into one rock give us an igneous rock granodiorite (similar to granite but contains more plagioclase than K-feldspar). The width of the view is 10 mm.

Sand composed of rock fragments (lithic sand) is definitely the most immature sand type in existence but I’m not focusing on that here. This post is devoted to immature sand that is largely composed of mineral grains.

Many minerals are unstable in the weathering environment. Therefore the immature sands can not be located far from their parent rocks. In addition to time, the most important variables that determine the rate of weathering are temperature and the availability of moisture. Hotter climate makes chemical reactions faster and water is needed to generate these reactions. Maybe you have heard that weathering is the disintegration process of rocks which is driven by temperature changes, frost-thaw cycles, etc. That is true to some extent but chemical weathering is by far the most important agent of weathering.

So, where do we have the best chances to see this type of sand? We need cold and/or dry climate plus short transport distance. Our chances are pretty good if we start looking for this type of sand in the riverbanks because the transport distance there is often very short. Sand grains on the beach are often evolved, especially in hot and humid climates. Beaches of Florida, for example, are predominantly composed of almost pure quartz (some contain significant amount of biogenic grains also, but this is not important here). Beaches of Canadian Arctic on the other hand are not mature at all. They generally contain lots of feldspar and other minerals in addition to quartz.

This is beach sand from the Canadian Arctic. Coronation Gulf, Nunavut. Sand is composed mostly of quartz (transparent), feldspars (red), and hornblende (black). Its source area could be a metamorphic terrane composed of granitic gneiss and amphibolite. The width of the view is 10 mm.

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