Everybody knows sand from the summer time on the beach, as construction material or raw material for glass making. Talking about beach sand, did you ever notice that not all the beaches are the same? Some are really wonderful to walk onto while others can be a pain for the feet. And some are white like the snow and others are dark like the night. What about the sand from the desert, is it the same like the one from the beach? What makes them different, why they have so many colors…? Without pretending to be an expert I will try to answer these questions here from the point of view of a sand collector and not of a scientist. So…
…what is sand?
Sand can be defined as naturally occurring granular material having the grain diameter falling in the interval of 0.063 (1/16) to 2.0 mm. In the USA sand is usually classified by grain size into five main categories: very fine (1/16 – 1/8 mm), fine (1/8 – 1/4 mm), medium (1/4 – 1/2 mm), coarse (1/2 – 1 mm) and very coarse (1 – 2 mm).
…where does the sand come from?
Sand can be found on a variety of relief forms from beaches to deserts, lake shores, rivers, but also mountains, quarries, dunes along sea shores, sometimes also the side of the road. But how the sand arrives there depends on the processes that lead to the formation of sand grains. The most important is represented by weathering of the rocks caused by external factors like wind, precipitations, freezing – unfreezing cycles etc.
This method is basically mechanical and the sands resulted bear the physico-chemical characteristics of the mother rock they originate from. Explosive volcanism (pyroclastic process) is the reason for a more ‘dramatic’ formation of sands associated with volcanic activity and formation of igneous rocks of altered chemical composition. The third class is given by rock crushing mechanisms, both caused by rock movements as well as caused by the impact between two rock massifs. Deposition of dissolved minerals from warm seas can be the reason for the formation of sand grains by pelletization using both chemical and biological precipitation processes.
…what is the composition of sand?
The sand composition depends directly on its origins. As an example, sands collected from volcanic islands often have in their composition volcanic tuffs and ashes while the beaches of single atolls are usually made of corals and skeletal remains of tiny sea animals. The mineralogical composition of sands does have also a direct connection to the sands’ color, the mother nature being here extremely generous: white sands with biologic origin, black sands from volcanic islands, reddish hues in arid areas (deserts) etc.
Weathered cross-bedding sandstone layers in Negev desert, Israel
Further classification of sands is based on the consideration whether the sand has resulted from processes related with animal and plants life (biogenic sands) or resulted from rocks transformations (mineral sands). Very often in nature we can find combinations of these two groups – in this case we can speak about mixed sands. Nevertheless, the term carbonate sands is also used when speaking about sands formed by precipitation of calcium carbonate. This term may be very conclusive but it does not make any difference between biological precipitation (calcium carbonate found in skeletons of sea animals) and chemical precipitation (calcium carbonate dissolved in warm sea waters).
A biogenic substance is generally defined as a substance resulted from life processes (either animals or plants or both). The term ‘biogenic sand’ refers to sand made of skeletal remains of plants and animals. Most sands of biogenic origin contain skeletal rests of corals, calcium-depositing algae and different small marine animals such as gastropod mollusks (snail-like shells or fragments), bivalve shells, barnacles, foraminifera, sponge spicules, worm tubes etc. The biogenic sands can be distinguished from mineral sands by their high content in calcium carbonate (CaCo3).
The most common constituent of mineral sands is silicon dioxide present in its quartz form. Silicate sands (based on quartz) are present on beaches, deserts but also rivers, mountains and lakes and, to some extend, on volcanic islands. Beside quartz, another popular group are the sands composed of immature sediments such as granitic rock fragments (which implies that original rock is not very far and the weathering agents did not have sufficient time to alter the original rock). These sands may be called lithic sands and they may contain a huge variety of rock fragments (granite, basalt, sandstone, limestone etc).
Heavy mineral sands contain those minerals with specific weight higher than 2.9 g/cm³. Due to their stable composition in time, the heavy minerals offer valuable information about the original rocks they are originating from. Eroded from these rocks and transported to the bottom of water bodies, the heavy mineral sands are too heavy to be transported to the shores by the regular waves but they can be found in thin layers on the beaches after some stormy nights. Very common minerals are magnetite (black color, with magnetic properties), garnets (usually pink or red), ilmenite (black) etc.
Another group of sands which is very spectacular and offers impressive macroscopic images is represented by the volcanic sands. This type of sand is generally made of unconsolidated fragments of volcanic debris, usually lava which is cooling off very fast after a volcanic eruption. Volcanic sands can be further classified according to the final characteristics of the lava. When the ejected solid material get into violent contact with gases and steam released by a volcano it is blown apart into pieces of diffeernt sizes. The very small sized material forms the volcanic ash which, by consolidation, lead to the formation of volcanic tuff. Further weathering of volcanic tuff can produce particles of diameters in sand range as depicted in the image below:
The mixed sands contain both mineral and biogenic sand grains and are very common especially on sea shores. However, the group is very difficult to define since the it consists of combinations in different ratios of any of the above described sands (the mineral part can be of igneous, sedimentary or metamorphic origin while the biogenic part can have animal or plant origins).
These are sands of special interest for sand collectors with very often only one major constituent. These sands do not form a scientific group by themselves (they can be either biogenic or mineral) but they are very appreciated for their visual characteristics (color, grain shape etc) so I decided to list them separately, also mentioning the group they belong to.
Olivine sand from Hawaii (mineral sand)
After definition, origins and classification, the physical characteristics of sand like grain size, roundness, sphericity, and sorting degree will be described here during the following days so stay tuned and visit again this page if you want to know more about the fascinating sand.