Sandatlas

Lithic sand

Siim — Sun, 07 Feb 2010 13:08:54 +0000

West Beach, Whidbey Island, Washington, USA.

There are mineral sands and biogenic sands. Mineral sands, as name says, are composed of minerals, but not exclusively. Sometimes they also contain significant amount of mineral aggregates which are called ‘rocks’ or in everyday language also ’stones’. Such sands and sandstones are said to be lithic.

Lithic sands are immature. It means that source rocks must be not very far and weathering agents have not yet had time to break these rocks down to their components. Lithic sands are often interestingly versatile, containing many unusual minerals and rock fragments.

There are very many rock types to be found in the world and consequently almost endless possibilities exist to categorize lithic sands into different types. Rock fragments that often occur in sand are amphibolite, sandstone (especially lithic sandstone), quartzite, chert, limestone, schist, phyllite, granite and basalt. Most lithic sands are dark in color.

Mixed carbonate-silicate sand

Siim — Sun, 07 Feb 2010 13:08:17 +0000

Jungmun Beach, Jeju-do island, South Korea.

As said earlier, world of sands can be divided into two major parts – mineral sands and biogenic sands. These sand types occur often separately but they can also mix in all proportions and form hybrid carbonate-silicate sand type.

This sand type is also found in low latitude beaches as biogenic sands and it is very common. It is usually a mixture of biogenic and volcanic or biogenic and continental sands.

Biogenic sand

Siim — Sun, 07 Feb 2010 13:06:15 +0000

Capdepera, Majorca, Baleares, Spain.

Biogenic sand is composed of exoskeleton or bone fragments of dead sea (usually) creatures. This sand is widespread in low latitude (less than 35°) beaches. There are myriad of different organisms that can contribute to the formation of this sand type and therefore one biogenic sand may greatly differ from the other. It is even possible to divide the world of sands into two parts – mineral sands and biogenic sands.

Most biogenic sands are light-colored and its components are usually made of carbonate material, although some organisms prefer silica. Important source material for biogenic sand are the remains of clams, snails, foraminifera, corals, sea urchins, sponges, etc. Sand may be composed up to 100 percent of biogenic grains but mixtures of mineral and biogenic grains are common also.

Garnet sand

Siim — Sun, 07 Feb 2010 13:03:17 +0000

Alma Gulch, Alaska, USA.

Garnet is important silicate rock forming mineral and present in many rock types, although it is almost never major component of a rock. Garnet sand is a sub-type of heavy mineral sand. Garnet is relatively stable mineral and therefore usual component in sands but almost never most abundant part of it.

There are some hard to find exceptions – red and very beautiful sands where garnet is indeed most abundant sand forming mineral. Source rocks of garnets are usually either metamorphic (garnet schist, rarely eclogite) or igneous (aluminium rich granite).

Olivine sand

Siim — Sun, 07 Feb 2010 13:02:35 +0000

Papakolea, Hawaii, USA.

Olivine sand is a rare sub-type of heavy mineral sand and also of volcanic sand. Olivine is important rock forming mineral and major component of mafic (rich in magnesium and iron) and especially ultramafic igneous rocks but it is rare in sand. There is one very good reason – it is very unstable in atmospheric conditions and decays quickly.

Good olivine containing sands can be found in Hawaii because basaltic lava, which makes up these islands, is rich in this mineral. There is one beach in Hawaii Island which is composed mainly of olivine. It is Papakolea on the southern tip of the island. Olivine is denser than other common minerals there and that allows waves to separate it from the rest and concentrate it on the beach. Source material for olivine is cinder cone which is situated right next to the beach.

Volcanic sand

Siim — Sun, 07 Feb 2010 13:01:59 +0000

Luciole Beach, Martinique.

Special type of heavy mineral sand is volcanic beach sand. It is usually black in color and may contain minerals like olivine, augite and hornblende which are major rock forming minerals but usually rare in sand because of weak resistance to weathering.

This sand type is especially common in volcanic islands because rocks that these islands are made of (mostly basalt) contains usually no quartz.

Some volcanic sands may be reddish because mafic (basalt and similar) volcanic rocks contain lots of iron which gives reddish appearance to weathered rocks and sand grains.

This sand type is beach sand. It does not include pyroclastic material like scoria, pumice or tuff found inland. Volcanic sand may be composed of minerals, fragments of volcanic rocks or both.

Heavy mineral sand

Siim — Sun, 07 Feb 2010 12:59:47 +0000

Seahurst, Washington, USA.

Like name says, heavy mineral sand contains lots of heavy minerals. Density of these minerals is generally above 2.85 grams/cm³. Important heavy minerals are magnetite, garnet, ilmenite, zircon, augite, hornblende, rutile, kyanite, tourmaline, biotite, titanite, apatite, etc.

These minerals appear often in sand but usually in low quantities (less than 1 percent). However, sometimes they are concentrated and form beautiful and unusual looking sands. There are many different looking heavy mineral sands but one which tends to appear most often contains garnet, magnetite and epidote as key minerals. Most heavy mineral sands contain quartz also but it isn’t as important as usually.

Source material for heavy mineral sands are metamorphic and igneous rocks. Metamorphic rocks are source material for kyanite and garnet for example, although latter may form in igneous rocks as well. Magnetite is of igneous origin mostly.

Heavy mineral sands occur almost always in seashore or paleo seashore. This sand type is natural mineral concentrate and it needs strong concentrating agent (sea waves) and right hydrodynamical conditions to form. Heavy mineral sand is not very rare but its area of occurrence is usually pretty limited.

Sands with hematitic pigment

Siim — Sun, 07 Feb 2010 12:56:18 +0000

Coral Pink Sand Dunes State Park, Utah, USA.

These sands and sandstones are composed mainly of quartz but they have intense pink, orange or red color. The coloring agent in this sand is volumetrically insignificant – it is hematite (oxide of iron) that forms very thin rust-colored pigment on larger silicate grains.

However, it’s influence on the appearance of sand or sandstone is very strong. A well known example of such sand is the Coral Pink Sand Dunes in the State of Utah, USA. Devonian sandstones in Estonia for example are even darker and are often said to be red. These sandstone formations are known throughout the world as “red beds”.

Hematite is not detrital mineral (formed as a result of disintegration of parent rock) in this sand type. It is mainly groundwater that carries iron and forms ferric gel on silicate grains. Complete dewatering of such gel results in a thin layer of iron oxide. Hematitic sands are very common in deserts.

Continental sand

Siim — Sun, 07 Feb 2010 12:54:36 +0000

Botany Bay, Sydney, Australia.

I’ve named this sand type ‘continental’ because it is the most characteristic sand type on beaches of continental landmasses. Quartz is dominating but it is usually accompanied by feldspar (usually K-feldspar) plus many other minerals, rock fragments or biogenic grains. However, all of these are not nearly as abundant as quartz.

This sand is usually yellow or light brown and often speckled with dark grains. It is common along passive continental margins and therefore the most widespread sand type on most of the European coasts for example.

Quartz sand

Siim — Sat, 06 Feb 2010 22:51:47 +0000

Tabina quarry, Estonia.

Quartz is most important as a sand forming mineral, and present in very many sand types but usually not exclusively. In this sand type, quartz is almost the sole component of sand. Similar sandstones are called quartz arenites or orthoquartzites.

Quartz is most resistant to chemical weathering among common minerals. It is also relatively hard and therefore resistant to physical weathering as well. This means that sands get more and more enriched in quartz as time goes by and sands composed almost solely of quartz are mature and often have travelled far from the source area, sometimes thousands of kilometers.

Needless to say the mineral particles that make up this kind of sand can be very old. They may have broken away from their source rocks millions or even hundreds of millions of years ago and have perhaps seen several lithification and weathering cycles.

Good examples of sands enriched in quartz can be found in Florida. Sand from Siesta Key beach is sometimes called the whitest in the world, although it has competitors. Quartz rich sands are found on the coasts of passive continental margins (the edge of a continent without active volcanism).