Sand samples

Sand from Bretignolles-sur-Mer

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Sand from Bretignolles-sur-Mer (France, the Bay of Biscay) is famous among sand collectors because of its unusual mineralogy and appearance. The sand is composed of spinel, magnetite, epidote, garnet, staurolite, ilmenite, K-feldspar, quartz, zircon, corundum, apatite, titanite, rutile, etc. It is reddish or even purplish in color.

Many sand samples contain these minerals but in the vast majority of cases all other minerals besides quartz and feldspar occur in very small quantities. Here they clearly form the majority. Such sand type is called heavy mineral sand. These sands tend to be fine-grained. Getting sharp images which still show every single grain is a tough task but I hope you can get a general overview what kind of beautiful and colorful crystals such sands contain.

I recommend to check out the gigapan of the same sand sample.

Heavy mineral sand
Sand from Bretignolles-sur-Mer. There are more interesting minerals in this sand but here I numbered those that should be recognizable in this picture: 1 — spinel (iron-bearing), 2 — almandine (garnet), 3 — staurolite, 4 — may be titanite (sphene), 5 — K-feldspar, 6 — epidote, 7 — quartz, 8 — magnetite. The width of the view is 5 mm.

Maerl from Ireland

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It seems to be a widespread misunderstanding that biogenic sand is a tropic phenomenon. It is true that corals prefer warm, clear, and shallow water but corals are not the only organisms that contribute to the formation of biogenic sand. Many so-called coral sands contain no coral reef fragments at all. One of them is a maerl which is not uncommon on the beaches of Ireland, Scotland, and Bretagne.

http://picasaweb.google.com/107509377372007544953/Chert#5807632592190233298
Fine-grained biogenic sand consisting of red algae, sponge spicules, sea urchin spines, clams, sea snails, and foraminifers that occur close to maerl deposits. Width of view 5 mm.

However, the term ‘maerl’ is probably not well known. Even the Glossary of Geology (1997 edition) doesn’t mention it. It is a sand or gravel (many maerl fragments are actually larger than 2 mm – the upper limit of a sand grain) that is predominantly composed of delicate aragonitic shells of a coralline red algae (Rhodophyta). Two of the most common species on the Ireland’s western coast are Phymatolithon calcareum and Lithothamnion corallioides.

The color of these algae is red or pink but the color vanishes when algae dies. Maerl sand is predominantly white. Red algae has important role in the coral reef ecosystem but they could successfully exist without the help of corals. Maerl is not composed of only red algae. It is interestingly versatile. There are lots of sponge spicules. These are clear siliceous (mostly) structural elements of sponges. They look very cool – just like the logo of Mercedes-Benz. There are also clams, foraminifers, snails, and sea urchins. Maerl is sometimes composed entirely of biogenic fragments. It could be fine- or coarse-grained.

Coralline red algae live in varying depths of water (less than 20 meters below the sea level in NE Atlantic) and get occasionally washed onshore with stronger waves. This sand is not without economic use. It has been used in agriculture (as a soil conditioner), cosmetics, bone surgery, water purification, and even as a food additive.

http://picasaweb.google.com/107509377372007544953/Chert#5807632589430685138
Coarse-grained maerl composed of red algae but also clams, sea snails, and quartz grains. Width of view 15 mm.

Ghana sand

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This sand sample is from the Western coast of Ghana. Sand sample is coarse-grained and is composed of several distinct and interesting components. Here is a short illustrated description of this sand sample.

This sand is pretty unusual because it is largely composed of lithic fragments. Most sands are composed of mineral fragments, majority of them being quartz of course. Quartz is present in this sample also but it is a relatively minor component. Lithic sands are immature sediments. It means that their source rocks are not far away. Volcanic rocks of bimodal character (basalt-rhyolite) are present on the coast in this particular case. Typically to ocean coasts at low latitudes, this sand contains many different biogenic grains. Most of them are clam shells and sea urchin spines.

Sand from Ghana
Sand sample itself. Most grains are about 1 mm in diameter.

Sea urchin spines
Sea urchin spines.

Clam shells
Clam shells.

Altered basalt
Altered basalt (green color is a result of chloritization and epidotization).

Rhyolite fragments
Rhyolite fragments.