Minerals of the Earth's Surface

Beach gravel is being ground into clay. Andrew Alden photo

Geologists know about thousands of different minerals locked in rocks, but when rocks are exposed at the Earth's surface and fall victim to weathering, just a handful of minerals remain. They are the ingredients of sediment, which in over geologic time returns to sedimentary rock.

Where the Minerals Go

When the mountains crumble to the sea, all of their rocks, whether igneous, sedimentary or metamorphic, break down.

Physical or mechanical weathering reduces the rocks to small particles. These break down further by chemical weathering in water and oxygen. Only a few minerals can resist weathering indefinitely: zircon is one and native gold is another. Quartz resists for a very long time, which is why sand, being nearly pure quartz, is so persistent. Given enough time even quartz dissolves into silicic acid, H4SiO4. But most of the silicate minerals that compose rocks turn into solid residues after chemical weathering. These silicate residues are what make up the minerals of the Earth's land surface.

The olivine, pyroxenes and amphiboles of igneous or metamorphic rocks react with water and leave behind rusty iron oxides, mostly the minerals goethite and hematite. These are important ingredients in soils, but they're less common as solid minerals. They also add brown and red colors to sedimentary rocks.

Feldspar, the most common silicate mineral group and the main home of aluminum in minerals, reacts with water too. Water pulls out silicon and other cations ("CAT-eye-ons"), or ions of positive charge, except for aluminum. The feldspar minerals thus turn into hydrated aluminosilicatesthat is, clays.

Amazing Clays

Clay minerals are not much to look at, but life on Earth depends on them. At the microscopic level, clays are tiny flakes, like mica but infinitely smaller. At the molecular level, clay is a sandwich made of sheets of silica tetrahedra (SiO4) and sheets of magnesium or aluminum hydroxide (Mg(OH)2 and Al(OH)3). Some clays are a proper three-layer sandwich, a Mg/Al layer between two silica layers, while others are open-face sandwiches of two layers.

What makes clays so valuable for life is that with their tiny particle size and open-faced construction, they have very large surface areas and can readily accept many substitute cations for their Si, Al and Mg atoms. Oxygen and hydrogen are available in abundance. From the viewpoint of living cells, clay minerals are like machine shops full of tools and power hookups. Indeed, even the building blocks of life—amino acids and other organic molecules—are enlivened by the energetic, catalytic environment of clays.

The Makings of Clastic Rocks

But back to sediments. With the overwhelming majority of surface minerals consisting of quartz, iron oxides and clay minerals, we have the ingredients of mud. Mud is the geological name of a sediment that is a mixture of particle sizes ranging from sand size (visible) to clay size (invisible), and the world's rivers steadily deliver mud to the sea and to large lakes and inland basins.

That is where the clastic sedimentary rocks are born, sandstone and mudstone and shale in all their variety. (See Sedimentary Rocks in a Nutshell.)

The Chemical Precipitates

When the mountains are crumbling, much of their mineral content dissolves. This material reenters the rock cycle in other ways than clay, precipitating out of solution to form other surface minerals.

Calcium is an important cation in igneous rock minerals, but it plays little part in the clay cycle. Instead calcium remains in water, where it affiliates with carbonate ion (CO3). When it becomes concentrated enough in seawater, calcium carbonate comes out of solution as calcite. Living organisms can extract it to build their calcite shells, which also become sediment.

Where sulfur is abundant, calcium combines with it as the mineral gypsum.

In other settings, sulfur captures dissolved iron and precipitates as pyrite.

There is also sodium left over from the breakdown of the silicate minerals. That lingers in the sea until circumstances dry up the brine to a high concentration, when sodium joins chloride to yield solid salt, or halite.

And what of the dissolved silicic acid? That too is extracted by living organisms to form their microscopic silica skeletons. These rain down upon the seafloor and gradually become chert. Thus every part of the mountains finds a new place in the Earth.