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Geologists know about thousands of minerals locked in rocks, but when rocks are exposed at the surface and weather away, less than 10 minerals remain. They are the ingredients of sediment, which in turn becomes sedimentary rock.
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. A very small number of minerals can resist 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, but given enough time even quartz dissolves into silicic acid, H4SiO4.
But most of the silicate minerals produce solid residues after chemical weathering. 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 hydroxides. These are an important ingredient in soils but uncommon 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 major cations (positive ions) except for aluminum. The feldspar minerals thus turn into hydrated aluminosilicates—that 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 (SiO4) tetrahedra 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 microbes, 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 quartz and clay, the overwhelming majority of surface minerals, we have the ingredients of mud. Mud is what geologists call a sediment that is a mixture of particle sizes ranging from sand (visible) to clay (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 were crumbling, much of their mineral content dissolved. 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 precipitates underground, from deeply buried fluids, as the silica mineral chalcedony. Thus every part of the mountains finds a new place in the Earth.
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