Richard Sanders, age 11, of Ottawa, Ontario, Canada, for his question:
How are concretions formed?
This topic belongs in the rock hound department. Mineral concretions are concreted or cemented nodules embedded in the rocks of the earth's crust. Any ordinary concretion is a collector's item. Some are surprisingly different from the rocks around them and some are treasured for their surprising shapes.
No two concretions are exactly alike, but every one of them is a rocky old timer. It was formed in the earth's crust by the chemical reaction of different minerals dis¬solved in ground water. For this reason, most concretions began to form when a region was submerged by an ancient swamp or sea. Through millions of years, the harder nodules were cemented molecule by molecule in layers of sedimentary rocks. As a rule a con¬cretion is formed from a typical impurity found in sandstone or shale, limestone or chalk. Often, but not always, it formed in concentric layers around a central nucleus, perhaps a leaf or a shell, a bone or a grain of sand.
Lime concretions tend to form in clay deposits and silica concretions in limestone. Nodules of iron oxide are found in sandstones and nodules of flint in beds of chalk. These patterns occur because a dissolved chemical tends to deposit its fragments when it contacts a different chemical. Very often the chemicals that started the process were organic materials such as decaying leaves, bones or shells. When a concretion is broken apart, a fossilized bone or the whole skeleton of a small animal may be revealed.
The most common concretions are claystones. Their odd shapes are found almost everywhere in deposits of limestone and softer clays. They may be ovals, rings or disks. Those found in limestone are usually flat because the water that deposits them flows more freely through the clay. These concretions grow in onionskin layers and often they grow large enough to touch each other. Later deposits cover them both, merging them into a dumbbell or other odd shape. Lime concretions usually are found in the shales or sandstones of ancient sea beds and almost always they formed around a shell or fish bone. They range in size from an inch to several feet.
Some of the most odd shaped concretions were cemented by iron oxide impurities in sandstone. These nodules of limonite and hematite are often unearthed from their bed¬rock by the wind in the western deserts. When cut apart, their concentric rings tell an age old story of mineral deposits by fluctuating ground water. The hardest concre¬tions are nodules of flint found in beds of soft, crumbly chalk. The flint is almost pure silica, hardest of the common minerals. Often a flint concretion is embedded with tiny fragments of bone and shell.
Septaria are very interesting lime concretions that have shrunk and cracked with time. Their networks of cracks were later filled with calcite, dolomite or other minerals dissolved in the ground water. Oolite concretions look like masses of little eggs and they are usually found in deep water. The cement may be lime, silica or hematite formed around grains of sand. But some oolites appear to have no hard nuclei and geologists suspect that they may be formed by the action of bacteria.