|MadSci Network: Earth Sciences|
Dear Jennifer, The uses to which rocks can be put depend on their physical properties and on the minerals they contain. As I am sure you have discovered, schist and conglomerate are two quite different types of rock. However, what they may have in common is that neither one is especially useful. For example, neither schist nor conglomerate is an ore containing desirable elements such as iron or gold, so neither is quarried on a large scale. This does not mean that they have no uses, however. Schist is a metamorphic rock, which means that it was formed by the transformation of other types of rock under conditions of great heat and pressure deep in the Earth’s crust. Metamorphic rocks come in a sequence of what are called metamorphic grades. Rocks of low metamorphic grade, such as slate, are rocks in which the process of metamorphism has been halted at a relatively early stage; and rocks of high metamorphic grade, such as gneiss, have been subjected to extensive metamorphism at very high pressures and at temperatures just short of being high enough to cause them to melt. Schist is intermediate between slate and gneiss, and is toward the higher end of metamorphic grade (closer to gneiss than to slate). Gneiss is widely used as an ornamental building stone because it takes a fine polish and can be quite beautiful. All metamorphic rocks show an alignment of their crystals because the high pressure in which they were formed tends to cause crystals to grow in the same alignment. With gneiss this alignment of the crystals can come out when polished as a lovely pattern of banding of the different minerals (such as pink feldspars and glassy quartz). Schist contains more mica than gneiss does, and since mica is a flaky mineral it is hard to work with. Schist would be more difficult to polish than gneiss, and would tend to stand up less well to weathering if placed, say, on the outside of a building. But, still, you might be able to find examples of schist used as a polished ornamental stone. Where I live, in the United Kingdom, one sees gneiss quite frequently as a polished ornamental stone, but I don’t recall ever seeing a schist used in this way. Another use for schist is that it is a source of the semi-precious gemstone called garnet. Garnet is a mineral that is nearly always found only in schists, and there are many schists throughout the world in which large crystals of garnet can be found. In the United Kingdom, for example, schists from Scotland are well known for their beautiful garnets. Conglomerate is a sedimentary rock, formed by the transportation and deposition of eroded fragments of other rocks by the action of running water, as in rivers and along shorelines. Conglomerate typically contains a wide range of sizes of “grains”, ranging from pebbles down to very fine material. Because of this range of grain sizes conglomerates probably don’t make very good building material – they would lack the strength to support heavy structures and would be vulnerable to weathering. But, like schist, they can make interesting and beautiful polished ornamental stone. I have even seen unpolished conglomerates used as ornamental stone, always inside of buildings. If you want to see examples of rocks used as ornamental stones in buildings, you can usually see plenty of examples in any town or city centre or in shopping malls, where the architects want to create beautiful and impressive surroundings. Most of the rocks you will see will be crystalline igneous rocks such as granite and basalt, or high-grade metamorphic rocks such as gneiss. You can tell them apart because the igneous rocks will be a mass of crystals without showing flowing or banding, whereas the metamorphic rocks will usually show flow patterns and banding. I hope this answers your question. Best wishes, David Scarboro
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