TOEFL Experts Reading Practice 24
When most people hear the word “crystal,” they think of something cherished and rare. In fact, however, crystals are common occurrences in nature. A crystal, by definition, is any solid whose constituent parts (atoms, molecules, or ions, for example) are arranged in a highly ordered structure. This structure repeats, forming what’s known as the “crystal lattice” that extends in all directions. Most inorganic solids are polycrystals—they are not each a single crystal. Rather, they are made up of many microscopic crystals fused together to make a single solid mass. Most metals and also ceramics fall into the polycrystal category. True crystals and polycrystals can be formed by a variety of natural forces, including heat and pressure deep underground, as well as cold on the earth’s surface and even the flow of water.
Gems and minerals are the most widely recognized crystals. Although most mineral crystals form under similar conditions, their structures can be very different. As a general rule, a crystal’s shape, perceptible to the naked eye, is a reflection of its microscopic structure. Because crystals are the repetition of a particular structure, they are defined by the “unit cell,” which is the smallest group of particles containing the pattern that is then repeated to form the crystal lattice. A crystal with cubic unit cells can form a crystal that is, itself, a cube. Pyrite, known as “fool’s gold” because of its resemblance to the precious metal, has a cubic structure and often forms in masses of interlocking cubes. Crystals with hexagonal unit cells, on the other hand, tend to form in barrel-shaped hexagonal prisms or pyramids. Corundum, the mineral best known for its gem varieties ruby and sapphire, is a hexagonal crystal. Some of the most prized specimens of corundum are bipyramidal sapphires, which are hexagonal prisms that are not barrel-shaped, but instead appear as though two pyramids are stuck together at the bottom. The middle of the crystal is its widest point, and it tapers sharply at each end, forming the tips of the pyramids.
Mineral crystals can form from magmatic processes. These processes occur when molten magma (also known as lava) solidifies and cools. The conditions under which the molten magma cools modify the type of crystal that is formed. Granite, for example, cools very slowly and under intense pressure, leading to complete crystallization. Many other magmatic rocks are formed when lava pours out onto the Earth’s surface and cools very quickly, leaving small amounts of glassy material and never achieving full crystallization.
Fluids can also be an agent of crystallization. Suspended in a fluid solution, microscopic molecules, or even individual atoms, can begin to gather into clusters within the solution, forming stable, though still microscopic, structures. Once a cluster is large enough to be stable, it is called a nucleus. This nucleus will contain at least one unit cell and thus will define the crystal’s structure, even in this microscopic state. Crystal growth will occur as more atoms or molecules are pulled out of the solution and join with the nucleus. In this way, crystallization is a solid-liquid separation technique.
When liquids evaporate, they form crystals from the sediment (solid materials) that remain after the evaporation has taken place. Water-based “aqueous” solutions often have suspended within them particles of minerals that can form crystals if the solution evaporates. Take, for example, the water of the ocean. Ocean water is saline, meaning that it contains relatively high concentrations of salt. When saline water evaporates, the salt cannot become gaseous. Instead, it is left behind in its solid form. When these solids make up a high enough volume of the evaporating fluid, they unite, forming the crystal halite, also known as rock salt. Vast expanses of halite can form when large bodies of saline water evaporate, giving rise to great deserts of salt like the one in Bonneville, Utah. The so-called “salt flat” is composed of 90 percent pure halite, with a surface so hard and flat that it has been used since 1914 to set land speed records for car and motorcycle racers.
When most people hear the word “crystal,” they think of something cherished and rare. In fact, however, crystals are common occurrences in nature. A crystal, by definition, is any solid whose constituent parts (atoms, molecules, or ions, for example) are arranged in a highly ordered structure. This structure repeats, forming what’s known as the “crystal lattice” that extends in all directions. Most inorganic solids are polycrystals—they are not each a single crystal. Rather, they are made up of many microscopic crystals fused together to make a single solid mass. Most metals and also ceramics fall into the polycrystal category. True crystals and polycrystals can be formed by a variety of natural forces, including heat and pressure deep underground, as well as cold on the earth’s surface and even the flow of water.
Gems and minerals are the most widely recognized crystals. Although most mineral crystals form under similar conditions, their structures can be very different. As a general rule, a crystal’s shape, perceptible to the naked eye, is a reflection of its microscopic structure. Because crystals are the repetition of a particular structure, they are defined by the “unit cell,” which is the smallest group of particles containing the pattern that is then repeated to form the crystal lattice. A crystal with cubic unit cells can form a crystal that is, itself, a cube. Pyrite, known as “fool’s gold” because of its resemblance to the precious metal, has a cubic structure and often forms in masses of interlocking cubes. Crystals with hexagonal unit cells, on the other hand, tend to form in barrel-shaped hexagonal prisms or pyramids. Corundum, the mineral best known for its gem varieties ruby and sapphire, is a hexagonal crystal. Some of the most prized specimens of corundum are bipyramidal sapphires, which are hexagonal prisms that are not barrel-shaped, but instead appear as though two pyramids are stuck together at the bottom. The middle of the crystal is its widest point, and it tapers sharply at each end, forming the tips of the pyramids.
Mineral crystals can form from magmatic processes. These processes occur when molten magma (also known as lava) solidifies and cools. The conditions under which the molten magma cools modify the type of crystal that is formed. Granite, for example, cools very slowly and under intense pressure, leading to complete crystallization. Many other magmatic rocks are formed when lava pours out onto the Earth’s surface and cools very quickly, leaving small amounts of glassy material and never achieving full crystallization.
Fluids can also be an agent of crystallization. Suspended in a fluid solution, microscopic molecules, or even individual atoms, can begin to gather into clusters within the solution, forming stable, though still microscopic, structures. Once a cluster is large enough to be stable, it is called a nucleus. This nucleus will contain at least one unit cell and thus will define the crystal’s structure, even in this microscopic state. Crystal growth will occur as more atoms or molecules are pulled out of the solution and join with the nucleus. In this way, crystallization is a solid-liquid separation technique.
When liquids evaporate, they form crystals from the sediment (solid materials) that remain after the evaporation has taken place. Water-based “aqueous” solutions often have suspended within them particles of minerals that can form crystals if the solution evaporates. Take, for example, the water of the ocean. Ocean water is saline, meaning that it contains relatively high concentrations of salt. When saline water evaporates, the salt cannot become gaseous. Instead, it is left behind in its solid form. When these solids make up a high enough volume of the evaporating fluid, they unite, forming the crystal halite, also known as rock salt. Vast expanses of halite can form when large bodies of saline water evaporate, giving rise to great deserts of salt like the one in Bonneville, Utah. The so-called “salt flat” is composed of 90 percent pure halite, with a surface so hard and flat that it has been used since 1914 to set land speed records for car and motorcycle racers.
- Crystals are solids with an orderly internal structure.