Science, Part 23

Question 1: What is a mineral?

Answer 1: A mineral is a naturally occurring, inorganic, solid physical substance. Minerals have regular atomic structures (crystallizations) and definite homogenous chemical compositions. Some minerals are elements, but many are chemical compounds. These materials are formed through the cooling and hardening of magma, sublimation of a gas (especially near volcanic vents), precipitation (parsing out of solids from a solution), or through metamorphism. Metamorphism is the process whereby a rock experiences changes in characteristics while remaining in a solid state. This occurs due to the presence of one or more of these agents: heat, stress, pressure, or chemically active fluids. This chemical reaction can produce minerals as by-products. Minerals are identified and classified based on certain physical characteristics; the study of minerals is called mineralogy.

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Question 2: How does the speed of cooling affect the crystallization of a liquid?

Answer 2: Nucleation is the process whereby clusters of atoms in a liquid arrange themselves in a solid configuration. At lower temperatures, these nuclei become stable. When this process occurs slowly (when the rate of growth is greater than the rate of nucleation), large, stable crystals form. When the rate of nucleation is greater than the rate of growth (during rapid cooling), the process results in the formation of many small crystals. Minerals with this crystalline composition are inherently unstable and will aggregate into a large, solid crystal over time. Rapid cooling usually produces anhedral crystals (crystals with internal regularity but without external crystal faces). Slower cooling processes generally result in the formation of euhedral crystals (those with well-formed faces) or subhedral crystals (which exhibit intermediate, rounded outlines).

Question 3: Explain three types of crystal growth.

Answer 3: The ideal type of crystal growth is two-dimensional nucleation. In this process, crystals are formed in separate layers. The crystal faces are built up through accumulation of these layers, as the mineral’s atoms arrange themselves in the configuration of highest stability (that with the lowest energy). Two-dimensional nucleation occurs slowly and necessitates high supersaturation. Spiral growth, on the other hand, is a quick process. Also called screw dislocation, spiral growth is the continuous growth of a single layer in the form of a spiral. This type of growth occurs in regions of low saturation. Branching growth is the rapid growth of a crystal in limited directions, resulting in a treelike structure. Heat transfer halts favorable growth and causes growth in an unfavorable direction. This process creates dendrites, formed from a single nucleus and with a single crystallographic orientation.

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