FALL - Meteorite seen to fall. Such meteorites are usually collected soon after falling and are not affected by terrestrial weathering.
FASSITE - Obsolete name for Ca- and Ti-rich clinopyroxene, properly termed augite.
FAULT - Fracture along which there has been movement or displacement.
FELDSPAR - Major rock building mineral containing Al-silicates. Feldspars include two groups: plagioclase (Na- and Ca-rich) feldspar and alkali (Na- and K-rich) feldspar. Feldspar compositions are often represented on a ternary diagram that plots Ca-, Na-, and K-feldspars at its apices. Alkali feldspars are rare in meteorites, which are depleted in the volatile element K.
FERMI ENERGY - Energy of the highest occupied state in a solid in its ground state. It represents the kinetic energy of electrons responsible for conduction.
FERMION - Class of elementary particles that have an odd half-integer (1/2, 3/2, and so forth) spin. Quarks and leptons, as well as most composite particles, like protons and neutrons, are fermions. Fermions obey the Pauli Exclusion Principle.
FILAMENT - Dark, thread-like feature seen on the Sun in the red light of hydrogen (H-alpha). Filaments are dense, omewhat cooler, clouds of material suspended above the solar surface by loops of magnetic field.
FIND - Meteorite not seen to fall, but found at some later date. For example, many finds from Antarctica fell 10,000 to 700,000 years ago.
FISSION - Breaking apart of a body into smaller fragments. In nuclear physics, fission refers to splitting of a heavy atomic nucleus into two or more lighter nuclei with an associated release of energy. The mass of the nucleus before fission is greater than the combined masses of the resulting fragments; the mass difference (Δm) is released as energy:
FLUX - Rate at which something is transferred through a surface. In astronomy, this is the quantity of energy per second (watts) passing through unit surface area (1 m2) perpendicular to the direction of the source and is expressed as W/m2. For example, the flux of radiation from the Sun at the Earth is 1367 W/m2.
FLUX DENSITY - Measure of the flux of radiant energy within a unit interval of frequency (a frequency band with a width of 1 Hz) and is expressed as W/m2Hz. The flux densities for most cosmic radio sources are extremely low: the unit of flux density used by radioastronomers is the jansky (Jy), where 1 Jy = 10-26 W/m2Hz.
FORBIDDEN LINE - Spectral line seen in emission nebulae but not seen in laboratory experiments, because under laboratory conditions, collisions kick the electron in question into some other state before emission can occur.
FORCE - That which produces acceleration. Intuitively, a push or a pull, an action of one thing upon another. Forces produce palpable effects. There are only four fundamental forces.
FORSTERITE - Mg-rich olivine, Mg2SiO4, common in meteorites. Fe substitutes for Mg with complete substitution yielding the Fe-rich olivine, fayalite, Fe2SiO4.
FORSTERITE (F) CHONDRITES - Material only known as inclusions in other meteorites and described by certain lithologies of the Cumberland Falls aubrite. They are thought to have derived from a small and primitive asteroid of F chondritic composition that collided with the aubrite parent body shortly after their formation in the early Solar System. Mineralogically, they consist primarily of forsterite. Forsterite chondrites are intermediate between the H and E chondrites in terms of their chemical makeup. No complete meteorites of this group have been found on Earth, and therefore this class should be considered hypothetical.
FRACTIONATION - Concentration or separation of one mineral, element, or isotope from an initially homogeneous system.
FREQUENCY (ν) - Rate at which specific events occur. For radiation and sound, this corresponds to the number of wave crests that pass a given point per second. Frequency is measured in cycles per second, called a hertz (Hz). Common multiples are kilohertz (1 kHz = 1000 Hz), megahertz (1 MHz = 106 Hz), and gigahertz (GHz = 109 Hz).
FUN CAIs - CAIs that show large mass fractionated and non-mass fractionated isotopic effects. FUN derives from Fractionated and Unidentified Nuclear effects.
FUNDAMENTAL PARTICLES - Particles considered basic, indivisible, building blocks of atoms and all forms of matter. These include stable leptons (electrons and neutrinos), and quarks and bosons. Also called "elementary particles."
FUNDAMENTAL FORCES - Forces that govern the various interactions between particles. The four fundamental forces, or interactions, are (in order of increasing strength): gravitational force, the weak nuclear force, the electromagnetic force, and the strong nuclear force. The strong and weak nuclear interactions are short-range forces that are effective only within atomic nuclei. The range of the strong force is ~10-15 m and that of the weak force ~10-17 m. In contrast, the electromagnetic and gravitational interactions are long-range forces, their strengths inversely proportional to the square of distance (1/r2). Although the effect of gravitation on particles is far weaker than the electromagnetic force, because matter tends to be electrically neutral, gravitation controls the overall dynamics of planets, stars, and galaxies. According to quantum theories, fundamental forces are conveyed between real particles by means of "virtual" particles called gauge bosons.
In the present-day universe, at common energy levels, the four forces are separate and have different strengths. However, at very high energies (more than ~1011 eV), the weak and electromagnetic forces merge into a single electroweak force. According to Grand Unified Theories (GUT), the strong nuclear and electroweak forces will behave as a single unified force at particle energies in excess of ~1024 eV (~1012 times higher than achievable experimentally). The fundamental forces may be summarized as follows:
FUSION - Process in which two lighter atomic nuclei combine to form a heavier atomic nucleus. Very high temperatures are normally required in order for atomic nuclei to collide with sufficient energy to overcome the Coulomb barrier (their mutual electrostatic repulsions). Fusion that occurs under high-temperature conditions is called thermonuclear fusion. Fusion reactions involving light elements release large amounts of energy. The mass of the resulting nucleus is less than the combined masses of the two original nuclei. The difference in mass, Δm (mass defect), is released as energy:
Fusion of elements up to 56Fe results in the release of energy. Thermonuclear fusion powers stars. Main sequence stars are dominated by hydrogen burning fusion reactions. In red giants, He is converted into C by the triple-alpha process and, in highly evolved high-mass stars, fusion reactions synthesize a succession of elements up to Fe by helium capture.
FUSION CRUST - Melted glassy exterior of a meteorite that forms when it passes through Earth's atmosphere. Friction with air can raise a meteorite's surface temperature to 4,800 K. Surface minerals melt at such temperatures and flow backwards over the surface as shown in the Lafayette meteorite photograph below. As the meteorite slows and the fireball is extinguished, the molten material cools and fuses to form a thin, glassy skin which envelopes the whole meteorite. Often the fusion crust is black or bluish-black and helps to make the meteorite stand out against the background of terrestrial rocks. Primary crust is the material that forms from the beginning of incandescent flight until dark flight. Secondary crust form if a piece breaks off the main mass during incandescent flight and a new crust forms on the broken surfaces.
