C CLASS – Common asteroid class in the outer part of the Main Belt. Compositions, based on reflectance spectra, are probably similar to carbonaceous chondrites containing hydrated minerals. B, F and G are subtypes of the C class.
C3 PLANTS – Plants that photosynthesize by the Calvin cycle. This process is called C3 because the CO2 is first incorporated into a 3-carbon compound. Examples are wheat, rice, and soybeans. C3 plants are more efficient than C4 plants and CAM plants under cool and moist conditions and under normal light because requires less machinery (fewer enzymes and no specialized anatomy). Most plants are C3.
C4 PLANTS – Plants that photosynthesize by the Hatch-Slack cycle. This process is called C4 because the CO2 is first incorporated into a 4-carbon compound. Examples include maize (corn), sorghum, millet, and sugarcane. C4 plants photosynthesize faster than C3 plants under high light intensity and high temperatures. C4 plants use water more efficiently because CO2 is brought in faster and it is not necessary to keep the stomata open as much (less water lost by transpiration) for the same amount of CO2 gain for photosynthesis. These are less efficient photosynthesizers in a carbon-enriched atmosphere than C3 plants.
CAI (CALCIUM-ALUMINUM INCLUSION) – White, millimeter-sized objects found, often with chondrules, in the most primitive carbonaceous chondrites, notably CV and CO. They consist of high-temperature minerals, including silicates and oxides of Ca, Al, and Ti. In 2002, an international team of scientists accurately dated CAIs at 4.567 Ga, making them the oldest known objects in the solar system. The same team found that chondrules, another of the earliest relics of the solar system, are 2-3 Ma younger than CAIs. Both types of object formed when dusty regions of the solar nebula were heated to high temperatures by shock waves. The dust melted and then crystallized, forming first CAIs and then chondrules.
There are three types of CAIs. Type A CAIs are melilite-rich, fine-grained and irregularly shaped. Type B CAIs are coarse-grained, consisting of Al-Ti-pyroxene (fassite), melilite, anorthite and spinel. Type C CAIs are anorthite-rich and coarse-grained.
CALCIUM MONOALUMINATE – Mineral, CaAl2O4, only observed in the CAIs of CH carbonaceous chondrites.
CALDERA – Large (>1 km) approximately circular volcanic depression, which may form by explosion or collapse, or a combination of the two.
CALICHE – Calcium carbonite, CaCO3, that often encrusts meteorites found in desert areas covered Caliche forms naturally during repeated wetting and drying in arid climates where calcium is present in the soil.
CAM PLANTS – Plants that utilize the CAM process, in which CO2 is stored in the form of an acid before use in photosynthesis. CAM plants include many succulents such as cactuses and agaves and also some orchids and bromeliads. The plants convert CO2 into an acid and store it during the night. During the day, the acid is broken down and the CO2 released for photosynthesis. CAM plants use water more efficiently than C3 plants under arid conditions because they open stomata at night when transpiration rates are lower.
In addition, when conditions are extremely arid, CAM plants can “CAM-idle” and leave their stomata closed night and day. Oxygen given off in photosynthesis is used for respiration and CO2 given off in respiration is used for photosynthesis. Although this appears to make them perpetual energy machines, there are costs associated with running the machinery for respiration and photosynthesis so the plant cannot CAM-idle forever. But CAM-idling does allow the plant to survive dry spells, and it allows the plant to recover very quickly when water is available again.
CAPACITOR – Device for storing electric charge, consisting of two conducting objects placed near each other but not touching. A voltage gradient placed between them can be kept stored until it needs to be discharged. A capacitor's capacitance (C) is a measure of the amount of charge (Q) stored on each plate for a given potential difference or voltage (V) which appears between the plates:
A capacitor has a capacitance of one farad when one coulomb of charge causes a potential difference of one volt across the plates. Since the farad is a very large unit, values of capacitors are usually expressed in microfarads (µF), nanofarads (nF) or picofarads (pF).
The capacitance is proportional to the surface area of the conducting plate (A) and inversely proportional to the distance between the plates (d). It is also proportional to the permittivity (e) of the dielectric (non-conducting) substance that separates the plates (when A >> d):
CARBON (C) – Element commonly found in meteorites, it occurs in several structural forms (polymorphs). All polymorphs are shown above (those starred have been found in meteorites and impact structures): a. diamond*; b. graphite*; c. lonsdalite*; d. buckminsterfullerene* (C60); e. C540; f. C70; g. amorphous carbon; h. carbon nanotube*.
Carbon has two stable, naturally-occurring isotopes: carbon-12, or 12C, (98.89%) and carbon-13, or 13C, (1.11%), and one unstable, naturally-occurring, radioisotope, 14C (t½ = 5730 y). Variations in carbon isotopic ratio, 13C/12C, are common and often large. They are expressed as δ13C (in parts per thousand ‰) relative to the Peedee Belemnite standard (PDB). This material has a higher 13C/12C ratio than nearly all other natural C-based substances and, for convenience, is assigned δ13C value of zero. This means that almost all other naturally-occurring samples have negative δ13C values.
A basic phase diagram of carbon, which shows the state of matter for varying temperatures and pressures, is shown below. The hashed regions indicate conditions under which one phase is metastable, so that two phases can coexist.
CARBON BURNING – Fusion processes within stars that consume carbon to form heavier elements. At temperatures reached only in the cores of stars much more massive than the Sun (~6 x 108 K), C nuclei fuse to form Mg. However, because of the rapidly mounting nuclear charges - that is, the increasing number of protons in the nuclei - fusion reactions between any nuclei larger than C require such high temperatures that they are actually quite uncommon in stars and formation of most heavier elements occurs by addition of He nuclei (the α process). For example, the repulsive force between two C nuclei is three times greater than the force between the nuclei of C and He. Thus, C-He fusion occurs at a lower temperature than C-C.
CARBON MONOXIDE – Molecule used to map the distribution of matter, especially molecular hydrogen, H2, in interstellar space. Molecular hydrogen is by far the dominant molecule in molecular clouds, but is very difficult to detect. One reason is that the strength of spectral lines from molecules is related to how asymmetric the molecule is. Since the hydrogen molecule is perfectly symmetric (containing two H atoms), its spectral lines are extremely weak. In addition, the energy required to change the rotational state of a molecule is dependent on its mass. Since the hydrogen molecule is the lightest of all molecules, a significant amount of energy (~500 Kelvin) must be absorbed to change its rotational state. In a cloud with an average temperature of ~10 Kelvin, this is an unlikely event and most of the hydrogen molecules remain in their ground state. In contrast, CO is asymmetrical and relatively heavy (28 vs. 2 amu). It has been shown that for every CO molecule there are about 10,000 H2 molecules meaning that we can trace molecular hydrogen through the emission from the CO molecule. This is the primary method use to locate molecular clouds.
In the Milky Way, the CO luminosity is closely correlated with the virial masses of the clouds (below), justifying the use of CO as a tracer of the mass of H2. The best fit to these data for clouds with masses between 105 and 2 x 106 Msun yields a constant of proportionality of 3.0 x 1020 H2 cm-2 (K km s-1)-1. The similarity of the clouds in other galaxies (M31, M33, and IC 10) to the Milky Way justifies the use of the same CO-H2 proportionality in external galaxies.
CARBON STAR – Red giant (or occasionally red dwarf) star whose atmosphere contains more carbon than oxygen, which combine in the outer layers of the star, forming carbon monoxide and other carbon compounds. The abundance of carbon is thought to be a product of helium fusion within the star. Carbon stars are cool (2000-3000 K), deep red or brown colored, and emit most of their energy at IR wavelengths (shorted wavelengths are absorbed by atmospheric carbon). Although very large, carbon stars are difficult to detect without specialized equipment. All carbon stars are irregular or semiregular variable stars.
Many carbon stars are actually binary stars, where one star is a giant star and the other a white dwarf. The giant star loses carbon to the surface of the white dwarf, resulting in a carbon enhanced spectra. Many carbon compounds (HCN, C2N2), Li, and Zr have been detected at high levels, which have circulated from the core of the star into its upper layers.
As much as half (or more) of the total mass of a carbon star may be lost in powerful stellar winds that eject carbon-rich “dust” into the interstellar medium. This dust provides the raw materials for the creation of subsequent generations of stars. The ablated material surrounding a carbon star may blanket it to the extent that the dust absorbs all visible light.
CARBONACEOUS CHONDRITE – Rare type of stony meteorite, carbonaceous chondrites are the most primitive and unaltered type of meteorite known, with elemental compositions probably similar to that of the nebula from which the Solar System formed. In addition to silicates, oxides, and sulfides, they contain, most distinctively, water or minerals that have been altered in the presence of water (serpentine), together with large amounts of carbon and organic compounds, including amino acids. There are two petrologic types observed in carbonaceous chondrites, indicating the degree of water processing or aqueous alteration. This alteration occurred in the parent body under very low temperatures, probably in the range of 20 to 50° C in a water-rich environment. This contrasts to the thermal metamorphism of ordinary chondrites that occurred in the range of 600 to 900° C under very dry conditions. Carbonaceous chondrites may contain up to 20 weight % water. Different groups of carbonaceous chondrites have been identified that came from parent bodies in different parts of the solar nebula. The six classes of carbonaceous chondrites are: CI chondrites, CM chondrites, CV chondrites, CO chondrites, CK chondrites, CR chondrites, CH chondrites, and CB chondrites.
CARBONATE – Mineral or compound containing carbon and oxygen (i.e. calcium carbonate, CaCO3, calcite).
CASSINI-HUYGENS MISSION – Planetary mission designed to explore in detail the Saturnian system (see http://saturn.jpl.nasa.gov/home/index.cfm and http://www.esa.int/esaMI/Cassini-Huygens/index.html). The spacecraft comprises the main craft (Orbiter) and the Titan Probe (Huygens). It is presently undertaking a detailed study of the planet Saturn, its rings, satellites and magnetosphere. The Huygens probe was successfully dropped onto Titan and sent back pictures during the descent and from the surface.
CATACLASTIC – Texture found in metamorphic rocks in which brittle minerals have been broken, crushed, and flattened during shearing.
CATALYST – Chemicals that are not consumed in a reaction, but, which speed up the reaction rate. Catalysts aid to form a transition state which is lower in energy than the transition state without the catalyst (essentially decreasing activationenergy). Since the barrier to the reaction is lower, the reaction rate increases in the presence of catalysts.
CATENA – Crater chain (q.v.).
CATHODOLUMINESCENCE – Emission of visible light in response to electron bombardment.
CATION – Positively charged ion.
CAUSALITY – Principle that a cause must precede its effect. More formally, if an event A ("the cause") somehow influences an event B ("the effect") that occurs later in time, then event B cannot have an influence on event A. That is, event B must occur at a later time t than event A, and further, all frames must agree upon this ordering.
CB CHONDRITES – Class also known as bencubbinites, are named after a meteorite found in 1930 near Bencubbin, Australia. Only a handful of these strange meteorites are known, all composed of >50 vol. % Ni-Fe metal, together with highly reduced silicates, and chondrules similar to those found in the CR group.
CENTER OF SYMMETRY – Point through which an inversion operation is performed, converting an object into its mirror image. Also called a "center of inversion"..
CENTRAL PEAK – Exposed core of uplifted rocks in center of a complex impact crater. Central peak material typically shows evidence of intense fracturing, faulting, and shock metamorphism.
CEPHEID VARIABLE – Star whose luminosity and other parameters vary in a characteristic way (below), with a rapid rise in brightness followed by a slower decline. The period of a Cepheid variable star is related to the luminosity of the Cepheid by the period-luminosity relationship: the more luminous the Cepheid, the longer the period. This property makes Cepheids useful for obtaining distances. One determines the pulsation period and uses the relationship to get the luminosity. The apparent brightness of the star then yields distance. Cepheids come in two types: Type I are metal rich; Type II are metal poor. Type I Cepheids are more luminous than Type II.
CERAMIC – Inorganic non-metallic materials. Up until the 1950s, the most important of these were the clays made into pottery, bricks, tiles along with cements and glass. A composite material of ceramic and metal is known as "cermet".
CH CHONDRITES – Carbonaceous chondrite class chemically very close to the CRs and CBs. The "H" stands for "high metal" since the CH chondrites contain up to 15 vol. % Fe-Ni metal. The first CH chondrite was found in the Antarctic Allan Hills (ALH 85085) and can be regarded as the type specimen. They also have many small fragmented chondrules (~0.05 mm diameter; 5-10 vol. %) and less abundant CAIs (<1 vol. %). All members recovered so far belong to petrologic types 2 or 3. As with the CRs, the CHs contain some phyllosilicates and other traces of aqueous alteration. Some CH chondrites are known to contain rare mineral phases, and in one member, NWA 470, the mineral Ca-monoaluminate, CaAl2O4, has been found for the first time in nature. However, unlike all other carbonaceous chondrites, the matrix of CHs is dominated by pyroxene (~70 vol. %). Some researchers have suggested that the CH chondrites formed in close to the Sun as reflected in the abundance of refractory trace elements and mineralogy. It is possible that the planet Mercury might have formed from similar, metal-rich material, which would explain its high density and gigantic metal core that makes Mercury unique among all other terrestrial planets in our solar system. Only ~12 meteorites are known to belong to the CH group (excluding probable pairings).
CHAIN SILICATES – Minerals composed of chains of silicon tetrahedra. These minerals are divided into single chain and double chain silicates. The former include pyroxenes and pyroxenoids and consist of linked (SiO4)4– tetrahedra each sharing 2 oxygens with its neighbours. Double chain silicates comprise the amphibole group and consist of two tetrahedral chains cross-linked by bridging oxygens. The cleavages are different, reflecting differences in underlying chain structures.
Generally, chemically similar pyroxenes and amphiboles have similar color, luster and hardness. However, amphiboles have (OH)– groups in their structures, which are lacking in pyroxenes, resulting in lower densities and refractive indices than pyroxenes with similar cation proportions.
CHALCOPHILE ELEMENT – Element in the Goldschmidt classification that tends to be concentrated in sulfide phases, e.g., Cu, Zn, Ag through Te, S, As, Se, and Hg through Bi.
CHANDRASEKHAR LIMIT – Maximum mass, ~1.4 Msun, above which an object has too much mass for electron degeneracy pressure to prevent collapse into a neutron star (the maximum mass of a white dwarf star). Named after Subramanyan Chandrasekhar, the astrophysicist who first derived the white dwarf mass limit.
CHAOS – Distinctive area of broken terrain on a planetary surface.
CHARACTERISTIC X-RAY – X-ray emitted from an excited atom when an outer-shell (e.g. L) electron jumps in to fill an inner-shell (e.g. K) vacancy. It has energy characteristic of the atom and can therefore be used for analytical purposes. Its energy is the difference between the energies of the atom with an inner-shell vacancy and the same atom with an outer-shell vacancy. The emission of the excess energy when an atom de-excites (decays or relaxes) can alternatively be achieved by the production of an Auger electron. The transitions that produce characteristic x-rays from barium are shown below.
CHARGE – Quantity carried by a particle that determines its participation in an interactions process. A particle with electric charge has electrical interactions; one with strong charge (or color charge) has strong interactions, etc.
CHARGE CARRIER – Particle or feature having electric charge that can move freely through a material. In a conductor, electrons are free to move, so they are the charge carriers. Holes, essentially missing electrons, are positive charge carriers in p-type semiconductors. Positive or negative ions can be charge carriers in liquids.
CHARGE CARRIER NUMBER DENSITY (n) – Number density (number per volume) ofcharge carriersin a material. In conductors, electrons are free to move, so n for a conductor equals the number density of conduction electrons in the material. Charge is carried by both electrons and holes in intrinsic semiconductors (some plasmas and solutions have both positive and negative charge carriers also). In this case, one must add the density of holes (or other positive charge carriers) to the density of conduction electrons (or other negative charge carriers) to obtain the total charge carrier number density.
CHASMA – Term applied to a deep, elongated, steep-sided depression (canyon) on a planetary surface (e.g., Candor Chasma on Mars).
CHASSIGNITE (CHA) – One of the SNC meteorites believed to have come from Mars. The group is named for its only known member, a meteorite seen to fall in Chassigny, France, in 1815; its subsequent recovery led to it being one of the first meteorites to be recognized as a genuine rock from space. Chassigny resembles a terrestrial dunite - a coarse-grained, deep-seated igneous rock - and consists of ~91% Fe-rich olivine, 5% clinopyroxene, 1.7% plagioclase, 1.4% chromite, 0.3% melt inclusions, and other minerals. Cracks within Chassigny are filled with carbonate and sulfate salts that point to chemical alteration by water before its arrival on Earth. Its crystallization age of 1.36 Ga and composition suggest a close relationship with nakhlites and an origin from the same parent magma on Mars. However, Chassigny contains noble gas values entirely different from those found in other Martian meteorites or the Martian atmosphere. If these gases came from the Martian mantle, as suspected, Chassigny must have originated within a pluton deep inside the Martian crust.
CHEMICAL BOND – Strong attractive force between atoms that holds the atoms close together in space.
CHEMICAL EQUATION – Method of expressing chemical reactions in a succinct way. The reactants involved are placed on the left, and the products are placed on the right. Usually an arrow separates the reactants and products. Specific reaction conditions (temperature, pressure, catalyst, etc.) are sometimes placed above or below the arrow symbol.
CHEMICAL LIFETIME – Length of time a chemical species can survive without reacting, photolyzing, dissociating, or otherwise changing into another chemical species. Highly reactive chemicals have short lifetimes.
CHEMICAL VAPOR DEPOSITION (CVD) – Method for growing solids in which a gaseous precursor (containing fragments of the desired solid) is decomposed and deposited onto a desired surface. CVD is one of the most powerful synthetic methods in material science due to its remarkable flexibility. A variety of surfaces can be coated, and very thin layers can be applied if necessary.
CHEMILUMINESCENCE – Emission of absorbed energy as visible light as the result of a chemical reaction. Examples include chemical glow sticks and fireflies are examples of this.
CHERENKOV RADIATION – Radiation emitted by when a massive particle moves faster than the speed of light in the medium through which it is traveling. No particle can travel faster than light in vacuum, but the speed of light in other media (water, glass, etc.) is considerably lower. When any charged particle moves through water it tends to polarize the water molecules in a direction adjacent to its path, thus distorting the local electric charge distribution. After the particle has passed, the molecules realign themselves in their original, random charge distribution, emitting a pulse of electromagnetic radiation. When the speed of the particles is less than the speed of the light in water, the pulses tend to cancel due to destructive interference; however, when the speed of the particle is greater than the speed of light in water, the light pulses are amplified through constructive interference. The phenomenon is analogous to the acoustic "sonic boom" observed when an object exceeds the speed of sound in air.
Cherenkov radiation was first observed by Marie and Pierre Currie in the early part of the 20th century. The effect was named after Pavel Alekseyevich Cherenkov, who won the 1958 Nobel Prize for being the first to rigorously characterize it. Cherenkov radiation is observed in nuclear reactors when fission products decay and produce high-energy β particles. The β particle velocities exceed the speed of light in water (2.3 × 108 m/s) producing blue Cherenkov radiation (below).
CHIXCULUB CRATER – Large (150-180 km wide), partially submerged, multiringed impact crater straddling the northwest coastline of the Yucatan Peninsula in Mexico. It is named after a village located near its center (21° 20' N, 89° 30' W).
The Chixculub Crater is believed to have formed by impact of an asteroid measuring some 10 to 20 km across. Evidence for impact includes shocked quartz grains, elevated Ir and other platinum-group-element (PGE) abundances, impact melt and suevite. Environmental effects resulting from the impact are thought to have been responsible for the mass extinction at the Cretaceous-Tertiary (K-T) boundary, now called the Cretaceous-Paleogene (K-P) boundary because of a recent revision in stratigraphic terminology.
CHONDRITE – Commonest kind of meteorite (accounting for ~86% of falls). Chondrites are made mostly of Fe- and Mg-bearing silicate minerals and have remained little changed from the origin of the Solar System. Except for the lightest elements (e.g., H, He), chondrites have the same elemental composition as the original solar nebula because they come from asteroids that never melted or underwent differentiation. Chondrites are so named because they nearly all contain chondrules - small round droplets of olivine and pyroxene - that apparently condensed and crystallized in the solar nebula. Variations in chemical composition among chondrites reflect formation of their parent bodies in different regions of the solar nebula. The main groups are ordinary (H, L, and LL), enstatite(E), and carbonaceous chondrites. Other, rarer chondrite groups are rumurutiites (R), kakangariites(K).
Each group is further subdivided into petrologic types 1 through 7. Types 1 and 2 show evidence of aqueous alteration to the extent that chondrules are either absent (Type 1) or rare (Type 2). Petrologic types 3 to 7 have experienced varying degrees of thermal metamorphism, which is reflected by modification of the chondrules and chemical homogenization. Type 3 samples show plentiful, unaltered and distinct chondrules; whereas the chondrules become increasingly indistinct due to recrystallization in types 4 to 6. Type 7 chondrites, better termed metachondrites, in which chondrules are absent, are transitional between chondrites and primitive achondrites.
CHONDRULE – Roughly spherical aggregate of coarse crystals formed from the rapid cooling and solidification of a melt at ~1400° C. Large numbers of chondrules are found in all chondrites except for the CI group of carbonaceous chondrites. Chondrules are typically 0.5-2 mm in diameter and are usually composed of olivine and pyroxene, with smaller amounts of glass and Ni-Fe metal. Together with CAIs, which predate chondrules by ~2-3 Ma, they are among the oldest objects in the Solar System with an age of ~4.56 Ga. They formed when "dust balls" were shock heated to very high temperatures, became molten, and resolidified as tiny droplets. Type I chondrules are FeO-poor; whereas, Type II are FeO-rich. Other chondrules are cryptocrystalline, or show barred or radial textures.
CHROMATIC ABERATION – Effect that occurs in a lens when radiation of different wavelengths is brought to focus at different focal points. In light microscopy this implies that, for instance, red and blue rays have different foci. In electron microscopy, the analogous effect occurs for electrons of different energies.
CHROMITE – Cr-Fe oxide, Cr2FeO4, found in many meteorite groups.
CHROMOPHORE – Part of a molecule responsible for its color. Chromophores almost always are one of two forms: conjugated pi systems and metal complexes. In the former, the electrons jump between energy levels of extended pi orbitals created by alternating single and double bonds, often in aromatic systems. In the latter, chromophores arise from the splitting of d-orbitals by bonding of a transition metal.
CHROMOSPHERE – Region immediately above a star’s photosphere.
CHRYSOTILE – Also called white asbestos, it is less friable (and therefore less likely to be inhaled) than the other types; it is the type most often used industrially. The chemical formulae is Mg3(Si2O5)(OH)4 with variable Fe2+ substituting for Mg2+. Chrysotile should not be confused with chrysolite, a synonym of olivine.
CI CHONDRITES – Very rare meteorite class named after the Ivuna meteorite that fell in Tanzania in 1938. They are among the most primitive, friable (crumbly), and interesting of all meteorites, having undergone extensive aqueous alteration. They lack chondrules and CAIs as a result of this alteration, but contain up to 20% water, as well as various alteration minerals, such as hydrous phyllosilicates (similar to terrestrial clays), oxidized Fe in the form of magnetite, and olivine crystals sparsely scattered in a black matrix. They also contain organic matter, including polycyclic aromatic hydrocarbons (PAHs) and amino acids, which make them important in the search for clues to the origin of life in the universe. CIs have never been heated above 50° C, indicating that they came from the outer part of the solar nebula.
CK CHONDRITES – Class of carbonaceous chondrite named for the Karoonda meteorite that fell in Australia in 1930, were initially regarded as members of the CV group. However, they are now grouped separately since they are more oxidized than all other carbonaceous chondrites. They appear dark-gray or black due to a high percentage of Cr-rich magnetite dispersed in a matrix of dark silicates, consisting of Fe-rich olivine (Fa28-33) and pyroxene (Fs22-29). The presence of Fe3+ indicates oxidizing conditions, yet there is no sign of aqueous alteration or phyllosilicates. Chondrules averaging ~0.7 mm comprise ~15 vol. %. Elemental abundances and oxygen isotopic signatures suggest that CKs are closely related to CO and CV types. Most CK chondrites contain large CAIs (~4 vol. %) and some show shock veins indicating a violent impact history.
CLAUSIUS-CLAPEYRON EQUATION – Relationship in thermodynamics, characterizing the phase transition between two states of matter, such as solid and liquid. On a pressure-temperature (P-T) diagram, the line separating the two phases is known as the coexistence curve. The Clausius-Clapeyron relation gives the slope of this curve:
where, dP/dT = slope of the coexistence curve, ΔH = latent heat, T = temperature, and ΔV = volume change.
CLAY MINERAL – Hydrous aluminium phyllosilicates, with variable amounts of Fe, Mg, Ca, Na, K, and other cations. The structure of clay minerals is similar to that of micas, consisting of alternating octrahetral and tetrahedral sheets. They are common weathering products of feldspar and low temperature hydrothermal alteration products.
CM CHONDRITES – Class named after the Mighei meteorite that fell in Ukraine in 1889, have about half the water content of CI chondrites, show less aqueous alteration, and retain some well-preserved small chondrules (diameter s <0.5 mm; 10-40 vol. %) and CAIs (~5 vol. %). Like CIs, however, they harbor a wealth of organic material: more than 230 different amino acids have been identified in the Murchison meteorite. Comparisons of spectra point to the asteroid 19 Fortuna or, possibly, the largest asteroid, 1 Ceres, as candidate parent bodies.
CNO CYCLE – Fusion process in stars contaminated by "metals," specifically carbon (C), nitrogen (N), and oxygen (O). The two branches of the CNO process cycle through a sequence that converts the elements carbon, nitrogen, and oxygen into each other's isotopes. The first cycle (shown) starts and ends with 12C, which may be considered a catalyst. For abundances characteristic of the Sun, the CNO cycle becomes important for core temperatures of roughly 15 million degrees (1.3 keV), and it provides virtually all of the conversion of 1H into 4He above 25 million degrees (2.2 keV).s
CO CHONDRITES – Meteorite class named after the Ornans meteorite that fell in France in 1868, are related in chemistry and composition to the CV chondrites and may, with them, represent a distinct clan of carbonaceous chondrites that formed in the same region of the early solar system. However, COs are usually blacker and have abundant smaller chondrules (60-70 vol. %; <1 mm in diameter) packed densely within a matrix representing over 70% of the meteorite. The matrix contains magnetite with <1.0 wt. % Cr2O3. CAIs are present but are commonly much smaller and less abundant (8-15 vol. %) than in the CV group. Also typical of COs are small inclusions of Ni-Fe metal that appear as tiny flakes on the polished surfaces of fresh, unweathered samples (1-6 vol. %).
CODON – Sequence of three consecutive nucleotides that specifies an amino acid or represents the starting or termination signals of protein synthesis.
COESITE – High pressure polymorph of silica produced during shock metamorphism at over 20 kilobars. Coesite was synthesized in 1953 before its discovery at Meteor Crater, Arizona. Its strucutre consists of silicon tetrahedras linked into four- membered rings that are cross-linked to other rings. Coesite has also been discovered in eclogite xenoliths from South African kimberlites.
COHENITE – Fe-Ni-Co carbide, (Fe,Ni,Co)3C, that occurs as an accessory constituent in several iron meteorites, and coarse octahedrites with < 7 wt. % Ni.
COHERENCE – Situation when electromagnetic waves are in-phase ("wiggle" up and down together). Light produced by a laser is a good example of coherent radiation.
COLD DARK MATTER (CDM) – Non-relativistic and non-luminous matter that likely contributes significantly to the density of galaxies and of the universe as a whole (but does not close the universe). In a cold dark matter cosmological model of structure formation, the CDM is primarily responsible for structure formation. CDM cosmologies produce a Bottom-Up hierarchy of structure formation.
COLLIMATION – Alignment of a beam so that the photons can be directed at a well-defined part of a target. This is accomplished by a mechanical device, often called a "slit," installed along the beam trajectory to reduce the size of the beam.
COLOR CENTER – Point defect in which an electron occupies an anion vacancy. Also called an "F center," after Farbe, German for color, the electron may absorb light and make some normally transparent crystals appear colored.
Color centers most often result from radiation damage: e.g., damage due to exposure to gamma rays. This irradiation may be natural (U, Th, K in minerals) or artificial. In rare cases, UV light can produce color centers. Once damaged by radioactive decay, electrons can be removed from their normal sites, bounce around, loose energy, and eventually come to rest in a vacant site in the structure (a trap). One crystal may have many different types of electron traps. Electrons in specific traps absorb only a certain range of wavelengths. The color that results is the color not absorbed by trapped electrons. If an electron is removed from its trap (by heating, e.g.) the color center is also removed.
COLOR INDEX – Parameter defining how a star's brightness varies with color. It is defined by taking the difference in magnitude (which is related logarithmically to intensity) at different wavelengths. For example, the B-V color index is defined by taking the difference between the magnitudes in the blue and visual regions of the spectrum and the U-B color index is the analogous difference between the UV and blue regions of the spectrum. The color index is directly related to the temperature of the star.
COMA – Spherical envelope of gas and dust surrounding the nucleus of an active comet, created when the ambient heat causes the vaporization of comet material.
COMBUSTION – Reactions involve elements or compounds burning in the presence of dioxygen (O2). The burning of the octane in gasoline is an example of combustion.
COMET – Conglomeration of frozen water and gases (methane, ammonia, CO2) and silicates which travels around the Sun. In recent years, the description of comets has shifted from dirty snowballs to snowy dirtballs with more dust than ice. However, the ratio is less than 10-to-1. Comets appear to contain a mixture of materials formed at all temperature ranges, at places very near the early sun and at places very remote from it. High-temperature silicates (e.g., olivine) formed near the protosun and were ejected, perhaps by strong bipolar jets, to the outer parts of the solar system. Recent data indicate that comets probably have porosities of ~75% and perhaps no solid core. It is likely that they consist of very loosely packed grains.
Comet orbits vary in eccentricity between an ellipse and a parabola and have known periods from 3 to 1000s of years. The names of such periodic comets are prefaces with “P/” (e.g., Comet P/Halley).
Near the Sun, a comet's materials are evaporated to produce a coma of gases and dust which may develop into a tail extending, in some cases, 80 millions km. Comets have two types of tails, of dust and ionized gas, which point in different directions (white and blue, respectively, in the photograph; the red colors are of an emission nebula in the background).
COMET P/HALLEY – Comet with 76-year orbital period. Images returned by the Giotto Mission revealed its nucleus was a dark (albedo 0.04-0.05), peanut-shaped body, ~15 km long and 7-10 km wide; the measured density (0.3 g/cm3) indicated a fluffy porous texture. The black surface is composed of organic compounds. Giotto imaged 7 jets on the warmer sunlit side (ejecting ~3 tonnes/s of material), but only ~10% of the surface was active. The jets gave the comet a strange, wobbling rotation. H2O accounted for ~80 % of the ejected material with substantial amounts of CO (10 %), CO2 (2.5 %), CH4 and NH3. Traces of other hydrocarbons, Fe, and Na were also found. Two major classes of dust particles were found: one dominated by the light elements (C, H, O, N); the other rich in mineral-forming elements (Na, Mg, Si, Fe, Ca), which must have been derived from closer to the Sun than its ices. Light elements (except N) occurred in the same relative abundances as in the Sun, confirming that Comet Halley is made of primitive solar nebular material.
COMPENSATION DEPTH – In isostasy, the depth (level) at which the overlying mass is independent of location. See ISOSTASY for more information.
COMPLEX CRATER – Craters with stepped terraces on the interior wall, a flat floor, and a single peak or group of peaks in the center. The interior wall terraces, which form due to landslides, result in a shallower overall inclination of the wall than in simple craters. The exterior rims are similar in appearance to simple craters. The photograph shows King Crater on the far side of the Moon, a typical lunar complex crater, which is 93 km in diameter and ~5 km deep. The central peaks are ~2 km high.
COMPOUND CHONDRULE – Object composed of two or more chondrules that were fused together.
COMPTON SCATTERING – Collision process between a γ-ray and a bound atomic electron where only part of the γ-ray energy is transferred to the electron. The probability for Compton scattering is approximately proportional to Z, and for energies greater than 500 keV approximately proportional to 1/Eγ.
CONDENSATION NUCLEI – Dust grains in the interstellar medium which act as seeds around which other material can coagulate. The presence of dust was very important in causing matter to clump during the formation of the solar system by allowing radiation of heat.
CONDUCTION – Transfer of heat as a result of collisions between molecules; when one end of an object is heated, the molecules vibrate faster and their energy is transferred sequentially to their neighbors. Also, the transfer of electrical current by movement of electrons in the conduction band.
CONDUCTION BAND – The unfilled top energy band in a solid. Since this band is not filled, electrons with energies in this band can move easily through the solid, creating an electric current. In semiconductors, electrons must first be promoted to this band before a current will flow. The valence band and the conduction band are separated by the band gap.
CONDUCTION ELECTRONS – Electrons free to move within a solid. The motion of these electrons can create an electric current through the solid.
CONDUCTIVITY (σ) – Measure of how freely current can flow through a material. Cu (conductivity of 5.95 × 107 Ω-1m-1) conducts electric current more freely than does Al (conductivity of 3.77 × 107 Ω-1m-1). Conductivity is the inverse of resistivity, r:
CONDUCTOR – Material with low resistivity. Conductors have a partially filled valence band, through which electrons can move freely. Thus, in a conductor, the conduction band is the same as the valence band, and the charge carriers are primarily electrons.
CONSERVATION LAWS - Expression of constancy within a system during change. Specifically, given an isolated system, a quantity is conserved if it remains constant despite whatever changes occur to the system. Some examples include: momentum, angular momentum, energy (which must be enlarged under special relativity to include mass-energy), matter (only approximately true, since it is violated by special relativity), and charge.
CONSTRUCTIVE INTERFERENCE – Phenomenon that occurs when two waves overlap such that they are in phase (the troughs and peaks of the two waves coincide). Note, for this to occur down the entire wave, the two waves must have the same wavelength. When constructive interference occurs, the amplitude of the resulting wave is the sum of the two waves. So, if the original two waves had the same amplitude, the resulting wave would have twice their amplitude.
CONTACT POTENTIAL – Potential difference that arises at the junction of two different conducting materials. When two materials with different work functions, F, are brought into contact with each other, electrons will flow from the material that has a lower work function (higher Fermi energy, EF) to the material with a higher work function (lower Fermi energy). This flow of electrons takes place until the Fermi energies of both materials become equal.
For example, Al has a higher Fermi energy than does Cu, so it is easier to remove an electron from it. When Cu and Al placed in contact, the discontinuity in Fermi energy at the boundary is quickly smoothed out: some of the conduction electrons from the Al flow into the Cu, until the Fermi energy is the same on either side of the boundary. This produces a net negative charge on the Cu and a net positive charge on the Al. The separation of charge sets up a potential difference, or voltage, across the boundary. The size of this "contact potential" difference is dependent upon the difference in Fermi energies between the materials.
CONTINUOUS – Smoothly varying; taking any value. For example, an electron moving through empty space can have any value of energy, so we say the allowed energies for this free electron are continuous. In contrast, an electron in an atom can only have certain discrete, or quantized, energies.
CONTINUUM – Smooth continuous spectrum without emission or absorption lines. Some sources like tungsten lamps or blackbodies are purely continuum sources, while in sources with lines the continuum is a smooth spectrum drawn through the points between the lines. In x-ray analysis, the continuum represents bremsstrahlung.
CONVECTION – Transfer of heat energy by moving material. Temperatures increases with depth in planetary objects. Deep hot less-dense material physically rises and cools, releasing heat and becoming denser. The now cooler denser material sinks back into deeper regions, where it will be reheated and rise again. Convection is an important mechanism for heat transport within Earth's mantle, the interiors of some types of stars, and within certain regions of other stars.
CONVECTIVE ZONE – Region within a star, where the change in temperature with increasing radius is so rapid that energy transfer occurs by convection (rapid up and down motion of large packets of gas).
CONVERSION ELECTRON – Alternate process to x-ray or γ-ray emission during de-excitation of an excited atom.
COORDINATION – The number of anions surrounding a cation (or vice versa) in a stable ionic structure. For example, the Si4+ cation in minerals is typically is surrounded by 4 O2- anions and has a coordination number of 4 (called 4-fold coordination). Note that the general relationships of coordination only apply if bonding is dominantly ionic.
COORDINATION NUMBER (CN) – Number of atoms closest to any given ion in a crystal. CNs in minerals typically range from 3 to 12. Coordination number is largely controlled by ionic size: more ions can fit around a larger central ion. The shape of resulting arrangement of ions (central plus 4+ surrounding) is called a coordination polyhedron. Ions with CN=2 are described as being in "linear" coordination; those with CN = 3, in "triangular" coordination.
It should be noted that 5-, 7-, 9-, and 10-fold coordinations are possible in complex structures where anions are not closely packed.
COORDINATION POLYHEDRON – Shape formed by a central ion and the surrounding ions of opposite charge. The radius ratio (ratio of the size of the central to the size of surrounding ions) determines the polyhedron formed. The arrangement of ions in 12-fold polyhedra may be either cubic closest packing (CCP) or hexagonal closest packing (HCCP).
The following coordination polyhedra are formed by common cations and oxygen: tetrahedron (Si4+, Al3+, Fe3+), octahedron (Mg2+, Fe2+, Al3+, Fe3+, Ti4+), distorted cube (Ca2+, Na1+, K1+).
CORE – The central region of a star, planet or moon usually made of denser materials than the surrounding regions (mantle and crust). Earth and the Moon are thought to have cores of Ni-Fe metal.
CORONA1 – Extended outer atmosphere of the Sun. The glow of the corona is a million times less bright than that of the photosphere; it can only be seen when the disk of the Sun is blocked during a total solar eclipse, or by using a cchoronagraph, which artificially blocks the disk of the Sun so that it can image the regions surrounding the Sun. The corona is very hot as indicated by emission lines corresponding to very highly ionized atoms (e.g., Fe16+). Such highly ionized atoms can only be produced at temperatures in the 106 degree range. The extremely high temperature of the corona is thought to due to the solar magnetic field, which can store and transport energy from lower regions of the Sun to the corona. However, the details of how this heating takes place are not yet fully understood.
CORONA2 – Circular to elliptical feature observed on Venus. Although these superficially resemble craters, they consist of deep, curving, trench surrounding an elevated plain. Plural: coronae.
CORUNDUM – Al oxide, Al2O3, found in CAIs.
COSMIC BACKGROUND EXPLORER (COBE) – Satellite developed to measure the diffuse infrared and microwave radiation from the early universe to high precision (see http://lambda.gsfc.nasa.gov/product/cobe/ and http://aether.lbl.gov/www/projects/cobe/). It carried three instruments: the Diffuse Infrared Background Experiment (DIRBE) to search for the cosmic infrared background radiation (CIBR), the Differential Microwave Radiometer (DMR) to map the cosmic radiation sensitively, and the Far Infrared Absolute Spectrophotometer (FIRAS) to compare the spectrum of the cosmic microwave background radiation with a precise blackbody. The FIRAS has shown that the cosmic microwave background spectrum matches that of a blackbody of temperature 2.726K ± 0.002K with a precision of 0.03%. The DMR had a nominal beam size of 7° and analysis its data revealed that the background radiation has an intrinsic anisotropy at a level of a part in 105.
COSMIC MICROWAVE BACKGROUND RADIATION (CMBR) – Relic photons remaining from the very hot, early phase of the Big Bang. The CMBR peaks in the microwave band, corresponding to blackbody radiation with a temperature of ~2.7 K.
The CMBR is also sometimes called the "microwave background," or "cosmic background radiation" (CBR). The first map of the anisotropy in the CMBR was provided by the Cosmic Background Explorer (COBE); this was significantly improved upon by results from the Wilkinson Microwave Anisotropy Probe (WMAP).
COSMIC INFRARED BACKGROUND RADIATION (CIBR) – Radiation predicted to account for the energy from production of metals in the nearby universe. The radiation was detected in the sub-millimeter and its average spectrum measured by two COBE experiments. The CIBR average surface brightness and spectrum are consistent with models where the bulk of the energy comes from the thermal emission from dust heated by UV from hot young stars.
COSMIC RAYS – Extremely high-energy subatomic particles that continuously bombard Earth from all directions. When they collide with atoms and molecules in the upper atmosphere, they generate cosmic ray "showers." The initial collision produces pions (π+,π0), which quickly decay into muons μ+) and γ-rays. Muons decay further into electrons (e-), positrons (e+), and neutrinos (n). Deceleration of the electrons and positrons in the atmosphere produces a flash of light that can be observed from the ground with special telescopes; however, most of the secondary cosmic-ray particles that reach sea-level are undecayed muons.
COSMIC-RAY EXPOSURE AGE – Interval for which a meteoroid was an independent body in space; in other words, the time between when a meteoroid was broken off its parent body and its arrival on Earth as a meteorite; also known simply as the "exposure age." It can be estimated from the observed effects on a meteorite by bombardment by cosmic rays from the Sun and the rest of the Galaxy. As these cosmic rays strike the meteoroid in space, they produce both radioactive isotopes, such as 3He, 21Ne, and 38Ar, and stable isotopes. The longer a meteoroid has been exposed to cosmic rays, the more of these new isotopes are formed. Further dating information comes from an analysis of the fission tracks (thin trails left in a substance by a fast-moving atomic nucleus) that cosmic rays cause. Exposure ages range typically from 1-100 Ma.
COSMOCHEMISTRY – Study of the origin and development of the elements and their isotopes in the universe.
COSMOLOGICAL CONSTANT (Λ) – Constant introduced by Einstein in 1917 to reconcile his theory of General Relativity with the prevailing assumption that the universe was static, later interpreted as the energy density of the vacuum. If positive (repulsive), it counteracts gravity and leads to an acceleration of the expansion of the Universe. If negative (attractive), it augments gravity. It is usually denoted by Λ when expressed with units of inverse length squared. The observational upper bound on the value of the vacuum energy density is 40-120 orders of magnitude smaller than that predicted from quantum field theories of elementary particles - the "cosmological constant problem." The cosmological constant may be the cause of the acceleration of the universe recently inferred from observations of Type Ia supernovae, but again there is as yet no theoretical understanding of why it would have the small, non-zero value needed to explain these observations
COSMOLOGICAL PRINCIPLE – Principle incorporating the axioms that there is no center to the universe, that the universe is the same in all directions (isotropic) and the same everywhere (homogeneous), when considered on the largest scales. This principle means that what we observe of the universe from our specific location will be representative of the true nature of the universe.
COSMOLOGICAL REDSHIFT – Redshift caused by the expansion of space. The wavelength of light increases as it traverses the expanding universe between its point of emission and its point of detection by the same amount that space has expanded during the crossing time.
COULOMB – SI unit of electric charge. One coulomb is a fairly large amount of charge, equaling the charge of 6.25 x 1018 protons.
COULOMB FORCE – Electrostatic force between charged particles. For charges, q1 and q2, separated by distance, d, the force between them is described by Coulomb's Law:
Opposite charges attract and like charge repel.
COULOMB BARRIER – Energy barrier due to electrostatic interaction that two nuclei need to overcome so they can get close enough to undergo nuclear fusion. This energy barrier is produced by electrostatic potential energy. In fusion of light elements to form heavier ones the positively charged nuclei must be forced close enough together to cause them to fuse into a single heavier nucleus. The force between nuclei is repulsive until a very small distance separates them, and then it rapidly becomes very attractive. Therefore, in order to surmount the Coulomb barrier and bring the nuclei close together where the strong attractive forces operate, the kinetic energy of the particles must be as high as the top of the Coulomb barrier. This requires extremely high temperatures, if temperature alone is considered in the process. In the case of the PP cycle in stars, this barrier is penetrated by quantum tunneling, allowing the process to proceed at lower temperatures.
COUPLED SUBSTITUTION – Substitution in which the charges of substituting ions are not same and charge balance is achieved by a second substitution on a different crystallographic site. The most common example in the solid solution series of the plagioclase feldspars: anorthite, CaAl2Si2O8, to albite, NaAlSi3O8. Here, there are two substitutions taking place: Na1+ « Ca2+ (in the A site) and Al3+ « Si4+ (in the tetrahedral site). In order to maintain charge balance these combine to yield:
Na1+ + Si4+ ↔ Ca2+ + Al3+
Other examples of common coupled substitutions include:
Ca2+ + Mg2+ ↔ Na1+ + Al3+ (in clinopyroxene)
2Al3+ + Mg2+ ↔ 2Fe2+ + Ti4+ (in spinel)
Coupled substitutions may also involve introducing vacancies (symbolized as □) into a crystal structure. For example the following substitution occurs in amphiboles:
□0 + Si4+ ↔ Na1+ + Al3+
COVALENT BOND – Interaction between atoms by which they "share" valence electrons in the outermost energy shells, thereby filling the outer shell of both atoms involved in the interaction. This occurs between atoms with similar high electronegativities (ΔX~0), resulting in highly localized bonding orbitals. For example, each hydrogen atom in a hydrogen gas molecule has a single electron. The valence energy level of a hydrogen atom is filled by two electrons; sharing their electrons allows each hydrogen atom to fill its energy shell.
CR CHONDRITE – Class named for the Renazzo meteorite that fell in Italy in 1824, are similar to CMs in that they contain hydrous silicates, traces of water, and magnetite. The main difference is that CRs contain Ni-Fe metal and Fe sulfide that occurs in the black matrix and in the large chondrules (>1 mm diameter; 50-60 vol. %). CRs are marked by Fe-poor olivine (Fe1-7) and pyroxenes (Fs2-5). A possible parent body is 2 Pallas. CH and CB chondrites are so closely related to the CRs that all three groups may have come from the same parent or at least from the same region of the solar nebula.
CRATER – Bowl-like depression ("crater" means "cup" in Latin) on the surface of a planet, moon, or asteroid. Craters range in size from a few centimeters to over 1,000 km across, and are mostly caused by impact or by volcanic activity, though some are due to cryovolcanism.
CRATER CHAIN – Several craters along a general line that may be overlapping, touching, or separate from one another. A crater chain, or catena, is typically the result of either secondary impacts or volcanic activity. Several unusual crater chains have been found on Ganymede (example below) and Callisto that appear to be the impact scars of tidally disrupted comets or asteroids. These features serve to record the characteristics of comets and support the rubble-pile model for comet nuclei and some asteroids, in which these objects are formed of many small, loosely bound fragments. A couple of crater chains have been found on Earth, including the Aorounga Craters.
CRATER DENSITY – Number of craters of a certain size and larger per unit area. Higher crater density reflects an older age for a surface.
CRETACEOUS-TERTIARY (K-T) BOUNDARY – Major stratigraphic boundary on Earth that marks the end of the Mesozoic Era. (Now also called the K-P boundary, because of a recent stratigraphic revision.) In many places the boundary is marked by a distinctive clay layer, often enriched in Ir relative to the layers above and below. The K-T boundary marks a global extinction event at 65 Ma famous for killing off the dinosaurs (except birds, of course!) and two-thirds of all species on Earth. However, small mammals, turtles, crocodiles, birds, redwood trees and many others survived. There is very good evidence that a giant asteroid hit Earth at the same time as the K-T extinction. The "smoking gun" is the 100-mile wide Chixculub Crater it left behind off the coast of Mexico, along with disturbed geologic deposits (iridium and shocked quartz) consistent with an asteroid impact. The impact probably caused tidal waves, earthquakes, and clouds of dust so thick that they blotted out the sun for months. Such a disaster is certainly capable of causing a mass extinction. Other mass extinction observed in the geologic record may also have been caused by impacts.
CRISTOBALITE – Silica group mineral occuring in terrestrial volcanic rocks, martian and lunar meteorites, and chondrites. Cristobalite has a very open structure consisting of sheets of tetrahedra in 6-fold rings with tetrahedra pointing alternately up and down. The α to β inversion occurs at 268 °C.
CRITICAL DENSITY (ρc) – Boundary value of mass density between universe models that expand forever (open models) and those that recollapse (closed models). The ratio of the actual density of the universe to the critical density is the cosmological density parameter Omega (Ω). Numerically the critical density is:
CRITICAL POINT – Point along a phase boundary on a phase diagram where the liquid and gas states cease to be distinct. For example, the critical point of water is 647 K (374 °C or 705 °F) and 22.064 MPa (218 atm). These two parameters are called “critical temperature” and "critical pressure".
CROSS-SECTION – In scattering, the cross-section is the apparent area which a particle (e.g. an atom) presents to the primary radiation (e.g. electrons). The scattering cross-section, σ, and mean free path, λ, are related by:
where N is the number of scatterers per unit volume. The cross-section for any scattering process may depend on the energy/wavelength of the primary beam and the range of scattering angles accepted by the measuring device.
CRUST – Outermost solid layer of a planet or moon, usually consisting of silicate rock and extending no more than 10s of km from the surface. The term is also applied to icy bodies, in which case it is composed of ices, frozen gases, and accumulated mateoritic material.
CRYOSPHERE – Portion of Earth which consists of the ice masses and snow deposits (continental ice sheets, mountain glaciers, sea ice, surface snow cover and lake/river ice). The volume of water tied up in the glaciers and ice sheets can affect sea level and hydrologic cycle.
CRYOMAGMA – Water or other liquid or vapor-phase volatiles together with gas-driven solid fragments produced by internal heating of a planetary body.
CRYOVOLCANISM – Eruption of water or other liquid or vapor-phase volatiles (collectively referred to as "cryomagma"), together with gas-driven solid fragments, onto the surface of a planet or moon due to internal heating. After eruption cryomagma condenses to a solid form when exposed to the very low surrounding temperature. Cryovolcanism is not known to exist on Earth. Ice volcanoes were first observed on Neptune's moon Triton during the Voyager 2 flyby. Indirect evidence of cryovolcanic activity has since been observed on several other icy moons of our solar system, including Europa, Ganymede, and Enceladus. The Cassini-Huygens mission has found a methane-spewing cryovolcano on Titan, and such volcanism is now believed to be a significant source of the methane found in Titan's atmosphere.
CRYPOCRYSTALLINE – Rock texture in which individual crystals are too small to be distinguished even using a standard petrographic microscope (less than a few μm in size).
CRYSTAL DEFECT – Variation in the regular arrangement of the atoms or molecules of a crystal. Such variations include point defects, line defects, plane defects and screw defects.
CRYSTAL LATTICE – Atoms or groups of atoms repeated at regular intervals in three dimensions with the same orientation. One may regard each atom or group of atoms as occurring at point and the resulting collection of points is the space lattice or lattice of the crystal. Each crystal lattice is a Bravais lattice.
CRYSTAL LATTICE ENERGY – Energy change when one mole of formula units of a crystalline solid is formed from its ions, atoms, or molecules in the gas phase; always negative.
CRYSTAL STRUCTURE – Mutual arrangement of atoms, molecules or ions that are packed together in a crystal lattice to form a crystal.
CRYSTAL SYSTEM – Atomic arrangement of the atoms of an element when it is in its solid state. In mineralogy, these are best classified in terms of their symmetry and correspond to the seven fundamental shapes for unit cells consistent with the 14 Bravais lattices. Six systems are recognized: isometric (cubic), hexagonal, tetragonal, orthorhombic, monoclinic, and triclinic.
Minerals of the isometric system (also known as the cubic system) have mutually perpendicular crystallographic axes of equal length. All crystals have four 3-fold axes of symmetry diagonally from corner to corner through the center of the unit cell. They may also have up to three separate 4-fold axes of rotational symmetry from the center of each face through the origin to the center of the opposite face; these correspond to the crystallographic axes.
The hexagonal system have three crystallographic axes which intersect at 120° and a fourth which is perpendicular to the other three. This fourth axis is usually depicted vertically. The hexagonal system may be subdivided into hexagonal or trigonal divisions based upon whether the vertical axis has 6-fold or 3-fold rotational symmetry.
The tetragonal crystal system has three mutually perpendicular axes. The two horizontal axes are of equal length, while the vertical axis is of different length and may be either shorter or longer than the other two. Crystals all have a single 4-fold symmetry axis.
The orthorhombic system has three mutually perpendicular axes, each of which is of a different length than the others. Crystals of this system possess three 2-fold rotation axes and/or three mirror planes.
The monoclinic system has three unequal axes. Two axes are inclined toward each other at an oblique angle; these are usually depicted vertically. The third axis is perpendicular to the other two. The two vertical axes do not intersect one another at right angles, although both are perpendicular to the horizontal axis. Monoclinic crystals demonstrate a single 2-fold rotation axis and/or a single mirror plane.
The triclinic system has three unequal axes, all of which intersect at oblique angles. None of the axes are perpendicular to any other axis. Crystals possess no symmetry at all.
CRYSTALLINE MATERIAL - Substance with a structure consisting of a systematically repeated pattern of atoms. This term is also applied to rock types made up of crystals, such as metamorphic rocks that recrystallized at high temperatures or pressures, or igneous rocks that formed from cooling of magma.
CRYSTALLIZATION – Physical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals.
CRYSTALOGRAPHY – Science of determining the arrangement of atoms in solids; the scientific study of crystals. Crystallography relies on the analysis of the diffraction patterns that emerge from a sample bombarded by a beam of some type. The beam is commonly electromagnetic radiation (usually x-rays; x-ray diffraction), but for some purposes electrons or neutrons are used (electron or neutron diffraction).
CUBIC CLOSEST-PACKED (CCP) – Way in which atoms (considered as hard spheres) pack together to fill space. In cubic closest-packing, there are three alternating hexagonal layers, a, b, and c, offset from one another so that the spheres in one layer sit in the small triangular depressions of neighboring layers. Each sphere is touched by 12 neighbors, 6 in the same layer, 3 in the layer above, and 3 in the layer below.
CUMULATE – Rock composed of crystals that have grown and accumulated (often by settling) in a cooling magma chamber.
CURIE – Unit used to describe the intensity of radioactivity in a sample. One curie equals 37 × 109 billion disintegrations per second (approximately the amount of radioactivity given off by 1 gram of radium).
CV CHONDRITES – Meteorite class named after the Vigarano meteorite that fell in Italy in 1910, more closely resemble ordinary chondrites. They have abundant large, well-defined chondrules of magnesium-rich olivine (>0.7 mm diameter; 40-65 vol. %), often surrounded by Fe sulfide. They also contain 7-20 vol. % CAIs. The dark-gray matrix is dominated by Fe-rich olivine (~60 vol. %). Allende meteorite is a very famous and well-studied CV meteorite.
CYTOSINE – One of the five nitrogen-containing bases occurring in nucleotides.