Every academic field has its share of confusing technical terms. Their redeeming feature is that they lend precision to scholarly discussion, but in some cases, a word may have different meanings in different contexts. For example, the word abduction has specific definitions in the legal realm (a kidnapping); formal logic (a syllogism with a major premise that is certain and a minor premise that is merely probable); and anatomy (the action of moving a part of the body away from the midline). Technical terms also bedevil meteoritics; some terms don’t have exactly the same meaning as in common parlance. In some cases, the following definitions differ from those approved by the International Astronomical Union (IAU), but the set presented here is precise and self-consistent. We recommend its adoption by astronomers, geologists, cosmochemists, and planetary scientists. Terms are arranged topically, not alphabetically.Meteorite. In the nineteenth century, meteorites were commonly defined as solid objects that fell to Earth, but since Sputnik 1 reentered the atmosphere in January 1958, man-made objects (or at least scorched debris derived from such objects) joined natural bodies falling from the sky. The identification of a couple of chondritic meteorites among Apollo samples and of some iron meteorites (and probably a few stony meteorites) photographed by rovers on the martian surface demonstrate that meteorites can fall onto bodies other than Earth. They can also hit spacecraft. In addition, lunar and asteroidal materials very similar to lunar meteorites and known groups of chondrites have been retrieved by manned and unmanned spacecraft, i.e., transported to Earth by artificial means. Such complications mandate a more comprehensive definition of “meteorite” (modified from Rubin and Grossman Reference Rubin and Grossman2010):
A meteorite is a natural solid object, larger than 1 μm in size, derived from a celestial body (e.g., asteroid, moon, comet, planet), that was transported by natural means (e.g., impact excavation) from the body on which it formed to a region outside the dominant gravitational influence of that body (i.e., to interplanetary space) and that later collided with a natural or artificial body larger than itself (e.g., Earth, Moon, Mars, an asteroid, a comet, or the International Space Station).
The most studied meteorite in history is named Allende (Figure 2.1); it fell in Mexico in 1969. Our current understanding of the history of the solar nebula was greatly enhanced by extensive studies of the components of this meteorite.
Micrometeorite. Because micrometeorites smaller than 4 µm have been observed (Figure 2.2), the lower limit for a micrometeorite has been set to 1 µm. A micrometeorite is a meteorite 1 µm to 2 mm in size.
Meteorwrong (a.k.a. pseudometeorite). A substance mistakenly thought to be a possible meteorite. This category includes all types of terrestrial rocks (e.g., basalt, quartzite, scoria, limestone, granite), iron ore (magnetite and hematite), chunks of sulfide, manganese nodules, metallic slag, ball bearings, machine parts, rusted spikes, bottle glass, petrified wood, even occasional twigs, pottery shards, and gelatin. Most meteorwrongs are found on the ground; a few are alleged to be observed falls. Some are family heirlooms. The Moon and every planet in the Solar System have been implicated as the parent body of one meteorwrong or another. The term “meteorong” (without the “w”) appears to have been coined by Fredrick C. Leonard of UCLA sometime before 1950. Discussions and some photos of meteorwrongs can be found in Olsen (Reference Olsen1979), chapter 9 of Norton and Chitwood (Reference Norton and Chitwood2008), and at the website https://sites.wustl.edu/meteoritesite/items/some-meteorite-realities/.
Dark flight. The final stage of descent of a meteoroid through an atmosphere after the object has slowed sufficiently so that it no longer produces light generated by frictional heating with the surrounding gas.
Meteoroid. A meteoroid is a natural solid object, 1 µm to 1 m in size, moving in interplanetary space. Meteoroids can be of any composition; for example, they may consist mainly of silicate, metallic Fe-Ni, sulfide or ice. A meteoroid or a meteoroid remnant that survives collision with a larger body becomes a meteorite.
Dust trail. An atmospheric train of small meteoroid particles that ablated but did not vaporize during atmospheric passage. The particles move along the meteoroid’s trajectory before dispersing and settling.
Dust. A dust grain is a particle larger than a molecule and less than 1 µm in size. A 1-µm- to 1-m–sized porous aggregate of dust particles is a micrometeoroid or meteoroid. Many small interplanetary dust particles do not melt and do not produce meteors when they pass through an atmosphere; they eventually settle to the body’s surface.
Asteroid. An asteroid is a natural subplanetary object, between 1 m and 1,000 km in diameter, in orbit around its central star; it does not show the characteristics of an active comet. It does not currently experience appreciable outgassing (sufficient to produce a coma and tail and cause orbital deviations); it would not undergo appreciable outgassing upon close approach to its central star; and it did not previously undergo appreciable outgassing. Older definitions of asteroids had placed the minimum size at 10 m, but the recent detection of smaller asteroids necessitated the change. These small interplanetary bodies include the meteorite-droppers 2008 TC3 (4.1±0.3 m) and 2018 LA (2.6–3.8 m), asteroid 2014 AA (2–4 m) that plunged into the Atlantic, as well as two objects that did not collide with Earth: 2008 TS26 (0.61–1.36 m) and 2011 CQ1 (0.8–2.6 m). Some asteroids can survive atmospheric passage and land as intact bodies with mean diameters exceeding 1 m; these large objects are considered to be meteorites as well as asteroids (e.g., the 2.7 × 2.7 × 0.9-m-sized IVB-iron-meteorite Hoba; the ~3 × 2 × 1.3-m-sized ungrouped iron Willamette). The upper limit on the diameter of an asteroid is rounded up from the mean diameter of Ceres (946 km). Bodies in interplanetary space with diameters exceeding 400 km are considered planets or dwarf planets; a body between 400 and 1,000 km in diameter (such as Ceres) can be both an asteroid and a dwarf planet. Asteroid Bennu (d ~492 m) is the target of a NASA sample-return mission (Figure 2.3).
Comet. A comet is a natural volatile-rich solid subplanetary object, at least 1 m in size, in orbit around its central star that exhibits (or would exhibit) extensive outgassing and develop a coma and tail upon close approach to the star; a comet is also a body that can be inferred to have been volatile rich and undergone extensive outgassing in the past. Icy planetesimals that remain far from the Sun are comets even though they do not exhibit outgassing. These include (a) Kuiper Belt Objects (KBOs) and bodies in the outlying scattered disk (all of which are sometimes called trans-Neptunian objects, TNOs); these regions are the sources of short-period comets with periods ≤ 200 years, and (b) Oort Cloud objects (the source of long-period comets, some of which have periods exceeding 1 million years). Most TNOs and Oort Cloud objects never enter the inner Solar System. In contrast, extinct comets that exhausted their supply of volatiles during numerous past approaches to the Sun are still considered comets, even though they have come to resemble asteroids.
Planet. With the discovery of the TNO Eris (mean diameter 2,326 km) in 2005, it became clear that the then-accepted-planet Pluto (mean diameter 2,377 km) was not the only substantial body orbiting the Sun in this region of the Solar System. The situation was analogous to one that had occurred two centuries earlier. The discoveries of the asteroids 2 Pallas (mean diameter 512 km) in 1802, 3 Juno (mean diameter 233 km) in 1804, and 4 Vesta (mean diameter 525 km) in 1807 indicated that the then-accepted-planet Ceres (mean diameter 946 km) was not the only substantial body orbiting the Sun between Mars and Jupiter. (Modern values for the sizes of these bodies are given here.) Just as Ceres lost its planetary status in the 1850s (along with Pallas, Juno, and Vesta) after dozens of bodies had been discovered in the asteroid belt, Pluto followed suit in the early twenty-first century after a handful of large TNOs were discovered. In 2006, the IAU redefined a planet in our Solar System as a body that orbits the Sun, is sufficiently massive for gravity to have molded it into a round shape, and has managed to clear its neighborhood of comparably sized objects. An analogous definition could be applied to exoplanets (although it is not known if all bodies so labeled have cleared their neighborhoods of siblings). The eight planets of our Solar System are illustrated in Figure 2.4.
In principle, a planet could be orbiting a star, a black hole, a pulsar or a white dwarf.Footnote 1 Planet-sized objects orbiting substellar objects should be considered dwarf planets, moons or substellar objects in a binary or multiple-object system. Planets do not lose their planethood if they are ejected from their system by gravitational forces.
However, planet-sized objects may have formed directly by gas collapse (perhaps with the aid of magnetic fields) in interstellar space, never having been part of a planetary system; these bodies should not be considered planets. It is often difficult to determine the origin of a low-mass isolated body. Such objects have been variously named “escaped planets,” “rogue planets,” “free-floating planets,” “Steppenwolf planets,” “sub-brown dwarfs,” and “planetary-mass brown dwarfs.” Brown dwarfs have been defined by the IAU as having masses ranging between 13 times that of Jupiter (13 MJ) and 0.08 times that of the Sun (0.08 Mʘ, equivalent to ~80 MJ). They may be undifferentiated, fully convective bodies. Those with >13 MJ may fuse deuterium (2D + p+ → 3He); those with >65 MJ may fuse lithium (7Li + p+ → 2 4He). One astronomer suggested that sub-brown dwarfs should be defined as free-floating bodies with masses between 1 and 13 times that of Jupiter. The smallest known star, a red dwarf labeled OGLE-TR-122b, has a mass of about 0.1 solar masses (equivalent to ~100 MJ). The estimate for the smallest mass of a star capable of the proton–proton chain reaction is ~75 MJ.
An important question we need address is the size range of planets. If Ceres had been alone in its region, it would have retained its planetary status; it seems likely that if Juno had been the sole substantial object in its neighborhood, it also would have been judged a planet. This suggests that the minimum diameter for a planet could be set at 200 km. Bodies with diameters less than 200 km should be considered asteroids, comets, or meteoroids even if there are no comparably sized bodies in their neighborhoods. [The smallest exoplanet found to date is PSR B1257+12A (a.k.a. Draugr), orbiting a pulsar in Virgo. It has a mass of ~0.02 M⊕ (where M⊕ = 1 Earth mass); if its bulk density is 3 g cm−3, it would be ~4,000 km in diameter.]
The maximum size of a planet is more difficult to define. Jupiter has a mean diameter of 139,822 km; the red dwarf star OGLE-TR-122b has a diameter of ~334,000 km. We suggest that a body can be categorized as a planet if it has a diameter up to about twice that of Jupiter, rounded up to 300,000 km; larger bodies should be considered stars or substellar objects. This definition can hold for bodies orbiting central stars, but there will still be ambiguity for bodies that are free floating in interstellar space; this is because of the overlap in sizes between escaped planets and objects formed by gas collapse.
We can now formulate a definition: A planet is a natural body that (1) is orbiting (or was previously orbiting) a star or former star, (2) has sufficient mass for gravity to have molded it into a round shape, (3) has cleared its neighborhood of comparably sized objects (if it is still in orbit around a star or substellar object), and (4) has a mean diameter between 200 km and 300,000 km.
Exoplanets can be classified into six groups by mass range (Abron Reference Abron2019): Mercurian, ≤0.1 M⊕; subterran, 0.1–0.5 M⊕; terran, 0.5–2 M⊕; super-Earths or mini-Neptunes, 2–10 M⊕; Neptunian, 10–30 M⊕; and Jovian, >30 M⊕. [The nearest star outside our Solar System is Proxima Centarui (4.2 light years away); it appears to shine on two exoplanets – one terran body and one super-Earth. The next closest star is Barnard’s Star, a red dwarf; it is orbited by a super-Earth (Barnard b) of about 3 M⊕.]
Our meteorite and rock collections currently contain specimens from only two planets: Earth and Mars. There is a small chance there are misidentified meteorites from Mercury in our collections (Love and Keil Reference Love and Keil1995) but almost no chance we have meteorites from Venus (Gladman et al. Reference Gladman, Burns, Duncan, Lee and Levison1996) and no chance of samples from the outer planets.
Dwarf planet. We follow here a modification of the IAU definition, taking into account the above definition of planet. A dwarf planet is a natural body that (1) is in orbit (or was once in orbit) around a star, former star, or substellar object, (2) has sufficient mass for gravity to have molded it into a round shape, (3) has a mean diameter exceeding 400 km, and (4) has not cleared its neighborhood of comparably sized objects (if it is still in orbit around a star or substellar object.) Smaller objects are asteroids and meteoroids. The minimum diameter of 400 km would allow asteroid 10 Hygiea (d ≈ 430 km) to be included in the definition; new images from the Very Large Telescope at the European Southern Observatory in Chile show Hygeia to be quasispherical. An object ≥400 km in diameter can be both a dwarf planet and an asteroid (e.g., 1 Ceres). There is no formal limit on the maximum size of a dwarf planet; however, if bodies are sufficiently massive (Earth-sized, for example), almost certainly they would have cleared their zone of comparably sized objects. In principle, a dwarf planet that has not cleared its region of comparably sized bodies could be larger than a planet in another, otherwise-unoccupied, region of the same planetary system.
Moon (a.k.a. natural satellite). A moon is a natural body, at least 1 m in diameter, which is in orbit around a larger planetary body, dwarf planet, asteroid, comet, or substellar object. Bodies smaller than 1 m are meteoroids even if they are in orbit around a planet. A moon can be larger than a planet (but not the one it is orbiting); in our Solar System, the moons Ganymede (diameter 5,268 km) and Titan (diameter 5,151 km) are both larger than the planet Mercury (diameter 4,879 km).
Our rock and meteorite collections currently contain material from only one moon – ours. Specimens include several hundred lunar meteorites, 382 kg of Apollo samples from six manned missions, 0.32 kg of material returned by three unmanned Soviet Luna spacecraft, and ~2 kg from the Chang’e-5 mission. Calculations indicate that it is possible that meteorites could reach Earth from the martian moons Phobos and Deimos (Wiegert and Galiazzo Reference Wiegert and Galiazzo2017), although none has yet been identified.
Meteor. A meteor is the light phenomenon produced by a solid object transiting an atmosphere at high velocity. Collisions with atmospheric molecules cause the outer layers of the incoming particle to sputter away, typically producing a vapor of metal atoms. Light is emitted when excited electrons in the orbits of these atoms fall back to lower-energy states. The color is dependent on the element: e.g., Na – orange-yellow; Fe – yellow; Mg – blue-green; Ca+ ions – violet. Ionized oxygen and nitrogen atoms in the atmosphere can produce red light. The solid object transiting the atmosphere could be natural or artificial; the atmosphere could be one surrounding any solid body (typically a planet, but also including a gas-shrouded moon like Titan). Meteors are commonly referred to as “shooting stars” or “falling stars” (Figure 2.5).
Meteor shower. Dust grains and larger particles lost by comets (primarily) or asteroids (in one or two cases)Footnote 2 typically escape at low relative velocities and spread out along their parent-body’s orbit. If the Earth passes through that orbital plane, our planet will encounter the ejected debris. As the dust particles, micrometeoroids, and meteoroids are frictionally heated by the Earth’s atmosphere, they produce meteors; if many particles enter the atmosphere within a short interval of time (hours to weeks), they create a meteor shower. Peak rates are typically 5–80 meteors per hour, but every 33 years, the Leonids can produce several thousand meteors per hour. Because nearly all the particles in a meteor shower are transiting the atmosphere at the same angle, they appear from the Earth’s surface to radiate from the same point in the sky.
Meteor showers are named after the constellation from which they appear to radiate; e.g., the Perseids (peaking around August 13) appear to emanate from Perseus (Figure 2.6), the Orionids (peaking around October 21) from Orion, and the Leonids (peaking around November 17) from Leo. There are also daytime meteor showers, detected mainly by radar observations and radio scatter (e.g., Arietids, peaking around June 8 and emanating from Aries).
The Perseids are derived from the debris of comet 109P/Swift-Tuttle, the Leonids from comet 55P/Tempel-Tuttle, the Orionids from the famous comet 1P/Halley, and the Arietids possibly from asteroid 1566 Icarus. The 33-year period of high Leonid-meteor counts corresponds to the period of comet 55P/Tempel-Tuttle and is due to large concentrations of dust in the comet’s proximity. A meteor shower is the occurrence within a short period of time (typically a few days or weeks) of numerous meteors appearing to radiate from a particular point in the sky. They can occur above any celestial body with a sufficiently thick and sufficiently transparent atmosphere; meteor showers should also be visible on Venus and Mars.
Meteoroid stream. A population of meteoroids in interplanetary space that have similar orbits and were derived from the same parent object. When a meteoroid stream enters an atmosphere, it can produce a meteor shower.
Fall. A meteorite can crash through your roof while you’re watching TV [Wethersfield (1982)] or taking a nap (Sylacauga); it can dent your mailbox (Claxton), break into your garage (Benld, Canon City), damage your car (Benld, Peekskill, Neagari, Orlando), land in your rice field (Kamiomi), plop into your cooking pot (Juancheng), hit your dog house (Aguas Zarcas), or plunk you on the head (Mbale). A meteoroid can explode in the atmosphere, creating a shock wave that can shatter window glass and masonry and send you to the hospital (Chelyabinsk). A fall is a recovered meteorite that was (a) responsible for a fireball observed by people (e.g., L’Aigle, Revelstoke), fireball-network cameras (e.g., Lost City, Bunburra Rockhole), Doppler radar (e.g., Mifflin), seismometers (e.g., Vilna), and/or satellites (e.g., Berduc); (b) responsible for thunderous sonic booms (e.g., Dong Ujimqin Qi, Nogata), whistling or hissing sounds (probably produced by very low frequency radio waves, i.e., electrophonic sounds) (e.g., Guangmingshan) or noise produced when the object strikes a structure or the ground (e.g., Novato); (c) inferred to have fallen recently from the damage it caused to structures (e.g., Tahara, Dunbogan), the ground (e.g., Woolgorong), or living creatures (Valera); or (d) found on a surface where recent prior observation had not indicated its presence (e.g., Sayama). Most falls belong to more than one observational category.
People who have picked up freshly fallen stony meteorites have often described the samples as cold. Four factors contribute to the low temperature: (1) The temperatures of chondritic meteoroids in near-Earth orbits are ~250 K: (2) the heated outer surface of the incoming meteoroid ablates away during atmospheric passage; (3) the period of surficial heating of the meteoroid as it transits the atmosphere is brief, typically a few seconds (too short a time to allow much heat to be conducted to the interior); (4) silicate material is a poor conductor of heat in any case.
Terms often associated with observed falls include fireball (a very bright meteor) and bolide (a fireball-producing meteoroid that breaks apart in the atmosphere).Footnote 3 The IAU defined bolides and fireballs to be brighter than absolute visual magnitude -4 at a distance of 100 km. A superbolide is a bolide brighter than absolute magnitude -17 at a distance of 100 km.
Find. A find is a meteorite that is discovered at the surface of a target body without having been observed to fall. It may have been discovered by accident or during an intentional search. There are 50 times as many meteorite finds as falls on Earth. A find could be the sole meteorite recovered from a site or could be one of several dozen to several thousand from a dense collection area such as Antarctica, the Sahara, or desert dry lakes in the southwestern United States. Meteorites are routinely named after the place where they were recovered: a geomorphological feature (e.g., a mountain, lake, river, or valley) or a geographic locale (e.g., a town, city, or county). Meteorites from dense collection areas share a name followed by a unique number (e.g., Allan Hills 84001; Northwest Africa 5000; Cayote Dry Lake 033). Dense collection areas have also been identified on Mars: Aeolis Mons and Aeolis Palus. Many finds on Earth have been altered texturally, mineralogically, and chemically by interaction with water, a process known as terrestrial weathering.
Meteorite Shower. Many meteoroids break apart in the atmosphere during deceleration. The fragments continue along the original object’s trajectory, but small fragments (with their greater surface/volume ratios) encounter more effective air resistance, decelerate more rapidly, and hit the ground sooner. The fallen fragments are distributed within an ellipse, known as a strewn field, with the larger fragments concentrated at one end. The orientation of the ellipse is governed by the trajectory of the original meteoroid. In the case of multiple fragmentation events, a series of overlapping strewn fields can be formed. A meteorite shower is the result of the collision with a large gas-shrouded body of a swarm of fragments produced in the atmosphere by the breakup of an incoming meteoroid or small asteroid. The 1912 Holbrook L/LL6 meteorite shower (Navajo County, Arizona) produced about 14,000 individuals with a total recovered mass of ~220 kg. By one estimate, the 1868 meteorite shower associated with the Pultusk H-chondrite regolith breccia (Warsaw, Poland) produced 180,000 individuals, although the recovered mass is only ~200 kg.
Pairs. Meteorites are collected as individual objects. In those cases where two or more objects can be shown to have originated from the same parent meteoroid that fragmented in the atmosphere, those objects are considered paired. Pairing among finds can be inferred from similarities in texture, patina, mineralogy, bulk chemical composition, bulk isotopic composition, cosmic ray–exposure (CRE) age, and proximity of discovery sites; in some cases, individual fragments fit together. Some meteorites, given separate names when first found, were later shown to be paired. Freshly fallen meteorites recovered from a strewn field can also be paired. Pairs are separate meteorites that are inferred to have been derived from the same parent meteoroid that fragmented in the atmosphere or from a larger individual meteorite that broke apart on the ground.
“Launch pairs” are meteorites (particularly lunar or martian meteorites) inferred to have been impact launched from the same crater at the same time even though they may have reached Earth at different times and landed in different locations. These inferences are based on similarities to one another in texture, mineralogy, bulk chemical composition, crystallization age and CRE age (and dissimilarities to other samples from the same meteorite group in these characteristics).
Fusion Crust. As a meteoroid traverses the atmosphere at high velocity, friction with the surrounding air heats the meteoroid’s surface to a temperature exceeding its melting point. The wind generated by the downward plunge of the meteoroid pushes the melt away from the projectile, exposing a fresh surface, which is then subject to additional frictional melting. As this process is repeated, a meteoroid can lose more than 95 percent of its initial mass during descent. At some point during deceleration, frictional heating has been greatly reduced and the temperature of the meteoroid surface falls below its melting temperature. The fusion crust of a meteorite is the final melt produced on the surface of its parent meteoroid during atmospheric passage. An individual meteorite can have different generations of fusion crust produced after fragmentation at different altitudes. Many amateurs confuse desert varnish with fusion crust, but these features are readily distinguishable. Desert varnish appears as a dark stain on the rock surface; it is generally only ~1 µm thick and is composed of clay particles along with iron and manganese oxides. Fusion crust is a distinct melt layer, typically ~1 mm thick, atop the surface of the unmelted meteorite (Figure 2.7).
Mineral. A mineral is a naturally occurring homogeneous solid, formed in most cases by inorganic processes, that has a definite (although typically not fixed) chemical composition and an ordered atomic arrangement. Although substances identical in form, appearance, and composition to minerals can be created in the laboratory, these synthetic products are not naturally occurring and are not considered minerals. Synthetic crystals include precious gemstones (diamond, emerald, ruby, sapphire), semiprecious gemstones (e.g., garnet, amethyst) as well as more mundane substances (e.g., ice, salt). Ice in your kitchen freezer is not a mineral, but winter frost on your car windshield is. Minerals are homogeneous solids in that they cannot be physically divided into separate crystalline substances. Although most minerals are formed by inorganic processes, i.e., without the interaction of living organisms, some mineralogists accept a few organically produced substances as genuine minerals. The most prominent example is calcium carbonate from oyster shells and pearls that is chemically, physically, and optically identical to aragonite. Because rocks are formed from minerals and some legitimate rocks are composed of organic materials (e.g., coal, coquina), consistency demands that legitimate minerals include those formed by organisms.
Minerals have a definite chemical composition that can be written with a specific chemical formula (e.g., SiO2; CaCO3; NaCl; Cu9S5; SiC; TiN; TiS; FeCrP; Fe3Si). However, most minerals do not have a fixed chemical composition – they may contain impurities that can vary from sample to sample; these impurities are additional elements not expressed in the ideal formula. For example, orthopyroxene in the enstatite–ferrosilite solid solution series (MgSiO3–FeSiO3) can contain up to ~10 mol% Al, Ca, Mn, Fe3+, Ti, Cr, and Ni; melilite (the intermediate portion of the gehlenite–åkermanite solid solution series: Ca2Al(Si,Al)2O7–Ca2MgSi2O7) can contain up to ~7 wt% total of Ti, Fe3+, Fe2+, Mn, Na, and K. These minor-element impurities substitute for the major elements in specific crystallographic sites. Minerals have an ordered atomic arrangement, indicating they are crystalline solids; their constituent atomsFootnote 4 or ions are arranged in a geometric structural framework. However, mercury is an exception; it is a liquid (and thus lacks atomic order) at room temperature.
The terms mineral species and mineral phase refer to the entire set of samples of a particular mineral (e.g., diopside; troilite; diamond; calcite; rutile; halite), defined by a unique set of compositional and structural properties. Although there are more than 5,670 approved minerals as of this writing, only about 470 have been identified in meteorites, and only about 100 minerals are relatively common on Earth. A number of reduced meteoritic minerals (e.g., some that are present in enstatite chondrites and aubrites) and some refractory minerals (e.g., panguite and kangite in carbonaceous chondrites) are not known to occur in terrestrial rocks.
Mineraloid. A mineraloid is a naturally occurring amorphous solid, lacking long-range atomic order, but possessing short-range order. Examples include maskelynite (a shock-produced glass that can have the same chemical composition and external form as its crystalline plagioclase precursor) and opal (a hydrated amorphous variety of silica, typically containing 6–10 wt% H2O). Terrestrial mineraloids also include obsidian (dark-colored, extrusive volcanic glass), tektites and related impact glasses (derived from fine-grained materials melted by meteorite impacts), jet (a form of coal derived from decaying wood under high pressure), amber (fossilized tree resin), and tar and related materials (pitch, asphalt, and bitumen) (highly viscous liquid or semisolid petroleum products formed as residues after volatilization of more-volatile hydrocarbons).
Crystal. A crystal is a largely homogeneous solid (either naturally occurring or synthetic) that possesses long-range three-dimensional internal atomic order. Its three-dimensional, periodically repeating atomic pattern is called a lattice. The internal symmetry of the crystal conforms to one of seven crystal systems, depending on the relative lengths of its three axes and the angles between the axes. Many individual coarse crystals exhibit planar faces arranged in geometric forms; they grace the covers of mineral magazines and are coveted by collectors. Because the internal atomic arrangements of these crystals do not differ from those of their malformed congeners (crystals that may have encountered external obstacles or unusual ambient conditions while growing), it is clear that the internal structure of a crystal is a more fundamental feature than the external shape (habit). Euhedral crystals are bounded by well-formed faces; anhedral crystals are bounded by no crystallographically formed faces; and subhedral crystals are bounded by some well-formed and some poorly formed faces or bounded completely by only partially developed faces.
A crystal form is the set of symmetrically equivalent crystal faces. Crystal forms may be closed (e.g., cubes and dipyramids, wherein the faces entirely enclose the crystal volume) or open (e.g., pinacoids and pyramids, wherein the faces do not enclose the entire volume). The number of faces characteristic of a particular crystal form is dependent on the crystal class to which the mineral belongs and on the orientation of particular crystal faces to the symmetry axes and planes.
In polyatomic crystals, the atoms of different elements generally occupy distinct locations within the lattice. For example, nesosilicates contain isolated SiO4 tetrahedra with four O2− anions located at the apices of a regular tetrahedron and one Si4+ cation at the center. The nesosilicate monticellite (CaMgSiO4) is composed of stacked layers, individually consisting of octahedra cross-linked by SiO4 tetrahedra; there are two distinct octahedral sites: M1 (a moderately distorted site) contains Mg, and M2 (a generally undistorted site) contains Ca.
Natural crystals are not perfectly homogeneous; they contain various defects. Point defects include vacant lattice sites, interstitial defects (wherein atoms occupy sites that are typically vacant), antisite defects (wherein atoms of different elements exchange sites in the lattice), and topological defects (wherein atoms are arranged in different patterns than that of the bulk of the crystal). An important parameter is the Burgers vector (b), which represents the magnitude and direction of the lattice distortion. Line defects include edge dislocations (wherein a plane of atoms abruptly terminates within the body of a crystal and the Burgers vector is perpendicular to the dislocation line) and screw dislocations (wherein the planes of atoms are arranged in a helix around a dislocation line and the Burgers vector is parallel to the dislocation line). Common planar defects include grain boundaries (typically responsible for changes in crystal orientation), stacking faults (changes in the normal sequence of adjacent layers), and twin boundaries (which represent domains related by a mirror plane, a rotation axis, or an inversion axis). Bulk defects include fractures, clusters of vacancies, inclusions, and pores.
Although crystals can contain different numbers of cations and anions, the total electrical charge of each unit cell is zero. In halite (NaCl), there is an equal number of Na+ cations and Cl− anions; in quartz (SiO2), there are twice as many O2− anions as Si4+ cations; in spinel (MgAl2O4), charge balance is maintained by four O2− anions, two Al3+ cations, and one Mg2+ cation. Real crystals also contain vacancies (where atoms are absent from the ideal crystal lattice); the number of vacancies tends to increase with temperature. Common types of vacancies in crystals are called Schottky defects; they comprise unoccupied cation and anion sites that are in a stoichiometric ratio, thereby maintaining charge neutrality.
The sizes of natural crystals vary enormously and are studied by different techniques: 10 nm to 100 nm–sized grains are routinely imaged with the transmission electron microscope (TEM); 0.05–500-µm-sized grains are observed with backscattered electrons (BSEs); ~0.2-µm- to 1 cm-sized grains with the optical microscope; and larger grains are observed visually or with hand lenses. Among the largest known crystals are those of selenite (CaSO4·2H2O) from the Giant Crystal Cave in Naica, Mexico; these enormous crystals range up to 12 m in length and 4 m in width. They crystallized slowly over a period of ~0.5 Ma from ground water heated by a magma chamber located a few kilometers below the cave. The largest known crystal may be a beryl (Be3Al2(SiO3)6) from the Malakialina pegmatite, Madagascar; the crystal is 18 m long and 3.5 m wide (Rickwood Reference Rickwood1981).
Among meteorites, crystals range from submicrometer-sized (e.g., every known grain of wassonite (TiS)) to a ~50-cm-sized low-Ni metallic iron (kamacite) grain in the Coahuila IIAB hexahedrite (Buchwald Reference Buchwald1975). (Hexahedrites are a structural type of iron meteorite discussed in Section 8.5). Some hexahedrites may contain single kamacite grains approaching 1 m in maximum dimension (Fesenkov Reference Fesenkov1958). The Agpalilik specimen of the Cape York IIIAB iron meteorite crystallized as a single crystal of taenite at least 2 m across, but the crystal developed a Widmanstätten pattern (consisting of numerous separate kamacite and taenite grains) during slow subsolidus cooling.
Quasicrystal. A quasicrystal is a solid substance (either naturally occurring or synthetic) that has an ordered atomic arrangement but is not periodic. The internal crystalline patterns of the material do not occur at regular intervals, i.e., there is no translational symmetry. They can display diffraction patterns with otherwise forbidden rotational symmetry, e.g., fivefold, eightfold, and more axes of symmetry (Socolar et al. Reference Socolar, Steinhardt and Levine1985). The internal structure of a quasicrystal is intermediate between that of an amorphous substance and a crystalline solid. Quasicrystals were first conceptualized and synthesized in 1984 (Shechtman et al. Reference Shechtman, Blech, Gratias and Cahn1984; Levine and Steinhardt Reference Levine and Steinhardt1984). Although three quasicrystals have been reported in the CV3 Khatyrka meteorite including two icosahedrites (Al63Cu24Fe13 and Al62Cu31Fe7) plus decagonite (Al71Ni24Fe5) (Bindi et al., Reference Bindi, Steinhardt, Yao and Lu2011, Reference Bindi, Yao, Lin, Hollister, Andronicos, Distler, Eddy, Kostin, Kryachko, MacPherson, Steinhardt, Yudovskaya and Steinhardt2015, Reference Bindi, Lin, Ma and Steinhardt2016) and approved by the IMA-CNMNC, their validity as natural species remains controversial.
Rock. A rock is a naturally occurring solid aggregate of one or more minerals and/or mineraloids. Concrete is a hard composite material but is not naturally occurring and is therefore not a rock. Although naturally occurring, some beachrocks contain artifacts such as coins and bottle glass; the presence of these man-made components does not disqualify beachrocks from being legitimate rocks.
The three principal rock types are igneous rocks (formed by the cooling and solidification of molten or partly molten material), sedimentary rocks (formed by the mechanical deposition and consolidation of material or by chemical precipitation from a fluid), and metamorphic rocks (formed by mineralogical, textural, and/or chemical alteration of preexisting rocks in response to significant ambient changes in temperature, pressure, chemical environment, and/or stress). Terrestrial sedimentary rocks include some composed of inorganic material (e.g., sandstone, claystone, evaporites), some with high concentrations of organic material (e.g., coal, oil shale), and some made mainly of fossilized organic material (e.g., coquina – bioclastic limestone; chert – derived ultimately from the skeletal remains of diatoms, silicoflagellates, and radiolarians).
All three major rock types are present among meteorites: igneous meteorites include basalts, dunites, orthopyroxenites, magmatic irons, and impact-melt rocks; sedimentary meteorites include primitive chondrites (e.g., LL3.00 Semarkona, which formed principally by the agglomeration of individual constituents in the solar nebula); metamorphic meteorites include type 4–6 H, L, LL, EH, EL, R and CK chondrites, and many eucrites.
Common terrestrial rocks that are mainly monomineralic include sedimentary rocks (e.g., salt, ice, limestone, and some sandstones such as the St. Peter Sandstone), metamorphic rocks (e.g., quartzite and marble), and igneous rocks (e.g., dunite and anorthosite), although essentially all of them contain minor amounts of additional minerals. Common terrestrial rocks that are polymineralic include shale (sedimentary rock composed mainly of clay minerals such as kaolinite, montmorillonite, and illite, with minor to accessory quartz, calcite, hematite, goethite, and/or mica), schist (metamorphic rock commonly composed of mica, quartz, and feldspar, and in some cases containing minor garnet, hornblende, talc, chlorite, graphite, or glaucophane), granite (igneous rock composed of major quartz, K-feldspar, and plagioclase with minor mica and amphibole), and basalt (igneous rock composed of major plagioclase and pyroxene with accessory to minor olivine, magnetite, ilmenite, and/or ulvöspinel).
Igneous rocks made of crystals large enough to be seen by the unaided human eye are labeled phaneritic. Rock textures can be described as very-coarse-grained (>3-cm-sized grains), coarse-grained (5 mm to 3 cm), medium-grained (1–5 mm), and fine-grained (<1 mm). By these criteria, nearly all meteorites are mostly fine grained. If a rock contains numerous grains, too small to be seen visually, it is called aphanitic.
Breccia. A breccia is a rock composed of rock, mineral, and/or mineraloid fragments held together by a fine-grained or glassy matrix. Breccias can be monomict (consisting of a single lithology), dimict (two lithologies), polymict (more than two lithologies), or genomict (consisting of clasts of a single rock type but with different metamorphic grades) (e.g., Bischoff et al. Reference Bischoff, Scott, Metzler and Goodrich2006). Most breccias are fragmental breccias: cemented assemblages of angular clasts. Terrestrial fragmental breccias include those formed in sedimentary, tectonic, volcanic, and hydrothermal environments as well as those associated with impact craters. Impact-melt breccias contain shocked but unmelted rock fragments surrounded by a major glassy or fine-grained component that solidified from a cooling impact-derived melt. Regolith breccias, known from the Moon, Mars, and asteroidal meteorites, formed in the near-surface regions of these bodies; they are commonly composed of rock fragments admixed with glass and/or fine-grained material and typically contain relatively high concentrations of solar-wind-implanted noble gases. They include some mineral grains whose crystal lattices were damaged by energetic cosmic ray particles. Many regolith breccias also contain projectile fragments (unrelated to the principal rock components) that survived collision with the parent-body’s surface. Examples include (1) CM carbonaceous-chondrite clasts constituting ~5 vol% of the Kapoeta howardite and ~1–3 vol% of the Abbott H-chondrite regolith breccia, (2) a shocked LL5 clast in the Dimmitt H-chondrite regolith breccia, (3) a 7 mm eucritic clast in the NWA 869 L-chondrite regolith breccia, and (4) two dark H-chondrite clasts in the St. Mesmin LL-chondrite regolith breccia.