Meteorites are delivered to Earth from at least 100 different asteroids as well as from the Moon and Mars. A significant fraction of micrometeorites and interplanetary dust particles are from comets. Some of the meteorite parent bodies melted and differentiated; they developed metal cores and basaltic crusts. Other bodies never melted, preserving materials formed in the solar nebula. Some unmelted asteroids were thermally metamorphosed; many were aqueously altered. Meteorites suffered shock damage on their parent bodies, ranging from fracturing and brecciation to impact melting and devolatilization. After their formation, many samples experienced thermal metamorphism, aqueous alteration, and shock metamorphism to different extents; some were altered and/or shocked more than once.

As discussed in Chapter 2, quasicrystals are an unusual form of matter with orderly, but nonperiodic atomic arrangements. They can display diffraction patterns with otherwise forbidden rotational symmetry, e.g., fivefold, eightfold, and higher axes of symmetry (Levine and Steinhardt 1984; Shechtman et al. 1984). Princeton physicist Paul Steinhardt spent decades searching for natural quasicrystals, a quest documented in his book, The Second Kind of Impossible (Steinhardt 2019).

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.

It is not the purpose of this chapter to present a comprehensive overview of crystallography and crystal chemistry. Excellent discussions of those topics can be found in the classic Crystallography and Crystal Chemistry: An Introduction by F. D. Bloss (1971) as well as in several more recent texts including those by Klein and Dutrow (2007) and Nesse (2012). Here we cover only the most salient points about crystallography to facilitate understanding of topics and terms presented elsewhere in this book without explanation.

More than half a century ago, Buzz Aldrin surveyed the lunar landscape, noted its starkness and ash-colored hues, and declared it a site of “magnificent desolation.” Extraterrestrial mineralogy is the study of minerals from our parental molecular cloud as well as from the Moon, Mars, asteroids, and similarly desolate places. The information gathered from detailed analyses of these phases has provided a deeper understanding of the myriad processes that can produce minerals in the Solar System and its erstwhile stellar neighborhood.

During the earliest stages of thermal metamorphism on parent asteroids, primary feldspathic chondrule glass in primitive chondrites began to devitrify to form fine-grained clinopyroxene and crystalline feldspar (Figure 11.1). This resulted in a significant increase in the thermoluminescence (TL) sensitivity of the whole rocks. In some cases, the reaction leading to the crystallization of feldspar may have been catalyzed by water, consistent with the evidence for aqueous alteration in the most primitive ordinary and carbonaceous chondrites.

Many meteoritic minerals can be identified microscopically in reflected light; these include both opaque and nonopaque phases. The appearance of numerous opaque minerals is discussed in detail in two comprehensive books by Paul Ramdohr (1969, 1973).

The first significant step in meteorite classification was taken in 1802 by British chemist Edward Howard who showed that the meteoritic stones and irons he analyzed all contained Ni. This criterion was used to distinguish meteorites from terrestrial rocks, but the reliability of this distinction was soon undermined by two essentially metal-free falls: the Stannern eucrite in the Czech region of the Austrian Empire in 1808 and the Luotolax howardite in Finland in 1813. It became apparent to researchers that, although most stony meteorites contained metallic Fe-Ni, a few did not. This realization somewhat hampered subsequent classification efforts because a few large collections included a handful of metal-free terrestrial rocks posing as meteorites. Some of these “meteorwrongs” were covered with dark crusts, probably desert varnish or weathering rinds.

Mercury is the smallest and innermost major planet (Figure 9.1); from the perspective of Earth-bound observers, it never appears more than 28° from the Sun. It is locked in a state of spin-orbit coupling: its rotation period (58.646 Earth days) is exactly two-thirds of its year (87.969 Earth days). Mercury is named after the Greco-Roman messenger to the gods, perhaps because of its rapid apparent day-to-day movement with respect to the Sun. Compared to the other major planets, Mercury has the most eccentric orbit (e = 0.2056), the highest inclination relative to the ecliptic (7.007°), and the smallest axial tilt (0.01°). It has a weak magnetic field (~1.1 percent as strong as Earth’s) and no natural satellites.

A typical meteoroid enters the Earth’s atmosphere at 18–20 km s−1. During the traverse, the surface of a chondritic meteoroid reaches temperatures of 1,180–1,410°C and melts to depths of ~0.3–1.0 mm due to friction with the surrounding air. The surface of the meteoroid ablates, exposing a new surface that also melts and ablates; this process can be repeated several times.

Bob Hazen and colleagues have pointed out that as the universe evolved and as stars, planets, and life developed, the number of diverse mineral-forming environments increased (Hazen et al. 2008; Hazen and Morrison 2020; Morrison and Hazen 2020). This resulted in more complex minerals and an ever-increasing set of mineral species. The focus of the following chapters is narrower – we largely restrict the discussion to mechanisms that formed minerals within meteorites.

Chondrules are among the most abundant components of chondrites, constituting, for example, 55 vol% of CR carbonaceous chondrites and 65–75 vol% of type-3 OC (Table 7.1). Their average apparent diameters range from ~150 µm in CO3 chondrites to ~900 µm in CV3 chondrites; the overall range in diameter is 0.25 µm to ~5 cm. The most common chondrules in all chondrite groups are ferromagnesian objects, rich in olivine and/or low-Ca pyroxene. There are five common textural types of ferromagnesian chondrules.

One broad-brush approach to systematizing minerals is to divide them into two categories: (1) native elements and their alloys (e.g., Au, Ag, Cu, C, S, Ni3Fe, Fe5C2) and (2) compounds (e.g., sulfides, sulfates, oxides, halides, silicates, carbides, carbonates, phosphides, phosphates, tungstates, nitrides). Compounds are grouped together based on their monatomic or polyatomic anions. This is a natural classification system (first adopted by Jacob Berzelius) because the chemical properties of minerals within these groups tend to be similar. For example, the properties of anhydrite (CaSO4) more closely resemble those of other sulfates such as barite (BaSO4) and celestine (SrSO4) than those of other Ca-rich minerals, e.g., calcite (CaCO3), oldhamite (CaS), fluorite (CaF2), antarcticite (CaCl2·6(H2O)), lime (CaO), apatite (Ca5(PO4)3(F,Cl,OH)), scheelite (CaWO4), portlandite (Ca(OH)2), powellite (CaMoO4), and wollastonite (CaSiO3).

In this book, minerals in chondrites are considered primary if they formed in the solar nebula (even if they formed by reaction of a different precursor phase with nebular gas) and secondary if they formed on their parent body. Minerals in differentiated meteorites are considered primary if they formed by igneous processes or upon initial cooling, and secondary if they formed during subsequent alteration.