Classification: Chemical, Isotopic and Structural Trends

PART VI

CHEMICAL, ISOTOPIC, & STRUCTURAL TRENDS FOR CLASSIFICATION

CONTINUE TO
[PART I] ChondritesChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such Click on Term to Read More
[PART II] Achondrites
[PART III] Irons
[PART IV] Stony-Irons
[PART V] Refractory Phases
[APPENDECTOMY]

1. OxygenElement that makes up 20.95 vol. % of the Earth's atmosphere at ground level, 89 wt. % of seawater and 46.6 wt. % (94 vol. %) of Earth's crust. It appears to be the third most abundant element in the universe (after H and He), but has an abundance only Click on Term to Read More Content:

Carbonaceous chondrites are oxygen-rich with most of the iron combined as silicates, magnetiteFe oxide, Fe2+Fe3+2O4, containing oxidized iron (Fe3+) found in the matrix of carbonaceous chondrites and as diagnostic component in CK chondrites. In CK chondrites, magnetite is typically chromian, containing several wt. % Cr2O3. Click on Term to Read More, or water. Ordinary chondrites have about half their iron in the oxide form with the rest in troiliteBrass colored non-magnetic Fe sulfide, FeS, found in a variety of meteorites. or metalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn Click on Term to Read More. EnstatiteA mineral that is composed of Mg-rich pyroxene, MgSiO3. It is the magnesium endmember of the pyroxene silicate mineral series - enstatite (MgSiO3) to ferrosilite (FeSiO3). Click on Term to Read More chondrites are oxygen-poor with all the iron in metal or sulfide form. R chondrites have a high degree of Fe oxidationOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons,  while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants Click on Term to Read More.

2. CI-normalized refractory-lithophile abundance ratioLogarithm of the ratio of two metallic elements in a star relative to their ratio in the Sun. This is used to quantify the relative amounts of individual elements present in a star. For example, the abundance ratio of Mg to Fe, written [Mg/Fe], is: Chemical elements are produced through Click on Term to Read More:

Carbonaceous chondrites have a ratio of 1.00–1.35 (CV = 1.33, CK = 1.11–1.33, CO,CM = 1.11, CR = 1.00), R chondrites 0.85, ordinary chondrites 0.77–0.82, and enstatite chondrites ~0.6.

3. Fine-grained matrixFine grained primary and silicate-rich material in chondrites that surrounds chondrules, refractory inclusions (like CAIs), breccia clasts and other constituents. Click on Term to Read More/chondrule modal abundance ratio:

Carbonaceous chondrites have a ratio of 0.5–7.0, ordinary chondrites ~0.3, enstatite chondrites essentially none, and R chondrites 1.6 (±0.9).

4. Abundance of isotopically heterogeneous refractory inclusionsInclusions found predominantly in carbonaceous chondrites and are rich in refractory elements particularly calcium, aluminum and titanium that in various combinations form minerals such as spinel, melilite, perovskite and hibonite. There are two types of refractory inclusion: • Ca Al-rich inclusions (CAIs) • Amoeboid olivine aggregates (AOAs) Refractory inclusions were Click on Term to Read More:

Carbonaceous chondrites have a ratio of ~0.5–5.0 vol%, ordinary chondrites have a negligible amount, enstatite chondrites a negligible amount, and R chondrites essentially none.

5. Whole rock O-isotope composition:

Carbonaceous chondrites are much below the terrestrial fractionationConcentration or separation of one mineral, element, or isotope from an initially homogeneous system. Fractionation can occur as a mass-dependent or mass-independent process. Click on Term to Read More line (TFL), ordinary chondrites are above the TFL, enstatite chondrites are on the TFL, and R chondrites are much above the TFL. The CO and CK groups share a common O-isotopic composition.

6. CI-normalized Se/Sb concentration ratios:

Carbonaceous chondrites have a ratio of 0.6–0.9, ordinary chondrites 0.9–1.1, enstatite chondrites 1.0–1.2, and R chondrites 1.5 (±0.2).

7. Molar An of plagioclaseAlso referred to as the plagioclase feldspar series. Plagioclase is a common rock-forming series of feldspar minerals containing a continuous solid solution of calcium and sodium: (Na1-x,Cax)(Alx+1,Si1-x)Si2O8 where x = 0 to 1. The Ca-rich end-member is called anorthite (pure anorthite has formula: CaAl2Si2O8) and the Na-rich end-member is albite Click on Term to Read More:

Carbonaceous chondrites have a high molar, ordinary chondrites have a low molar, enstatite chondrites have a very low molar, and R chondrites have a low molar.

8. Abundance of opaque mineral-rich porphyritic chondrulesRoughly 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 Click on Term to Read More:

Carbonaceous chondrites have a large abundance, ordinary chondrites have few, enstatite chondrites have a low to moderate abundance, and R chondrites have few.

9. Ratio of Iron to Silicon:

Subdivides ordinary chondrites into subgroups H, L, and LL, and the enstatite chondrites into subgroups EH and EL.

10. Ratio of Magnesium to Silicon:

Separates the carbonaceous, ordinary, and enstatite chondrites. Carbonaceous chondrites have average ratios of 1.05. Ordinary chondrites have ratios of 0.97, 0.92, and 0.92 for H, L, and LL groups, respectively. Enstatite chondrites have ratios of 0.73 and 0.88 for EH and EL groups, respectively.

11. Ratio of Calcium to Silicon:

Subdivides carbonaceous chondrites into subgroups CI, CM, CR, CO, CK, CV, and CH.

12. CV oxidizedOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons,  while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants Click on Term to Read More Bali-like and Allende-like, and reducedOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons,  while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants Click on Term to Read More subgroups are separated by FeNi-metal, sulfide, and magnetite abundances (after Bland et al., 2000):
• Allende-like have low FeNi-metal, sulfides, and magnetite; metal and sulfides are Ni-rich.
• Bali-like have variable to high magnetite, low to absent metal, and low sulfides; metal and sulfides are mostly Ni-rich.
• reduced CVs have intermediate magnetite, high metal, and high sulfides; metal and sulfides are mostly Ni-poor.
13. Ratio of low-temperature metals (e.g., Pd and AuThe astronomical unit for length is described as the "mean" distance (average of aphelion and perihelion distances) between the Earth and the Sun. Though most references state the value for 1 AU to be approximately 150 million kilometers, the currently accepted precise value for the AU is 149,597,870.66 km. The Click on Term to Read More) to Ir:

Ratios are higher in EL than in EH chondrites.

14. ChondruleRoughly 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 Click on Term to Read More Size (Brearley and Jones, 1998):

Average diameter (mm) decreases in the order CV>LL>CR=CK=L>EL=K>R>CM=H>EH>CO>CH

ORDINARY CHONDRITEWork in Progress Ordinary chondrites (OCs) are the largest meteorite clan, comprising approximately 87% of the global collection and 78% of all falls (Meteoritical Society database 2018)1. Meteorites & the Early Solar System: page 581 section 6.1 OC of type 5 or 6 with an apparent shock stage of S1, Click on Term to Read More CHONDRULE SIZES (µm) after K. Metzler (2018)
	Mean 2D (3D)	Median 2D (3D)	Min 2D (3D)	Max 2D (3D)
H4 (NWA 2465)	450 (490)	370 (420)	95 (90)	5400 (2360)
L4 (Saratov)	500 (610)	450 (530)	130 (180)	2160 (2520)
LL4 (NWA 7545)	690 (830)	580 (730)	190 (245)	3810 (2880)
after D. W. Hughes (1978)
L/LL4 (Bjurböle)	— (750)	— (688)	200 (250)	— (—)

15. Petrologic TypeMeasure of the degree of aqueous alteration (Types 1 and 2) and thermal metamorphism (Types 3-6) experienced by a chondritic meteorite. Type 3 chondrites are further subdivided into 3.0 through 3.9 subtypes.:

Type 3 represents unaltered material, while lower numbers represent progressive alteration by aqueous conditions and higher numbers represent progressive alteration by thermal metamorphism. As metamorphicRocks that have recrystallized in a solid state due to changes in temperature, pressure, and chemical environment. Click on Term to Read More alteration increases to type 7, a number of chemical and physical changes occur:

• olivineGroup of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in all chondrites within both the matrix and chondrules, achondrites including most primitive achondrites and some evolved achondrites, in pallasites as large yellow-green crystals (brown when terrestrialized), in the silicate portion Click on Term to Read More and pyroxeneA class of silicate (SiO3) minerals that form a solid solution between iron and magnesium and can contain up to 50% calcium. Pyroxenes are important rock forming minerals and critical to understanding igneous processes. For more detailed information, please read the Pyroxene Group article found in the Meteoritics & Classification Click on Term to Read More become more homogenous
• pyroxene changes from low-temperature clinopyroxene to high-temperature orthopyroxeneOrthorhombic, low-Ca pyroxene common in chondrites. Its compositional range runs from all Mg-rich enstatite, MgSiO3 to Fe-rich ferrosilite, FeSiO3. These end-members form an almost complete solid solution where Mg2+ substitutes for Fe2+ up to about 90 mol. % and Ca substitutes no more than ~5 mol. % (higher Ca2+ contents occur Click on Term to Read More
• amount of crystalline feldsparAn alumino-silicate mineral containing a solid solution of calcium, sodium and potassium. Over half the Earth’s crust is composed of feldspars and due to their abundance, feldspars are used in the classification of igneous rocks. A more complete explanation can be found on the feldspar group page. Click on Term to Read More increases at the expense of glass, then decreases after vitrification
• feldspar coarsens from type 6 to 7, becoming >0.1 mm in size
• fine-grained matrix becomes transparent and recrystallizes into a coarser texture
• bulk carbonElement commonly found in meteorites, it occurs in several structural forms (polymorphs). All polymorphs are shown to the left with * indicating that it 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*. Click on Term to Read More and bulk water content decrease
• Ca range in Wollastonite increases from 0.4–1.2 in types 3 and 4, to 1.2–1.6 in type 5, and to 1.6–2.2 in type 6
• CaO in low-Ca pyroxenes increases from <1.0 wt% to >1.0 wt% from type 6 to 7
• chondrules merge into surrounding material and become only relics by type 7, while like-metal grains coalesce and exhibit an even dispersion
• TL sensitivity increases with higher petrologic type, while decreasing at higher shock levels

CO3 Petrographic Types after Sears et al., 1991
	TL sens. (120°C)
3.0	<0.017
3.1	0.017–0.030
3.2	0.030–0.054
3.3	0.054–0.100
3.4	0.100–0.170
3.5	0.170–0.300
3.6	0.300–0.540
3.7	0.540–1.000
3.8	1.000–1.700
3.9	>1.700

It is considered that the TL sensitivity technique is not applicable below subtype 3.2 because feldspar could have been dissolved during the aqueous alteration process. This classification method has now been superceded by more sensitive techniques that are able to measure the peak metamorphic temperatures, and thus enable the direct comparison of petrologic types among chemical classes. To discriminate among subtypes below type 3.2, it has been shown that the Cr content of ferroan olivine is an excellent indicator of metamorphism. ChromiteBrownish-black oxide of chromium and iron (Cr-Fe oxide), Cr2FeO4, found in many meteorite groups. Click on Term to Read More exsolves from olivine in the incipient stages of metamorphism, initially producing heterogeneous Cr2O3 contents, and eventually low-Cr olivine. In a study by Chizmadia and Bendersky (2006), they determined that this sequence progresses from type 3.0, corresponding to high Cr2O3 contents of 0.3–0.4 wt%, to type 3.2, in which Cr2O3 constitutes less than 0.1 wt%. The gap between these subtypes represents type 3.1. A detailed petrographic study of CO3 chondrites was conducted by Davidson et al. in 2014 in order to better define a metamorphic trend for this group (see following diagram).
Diagram credit: J. N. Grossman & A. J. Brearley, MAPS, vol. 40, p. 87 (2005)
‘The onset of metamorphism in ordinary and carbonaceous chondrites’
(http://dx.doi.org/10.1111/j.1945-5100.2005.tb00366.x) <!–
Diagram credit: J. N. Grossman & A. J. Brearley
MeteoriticsScience involved in the study of meteorites and related materials. Meteoritics are closely connected to cosmochemistry, mineralogy and geochemistry. A scientist that specializes in meteoritics is called a meteoriticist. Click on Term to Read More & Planetary Science, vol. 40, p. 87 (2005) –>

CO3 Chondrites

Image courtesy of Davidson et al., 45th LPSC, #1384 (2014)

A Chemical-Petrologic Classification for the Chondritic Meteorites (Van Schmus and Wood, 1967)
Revised by Tait et al. (2015) to include criteria for petrologic type 7, incorporating updates from Sears and Dodd (1998), Brearley and Jones (1998), #Dodd (1981), and *Tait et al. (2014)

Diagram credit: A.W. Tait et al., GCA, vol. 134, p. 192 (2014)
‘Investigation of the H7 ordinary chondriteChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such Click on Term to Read More, Watson 012: Implications for recognition and classification of Type 7 meteorites’ (http://dx.doi.org/10.1016/j.gca.2014.02.039) In a manner similar to that employed for the chondrite groups, the eucritesMost common type of achondrite meteorite and a member of the HED group. Eucrites are basalts composed primarily of pigeonite and anorthite (An60-98). Eucrites have been placed into three subgroups based on mineralogical and chemical differences. • Non-cumulate eucrites represent the upper crust that solidified on a magma ocean after Click on Term to Read More have been petrologically divided into a metamorphic sequence comprising seven types (after Takeda and Graham, 1991; Yamaguchi et al., 1996):

1. Type 1—Most rapidly cooled within the sequence; mesostasis-rich with a glass phase and original chemistry preserved; exhibits pronounced Mg–Fe zoning in pyroxenes; represents the least altered basaltBasalt is the most common extrusive igneous rock on the terrestrial planets. For example, more than 90% of all volcanic rock on Earth is basalt. The term basalt is applied to most low viscosity dark silicate lavas, regardless of composition. Basalt is a mafic, extrusive and fine grained igneous rock Click on Term to Read More studied; e.g., clasts in Y-75011, Y-75015, and Y-74450

2. Type 2—Metastable Fe-rich pyroxenes are absent; mesostasisLast material to crystallize/solidify from a melt. Mesostasis can be found in both chondrules, in the matrix around chondrules, and in achondrites as interstitial fine-grained material such as plagioclase, and/or as glass between crystalline minerals.   Click on Term to Read More glass is no longer clear; e.g., Pasamonte

3. Type 3—Zoning from coreIn the context of planetary formation, the core is the central region of a large differentiated asteroid, planet or moon and made up of denser materials than the surrounding mantle and crust. For example, the cores of the Earth, the terrestrial planets and differentiated asteroids are rich in metallic iron-nickel. Click on Term to Read More to rim is less defined with an increase in Ca towards the rim; pyroxenes becoming cloudy; coarsening of pyroxenes resulting from augiteHigh-Ca clinopyroxene, (Ca,Mg,Fe)SiO3, that occurs in many igneous rocks, particularly those of basaltic composition. In order to be considered augite, the clinopyroxene must contain 20 to 45 mol % of calcium (Wo20 - 45). An important and unique Martian meteorite is NWA 8159, that has been classified as an augite Click on Term to Read More exsolutionSegregation, during cooling, of a homogeneous solid solution into two or more different solids. Click on Term to Read More lamellae; e.g., clastA mineral or rock fragment embedded in another rock. Click on Term to Read More in Y-790266

4. Type 4—Only remnants of zoning still visible; cloudy pyroxenes present; mesostasis glass is recrystallized or absent; augite exsolution lamellae becoming resolvable in microprobe; e.g., Stannern, Nuevo Laredo

5. Type 5—Homogenous host composition with readily resolvable exsolved pigeoniteLow-Ca clinopyroxene, (Ca,Mg,Fe)SiO3, found as a major mineral in eucrites and shergottites. In order to be considered pigeonite, the clinopyroxene must contain 5 to 20 mol % of calcium (Wo5 - 20). Chondrites of petrologic types 4 and below contain significant low-Ca clinopyroxene. During metamorphism to higher temperatures, all existing lamellae; pigeonites extensively clouded by reheating; mesostasis glass recrystallized or absent; e.g., Juvinas, Sioux Co., Lakangaon

6. Type 6—Most slowly cooled eucrites in the sequence; the clinopyroxene pigeonite is partly inverted to orthopyroxene through slow cooling processes; pyroxenes contain Mg-rich cores and coarse augite exsolution lamellae; original mesostasis is absent; Ca is enriched in the rims; often have a brecciated texture; e.g., Millbillillie, Y-791186

7. Type 7—Recognized as the most metamorphosed in the sequence (Yamaguchi et al., 1996); e.g., Palo Blanco Creek, Jonzac, Haraiya, A-87272, NWA 3152

Weathering Grade (Cassidy, 1980; Otto, 1992)
(based on surficial rust and evaporites, used for Antarctic meteorites)

• A–Minor rustiness; rust haloes on metal particles and rust stains along fractures are minor
• B–Moderate rustiness; large rust haloes occur on metal particles and rust stains on internal fractures are extensive
• C–Severe rustiness; metal particles have been mostly stained by rust throughout
• e–Evaporite minerals visible to the naked eye

Weathering Grade (Wlotzka, 1993)∗
(based on oxidation of FeNi-metal and FeS; primarily used for ordinary chondrites)

• W0–No visible oxidation of metal or sulfide but a limonitic staining might be noticeable in transmitted light. Fresh falls are usually of this grade, although some are already W1
• W1–Minor oxide rims around metal and troilite, with minor oxide veins
• W2–Moderate oxidation of metal, about 20–60% being affected
• W3–Heavy oxidation of metal and troilite, 60–95% being replaced
• W4–Complete (>95%) oxidation of metal and troilite, but no alteration of silicates
• W5–Beginning alteration of maficOne of the two broad categories of silicate minerals, the other being felsic, based on its magnesium (Mg) and/or iron (Fe) content. Mafic indicates silicate minerals that are predominantly comprised of Mg and/or Fe.The term is derived from those major constituents: Magnesium + Ferrum (Latin for iron) + ic (having Click on Term to Read More silicates, mainly along cracks
• W6–Large scale replacement of silicates by clay minerals and oxides

∗Revised by Zurfluh et al. (2016)
(based on improved weathering parameters applied to a greater number of meteoriteWork in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and Click on Term to Read More samples)

• W0.0–Fresh, some iron hydroxide staining possible
• W1.0–Minor oxide rims around metal and troilite, small iron oxides and iron hydroxide veins might be already present
• W2.0–Onset of veining with iron oxides and iron hydroxides
• W3.0–Strong oxidation of metal, troilite shows only minor alteration
• W3.3– Strong oxidation of metal, troilite moderately altered. Usually a few troilites are completely oxidized
• W3.6–Strong oxidation of metal and troilite. Most troilites are oxidized or show reduced reflectivity
• W4.0–Nearly complete oxidation of metal and troilite, usually some troilite remnants are visible
• W4.5–All metal and troilite oxidized, only minor remnants of metal and troilite as inclusions in silicates; some silicateThe most abundant group of minerals in Earth's crust, the structure of silicates are dominated by the silica tetrahedron, SiO44-, with metal ions occurring between tetrahedra). The mesodesmic bonds of the silicon tetrahedron allow extensive polymerization and silicates are classified according to the amount of linking that occurs between the alteration (mainly olivine) possible
• W5.0– Metal and troilite 100% oxidized, major alteration of silicates, mainly olivine
• W6.0–Massive replacement of silicates by clay and oxides

When metal and troilite each are oxidized >95 vol%, the sample is classified as W4.0, while a sample with 100 vol% metal alteration and 90 vol% troilite alteration is still a W3.6.

Weathering Index (Rubin and Huber, 2005)
(based on the amount of staining in silicates, used for oxidized CK and R chondrites)

• wi-0–Unweathered; <5 vol% of silicates stained brown
• wi-1–Slightly weathered; 5–25 vol% of silicates stained brown
• wi-2–Moderately weathered; 25–50 vol% of silicates stained brown
• wi-3–Significantly weathered; 50–75 vol% of silicates stained brown
• wi-4–Highly weathered; 75–95 vol% of silicates stained brown
• wi-5–Severely weathered; >95 vol% of silicates stained brown
• wi-6–Extremely weathered; nearly complete staining of silicates, and significant replacement of mafic silicates by phyllosilicatesClass of hydroxyl-bearing silicate minerals with a sheet-like structure. They result from aqueous alteration are dominantly serpentine and smectite in meteorites; found in the matrixes of carbonaceous chondrites. Phyllosilicates consist of repeating sequences of sheets of linked tetrahedra (T) and sheets of linked octahedra (O). The T sheet consists of Click on Term to Read More

Fracturing Scale

• A–Minor cracks; few or no cracks are conspicuous to the naked eye and no cracks penetrate the entire specimen
• B–Moderate cracks; several cracks extend across exterior surfaces and the specimen can be readily broken along the cracks
• C–Severe cracks; specimen readily crumbles along cracks that are both extensive and abundant

Age-Dependent Fragmentation and Dispersion in Hot Deserts (A. Al-Kathiri et al., 2005)

Fragmentation is considered to result from daily thermal fluctuations, volume increase, and penetration of sand and water into cracks. Dispersal occurs through natural means, i.e., wind and water, as well as by animals.

Fragmentation Index (FI):

the mass of the largest fragment ÷ total mass of all fragments

Fragment Dispersion (FD):

the maximum distance between different fragments of a single fallMeteorite seen to fall. Such meteorites are usually collected soon after falling and are not affected by terrestrial weathering (Weathering = 0). Beginning in 2014 (date needs confirmation), the NomComm adopted the use of the terms "probable fall" and "confirmed fall" to provide better insight into the meteorite's history. If Click on Term to Read More

Shock StageA petrographic assessment, using features observed in minerals grains, of the degree to which a meteorite has undergone shock metamorphism. The highest stage observed in 25% of the indicator grains is used to determine the stage. Also called "shock level".               Click on Term to Read More* (olivine-bearing meteorites) (Stöffler et al., 1991; revised by Schmitt et al., 1994, Schmitt and Stöffler, 1995, and Rubin, 2004)

• S1–S2: Unshocked (sharp extinctionIn astronomy, the dimming of starlight as it passes through the interstellar medium. Dust scatters some of the light, causing the total intensity of the light to diminish. It is important to take this effect into account when measuring the apparent brightness of stars. The dark bands running across portions Click on Term to Read More of olivine grains), peak shock pressure <4–5 GPa, where 1 GPa = 10kb or 10,000 bars; min. temp. increase 10°C
• S2–S3: Very weakly shocked (weak undulose extinction of olivine grains), peak shock pressure 5–10 GPa; min. temp. increase 20°C
• S3–S4: Weakly shocked (strong undulose extinction in olivine grains with planar fractures and melt pockets; silicate darkening; irregular FeS in FeNi-metal; chromite veinlets and chromite–plagioclase assemblages; metallic Cu grains), peak shock pressure 10–15 GPa; min. temp. increase 100°C
• S4–S5: Moderately shocked (mosaicism in olivine grains; some maskelynitization of feldspar; mobilization of metal and FeS in shock veins; narrow silicate melt veins; metal and sulfide nodules; polycrystalline troilite; melt pockets; mechanical twinning in Ca-rich clinopyroxene; martensite/plessite; high-pressure minerals), peak shock pressure 25–30 GPa; min. temp. increase 300°C
• S5–S6: Strongly shocked (large impact-melt clasts present; high-pressure minerals), peak shock pressure 45–60 GPa; min. temp. increase 600°C
• S6: Very strongly shocked (localized melt veins and maskelyniteNatural glass composed of isotropic plagioclase produced during shock metamorphism (not melting) at pressures of ~30 GPa. Maskelynite is commonly found in shergottites though also found in some ordinary chondrites, HED and lunar meteorites. It is also found in association with meteorite impact craters and crater ejecta. Named after British Click on Term to Read More present; high-pressure minerals), peak shock pressure 60–75 GPa; min. temp. increase 1500°C (whole rock impact melting occurs at 75–90 GPa; temp. increase >1500°C)

*Shock stage is determined by the highest indicated stage by at least 25% of the indicator grains. In actuality, shock stage is determined by factors in addition to equilibriumTerm used to describe physical or chemical stasis. Physical equilibrium may be divided into two types: static and dynamic. Static equilibrium occurs when the components of forces and torques acting in one direction are balanced by components of forces and torques acting in the opposite direction. A system in static Click on Term to Read More shock pressure, including shock duration, pre-shock porosityThe volume percentage of a rock that consists of void space. Vesicular porosity is a type of porosity resulting from the presence of vesicles, or gas bubbles, in igneous rock such as the pumice presented here. Vesicular porosity is very rare in meteorites and is often associated with slag, one Click on Term to Read More, and stress-strain history (Xie et al., 2006).

Shock Stage (orthopyroxene-bearing meteorites) (Rubin et al., 1997), in conjunction with olivine and plagioclase data characterized by Stöffler et al., 1991; Schmitt et al., 1994; Schmitt and Stöffler, 1995; and Izawa et al., 2011)

• S1: Sharp optical extinction of orthopyroxene grains, peak shock pressure <5 GPa
• S2: Undulose extinction of orthopyroxene grains, irregular fractures, peak shock pressure 5–10 GPa
• S3: Clinoenstatite twinning parallel to (100), planar fractures, peak shock pressure 10–15 GPa
• S4: Weak mosaicism in orthopyroxene grains, peak shock pressure 15–30 GPa
• S5: Strong mosaicism, peak shock pressure 30–60 GPa
• S6 (only localized regions identified in enstatite chondrites): Melting or transformation to majorite, peak shock pressure 75–90 GPa; (whole rock impact melting occurs at >90 GPa)

Shock Pressure Calibration (Schmitt, 2000)

A. SILICATES

• <15 GPa: Undulatory extinction of olivine
• 10–15 GPa to 20–25 GPa: Weak mosaicism of olivine
• 20–25 GPa: Onset of strong mosaicism of olivine
• 20–25 GPa (high-temperature conditions); 25–30 GPa (low-temperature conditions): transformation of oligoclase to diaplectic glassNatural glass formed by shock transformation from any of several minerals (more commonly feldspar) without melting. It is found in some meteorites and meteorite impact craters. Diaplectic plagioclase glass is called maskelynite and is found in martian shergottites. Click on Term to Read More is complete
• 25–30 GPa (high-temperature conditions); 30–35 GPa (low-temperature conditions): onset of weak mosaicism in orthopyroxene
• 35–45 GPa (high-temperature conditions); 45–60 GPa (low-temperature conditions): start of recrystallization or melting of olivine
• >45–60 GPa (high-temperature conditions): Recrystallization of olivine complete

B. TROILITE

• <25 GPa: Undulatory extinction
• 25–45 GPa: Twinning
• 30–60 GPa: Partial recrystallization
• 10–45 GPa (high-temperature conditions); >35 GPa (low-temperature conditions): Complete recrystallization
• >45 GPa (high-temperature conditions): Melting and crystallizationPhysical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals. Click on Term to Read More

C. LOCALIZED SHOCK-INDUCED MELTING

• >15 GPa (high-temperature conditions) or >30 GPa (low-temperature conditions): FeNi–FeS melt veins with pockets and veins of whole-rock melt

When utilizing shock-melt veins, pressure calibrations for stage S6 of Stöffler et al. (1991) may be too high by a factor of at least two (Xie et al., 2006).

Chromite as a Shock Indicator (Rubin, 2003)

• Shock Stage 1: Unmelted, unfractured chromite grains
• Shock Stage 2: Unmelted, fractured chromite grains
• Shock Stage 3: Chromite grains transected by opaque veins
• Shock Stage 4: Chromite–plagioclase assemblages
• Shock Stage 5: Veinlets containing chromite needles and blebs
• Shock Stage 6: Chromite-rich chondrules

Rubin (2004) proposed that all equilibrated (type 4–6) ordinary chondrites were impact shocked to stage S3–S6, with subsequent annealing to an apparent shock stage of S1; some OCs experienced further shock events to acquire their present shock stage of S2–S6.

Shock Indicators in IVA Irons (Yang et al., 2011)

• Shock Stage 1: M-shaped Ni profiles in taeniteLess common than kamacite, both taenite and kamacite are Ni-Fe alloys found in iron meteorites. Taenite, γ-(Fe,Ni), has 27-65 wt% Ni, and forms small crystals that appear as highly reflecting thin ribbons on the etched surface of a meteorite; the name derives from the Greek word for "ribbon." Click on Term to Read More, Neumann twins in kamaciteMore common than taenite, both taenite and kamacite are Ni-Fe alloys found in iron meteorites. Kamacite, α-(Fe,Ni), contains 4-7.5 wt% Ni, and forms large body-centered cubic crystals that appear like broad bands or beam-like structures on the etched surface of a meteorite; its name is derived from the Greek word Click on Term to Read More, cloudy taenite, monocrystalline troilite (e.g., Gibeon)
• Shock Stage 2: M-shaped Ni profiles in taenite, shock-hatched kamacite, shock-melted troilite, cloudy taenite mostly lacking (e.g., Muonionalusta)
• Shock Stage 3: Recrystallized lamellae, taenite microprecipitates on grain boundaries, completely shock-melted troilite (e.g., MariaBroad low plains surrounded by basin-forming mountains, originally thought to be a sea (pl. maria). This term is applied to the basalt-filled impact basins common on the face of the Moon visible from Earth.   Click on Term to Read More Elena)
• Shock Stage 4: Complete recrystallization of kamacite and taenite lamellae, lack of Thomson (Widmanstätten) structure, cloudy taenite completely absent (e.g., Fuzzy Creek)

Yang et al. (2011) proposed that IVA irons experienced three impacts: 1) a glancing impact 4.5 b.y. ago that ejected the silicate mantleMain silicate-rich zone within a planet between the crust and metallic core. The mantle accounts for 82% of Earth's volume and is composed of silicate minerals rich in Mg. The temperature of the mantle can be as high as 3,700 °C. Heat generated in the core causes convection currents in Click on Term to Read More and produced a 300 km-diameter molten metallic body; 2) a head-on collision occurred upon cooling to 200°C, likely producing a rubble-pile >30 km in diameter which resulted in shock reheating; 3) a final severe impact 400 m.y. ago producing a swarm of m-sized fragments and their delivery to Earth.

Meteorite Pairing (P. Benoit et al., 2000)

PAIRING CRITERIA:

Parent bodyThe body from which a meteorite or meteoroid was derived prior to its ejection. Some parent bodies were destroyed early in the formation of our Solar System, while others like the asteroid 4-Vesta and Mars are still observable today. Click on Term to Read More history

Bulk elemental and isotopic concentrations

MineralInorganic substance that is (1) naturally occurring (but does not have a biologic or man-made origin) and formed by physical (not biological) forces with a (2) defined chemical composition of limited variation, has a (3) distinctive set of of physical properties including being a solid, and has a (4) homogeneous Click on Term to Read More abundance and compositions

Petrography (shock, metamorphic, and igneous textures)

Stable isotopeOne of two or more atoms with the same atomic number (Z), but different mass (A). For example, hydrogen has three isotopes: 1H, 2H (deuterium), and 3H (tritium). Different isotopes of a given element have different numbers of neutrons in the nucleus. Click on Term to Read More abundance and formation ages

MeteoroidSmall rocky or metallic object in orbit around the Sun (or another star). space history

Cosmogenic noble gasElement occurring in the right-most column of the periodic table; also called "inert" gases. In these gases, the outer electron shell is completely filled, making them very unreactive. Click on Term to Read More ratios (cosmic-ray exposure ageTime interval that 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 Click on Term to Read More, shielding, solar gases, thermal history)

Natural TL (reheating)

Meteorite terrestrial history

Geographic proximity

Shape and size

Number of specimens

Terrestrial age

Weathering grade

Natural TL levels

Applying data from these criteria to the formula below, a pairing score and its associated pairing likelihood is obtained.

P_rel = P_rel* × P_ss × P_brec × P_cre × P_solar × P_3He × P_tage × P_weath × P_NatTL where…
P_rel* = relative abundance by classification
P_ss = relative abundance by shock stage
P_brec = relative abundance by brecciationThe formation of a breccia through a process by which rock fragments of of various types are recemented or fused together. Click on Term to Read More
P_cre = relative abundance by cosmic-ray exposure age
P_solar = relative abundance by solar-gas-bearing meteorites
P_3He = relative abundance by light noble gas depleted meteorites
P_tage = relative abundance by terrestrial age
P_weath = relative abundance by weathering factor
P_NatTL = relative abundance by natural TL levels
Pairing score (%) / Pairing likelihood

>90_______Likely
80–90_____Probable
70–80_____Possible
50–70_____Potential
<50_______candidate or unlikely

Compositional Relationships Among Bodies
(Lindstrom et al., 1994; Mayne et al., 2008)

MINERAL COMPOSITIONS
	*Fe/Mn (pyx)	plag.	Fe-metal	sulfide	secondary alter.
HED	20–40	An90	Y	Troilite	none
Angrites	60–90	An99	N	Troilite	none
Moon	70	An92	Y	Troilite	none
Earth	60	An50	N	Pyrrh.	hydrous phases
Mars	35	An50	N	Pyrrh.	hydrous phases
Venus	55	—	N	Pyrrh.	anhydrous phases

*Differences in Fe/Mn ratios are attributed to initial accretional abundances. The primordial value of the Fe/Mn ratio in pyroxenes remains constant regardless of differentiationA process by which a generally homogeneous chondritic body containing mostly metal, silicates and sulfides will melt and form distinct (differentiated) layers of different densities.   When the melting process continues for a long enough period of time, the once chondritic body will re-partition into layers of different composition including Click on Term to Read More processes, and is considered to be diagnostic for the origin of each planetary body.

BULK CHEMICAL COMPOSITIONS
	Fe/Mn	K/U	K/La (×CI)	Rb/La (×CI)
HED	30 (±2)	2,000	0.03	0.002–0.02
Ibitira	34–36	—	—	—
Angrites	80–95	150	0.002–0.03	0.001
Moon	62 (±18); 67 (±9)	1,700	0.03	0.016
Earth	40 (±11)	12,500	0.15	0.09
Mars	35–50	15,000	0.2	0.3
Venus	55 (±30)	12,500	0.1–0.2	0.1

*O-ISOTOPIC COMPOSITIONS
	Δ17O (‰)
CR chondrites	–0.96 to –2.42
Lodranites	–0.85 to –1.49
AcapulcoitesPrimitive achondrite that belongs to a small group named after the Acapulco meteorite that was observed to fall in Mexico in 1976. Acapulcoites are made mostly of fine-grained olivine (Fo3-14), orthopyroxene(En86-97), Ca-rich pyroxene (En51Wo44), plagioclase (An12-31), Ni-Fe metal, and troilite. They are transitional between primordial chondritic matter and more differentiated Click on Term to Read More	–0.85 to –1.22
Winonaites	~ –0.4 to –0.80
HED	–0.24
Brachinites	–0.13 to –0.30
Angrites	–0.125
Moon	0.00
Earth	0.00
Mars	+0.3
Venus	+0.0 to +0.3
H (4–6)	+0.73
L (4–6)	+1.07
LL (4–6)	+1.26

*Δ17O is a convenient measure of the vertical displacement of a data point from the terrestrial fractionation line:
Δ17O = δ17O – (0.52 × δ18O)
where δ17O = 17O/16O, and δ18O = 18O/16O, expressed as parts per thousand (per mil [‰]) and measured in terms of deviations from a standard (Standard Mean Ocean Water [SMOW])

e.g., δ18O = [(18O_sample ÷ 16O_sample) ÷ (18O_SMOW ÷ 16O_SMOW)] – 1
See an oxygen 3-isotope plot A report by E. Young (2007) concludes that optically thin photoactive regions in the outer disk were the site of CO photochemical conversion to 16O-poor, high Δ17O water ice. This 16O-poor water was transferred to the inner Solar SystemThe Sun and set of objects orbiting around it including planets and their moons and rings, asteroids, comets, and meteoroids. during the infall phase, or within a wavefront, on a timescale of 100 t.y. to 1 m.y., exchanging with the silicates and other chondrite constituents which formed subsequently in the inner Solar SystemDefinable part of the universe that can be open, closed, or isolated. An open system exchanges both matter and energy with its surroundings. A closed system can only exchange energy with its surroundings; it has walls through which heat can pass. An isolated system cannot exchange energy or matter with; the igneous, refractory-rich CAISub-millimeter to centimeter-sized amorphous objects found typically in carbonaceous chondrites and ranging in color from white to greyish white and even light pink. CAIs have occasionally been found in ordinary chondrites, such as the L3.00 chondrite, NWA 8276 (Sara Russell, 2016). CAIs are also known as refractory inclusions since they Click on Term to Read More material that condensed prior to this water transfer remains 16O-rich.

Nucleosynthetic Anomalies Among Parent Bodies For O, Cr, Ca, Ti, Ni, Mo, Ru, and Nd
Burkhardt et al. (2017)

Diagrams credit: Burkhardt et al., MAPS, vol. 52, #5, pp. 815-817 (2017)
‘In search of the Earth-forming reservoir: Mineralogical, chemical, and isotopic characterizations of the
ungroupedModifying term used to describe meteorites that are mineralogically and/or chemically unique and defy classification into the group or sub-group they most closely resemble. Some examples include Ungrouped Achondrite (achondrite-ung), Ungrouped Chondrite (chondrite-ung), Ungrouped Iron (iron-ung), and Ungrouped Carbonaceous (C-ung). Click on Term to Read More achondriteAn achondrite is a type of stony meteorite whose precursor was of chondritic origin and experienced metamorphic and igneous processes. They have a planetary or differentiated asteroidal origin where the chondritic parent body reached a sufficient size that through heating due to radioactive decay of 26Al (aluminum isotope) and gravitational Click on Term to Read More NWA 5363/NWA 5400 and selected chondrites’ (http://dx.doi.org/10.1111/maps.12834)

Formational Relationships Among Ureilites (Cohen et al., 2004)

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TOPICAL, CHEMICAL, AND ISOTOPIC PARAMETERS
Surface	Depth
high formation temp	low formation temp
low formation pressure (~10–25 bars)	high formation pressure (> ~100–125 bars)
highly reduced (smelting)	little reduced (smelting suppressed)
low δ18O & Δ17O	high δ18O & Δ17O
low C content	high C content
magnesian (high Mg#; ~75–95)	ferroan (low Mg#; ~60–90)
albitic melt clasts (<An25)	labradoritic melt clasts (An39–58) ol–aug melt clasts (An39–54)
ol–pig ureilite residues	ol–aug ureilite residues

Accretion Process

AccretionAccumulation of smaller objects into progressively larger bodies in the solar nebula leading to the eventual formation of asteroids, planetesimals and planets. The earliest accretion of the smallest particles was due to Van der Waals and electromagnetic forces. Further accretion continued by relatively low-velocity collisions of smaller bodies in the Click on Term to Read More from dust to gas-giant planetThe term "planet" originally comes from the Greek word for "wanderer" since these objects were seen to move in the sky independently from the background of fixed stars that moved together through the seasons. The IAU last defined the term planet in 2006, however the new definition has remained controversial. Click on Term to Read More occurred within 3 m.y. (‘Pebble Accretion and the Diversity of Planetary Systems’, J. E. Chambers, The Astrophysical Journal, 825:63, 2016): In the beginning, µm-sized dust particles are embedded in a gaseous protoplanetary diskFlattened and rotating disk of dense gas and dust/solids orbiting a young star from which planets can eventually form.   Click on Term to Read More. By 0.02 m.y., mutual collisions between dust grains result in the formation of mm- to cm-sized pebbles. By 0.15 m.y., pebbles inside the ice line (~2.5–4.5 AU) have aggregated into planetesimalsHypothetical solid celestial body that accumulated during the last stages of accretion. These bodies, from ~1-100 km in size, formed in the early solar system by accretion of dust (rock) and ice (if present) in the central plane of the solar nebula. Most planetesimals accreted to planets, but many – Click on Term to Read More with diameters of 30–300 km. Just outside the ice line, aggregation of the larger ice-rich pebbles is more efficient, and larger planetesimals with diameters of ~1,500 km are formed. By 0.5 m.y., some of the larger planetesimals located within ~5 AU have become planetary embryos with diameters of 2,000 km. The largest embryos located just beyond the ice line begin to grow by ‘pebble accretion’ due to the inward drift of pebbles, reaching sizes of a few Earth masses (M_⊕). By 3 m.y., the largest of these embryos exceed a critical mass (3 × M_⊕) and undergo runaway gas accretion to form gas-giant planets. A large protoplanetary disk (radius = ~100 AU) and a small turbulence strength (α = 0.0005) help promote the formation of these gas giants, which ultimately clear their orbits. Inside the ice line, the growth of terrestrial planetsRocky planets: Mercury, Venus, Earth, and Mars. These planets have physical characteristics, chemical composition and internal structure similar to the Earth. The terrestrial planets have 0.4% of the total mass of all the planets in the Solar System. Some large satellites of planets are also similar to the characteristics of (0.02–1.4 M_⊕) ceases due to pebble depletion in the disk.

Accretion ages, in m.y. after CAIsSub-millimeter to centimeter-sized amorphous objects found typically in carbonaceous chondrites and ranging in color from white to greyish white and even light pink. CAIs have occasionally been found in ordinary chondrites, such as the L3.00 chondrite, NWA 8276 (Sara Russell, 2016). CAIs are also known as refractory inclusions since they Click on Term to Read More (Sugiura and Fijiya, 2011, 2012; Budde et al., 2018): magmatic irons=0.0 (±0.9)
pallasites=0.0 (±0.9)
mesosiderites=0.0 (±0.9)
angrites=0.5 (±0.4)
HED=0.8 (±0.3)
aubrites=<1.5
NWA 011=<1.5
ureilites=~1.0
acapulcoites=~1.2
Tafassasset=~2.0
chondrites: E=1.8; O=2.1; R=2.4; CK=2.6; CO=2.8; CV=3.0; CH/CB=3.3; CM=~3.5; CI=~3.5; CR=~3.6
Model Plot Predicting When and Where Meteorite Types Formed

click on photo for a magnified view
Data Key

Diagram credit: Desch et al., The Astrophysical Journal Supplement Series, vol. 238, #1, p. 23 (2018 open access version link)
‘The Effect of Jupiter’s Formation on the Distribution of Refractory ElementsUsing research by Wood (2019), any of the elements with a relatively high condensation temperature of 1291 K < TC,50 < 1806 K in the solar nebula1. They are the first elements to condense out of a cooling gas. Refractory elements are the main building blocks of rocky planets, dwarf Click on Term to Read More and Inclusions in Meteorites’
(https://doi.org/10.3847/1538-4365/aad95f)

Melting/Differentiation Process

According to Sanders and Scott (2007), any body that accreted to a diameter >60 km (i.e., large enough to minimize heat loss from the surface through conductionTransfer of heat as a result of collisions between molecules; when one end of an object is heated or excited, the molecules vibrate faster and their energy is transferred sequentially to their neighbors. Click on Term to Read More) within ~2 m.y. after CAI formation (the oldest known objects dating to 4.567 b.y. ago) as the angrites did, must contain enough 26Al to produce global melting and differentiation. In contrast, Senshu and Matsui (2007) determined that accretion to a diameter of only ~14 km occurring within 2 m.y. after CAI formation was all that was required for global differentiation to occur, while accretion to a diameter of 40–160 km within 1.5 m.y. after CAI formation was cited by Hevey and Sanders (2006) and Sanders and Taylor (2005) as the minimums for differentiation. Sanders and Scott (2011) later revised that to suggest radiogenic melting proceeded in bodies >20 km in diameter that accreted within 1.5 m.y. after CAI formation, while bodies accreting later than 1.5 m.y. after CAIs were heated but not melted. Furthermore, they found that bodies which accreted later than 2.2 m.y. would not have melted at all. In addition, at large heliocentricCentered around a sun. Our own Solar System is centered around the Sun so that all planets such as Earth orbit around the Sun. Note that 25% of Americans incorrectly believe the Sun revolves around the Earth. Click on Term to Read More distances (>~2.8 AU) accretion would proceed too slowly for sufficient 26Al to accumulate and initiate global melting prior to a body growing too large (~200 km diameter) for melting to be possible (Nyquist and Bogard, 2003).

Be that as it may, John T. Wasson (2016) presented evidence that the slow heating generated entirely by the decay of 26Al is insufficient to melt asteroids, and that an additional heat source would have been required; e.g., the rapid heating incurred from major impact events. He determined that the canonical 26Al/27Al ratio of 0.000052 is much too low to cause any significant melting, and that a minimum ratio of 0.00001 would be required to produce a 20% melt fraction on a well-insulated body having a significant concentration of 26Al. For example, the initial ratio of 0.0000004–0.0000005 calculated for the angrites Sah 99555 and D’Orbigny based on their 26Al–26Mg isochrons is too low to have generated any significant melting, and therefore impacts provided a major source of heat in early solar system history.

Oxygen Buffer Systems

Fugacity-temperature diagram

Log oxygen fugacityUsed to express the idealized partial pressure of a gas, in this case oxygen, in a nonideal mixture. Oxygen fugacity (ƒO2) is a measure of the partial pressure of gaseous oxygen that is available to react in a particular environment (e.g. protoplanetary disk, Earth's magma, an asteroid's regolith, etc.) and Click on Term to Read More vs. temperature at 1 barUnit of pressure equal to 100 kPa. pressure for common buffer assemblages, plotted from algorithms compiled by B. R. Frost in
Mineralogical Society of America ‘Reviews in Mineralogy’, vol. 25, ‘Oxide Minerals: Petrologic and Magnetic Significance’ (D. H. Lindsley, ed., 1991).
MH: magnetite-hematite; NiNiO: Nickel-nickel oxide; FMQ: fayalite-magnetite-quartz; WM: wustite-magnetite; IW: iron-wustite; QIF: quartz-iron-fayalite

Cosmochronology

SOME SHORT- AND LONG-LIVED CHRONOMETERS Norris et al., 1983; Hibiya et al., 2014; Pravdivtseva et al., 2016
Parent → Daughter	Half-lifePeriod of time required for 50% (½) of the atoms of a radioactive nuclide in a sample to decay. After two  half-lives, 25% ( ½ x ½ = 1/4) of the original radioactive nuclide will remain. After three half-lives, 12.5% ( ½ x ½ x ½ = 1/8) of the Click on Term to Read More	Minerals dated
26Al → 26Mg	717 t.y.	Plag – Olv,Pyx
60Fe → 60Ni	2.6 m.y.	Chr – metal
53Mn → 53Cr	3.74 m.y.	Olv – Chr
107Pd → 107Ag	6.5 m.y.	metal – metal
182Hf → 182W	8.9 m.y.	Pyx – metal
129I → 129Xe	≤15.7 m.y.	Pyx – Pyx
92Nb → 92Zr	36 m.y.	Pyx – Chr
146Sm → 142Nd	68 m.y.	Chr,Pyx – Plag
176Lu → 176Hf	3.54 b.y.	Pyx – Chr
238U → 206Pb	4.47 b.y.	ZirconOrthosilicate mineral, Zr(SiO4), observed in all terrestrial rocks type and in ordinary chondrites, eucrites, mesosiderites, and lunar rocks. – Zircon
87Rb → 87Sr	49.61 (±0.16) b.y.	metal – Plag
147Sm → 143Nd	106 b.y.	Chr,Pyx – Plag

Visit Jim Hurley’s informative webpage on the subject of Radiometric Dating.

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CONTINUE TO
[PART I] Chondrites
[PART II] Achondrites
[PART III] Irons
[PART IV] Stony-Irons
[PART V] Refractory Phases
[APPENDECTOMY]

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© 1997–2019 by David Weir

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