METEORITICAL GLOSSARY

 

CHONDRITES

 

Stony meteorites with compositions reflecting solar abundances of nonvolatile elements are regarded as chondrites. Further division based on chemical trends gives us the carbonaceous, ordinary, enstatite, and other chondrite groups. Still further division based on petrologic type gives us types 1-7; type 3 chondrites have remained unaltered, with lower types experiencing progressive aqueous alteration and higher types experiencing progressive thermal or shock alteration. Type 7 chondrites are transitional to an achondrite classification (see below). Chondrules are found in all petrologic types except types 1 and 7 where alteration has left them indistinct from the matrix. Chondrites represent greater than 85% of all meteorite falls.

 

CARBONACEOUS CHONDRITES

 

These are primitive, undifferentiated, stony meteorites composed of silicate chondrules set in a fine-grained silicate matrix. Within the matrix can be found calcium-aluminum inclusions that represent the earliest material that condensed from the hot nebula, while certain isotopes originated in interstellar grains that predate the formation of the solar system. Also found in these meteorites are carbon compounds including long-chain hydrocarbons and amino acids similar to those used in protein synthesis in living organisms. Carbonaceous chondrites formed in an oxygen-rich environment with most metal combined into silicates, sulfides, or other oxides. They formed on the smaller asteroids that retain the oldest record of the solar nebula, containing solar abundances of non-volatile elements. Carbonaceous chondrites have been divided into individual chemical groups including the CI, CM, CR, CO, CK, CV, and CH groups, along with the three-member Coolidge-grouplet and a few rare ungrouped members. The CI subgroup contains up to 20% water locked into hydrated minerals. The discovery of new and unique CCs helps us to continually revise the record of processes ocurring in the early solar system.

 

The CV subgroup has recently been further divided into three subgroups:

   reduced - CV3_R           Oxidized- Bali CV3_OxB           Oxidized- Allende CV3_OxA

 

The following mineralogical relationships have been found to exist among these three subgroups:

 

   matrix abundance: R < OxA < OxB

   metal to magnetite ratio: OxB < OxA < R

   fayalitic olivine range: R (Fa_32-60), OxA (Fa_32-60), OxB (Fa_10-90+)

 

Near-pure fayalite is found only in OxB; metal in R and OxB are low-Ni kamacite, while that in OxA is high-Ni awaruite phyllosilicates are found only in OxB. low-Ca pyx is found in R while Ca-Fe pyx is found in OxA and OxB magnetite, pentlandite, and NiFe metal is found only in OxA and OxB  nepheline, sodalite, wollastonite, andradite, kirschsteinite, and grossular are found only in OxA

 

ORDINARY CHONDRITES

 

The ordinary chondrites are composed of varying ratios of olivine and pyroxene with spheroidal chondrules that represent the early condensates of the presolar nebula. The group is subdivided into the H (olivine-bronzite), L (olivine-hypersthene), and LL (amphoterite) groups based on chemical trends, mainly their iron to silicon ratio. The "H" refers to a high-iron content of 27 wt%, the "L" to a low-iron content of 23 wt%, and the "LL" to both a low-iron content of 20 wt% along with a low-metal content of only 2 wt%. The H subgroup, having the lowest oxygen content, formed nearest the sun, with the L's and LL's at increasing heliocentric distances, but all ordinary chondrites accumulated at ~2 AU from the sun. The petrologic types of the ordinary chondrites range from 3 to 7, forming an onion shell internal structure reflecting an increased depth and a reduced cooling rate for a higher petrologic type. Thermal history constraints predict a diameter for the ordinary chondrite parent bodies of between 160 and 180 km. Based on remote sensing data, a likely asteroid for the parent body of the H chondrites is the S,IV-type, 6 Hebe. Most L chondrites have been severely shocked and their radiogenic ages have been reset ~20 m.y. ago, recording a disruptive impact of the parent body. Ordinary chondrites represent about 79% of all falls.

 

OTHER CHONDRITES

 

B Chondrites-

This is a newly designated grouplet of four meteorites which are members of the CR clan. The bencubbinites consist of Bencubbin, Weatherford, HaH 237, and GRO95551. The group has a  metal-silicate chondritic composition with highly reduced silicates and over 50% FeNi. Barred or cryptocrystalline chondrules are present, as are CAIs in HaH 237. Oxygen isotopes suggest a very close relationship between the bencubbinite grouplet and the CR and CH chondrites.

 

R Chondrites

This group of thirteen unpaired and three probable paired meteorites, formerly known for the Carlisle Lakes specimen, is now known for the only fall of the group, Rumuruti. The group is highly oxidized, olivine-rich, and metal-poor. They differ greatly in oxidation state, oxygen isotope composition, and mineralogy from ordinary, carbonaceous, or enstatite chondrites, or silicate inclusions in IAB and IIE irons. The parent body was originally highly unequilibrated but was subsequently thermally metamorphosed and impact-melted to a moderate degree. Most members are highly brecciated and contain implanted solar wind gases.

 

K Chondrites-

The type specimen of this chondrite grouplet, Kakangari, along with two other members, have unique petrologic, bulk chemical,  and O isotopic characteristics that distinguish it from other chondrite groups. The grouplet also does not fit into the existing systematics of the E, O, R, or C chondrites as their characteristics relate to heliocentric distance of formation. K chondrites therefore represent a unique, primitive, parent asteroid.

 

F Chondrites-

Forsterite chondrites are intermediate in composition, mineralogy, and oxidation state between the H-group ordinary and enstatite chondrites. They represent a highly unequilibrated distinct chondritic suite that underwent nebula condensation/accretion before colliding with the aubrite parent body. The highly-shocked chondritic fragments were incorporated into the aubrite meteorites Cumberland Falls and ALH78113 forming a breccia.

 

ENSTATITE CHONDRITES

 

These chondrites are highly reduced with all of the iron visible as metal or troilite (FeS). The silicate consists mainly of the iron-free pyroxene, enstatite. As with the ordinary chondrites, a subdivision is made based on the bulk iron content.

 

The following mineralogical relationships have also been found to differentiate these subgroups:

 

EH subgroup has a higher Si content in FeNi (1.9-3.8 vs. 0.3-2.1 wt%)

EH subgroup has a lower Mn content in daubreelite (0.4-1.1% vs. 1.4-4.0%)

EH subgroup has a lower An content in plagioclase (<3 mol% vs. 13-17 mol%)

 

Current theories incorporating multiple lines of evidence indicate that the EL and EH chondrites originate from different layers on the same parent body. A third grouplet with intermediate mineralogy has recently been identified, but it is not derived from the EH or EL groups through any metamorphic proccesses. From studies of rare-gas fractionation patterns, some researchers believe they may have formed inside the orbit of Venus, while the identification of E-type asteroids in the inner asteroid belt suggests that this was their actual location of origin. These meteorites are rare, representing about 1% of all meteorite falls.

 

ACHONDRITES

 

All members of this classification originated on chondritic bodies that underwent igneous melting and recrystallization. Their parent bodies were large enough to melt and segregate the denser metals from the lighter silicates, generally forming a metallic core, a magnesium-rich mantle, and a calcium-rich crust. Of the various achondrites, three are believed to have originated on the asteroid 4 Vesta. These represent the brecciated surface materials (howardites), the extrusive/intrusive basalts (eucrites), and the plutonic cumulates (diogenites). In addition, thirteen meteorites comprising three groups originated on Mars (8 shergottites, 4 nakhlites (including 1 orthopyroxenite), and 1 chassignite), and thirteen meteorites found are of lunar origin. The winonaites formed in the same nebula locality as that in which the iron group IAB silicate inclusions formed. There are still many theories proposed to explain the origins of the other groups including the angrites, brachinites, acapulcoites, lodranites, ureilites, and the aubrites. Achondrites represent about 8% of all meteorite falls.

 

IRONS

 

This is a chemically varied group of meteorites composed mainly of FeNi metal with small amounts of other minerals and trace elements. Most irons were formed in the cores of small, < ~400 km diameter, differentiated asteroids, although some are more consistent with formation in small melt pools distributed within very small parent bodies. Iron meteorites have been resolved into 13 distinct chemical groups (see Wasson et al.), with an "anomalous" designation given to those with certain elemental abundances significantly outside the normal ranges of the main groups. Groups are resolved based on a congruence in the percentages of nickel (Ni) and certain trace element contents, mainly germanium (Ge) and gallium (Ga), due to their wide ranges across the entire iron spectrum, but narrow ranges within specific iron groups. Other elements used to resolve groups include iridium (Ir), copper (Cu), cobalt (Co), arsenic (As), gold (Au), antimony (Sb), and tungsten (W); each resolved group representing an origin on a unique asteroid accreted in a unique nebular region. The occurrence of either positive or inverse correlations existing among the ratios of Ge, Ga, Ni, and Ir in iron meteorites serves as a useful grouping determinant. Likewise, the consistent variation of certain elemental concentrations in the irons also serves as a grouping determinant. Another useful, but less precise, grouping method is based on similarities in the macrostructure of etched irons, and is influenced by the bulk nickel content of taenite in association with the cooling rate. It categorizes irons as either hexahedrites, octahedrites, or ataxites.

 

IAB

This group is one of the most well represented groups of iron meteorites including the majority of meteorites that contain silicate inclusions, these having nearly chondritic compositions. As evidenced by a Rb-Sr age of ~4.7 b.y., a high retention of planetary-type noble gases including Xe, and a small size of the precursor taenite crystals, group IAB irons cooled rapidly below temperatures necessary for isotopic exchange a very short time after formation. This suggests a non-magmatic origin without undergoing fractional crystallization. The implication is that the metal-silicate fractionation occurred in the solar nebula and/or during accretion, and that the parent body was never in a molten state. Otherwise, the silicates would have segregated out during convective proccesses. Trace element studies show that Ge is positively correlated with Ga, and inversely correlated with Ni. Another resolving characteristic is the proportionally higher contents of Ge and Ga for a given Ni content compared to other meteorites. The Widmanstätten structure is commonly coarse and inclusions of troilite, graphite, and cohenite can be abundant. The similarity of group I Ge/Ga ratios with that of carbonaceous chondrites suggests that this iron group was originally formed from similar material.

 

(IB)

An arbitrary boundary has been established to separate the IA and IB subgroups; those members with low Ni contents and Ge contents above 190 ppm are designated group IA, while those members with high Ni contents and Ge contents below 190 ppm are designated group IB. Additionally, extrapolation of Ni and other elemental trends defines a continuum for the IAB and IIICD groups, suggesting they originated from different sections of a common parent asteroid. The silicate inclusions in this group are closely related to the group of meteorites known as the winonaites, and they probably originated on a common parent body.

 

IC

This is a small, structurally diverse group that resolves itself close to the IAB field, but with a lower elemental abundance of Au and As. Most members of this igneous group contain abundant cohenite.

 

IIAB

Group IIA consists of single-crystal hexahedrites in which Ni, Ge, and Ga are positively correlated with each other, and inversely correlated with Ir. Continuity in composition between groups IIA and IIB, along with similar cosmic-ray ages suggests that groups IIA and IIB are genetically related. An arbitrary boundary separating these two igneous subgroups has been proposed to be an Ir content of 1 microgram/gram. The similarity of group IIA Ge/Ga ratios with that of carbonaceous and enstatite chondrites suggests that this iron group formed from one of these types of material.

 

(IIB)

In contrast to the correlations in group IIA, Ge, Ga, and Ir are positively correlated with each other, and inversely correlated with Ni. Continuity in composition between groups IIB and IIA, along with similar cosmic-ray ages  suggests that groups IIB and IIA are genetically related. An arbitrary boundary separating these two subgroups has been proposed to be an Ir content of 1 microgram/gram. An inverse correlation exists between the cosmic-ray exposure (CRE) age and the Ir content. This can be explained through a senario whereby Ir is concentrated at depth, and members of group IIB were closer to the surface where they were stripped from the asteroid first, thus establishing a higher CRE age for a lower Ir content.

 

IIC

Group IIC consists of plessitic octahedrites in which Ni, Ge, and Ga are positively correlated with each other. Ni is inversely correlated with Ir. This group appears to have undergone igneous fractionation in the core.

 

IID

Ni, Ge, and Ga are positively correlated with each other, and inversely correlated with Ir. This group appears to have undergone igneous fractionation in the core, resulting in a very high Ga/Ge ratio and a schreibersite-rich composition, comfortably resolving this group. Based on cooling rate comparisons, the IID parent body was about 10 times larger than the IIC parent body.

 

IIE

Just as for group IAB-IIICD, the IIE parent body was never in a completely molten state, and the globular, Fe-rich silicates that most members of this group contain were probably mixed with the metal during impact-heating events. These events produced rapid heating of the chondritic material, with temperatures reaching higher levels than in IAB-IIICD melts due to a lower content of the elements C and S (having melt-inducing properties) in IIE. Consistent with these higher melting temperatures for group IIE are the differentiated and completely segregated silicates that form droplets rather than clasts. The higher temperatures are also responsible for the much lower levels of planetary-type rare gases than those found in group IAB-IIICD. A wide variation in cooling rates is also found among IIE members, indicative of cooling at a broad range of depths within the parent body. Based on bulk metal compositions and O-isotopic evidence, it is thought that the IIE irons may have originated on the same parent body as the H-chondrites.

 

IIF

Resolution of this group is based on several factors including structure, which ranges from ataxitic to plessitic, an unusually high Ge/Ga ratio, a high Ni content, a high Co and Cu content, and a low P content. The compositional trends are most consistent with igneous fractional crystallization of a core. Contrary to accepted theory, the Ni content is positively correlated with the Widmanstätten bandwidth. This occurrence could be explained by either differences in the cooling rate, or the effects of bulk P content on bandwidth growth. O-isotopic data indicates a relationship exists between the IIF irons, the Eagle Station pallasites, and the CO-CV carbonaceous chondrites, with formation occurring in the outer solar system.

 

IIIAB

Group III irons are the most represented group of iron meteorites in our collections, accounting for almost a third of known irons. Compositional, structural, cosmic-ray age, and cooling rate data provide evidence to believe groups IIIA and IIIB originated on the same parent body. The Ni content of group IIIA and IIIB members forms a continuous sequence between groups. The Ge content of IIIA members have ranges from 33 to 46 ppm, while that of IIIB members have ranges from 28 to 38 ppm, creating a small overlap. In spite of their differing structures (IIIA has mostly coarse textures, while IIIB has mostly medium textures), and their oppositely correlated Ge and Ni concentrations, they are considered to be genetically related. The compositional differences are likely due to late-stage chemical interactions during fractional crystallization involving mixing of different zones of the core. The IIIAB group members share a similar cosmic-ray exposure age of ~650 m.y., signifying a single breakup event at this time.

 

(IIIB)

An arbitrary boundary has been established dividing the IIIA and IIIB subgroups. Group IIIA members include those with a Ni content at the low-end range from 7.1 - 8.6%, while group IIIB members include those with a Ni content at the high-end range from 9.2 - 10.5%.

 

IIICD

Considering the wide range of Ge and Ga contents among members of groups IIIC and IIID, similarities in their cooling rates make it likely that they originated on a common parent body. The fractionation trends for the two groups suggest a non-magmatic origin without fractional crystallization as in the IAB group. Cosmic-ray exposure ages of IIIC members are ~700 m.y., while those of group IIID are ~200 m.y., suggesting a two-stage breakup event. A defining characteristic of group IIICD members is the presence of the carbide haxonite. The members of the combined IIICD group are best resolved from the low-Ni IAB members based on Ni/Ir relationships since Ir shows more variation at lower Ni concentrations. This resolves the two groups well at higher Ni concentrations but fails to resolve them at the lower concentrations. Extrapolation of Ni and other elemental trends defines a continuum for the IAB and IIICD groups and it is probable they originated on a common asteroid. The occurrence of certain very high Ni members within the resolved IAB-IIICD group is explained by crystal settling in small, meter-size, Ni-rich impact-melt pools. However, most impact-melt pools cooled without experiencing such fractionation.

 

(IIID)

The members of this igneous or magmatic group are resolved based on their high Ni contents, between 16.6 and 22.6%, as well having a balanced ratio of Ge and Ga. The structure of  these meteorites is that of a finest octahedrite.

 

IIIE

In spite of their similarities to the IIIAB group, members of this small group have been resolved based on their unique Ge,Ga/Ni and Ga,Co/Au ratios, swollen kamacite band structure, and inclusions of the carbide, haxonite.

 

IIIF

This low-Ni, low-Ge group has an inverse correlation of Ni with Ge, a very low Co and P content, and uniform cooling rates, all of which help resolve this group from the others. Inclusions of  S and P are rare.

 

IVA

There is a positive correlation in the concentrations of Ge with Ga, and Ge with Ni in this group which supports its resolution as an independent group. The compositional trends are most consistent with igneous fractional crystallization of a core. A lower than normal Ge/Ga content is also consistent with formation in the core of a small asteroid that experienced a high cooling rate. Following a catastrosphic impact that disrupted the IVA parent body, large core fragments were mixed with cooler mantle fragments; this reaccretion of the asteroid was complete in just 2 hours. Members of group IVA can be separated into a high-Ni or a low-Ni subset, with almost all members exhibiting a fine structure. Most members of the high-Ni subset experienced an initial rapid cooling followed by slow cooling, consistent with thermal equilibration with cooler silicates and dust material at depth. Conversely, the diverse range in cooling rates within the low-Ni subset is indicative of a less insulated position on the reassembled asteroid for some members. The cosmic-ray exposure history of the low-Ni members supports the occurrence of a catastrophic disruption of the reaccreted body occurring  ~450 m.y. ago. The lower CRE age of some high-Ni members can be interpreted as resulting from secondary breakups that exposed the more deeply buried material over time. Only one IVA member, Steinbach, contains significant silicates; these being bronzite and the rare mineral tridymite, the only silicon-containing mineral found in only a few other group IVA members. Steinbach was formed at the core-mantle boundary and then rapidly cooled at the edge of a core fragment following the earlier disruption and reaccretion of the IVA parent body. Other silicate fragments from the mantle were probably destroyed in the ensuing 450 m.y. space journey.

 

IVB

This group shows a positive correlation in the concentrations of Ge with Ga, and Ge with Ni which supports its resolution as an independent group. The members of group IVB have a high Ni content near 17% and have a plessitic composition with an ataxite structure. The compositional trends are most consistent with fractional crystallization involving mixing of different zones of a core. The absense of silicate inclusions is also consistent with a formation in the core of the parent body. The cosmic-ray history of this group suggests a multiple breakup of this parent body.

 

Many other irons occurring singly or in groups of less than 5 members remain "ungrouped". About 83% of the irons in our collections fall into one of the 13 main chemical groups, while another ~8% establish 20 small grouplets. The remaining ~8% of the irons originated on their own unique parent body. Of these perhaps 93 unique parent bodies, ~20 represent irons of impact melts origin on chondritic asteroids, while the others represent irons of igneous origin in the cores of differentiated asteroids.

 

IRONS, SILICATED

 

These irons probably formed from small melt pods of FeNi metal within the silicate mantle of their parent body, where complete melting did not occur. Silicate material was mixed with the viscous FeNi metal, cooling to form silicated irons. Most belong to the three non-magmatic groups IAB, IIICD, and IIE. Group IIE silicated irons are related to the H chondrites, while the unique silicated iron Steinbach is related to group IVA irons.

 

STONY-IRONS

 

These meteorites are mixtures of olivine and FeNi metal that formed deep in the core-mantle boundary of a small, differentiated asteroid. As the overlying cumulate olivine cooled and contracted, the still slightly molten metal was injected into the crystalline olivine forming a continuous matrix. Later collisions exposed this layer and delivered samples to Earth. There are three compositional clusters representing separate parent bodies.

 

Mesosiderites-

The mesosiderites are complex assemblages of FeNi metal with orthopyroxene, plagioclase, olivine, and eucritic clasts. They range from little recrystallized to melted (subgroups I-IV), which cooled slowly at great depth. There is not yet a consensus for the origin of the mesosiderites, and different theories currently exist to explain their formation. The standard theory calls for a large differentiated asteroid that underwent igneous activity to produce a basaltic crust. A large impactor mixed molten metal with the cooler silicates and was rapidly cooled. This was followed by burial in a deep regolith where slow cooling proceeded until excavation and delivery to Earth. Another theory has the basaltic crust of a molten parent body founder and sink through the mantle to the metallic core where mixing occurred. Subsequent collisions exposed this stony-iron layer and delivered fragments to Earth. There is also a theory that calls for the collisional disruption and gravitational reassembly of an asteroid to explain the mixing observed. Stony-irons represent only 2.8% of the total known meteorites.

 

CHEMICAL & STRUCTURAL TRENDS FOR CLASSIFICATION

 

Oxygen Content:

Carbonaceous chondrites are oxygen-rich with most of the iron combined as silicates, magnetite, or water. Ordinary chondrites have about half their iron in the oxide form with the rest in troilite or metal. Enstatite chondrites are oxygen-poor with all the iron in metal or sulfide form. R chondrites have a high degree of Fe oxidation.

 

CI-normalized mean-refractory-lithophile abundance ratio:

Carbonaceous chondrites have a ratio of 1.00-1.35, ordinary chondrites 0.77-0.82, enstatite chondrites ~0.6, and R chondrites 0.85.

 

Fine-grained matrix/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.

 

Abundance of isotopically heterogeneous refractory inclusions:

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.

 

Whole rock O-isotope composition:

Carbonaceous chondrites are much below the terrestrial fractionation (TF) line, ordinary chondrites are above the TF line, enstatite chondrites are on the TF line, and R chondrites are much above the TF line.

 

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.

 

Molar An of plagioclase:

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.

 

Abundance of opaque mineral-rich porphyritic chondrules:

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

 

Ratio of Iron to Silicon:

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

 

Ratio of Magnesium to Silicon:

Separates the ordinary, carbonaceous, and enstatite chondrites. 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.

 

Ratio of Calcium to Silicon:

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

 

Petrologic Type:

Type 3 represents unaltered material, while those of lower number are progressively altered by aqueous conditions, and those of higher number progressively altered by thermal metamorphism.  As metamorphic alteration increases, a number of chemical and physical changes occur:

 

   olivine and pyroxene become more homogenous

   pyroxene changes from low-temperature clinopyroxene to high-temperature

      orthopyroxene

   amount of crystalline feldspar increases at the expense of glass, then

      decreases after vitrification

   fine-grained matrix becomes transparent and recrystallizes into a coarser

      texture

   Ca range in Wollastonite increases from 0.4-1.2 in types 3 & 4 to 1.2-1.6 in

      type 5 to 1.6-2.2 in type 6

   chondrules merge into surrounding material

   TL sensitivity increases with higher petrologic type, while decreasing at

      higher shock levels

 

Weathering Grade (A)

  W1-minor oxide rims around metal and troilite. 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 mafic silicates, mainly along cracks.

  W6-massive replacement of silicates by clay minerals and oxides.

 

Weathering Grade (B)

  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.

 

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.

 

Shock Stage

  S1-unshocked, peak shock pressure <5 GPa  (1 GPa = 10,000 bars)<br>

  S2-very weakly shocked, peak shock pressure 5-10 GPa

  S3-weakly shocked, peak shock pressure 10-20 GPa

  S4-moderately shocked, peak shock pressure 20-35 GPa

  S5-strongly shocked, peak shock pressure 35-55 GPa

  S6-very strongly shocked, peak shock pressure 55-75 GPa; whole rock impact

     melting occurs at 75-90 Gpa