SAH 98034

H5, highly porous

Found September 1998
z+0° 07′ 08′ w+0° 08′ 58′ Five fragments of a stony meteoriteMeteorite composed of silicate minerals, but that may have up to 25% Ni-Fe metal by weight. Stony meteorites are extremely heterogeneous as a group, ranging from samples of primordial matter that have remained more or less unchanged for the last 4.56 Ga (chondrites) to highly evolved younger rocks from differentiated were found in the Sahara by a French team. These fragments fit together to constitute a single stone weighing 10.345 kg. The stone was subsequently purchased by Astronomical Research Network, and classification was completed at the Institut für Planetologie in Münster (A. Bischoff). Sahara 98034 was determined to be an H5 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 (Fa18, Fs17) composed of 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 and 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 fragments (along with some hollow chondrules) surrounded by an extremely porous groundmass. As is characteristic for members of the H-chondrite group, the fine-grained matrixFine grained primary and silicate-rich material in chondrites that surrounds chondrules, refractory inclusions (like CAIs), breccia clasts and other constituents. in Sahara 98034 is present in a lower abundance compared to all other 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, groups. This 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 has one of the highest total porosities known among ordinary 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, determined by X-ray microtomography (µCT) and heliumHelium (He) Second lightest and second most abundant element (after Hydrogen) in the universe. The most abundant isotope is 4He (99.9998%), 3He is very rare. Helium comprises ~8% of the atoms (25% of the mass) of all directly observed matter in the universe. Helium is produced by hydrogen burning inside pycnometry to be ~16% compared to the ~7% typical for most H-group ordinary chondrites (Sasso et al., 2009); Consolmagno et al., 2008).

Most of this 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 in Sahara 98034 and in the other highly porous ordinary chondrites represents primordial accretionary porosity in the form of intergranular voids. Similar to other highly porous ordinary chondrites, Sahara 98034 has a low 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".               now determined to be S1, although originally published in MetBull 88 as S2. Also like other highly porous ordinary chondrites, it contains petrographic evidence for relict shock and subsequent annealing (Friedrich et al., 2014). These shock indicators, as originally described by Rubin (2004), include 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 darkening, chromiteBrownish-black oxide of chromium and iron (Cr-Fe oxide), Cr2FeO4, found in many meteorite groups. veinlets in 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, chromite–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 assemblages, metallic Cu and irregular troiliteBrass colored non-magnetic mineral of iron sulfide, FeS, found in a variety of meteorites. grains within FeNi-metal, and polycrystalline troilite.

A proposed model for the development of high porosities in meteorites was presented in an article by Przylibski et al. in MAPS, vol. 38, #6 (2003). In PetrologyScience dealing with the origin, history, occurrence, chemical composition, structure and classification of rocks. of the Baszkówka L5 chondrite: A record of surface-forming processes on the 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., the authors describe how an early collision of two thinly crusted, molten 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 – occurred within the first two million years of Solar SystemThe Sun and set of objects orbiting around it including planets and their moons and rings, asteroids, comets, and meteoroids. history. This collision produced a hot cloud of low-density chondritic material, which thereafter slowly accreted onto the surface of the larger body. This homogeneous material was then loosely sintered together by hot, viscous 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 and sulfides. Material that remained near the surface of the body developed the highest porosities like those found in Baszkówka (L5), Miller (H5), NWA 2380 (LL5), and Sahara 98034 (H5). Those meteorites with somewhat less porosity, such as Mt. Tazerzait (L5) and Tjerebon (L5), were more deeply buried. These meteorites did not experience further compaction or recrystallization, and therefore their petrography reflects the conditions that existed during the earliest period of 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 history. A slightly different scenario was presented by Friedrich et al. (2008) to explain the range of porosity retention. They conclude that the highest-porosity material was located farther from the impact craterCrater formed by high-speed impact of a meteoroid, asteroid, or comet on a solid surface. Craters are a common feature on most moons (an exception is Io), asteroids, and rocky planets, and range in size from a few cm to over 1,000 km across. There is a general morphological progression and experienced less impact-induced compression and/or lower-energy forces. Since these highly-porous meteorites also exhibit features consistent with relatively high petrologic types, they calculated that these low-energy impacts must have occurred while the rock was still hot, during the early thermal metamorphism stage, i.e., cooling from radiogenic and collisional heating.

An alternative theory for the formation of porosity across the range of meteorite types was proposed by Strait and Consolmagno (2004). They suggest that the decompression that follows the passage of an impact-generated shock waveAbrupt perturbation in the temperature, pressure and density of a solid, liquid or gas, that propagates faster than the speed of sound. could have created the observed range of porosities. However, later studies failed to show any correlation between porosity and metamorphicRocks that have recrystallized in a solid state due to changes in temperature, pressure, and chemical environment. type, shock stage, brecciationThe formation of a breccia through a process by which rock fragments of of various types are recemented or fused together., or even terrestrial weathering. In their study of densityMass of an object divided by its volume. Density is a characteristic property of a substance (rock vs. ice, e.g.). Some substances (like gases) are easily compressible and have different densities depending on how much pressure is exerted upon them. The Sun is composed of compressible gases and is much and porosity utilizing a broad range of meteorite types, Consolmagno et al. (2008) determined the porosity of a significant number of meteorites, including ordinary chondrites (8.6 [±5.4]%), 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). chondrites (0.3–12.6%), primitive achondrites (11.5 [±3.6]%), and basaltic achondrites (similar to OC and EC), and they found that they all have very similar porosities. Only the carbonaceous chondrites (e.g., Murchison–CM: 22 [±2]%; Murray–CM: 28%; Warrenton–CO: 26%; Allende–CV: 23.0 [±3.6]%; Axtell–CV: 21 [±2]%) were found to have different, significantly higher porosities than the other meteorite groups. This is likely the result of unique 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/lithification/compaction histories on their parent bodies at their particular formation regions in the nebulaAn immense interstellar, diffuse cloud of gas and dust from which a central star and surrounding planets and planetesimals condense and accrete. The properties of nebulae vary enormously and depend on their composition as well as the environment in which they are situated. Emission nebula are powered by young, massive.

In a study by Wilkison et al. (2003), a formula was developed to quantify the microporosity of meteorites:

%porosity = [1-(bulk density ÷ grain density)] × 100
where bulk density utilizes the full volume enclosed by the outer surface, and grain density utilizes only the volume occupied by solid matter, disregarding cracks and voids (Macke et al., 2011) In their study of 30 ordinary chondrites, it was determined that typical microporosities range from 0% to 27%, with an average of ~6.4%; 95% of the samples had porosities below 20%. In an earlier study by Consolmagno et al. (1998), which utilized a larger data set representing 130 different chondrite porosity values, they demonstrated that the pre-weathering porosity for ordinary chondrites averaged ~10%. The CI chondriteRare meteorite class named after the Ivuna meteorite that fell in Tanzania in 1938. They are among the most primitive, friable (crumbly), and interesting of all meteorites, having undergone extensive aqueous alteration. They lack chondrules and CAIs as a result of this alteration, but contain up to 20% water, as OrgueilA large carbonaceous Ivuna-like (CI1) chondrite that disintegrated and fell in fragments near the French town of Orgueil on May 14, 1864. About 20 pieces, totaling ~12 kg in mass, were subsequently recovered from an area of several square km, some head-sized but most were smaller than a fist. Specimens was determined to have the highest porosity of ~35%, and also the lowest measured bulk density (Consolmagno and Britt, 1998).

Although they have different bulk compositions, the bulk density of Orgueil compares well to that of certain asteroids, including Phobos, Deimos, and Mathilde. Wilkison et al. (2003) found that the 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., which corresponds to burial depth, is not correlated with porosity; however, a weak trend for the reductionOxidation 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 of porosity as petrologic type increases was found by Macke (2010). Porosity was found to not be correlated with chemical group (e.g., H, L, LL, CM, LUN, AUB, CHA; Strait and Consolmagno, 2004), bulk density, grain density (possibly weakly correlated), brecciation, shock stage (at least not below very strong shock or shock-melt pressure levels), or permeability (Corrigan et al., 1997). Consolmagno et al. (1998) found that terrestrial weathering leads to the filling of the pore spaces on a time scale of hundreds of years, but Coulson et al. (2007) found no relationship between porosity and terrestrial residency time. Furthermore, they found that porosity is not obviously correlated with crystallizationPhysical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals. age.

Utilizing He pycnometry and microtomography (µCT) techniques, as well as determination of metal particle size distribution, Sasso et al. (2009) investigated the nature of the internal pore spaces in several ordinary chondrites including Sahara 98034. They argue that these meteorites accreted with incomplete compaction and preserve a significant degree of primary accretionary porosity. Although they were less affected by later compaction events, ubiquitous impact-generated microcracks were introduced over time. The lack of any solar windSupersonic flow of high-speed charged particles continuously blowing off a star (mostly e- and p+). When originating from stars other than the Sun, it is sometimes called a "stellar" wind. The solar wind may be viewed as an extension of the corona into interplanetary space. The solar wind emanates radially 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. enrichment suggests that these porous stones were not part of a fine-grained regolithMixture of unconsolidated rocky fragments, soil, dust and other fine granular particles blanketing the surface of a body lacking an atmosphere. Regolith is the product of "gardening" by repeated meteorite impacts, and thermal processes (such as repeated heating and cooling cycles).  . They calculated a total porosity for Sahara 98034 (H5) of 16.1% (±2.0%), while NWA 2380 (LL5) had a higher total porosity of ~18.7% and Miller (H5) had the highest total porosity in the study of ~20%. A study by Wittmann et al. (2010) of the unusual type-5 ordinary chondrite LAP 031047, which has an O-isotopic composition and petrographic features intermediate to H and L chondrites, revealed a very high porosity of ~25–27 vol%. Its porosity is consistent with its weak shock metamorphismMetamorphism produced by hypervelocity impact between objects of substantial size moving at cosmic velocity (at least several kilometers per second). Kinetic energy is converted into seismic and heat energy almost instantaneously, yielding pressures and temperatures far in excess those in normal terrestrial metamorphism. On planetary bodies with no atmosphere, smaller (S2–3) given the likelihood that it experienced lithification without compaction, probably reflecting low energy impact events below ~10 GPa.

An extensive listing of individual meteorite densitiesMass of an object divided by its volume. Density is a characteristic property of a substance (rock vs. ice, e.g.). Some substances (like gases) are easily compressible and have different densities depending on how much pressure is exerted upon them. The Sun is composed of compressible gases and is much and porosities for all chondrite groups and many 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 groups is presented by Britt and Consolmagno in MAPS, #8 (2003). Interestingly, they found that porosities of L chondrites (~6%) are significantly different from those of both H and LL chondrites (~10%). Low-density, porous material should be present in regolith breccias of asteroids, created by impact lithification of disordered material, as well as in deep fractures and faultFracture along which there has been movement or displacement. zones; the S-type asteroids 433 Eros and 243 Ida, with ~10% microporosity plus ~20% macroporosity, are examples which exhibit both environments. While this may be true, infrared spectra data obtained by the Galileo spacecraft indicate that 243 Ida is most probably an L- or LL-type chondritic asteroid (Granahan, 2013).

Results of an Ar–Ar age study by Friedrich et al. (2008) give a best crystallization age for Sahara 98034 of 4.229 (±0.044) b.y., and indicate that the source material was partially reset by an impact-heating event <600 m.y. ago. Although FeNi-metal grains are abundant throughout, some terrestrial weathering effects are evident in Sahara 98034 such as its lower content of metal, 40% less than typical H chondrites; it has a weathering grade of W1 (Sasso et al., 2009). The slice of Sahara 98034 pictured above weighs 56.9 g. The magnified image presented below illustrates the exceptional porosity of this meteorite.

click on photo for a magnified view