Five fragments of a stony meteorite 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 chondrite (Fa18, Fs17) composed of chondrules and chondrule 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 matrix component in Sahara 98034 is the least abundant among that of all ordinary chondrite groups. This meteorite has one of the highest total porosities known among ordinary chondrites, determined by X-ray microtomography (µCT) and helium 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 porosity 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 stage now determined to be S1, originally published in MetBull #88 to be S2. Also like the 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 silicate darkening, chromite veinlets in olivine, chromiteplagioclase assemblages, metallic Cu and irregular troilite 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 Petrology of the Baszkówka L5 chondrite: A record of surface-forming processes on the parent body, the authors describe how an early collision of two thinly crusted, molten planetesimals occurred within the first two million years of Solar System 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 metal 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 System 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 crater 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 wave could have created the observed range of porosities. However, later studies failed to show any correlation between porosity and metamorphic type, shock stage, brecciation, or even terrestrial weathering. In their study of density 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]%), enstatite chondrites (0.312.6%), 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., MurchisonCM: 22 [±2]%; MurrayCM: 28%; WarrentonCO: 26%; AllendeCV: 23.0 [±3.6]%; AxtellCV: 21 [±2]%) were found to have different, significantly higher porosities than the other meteorite groups. This is likely the result of unique accretion/lithification/compaction histories on their parent bodies at their particular formation regions in the nebula.
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 chondrite Orgueil 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 type, which corresponds to burial depth, is not correlated with porosity; however, a weak trend for the reduction 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 crystallization 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 wind noble gas enrichment suggests that these porous stones were not part of a fine-grained regolith. 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 ~2527 vol%. Its porosity is consistent with its weak shock metamorphism (S23) given the likelihood that it experienced lithification without compaction, probably reflecting low energy impact events below ~10 GPa.
An extensive listing of individual meteorite densities and porosities for all chondrite groups and many achondrite groups is presented by Britt and Consolmagno in MAPS, no. 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 fault 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 ArAr 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.