The Gao-Guenie meteorite shower, H5 chondrite fall, west Africa

The Gao-Guenie H5 meteorite shower, Burkina Faso

--- remarkable event in 1960

Figure 1. Here we have 67 little individuals of the Gao-Guenie meteorite shower, each roughly 1-2 cm in maximum dimension, average weight circa 7.25 grams, for a total weight of 486 grams. Material from Eric Wichman in 2012. Two of the individuals were sawn for polished section preparation. A sawn slice of a small individual is shown in Fig. 2. The photo above, of the ranks of small meteorites, is taken from (Wilson, 2012, p.6). The 2012 report (available on this web site, here) compares these dark stones, which, though variably weathered, display a black fusion crust, with small rock chips (such as commercial aggregate) that may at a glance seem similar, to an optimist.

"Rock of the Month #227 posted for May 2020" ---

The Gao-Guenie H5 ordinary-chondrite meteorite shower

was a significant event, in which an unknown number of meteorites (of unknown total weight) fell to Earth. Many, presumably thousands, were recovered at the time, and in the following decades. Note that meteorite showers are NOT at all the same thing as meteor showers. The latter are familiar, annual events in which tiny particles of cometary dust, following the elliptical orbits of existing comets or the remains of one which has broken up, fall to Earth, burning up in the atmosphere, to produce luminous flashes (and sometimes trails) across the sky, as we see inthe Perseid and Leonid showers, for example. A meteorite shower can occur at any time of year, when a body deflected by gravity has left its home in the asteroid belt, and heads toward the Sun, by chance encountering the Earth's gravitational field and descending. If it survives, the result is a meteorite or, if the body disintegrates into many parts (there may not be a quantitative definition) then we can say a meteorite shower has occurred.

It was in 1960 that two new ordinary-chondrite meteorite falls were recognized in west Africa: "Gao (Upper Volta)", soon truncated to "Gao", and "Guenie" (Graham, 1980). The early Gao finds included some larger pieces, weighing as much as 2.5 kg. However, the fall became known as a prolific source of small, rounded to elongate stones, each weighing a few grams, to the point where some dealers sold the material by the bag, like candy!

It took some time, but eventually it was realised, and demonstrated through comparison of material, that two meteorite falls in west Africa, named Gao and Guenie, were one and the same event, the meteorite shower now known as Gao-Guenie. Two H4-H5 chondrites from Burkina Faso, Guenie and Gao were originally said to have fallen one month apart. It seems that a single large meteorite shower fell in the area on 05 March 1960. Samples of the abundant material are mineralogically very similar (Bourot-Denise et al., 1998). This argument was accepted, and as of Met.Bull. 83 (Grossman, 1999), the name Gao-Guenie was officially applied to all material formerly referred to either Gao or Guenie, which fell in Burkina Faso (Upper Volta, west Africa) The Met.Bull. does not quote a total known weight for Gao-Guenie, but it would not be too surprising if 1 tonne or more has been recovered over the years.

As a postscript, there is also a Gao-Guenie (b), a small, rare CR chondrite, recognized by Eric Twelker as being different from the abundant ordinary chondrite of the Gao-Guenie strewnfield. It has been studied in the context of refractory grains and calcium-aluminium inclusions (Krot et al., 2012).

Samples of the prolific Gao-Guenie H5 shower have been studied in various contexts, e.g.:

Physical properties of meteorites (Gattacceca et al., 2004).
The study of xenoliths in meteorites (Briani et al., 2012).
Small examples of oriented stones (Warin and Kashuba, 2014).

Figure 2. A rough-sawn face of a small elongate individual, "Gao-B", cut for the preparation of polished thin sections. 8X magnification, long-axis field of view 16 mm. The exposed section through a large, sharply-defined chondrule is 2.8 mm in diameter. Note also the abundant fine-grained nickel-iron alloy (mainly the variety meteoriticists term kamacite) in the slightly rusty, fine-grained silicate-dominated groundmass.

The following table shows a small selection of some of the better-documented meteorite showers, two of which have previously been written up in these pages.

TABLE OF SELECTED METEORITE SHOWERS

Class	Fall	TKW (kg)	Year	Country
Chelyabinsk	LL5	1000	2013	Russia
Gao-Guenie	H5	(?)	1960	Burkina Faso
Holbrook	L6	220	1912	Arizona, USA
Knyahinya	L5/LL5	500	1866	Ukraine
L'Aigle	L6	37	1803	France
Pultusk	H5	250	1868	Poland
Tatahouine	DIO	12	1931	Tunisia

All these meteorites are very well studied (the total known weight is generally approximate, and - coming from the Meteoritical Bulletin, may be a minimum since, in the case of showers, detailed follow-up may add to the total inventory). All these showers are substantial, >10 kg in each case, meaning that the samples are often available in museums and universities worldwide.

Figures 3-4. Two views of a polished section of "Gao-B", the sawn individual in Figure 2. Left: chondrules, seen in binocular microscope view, 16X, unpolarized transmitted light, long-axis field of view 8 mm. Right: An excentroradial pyroxene chondrule seen at 50X, crossed-polarized transmitted light, long-axis field of view 1.7 mm.

As a major fall, Gao-Guenie is well-represented in the meteoritics literature. It figures in various guides to meteorites (e.g., Norton, 2002; Carion, 2009) and is well-represented in many collections (e.g., Killgore et al., 2002; Haag, 2003).

Micrometeorites

This article is concerned with those small, generally stony meteorite fragments that fall in rare meteorite showers. It may be helpful to put these in context. Roughly 100 tonnes per day, 40,000 tonnes of per year, of extraterrestrial material lands on Earth, mostly as micrometeorites (Folco and Cordier, 2015). The macroscopic meteorites, from the pea-sized Gao-Guenie and Holbrook individuals to the rare multi-tonne survivor, are a very small proportion of all this. Some of the micrometeorites are relatively large, say 0.25-1 mm in diameter. Their response to atmospheric passage, heating and partial melting, and subsequent weathering on the ground have been investigated (e.g., Suttle et al., 2019). Norwegian citizen scientist Jon Larsen has found out how to recover micrometeorites from roof gutters - his samples are quite small, typically 0.3 mm - but very beautiful. He has published his findings, with superb illustrations, including both the real thing and also the many natural and man-made materials that can fool the unwary, and excellent contribution of both scientific and artistic merit (Larsen, 2017). Larsen's success was duly noted in National Geographic (Williams, 2017).

References

Bourot-Denise,M, Wenmenga,U and Christophe,M (1998) The Guenie and Gao chondrites from Burkina Faso: probably a single shower of stones. Meteoritics & Planetary Science 33, supplement, A181-182.

Briani,G, Gounelle,M, Bourot-Denise,M and Zolensky,ME (2012) Xenoliths and microxenoliths in H chondrites: sampling the zodiacal cloud in the asteroid Main Belt. Meteoritics & Planetary Science 47, 880-902.

Carion,A (2009) Meteorites. Alain Carion, Paris, 3rd edition, translated from the French by Anne Black, 72pp.

Folco,L and Cordier,C (2015) Micrometeorites. In `Planetary Mineralogy' (Lee,MR and Leroux,H editors), European Mineralogical Union Notes in Mineralogy 15, 301pp., 253-297.

Gattacceca,J, Eisenlohr,P and Rochette,P (2004) Calibration of in situ magnetic susceptibility measurements. Geophys.J.Int. 158, 42-49.

Graham,AL (1980) The Meteoritical Bulletin, No.57. Meteoritics 15, 93-103.

Grossman,JN (1999) The Meteoritical Bulletin, No.83, 1999 July. Meteoritics & Planetary Science 34, A169-186.

Haag,R (2003) The Robert Haag Collection of Meteorites. Robert Haag Meteorites, Tucson, AZ, private collection edition, 126pp.

Killgore,K, Killgore,M and Killgore,E (2002) Southwest Meteorite Collection, a Pictorial Catalog. Southwest Meteorite Press, 201pp.

Krot,AN, Makide,K, Nagashima,K, Huss,GR, Ogliore,RC, Ciesla,FJ, Yang,L, Hellebrand,E and Gaidos,E (2012) Heterogeneous distribution of ²⁶Al at the birth of the solar system: evidence from refractory grains and inclusions. Meteoritics & Planetary Science 47, 1948-1979.

Larsen,J (2017) In Search of Stardust: Amazing Micrometeorites and their Terrestrial Impostors. Quarto Publishing Group USA Inc. / Voyageur Press, 152pp.

Norton,OR (2002) The Cambridge Encyclopedia of Meteorites. Cambridge University Press, New York, xx+354pp.

Suttle,MD, Genge,MJ, Folco,L, Van Ginneken,M, Lin,Q, Russell,SS and Najorka,J (2019) The atmospheric entry of fine-grained micrometeorites: the role of volatile gases in heating and fragmentation. Meteoritics & Planetary Science 54, 503-520.

Warin,R and Kashuba,J (2014) Incoming and in hand. Meteorite 20 no.2, 22-26, summer.

Williams,AR (2017) Where stardust hides on Earth. National Geographic 232 no.2, 12, August.

Wilson,GC (2012) Bibliography of Canadian meteorites. TGSL Report 2012-07, ii+52pp., 22 June.

Graham Wilson, 22-23 April 2020, format tweaks on 13 May 2020

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