"Rock of the Month # 7, posted January 2002" --- Samples 1775-1776.

Holbrook is a well-known meteorite, classified as an L6 ordinary chondrite. The Holbrook stone arrived on Earth in spectacular fashion on 19 July 1912. The fall was heralded by a loud blast followed by smaller explosions and a protracted rumble which lasted roughly 2 minutes, in the early evening (Foote, 1912; Merrill, 1912; Banks and Benny, 2001). Many stones fell in a large strewnfield (area from which meteorite fragments are recovered) along the Santa Fe railway line, extending for at least 6 miles (10 km) to the east of the town of Holbrook in Navajo county, Arizona. The heaviest concentration of fragments was recovered around the Aztec railway yard. After the meteorite "shower" had landed, rather more than 14,000 fragments were eventually recovered, varying in size from sand grains to masses weighing 6.6 kg (Mason and Wiik, 1961; Norton, 1994; Grady, 2000, pp.240-241), for an estimated total mass near 218 kg.

The illustrated samples, obtained from David New, are modest in scale, each roughly 1 cm in diameter (note mm scale in photograph). They are small (4.7 and 4.1 g) subangular to rounded stones. They contain small chondrules and metal/sulphide nodules, the latter sometimes projecting through the very thin (much less than 1 mm) fusion crust. In terms of meteorite recognition, note (1) the thin, shiny black and somewhat cracked fusion crust, (2) the pale interior, dominated by silicate minerals such as olivine and pyroxenes, and (3) the rusty spots, formed quite rapidly by terrestrial weathering due to the oxidation of specks of nickel-iron alloys and troilite (iron sulphide).

Holbrook is one of the more famous meteorite showers, a century prior to the Chelyabinsk fall (Warin and Kashuba, 2013). Nininger (1952), in his review of many meteorites, wrote descriptions of 14 "typical falls", all but one in the USA. These included Holbrook, and other examples such as: Richardton, N.Dakota (H5, 90 kg, 1918); Johnstown, Colorado (diogenite, 1924, 40.3 kg); Paragould, Arkansas (LL5, 408 kg, 1930); Pantar, Mindanao, Philippines (H5, 2.13 kg, 1938); Leedey, Oklahoma (L6, 51.5 kg, 1943); and Norton County, Kansas (aubrite, 1100 kg, 1948).

The large number of pieces and the volume and freshness of material have ensured that Holbrook has been analysed in many ways by many experts over the past 90 years. This is the case although it is a representative of the most common (L6 chondrite) meteorite class. Nine L6 stones have been recovered in Canada alone, including the largest Canadian fall (Bruderheim), Peace River, and the 1998 fall, Kitchener. Over 4,000 L6 meteorites are known (Grady, 2000, p.7). The mineralogy, chemistry, radiocarbon content (Jull et al., 2000) long-term history and recent weathering behaviour have all been examined in detail, making Holbrook a significant name in the study of meteorites. Further research involving Holbrook samples in the past 20 years also includes:

• Physical properties of meteorites (Britt and Consolmagno, 2003; Consolmagno et al., 2006).
• Terrestrial weathering processes (Lee and Bland, 2004).
• Chromite chemistry as a geothermometer (Wlotzka, 2005).
• Microtomography of chromite grains (Alwmark et al., 2011).

It is noteworthy that, for meteorite hunters, the passage of a century does not rule out new finds, especially in falls where many individuals have been recovered. Metal detectors can help in many but not all meteorites: in the case of Holbrook, a large (922-gram) fragment was the largest recovered in 41 years (Atkins, 2007).

References

Alwmark,C, Schmitz,B, Holm,S, Marone,F and Stampanoni,M (2011) A 3-D study of mineral inclusions in chromite from ordinary chondrites using synchrotron radiation X-ray tomographic microscopy - method and applications. Meteoritics & Planetary Science 46, 1071-1081.

Atkins,L (2007) The find of a lifetime. Meteorite 13 no.4, 27-29, November.

Banks,LW and Benny,M (2001) The day it rained rocks: a rare meteor shower over Holbrook in 1912 hurled fragments up to six miles across the desert. Arizona Highways, 20 August.

Britt,DT and Consolmagno,GJ (2003) Stony meteorite porosities and densities: a review of the data through 2001. Meteoritics & Planetary Science 38, 1161-1180.

Consolmagno,GJ, Macke,RJ, Rochette,P, Britt,DT and Gattacceca,J (2006) Density, magnetic susceptibility, and the characterization of ordinary chondrite falls and showers. Meteoritics & Planetary Science 41, 331-342.

Foote,WM (1912) Preliminary note on the shower of meteoric stones near Holbrook, Navajo county, Arizona, July 19th, 1912, including a reference to the Perseid swarm of meteors visible from July 11th to August 22nd. Amer.J.Sci. 34, ser. IV, 437-456.

Grady,MM (2000) Catalogue of Meteorites. Natural History Museum, London / Cambridge University Press, 5th edition, 690pp. plus CD-ROM.

Jull,AJT, Lal,D, Burr,GS, Bland,PA, Bevan,AWR and Beck,JW (2000) Radiocarbon beyond this world. Radiocarbon 42, 151-172.

Lee,MR and Bland,PA (2004) Mechanisms of weathering of meteorites recovered from hot and cold deserts and the formation of phyllosilicates. GCA 68, 893-916.

Mason,B and Wiik,HB (1961) The Holbrook, Arizona, chondrite. Geochim.Cosmochim.Acta 21, 276-283.

Merrill,GP (1912) A recent meteorite fall near Holbrook, Navajo county, Arizona. Smithsonian Miscellaneous Collections 60 no.9 / Publ. 2149, 4pp.

Nininger,HH (1952) Out of the Sky: an Introduction to Meteoritics. Dover Publications Inc., New York, 336pp.

Norton,OR (1994) Rocks from Space: Meteorites and Meteorite Hunters. Mountain Press Publishing Co., 449pp.

Warin,R and Kashuba,J (2013) Big booms, small stones. Meteorite 19 no.2, 24-28, summer.

Wlotzka,F (2005) Cr spinel and chromite as petrogenetic indicators in ordinary chondrites: equilibration temperatures of petrologic types 3.7 to 6. Meteoritics & Planetary Science 40, 1673-1702.