"Rock of the Month #129, posted March 2012" ---

Toluca is an historic meteorite find from central Mexico. Recognized in 1776, at which time it was being forged into agricultural implements (Grady, 2000, pp.492-493), utilizing some of the numerous fragments, as large as 300 pounds (136 kg) recovered in the area. It is classified as a IAB coarse octahedrite, kamacite bandwidth 1.40 ± 0.20 mm (Kissin et al., 1999), nickel content circa 8.05 weight percent. More recently, Toluca has been placed in the sLL (low gold, low nickel) subgroup of the IAB complex, along with Annaheim, Goose Lake and other irons (Wasson and Kallemeyn, 2002).

Toluca is one of the numerous large irons known from the western regions of the Americas (Lemaire, 1980) and it has sundry synonyms, most notably Xiquipilco. Also known as Morelos, Ocatitlan, Tacubaya, Tejupilco, Tennant's Iron, etc. One specimen was for many years mistakenly attributed to a now-discredited Canadian iron named Leeds (Kissin et al., 1999).

Toluca is one of the most-researched and referenced iron meteorites after Canyon Diablo, the impactor responsible for the Barringer Crater (Meteor Crater) in Arizona. In my own database of references *, these two are followed in turn by Gibeon, Campo del Cielo, Coahuila, Sikhote-Alin and Hoba. Notably, these seven irons also include some of the largest known meteorites, with total known weights (TKW) in the range of two tonnes (Toluca, Coahuila) and up to 15 to 60 tonnes (the other five). Willamette is another "giant" iron that is quite well-studied, in part for its intriguing human history. Smaller meteorites may be well-known and well-studied because of intrinsic interest (e.g., Kodaikanal, Negrillos). The largest Canadian irons are an order of magnitude smaller than Toluca, and include the recent Whitecourt find, plus the historic Madoc and Iron Creek masses. Availability, as well as sheer size, is also an influence, with readily-available material (e.g., Gibeon, Campo, and stony meteorites such as NWA 869) an invitation to closer scrutiny, individually and as part of wider synoptic studies of meteorite properties.

• the occurrence of graphite in meteorites (Farrington, 1901),
• the abundance of platinum group elements (PGE) in meteorites,
• occurrence of native copper,
• kamacite bandwidth (Goldstein and Short, 1967),
• the presence of silicate inclusions,
• iron-nickel alloys such as kamacite and tetrataenite,
• phosphides, and
• the silicon content of meteoritic metal (Pack et al., 2011)

* A casual acquisition of articles has so far resulted in the collection of about 220 items on Canyon Diablo, 110 on Toluca, 100 on Gibeon, and 55-75 items on each of the other four large irons mentioned above.

References

Farrington,OC (1901) The constituents of meteorites: I. J.Geol. 9, 393-408.

Goldstein,JI and Short,JM (1967) The iron meteorites, their thermal history and parent bodies. Geochim.Cosmochim.Acta 31, 1733-1770.

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

Kissin,SA, Plotkin,H and Bordeleau,A (1999) The Leeds, Quebec meteorite: its strange history and a re-evaluation of its identity. J.Roy.Astron.Soc.Canada 93, 135-139.

LeMaire,TR (1980) Stones from the Stars: The Unsolved Mysteries of Meteorites. Prentice-Hall, Inc., 185pp.

Pack,A, Vogel,I, Rollion-Bard,C, Luais,B and Palme,H (2011) Silicon in iron meteorite metal. Meteoritics & Planetary Science 46, 1470-1483.

Wasson,JT and Kallemeyn,GW (2002) The IAB iron-meteorite complex: a group, five subgroups, numerous grouplets, closely related, mainly formed by crystal segregation in rapidly cooling melts. Geochim.Cosmochim.Acta 66, 2445-2473.