Iron, IAB complex, sLL subgroup
Found 1776, known earlier
19° 34' N., 99° 34' W.
Many large masses were found near Xiquipilco, Mexico, the largest of which weighed 300 pounds. Recently, a taxonomic revision was proposed by Wasson and Kallemeyn (2002) that includes iron meteorites from the IAB-IIICD group, along with numerous IAB-related meteorites. On a Ni–Au diagram, Toluca and other similar irons resolve a low-Au, low-Ni subgroup (sLL).
The I–Xe closure age was determined by Pravdivtseva et al. (2009) for both high-Mg and low-Mg pyroxenes, the two closure times differing by 8.5 (±4.4) m.y. An absolute closure age based on the Shallowater standard was calculated to be 4,560.5 (±2.4) m.y. for high-Mg pyroxenes and 4,552.0 (±3.7) m.y. for low-Mg pyroxenes, a range similar to that of other IAB irons. The earlier closure age is likely commensurate with the catastrophic disruption of the IAB parent body. They also determined the cooling rate of Toluca following its parent body breakup and reassembly. This was calculated as a function of the difference between the crystallization temperatures and closure ages of the two pyroxenes to be 14.5 (±10.0)°C/m.y.
Further information on the formation of the IAB iron complex can be found on the Caddo County page. The specimen of Toluca shown above and below is a 2003 Harvey Award—"New Technology Award"—which was presented in recognition of the Meteorite Studies website.
