Iron, ungrouped
(silicated, possibly unique member of CR chondrite clan)
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Found 1845
31° 51' N., 110° 58' W.

Two masses of the Tucson meteorite were found, the ring-shaped Irwin–Ainsa mass and the paired, slab-shaped, Carleton mass. No fusion crust or heat-affected zone remains on either mass. The meteorites consist of a refractory and reduced mixture of fine-grained, Si–Cr-bearing FeNi-metal (92 vol%) and nearly alkali-free silicates (8 vol%), both having a nebular rather than an igneous origin; the FeNi-metal is considered to have condensed first.

The small (0.1–2 mm) silicate inclusions occur as curvy-linear arrangements suggestive of a flow alignment. They consist primarily of forsteritic olivine (66.4%) with both pure and high-Al enstatite (30.2%), with minor low- and high-Al pyroxene (diopside, 2.7%), pure anorthite and mesostasis glass (0.7%), and trace spinel and brezinaite (Nehru et al., 1982 and references therein). It was determined that the olivine crystallized from a liquid, of which the latter is now present as glass inclusions within olivine grains.

Tucson is thought to have formed from a metal–silicate mixture that was co-precipitated from the solar nebula at high temperatures (~1500°C) and high pressures (~ >1 bar) under highly reducing and turbulent conditions. This was followed by rapid cooling (~1,000°C/m.y.) and annealing as evidenced by the high-Al pyroxene and the presence of quenched clear glasses, thus preserving the early-condensed, chondrule-like form of the silicate inclusions. The occurrence of rare Ca-rich plagioclase in Tucson instead of the alkali feldspar present in most other silicated irons is consistent with volatile loss during high temperature conditions (Ruzicka, 2014 and reference therein). Due to this rapid cooling, no Thomson (Widmanstätten) structure is present upon etching, and Tucson is classified structurally as an ataxite. Tucson is highly reduced and might be related to the similarly reduced and Ge-depleted meteorites Santiago Papasquiero or Nedagolla. Nehru et al. (1982) consider the likely precursor material of Tucson to have been a unique forsterite–enstatite silicate assemblage thus far unsampled as a meteorite, or perhaps it was a more forsterite-rich E chondrite-like body, although it has also been more recently proposed that Tucson may represent the most metal-rich and volatile-element-poor member of the CR chondrite clan.

A study of glass inclusions within and between olivines was conducted by Varela et al. (2008, 2010). Olivine inclusions exhibit rounded surfaces in contact with metal, and crystal faces in contact with glass. Some investigators interpret the flow-like arrangement of the inclusions as indicative of a metallic melt intruding a silicate assemblage as a result of impact forces. On the other hand, ballistic aggregation is considered by some to be responsible for the elongated shapes and preferred direction of the silicates (Kurat et al., 2010). In their studies of Tucson based on new petrographic evidence, Kurat et al. (2010) found that metal and the Ca–Al–Si-rich liquid are early nebular condensates, which precipitated prior to the formation of the silicates. The silicates formed later from the Ca–Al–Si-rich liquid by vapor–liquid–solid condensation, in accord with the 'primary liquid condensation' model (Varela et al., 2005; Varela and Kurat 2006, 2009). The glass phase is consistent with rapid quenching from the liquid phase, with a mineralogical composition consistent with derivation from carbonaceous chondrite material, especially CR chondrites; the composition is much different from that of enstatite chondrites.

In a similar manner, the O-isotopic composition of Tucson silicates and glass is similar to the CR clan and to Kakangari, and the low volatile element abundances are also consistent with the CR clan. The Ca–Al–Si-rich glass in Tucson is also similar in trace element contents to those of carbonaceous chondrites, and they show unfractionated REE patterns. In addition, Tucson glass inclusions show many similarities to the glass in C chondrites and CR chondrites in particular. Beyond that, the metal component in Tucson has a highly refractory nature, in many ways similar to that of the IVA irons (Humayun, 2010). It is basically unfractionated and has probably inherited its trace element abundances and highly depleted volatile element content by direct condensation from an early solar nebula gas, such as with CB and CH chondrites (Kurat et al., 2010). A high-temperature nebular condensation origin is considered most plausible by investigators. A possible formation relationship between Tucson glasses and the glasses in IIE irons has also been conjectured.

A History Revealed

Each of the Tucson masses has a unique convoluted history. The first recovered and the largest of the two is the 1,400 pound (688 kg) ring-shaped mass, alternately called the Ring, Signet, Ainsa, and Irwin–Ainsa Meteorite at various times in history. The other mass, originally weighing 633 pounds (287 kg), is named the Carleton Meteorite for the Civil War general who appropriated the piece for public display.

The first written description of the Ring dates back to 1845. It was written in Spanish by a respected official of Sonora, Mexico, named José Velasco. From a section of his treatise concerning the state of Sonora, titled Mines of Iron, Lead, Copper, and Quicksilver, he described a mountain pass (known today as Box Canyon) within the Sierra de la Madera range (now the Santa Rita Mountains). This pass, located between Tucson and Tubac, contained many large masses of pure iron, lying at the foot of the mountains. He wrote of a medium-sized mass that was taken to Tucson, a journey of over thirty rugged miles, where it had resided for many years [before 1845], serving as an anvil for the garrison armorer/blacksmith.

Writing in his diary for May 31, 1849, the '49er A. Clarke clearly described the find circumstances and provided details of the appearance of the meteorite anvil used by the shoer of his mule. Shortly thereafter, in his article of 1852, Notice of Meteoric Iron in the Mexican Province of Sonora, Dr. John LeConte described the appearance and recovery information of two meteoric anvils being used by blacksmiths in Tucson. That same year, in his diary entry for July 17, boundary commissioner John Bartlett described the origin and dimensions of the Ring mass and alluded to a second large mass located within the garrison in Tucson. He also made a detailed sketch of the celestial anvil, brought to light only in 1978.

Perhaps the most thorough description of the two masses was written by John Parke, lieutenant in charge of a survey expedition. He indicated that with much effort some small samples were acquired and sent to the east for analysis. An analysis was performed by Dr. Charles Shepard and published in 1854 in the American Journal of Science. He reported the lack of crust and the oxidized nature of the meteorite sample, along with its chemical composition.

It was a blacksmith named Ramón Pacheco, who recovered the slab-like mass on or about 1850, and put it to use as an anvil in Tucson. In 1856, the other blacksmith anvil, the Ring, was abandoned leaving all the blacksmith duties to Pacheco and his anvil. In 1862, Colonel James Carleton confiscated the Pacheco anvil and had it shipped to San Francisco where permission was obtained to saw off a specimen for analysis. The mass remained on display at the Society of California Pioneers until 1939 when it was purchased by the Smithsonian to be displayed alongside the Ring mass.

During the year 1860, a medical officer named Bernard Irwin found the abandoned Ring mass and took possession of it on behalf of the Smithsonian. The following year, the meteorite was contracted to begin its journey from Arizona to Washington D.C. via Guaymas by Augustin Ainsa. He took two years to haul the mass to the coast, where his brother, Santiago Ainsa, took over the remaining leg to New York. Santiago was primarily interested in glorifying the family name and contrived a false history of the Ring mass in correspondence with the Smithsonian. In part, he claimed the mass was recovered by his famous great grandfather, Juan Bautista de Anza, in 1735 at a known location, and transported to Tucson. This legend, along with his other claims, have been proven to be totally fabricated; but not before the credit for the presentation of the Ring Meteorite to the Smithsonian was given to the Ainsas, including naming the Ring meteorite the Ainsa Meteorite.

When Irwin learned of this appalling turn of events, he sent a letter to the Smithsonian, debunking Ainsa's fabricated story and protesting their choice of names for the mass. He stated he would rather they rename it the Tucson Meteorite rather than honor the fraudulent claims of Santiago Ainsa. After all, the Ainsas had only contracted to carry it to Washington for Irwin, the original donator to the Smithsonian. The name was subsequently changed to the Irwin–Ainsa Meteorite, but Irwin was intent on removing the name of Ainsa from the meteorite and publishing the correct history of the mass. It took twelve years for the name to be changed at Irwin's insistence to the Tucson Meteorite.

The 3.6 g specimen pictured above was originally part of the inner nodule of the ring mass, and shows a polycrystalline structure with flow patterns of silicate inclusions. The Tucson Ring can be viewed today at the Smithsonian National Museum of Natural History in the Hall of Geology, Gems, and Minerals.

Portions above excerpted from The Tucson Meteorites by Richard R. Willey (1987)

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A large part slice of Tucson.
Photo courtesy of the J. Piatek Collection
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Tucson Ring and Carleton masses on display at the Smithsonian.
Photo courtesy of M. Horejsi