HISTORY AND SCIENCE OF THE CAMPO DEL CIELO METEORITE

The following material is quoted from:

The Handbook of Iron Meteorites
Their History, Distribution, Composition and Structure
Volume 2

by Vagn F. Buchwald

Department of Metallury, Technical University, Lyngby, Denmark
Published for the Center for Meteorite Studies, Arizona State University
By the University of California Press
Berkeley • Los Angeles • London

CAMPO DEL CIELO
Gran Chaco Gualamba, Argentina
27 degrees 39 minutes south, 61 degrees 44 minutes west; 150 m
Polycrystalline, coarse octahedrite, Og.
Group I. 6.68% Ni, 0.43% Co, 0.25% P, 87 ppm Ga, 407 ppm Ge, 3.6 ppm Ir.

HISTORY
According to the excellent work of Alvarez (1926), the history of this interesting shower of irons goes back to the Spanish exploration in about 1576. At that time, an expedition under the leadership of Captain de Miraval traversed the Gran Chaco Gualamba and brought back a few fragments of a large mass called Meson de Fierro (large table of iron). This was appropriately enough located in a district called Campo del Cielo (Field of the Sky) — an open, brush-covered plain which, although stretching for some hundreds of kilometers, was devoid of stones and water and had a very porous topsoil. Alvarez (1926: 19) stated that the Governor ofSantiago del Estero exprssly sent Miraval to locate the iron which was known to the Indians and was regarded as having fallen from heaven. Later expeditions, in 1774, 1776 and 1779, brought back more material. For a time it was considered to be a rich silver ore (1/5 silver, 4/5 iron) and in 1778 a large amount of mercury was dispatched from Almaden, Spain, in order to avoid delay in extracting the silver from the supposed ore (Alvarez: 33). However, a commission of blacksmiths, and the analyst O'Gorman, independently deduced from their results that the mass was absolutely devoid of silver and was an iron of superior quality.

In 1783 Rubin de Celis, a lieutenant of the royal Spanish Navy, relocated the mass near Otumps and excavated its "clay and ash bed" (Celis 1788). In order to ascertain whether the iron had roots of was part of a metallic vein, as assumed by many early visitors, de Celis dug trenches around the mass and tipped it over. He made a curious remark that the iron was sustained upon two, thin corroded pillars of the same material but roots were not found. He presented two drawings, one from above and one from the side, and estimated the weight to be 14 – 18 tons. (Alvarez: 45f). He concluded that the block originated from some volcanic eruption and found the economic value to be infinitesimal. Unfortunately, he left the iron in the overturned position in the hole, and Celis was the last man to see it. Apparently soil erosion has since covered all traces of everything. Small, detached fragments amounting to 10 – 15 kg reached several collections, and were discussed by Chladni (1794: 40, 1819: 318, 341) and others. Proust (1799) and Howard (1802) were among the first to determine nickel in meteorites and agreed on finding a quantity of about 10% in Rubin de Celis' Campo del Cielo material. Stromeyer (1824) claimed to have analyzed an olivine crystal isolated from the material but was disbelieved by his contemporaries who thought he had worked with mislabeled Krasnojarsk material.

[above] A very rare Campo del Cielo meteorite with two natural holes

Another mass of about 1,000 kg, was found in 1803 at Runa Pocita and called Otumpa, was transported via Santiago del Estero to Buenos Aires. Here an attempt was made to forge weapons from it, because there was a shortage of iron in the republic which was at war with Spain at that time. A report of the experiments in the armory by its director, De Luca, is preserved (Alvarez: 158). Two pistols with gun barrels of "Otumpa" iron were presented to President James Monroe of the United States and another to General Manuel Belgrano, but the remainder of the block was presented by the newly founded Argentinean Republic to the British Consul General Sir Woodbine Parish who, in 1826, transferred it to the British Museum where it became the first large meteorite on display.

The two pistols are now on display in the James Monroe Law Office, Museum and Memorial Library in Fredericksburg, Virginia. They constitute a pair of almost identical, short-barrelled, flintlock equestrian pistols with beautiful Spanish style ornamentation. By the kind cooperation of Mr. Lawrence G. Hoes, Keeper of the Museum, I was permitted to examine one of the pistols. A tiny bled of metal was removed from the flintlock, polished and examined. The microscopic examination showed the metal to be composed of equiaxial ferrite grains, with uniformly oriented slag particles. No meteoritic materials were present. The electron microprobe study disclosed less than 0.3% Ni. Independently, Daniel J. Milton of the U.S. Geological Survey analyzed the gun barrel by spectroscopy and also found negligible amounts of nickel. The material is thus typically wrought iron with no meteoritic admixture. Perhaps the pistols were originally intended to be produced from meteoritic iron, but the weapon smith may have found his work unsatisfactory and substituted the pistils with normal wrought iron pistols. At any rate, the pistols, formerly in President Monroe's possession and allegedly made from Campo del Cielo meteorites, contain little or probably no meteoritic iron at all.

The material analyzed and discussed in the nineteenth century probably more or less directly originated from the Rubin de Celis mass, while the Otumpa mass in London was only little cut. The specimens were heat treated, forged and investigated in many ways and then passed on to some other collection without heed to the sometimes rather destructive investigations. That is why the Otumpa or Campo del Cielo iron figured in catalogs as a nickel-poor ataxite for about 150 years. A reinvestigation of two old specimens in Copenhagen (Buchwald 1965; Buchwald & Munch 1965: 53) showed, however, that one specimen was a normal coarse octahedrite similar to Seelasgen, and that the other was a heated and forged specimen. In the latter, most of the phosphides were were resorbed, most of the Widmasntatten structure had disappeared. Unfortunately, the turned out to be the type specimen with which Cohen (1898a; 1905) had worked, and thus explains part of the long standing confusion about this famous meteorite.

Magera (1926) described the depressions ("hoyos") in the Campo del Cielo region and concluded that they were of artificial origin, but Spencer (1933) assumed that they were small craters connected with the large irons of which several more were now becoming known (Duclous 1928). Radice (1959) gave a review of the story and composition of the different masses.

In 1962 and following years, joint Argentinean-U.S. expeditions succeeded in locating a large number of meteorites ranging in size from the 1,998 el Taco mass down to minute fragments of oxidized masses (Cassidy et al. 1965; Cassidy 1967; Cassidy 1968). In the 1965 paper is a list of the 10 largest masses, headed by El Toba of 4,210 kg, found in 1923, and now in Buenos Aires. A table based mainly upon Cassidy's information is presented below. The last entry is an enormous fragment partly excavated in Crater No. 10, and preliminarily reported by Cassidy (1970). The field work showed that the strewnfield was about 75 km long and very narrow, stretching along a line bearing N.60 degrees E. From the northeastern part only scattered minor masses were reported; of these most are lost, but in the southwestern part at least 12 craters — or more correctly penetrations funnels — created by the impact of the larger masses were discovered. In each hole, 20 – 100 m in diameter, the main part of the imkpacting mass was found. Sometimes several masses hitting close to each other had created irregular holes. The distance between the extreme "craters," including thos almost destroyed by erosion, is about 20 km; the masses found in connection with these extremes are El Taco of 1,98 kg and El Mocovi of 732 kg. The total recovered weight since 1803 appears to be about 31,000 kg, and this is probably only a small fraction of the total meteorite.

An upright stump of charred wood excavated from one of the holes showed a C-14 age of 5,800 years, which may be the age of the impact (Cassidy et al. 1965). A more recent C-14 determination on another charcoal sample showed an age of 3,950 years (Cassidy 1973, personal communication). The theory (Cassidy et al. 1965) that the impacting mass was only part of a larger mass, in a decaying orbit, which later produced the North Chilean hexahedrites in untenable along for structural reasons. Campo del Cielo is coarse octahedrite with 6.7% Ni, while the Chilean masses are true hexahedrites with 5.6% Ni.

Th cosmic radiation age was found to be ~14 million years, which is very low compared to other odtahedrites (Nyquiest et al. 1967).

Perry (1944) presented several photomicrographs; one of them (plate 48) undoubtedly show cliftonite, a graphite crystal aggregate, and troilite as stated in the text. Curvello's (1958: 40) application of the aragonite twinning law in this supposedly troilite crystal is therefore invalid. Park et al. (1966) discussed some millimeter-sized silicate inclusions and showed that they consisted of forsterite, chrome diopside, enstatite and oligoclase, decreasing in frequency in that order, and further identified chromite, graphite, troilite and sphalerite, mostly in complex intergrowths. One of the first publications on dislocations in meteorites by thin-film transmission electron microscopy showed that Campo del Cielo had an immobile dislocation network of high density (Ashbee & Vassamillet 1966). Bunch & Cassidy (1968) discussed the numerous deformation structures and found one fragment in particular with indications of heavy plastic flow and recrystallized kamacite. Clarke & Jarosewich (1969) reported a number of excellent analyses (6.7% Ni), the first to be made since the misleading ones performed generations earlier and showing only 5.1–5.9% Ni (Cohen 1898a; Duclous 1929). Reed (1965; 1969) found 6.2–6.6% Ni and 670 ppm P in the kamacite.

In 1966, the 1,998 kg El Taco mass was cut in the Max Planck Institut, Mainz. The critical process of making such a large section (about 110 x 40 cm) was carried out under the direction of Professor H. Hintenberger, using a combination of close parallel 1.2 inch drilling holes and sawing. Two slices were prepared.

Nyquist et al (1967), Hintenberger et al (1969) and Schultz et al (1971) determined the rare gases, while Mason (1967) gave a photograph of the newly cut El Taco specimen and discussed the silicate inclusions of iron meteorites in general. Wlotzka & Jarosewich (1969) examined the silicate inclusions in detail. Podosek (1971) presented the argon analyses on a silicate inclusion, and potassium ages calculated by the 40Ar – 39Ar method. He estimate a K–Ar age of 4.6 x 10(9) years and found evidence of a later heating event 3.4 x 10(9) years ago.

[above] Detail of highly developed regmaglypts ("thumbprints")
on a large Campo del Cielo iron meteorite

COLLECTIONS
The 634 kg Otumpa mass is in London. At least 8 masses ranging from 100 to 4,210 kg are in different museums in Argentina (see table below). Of the newly excavated material 35 kg is in New York, while 44 kg is in Washington, where two large slices of the El Taco mass are also on display. Of the old and partly man-damaged material, small amounts are in Berlin, Copenhagen, London, Tubingen, Vienna and Canberra.

DESCRIPTION
It is not possible to describe all the fragments which are now in many different museums. (See table below). Typical important specimens will, however, be treated in the following passage.

El Toba, weighing 4,210 kg, measures 165 x 110 x 100 cm and displays a 32 x 16 cm cut and roughly polished surface. Chiseled in one place is, "Meteorito el Toba. Dadiva del Dr. Bartolome Vassallo — 1924 —." Chiseled in another place is, "IV — AV," several Roman numerals and an an illegible year, evidently an inscription of much older date. The mass is very well-preserved, the caliche deposits reveal that for a long period it was only partially buried, with 60 cm below and about 40 cm above the soil. The regmaglypts are fine and large; on the sides they are developed as subparallel grooves, indicating the flight direction. They reach sizes of 18 x 10 cm with depths of 1.5 cm. In several places there are secondary fracture surfaces where smaller fragments separated from the mass late in the flight. The secondary surfaces have only small and immature regmaglypts. Everywhere on the top side there are remnants of fusions crust which is only slightly weathered. the details of the sculpture on the under side are hidden beneath caliche deposits. On various occasions visitors have broken or chiseled material from El Toba, leaving scars of, e.g., 20 x 10 cm and 15 x 5 cm. Some of the detached samples are exhibited in the vitrines in Museo Bernardino Rivadaviva.

El Mocovi weights 732 kg, measures 72 x 40 x 40 cm and carries, on a 17 x 13 cm roughly polished surface, the chiseled inscription, "Meteorito El Mocovi. Dadiva del Senor D. Luis E. Zuberbuhler. 1925." It is well-preserved. Most of the regmaglypts are developed as subparallel grooves 4–9 cm wide, or occasionally as pists 5–5 cm across. In numerous places there are holes left by angular troilite aggregates that ablated away in the atmosphere. They attain sizes of 10–25 mm across with depths of 2–10 cm, very rough surfaces indicate where large fragments were torn from El Mocovi at a late stage in flight.

Al Abipon, or Charate, which weighs 460 kg, is rather pyramidal and measures 70 x 45 x 44 cm. One of the pyramid faces, which is 70 x 35 cm, is almost plane and is covered with shallow, immature regmaglypts, 2–3 cm across. In two places, the rough sculpture indicates late necking and fracturing. One is at an edge and measures 40 x 10 cm. The other is a depression, 12 x 8 x 5 cm, from which a fragment was plucked. The mass is well-preserved and shows fusion crust, albeit slightly weathered.

Otumpa, now weighing 634 kg, measures 80 x 50 x 50 cm. It has, just as the above-mentioned, a marked sculpture, but it is also indented in two or three places, thus showing the work of sledge hammers and chisels.

El Taco, which weights 1,998 kg, was rather shield-shaped and measured 120 x 110 x 40 cm before cutting. It is well-preserved and shows regmaglypts on the bottom of which even some slightly weathered fusion crust is present. The regmaglypts of the convex front side of the shield are 4–8 cm across, while those of the slightly concave opposite side are 12–30 cm across and may be further subdivided in 2–6 cm pits. Compare Hraschina, Henbury and Cabin Creek. The El Taco mass, as well as the other large masses, evidently had sufficiently long atmospheric flights to develop indepedently a sculpture characteristic of large single iron meteorites.

Several silicate inclusions, e.dl, 10 x 9 and 3 x 2.5 cm in size, are on the surface of El taco; they stand in marked contrast to the adjacent metal after the surface had been artificially sandblasted in 1966. Silicates are apparently absent from the surfaces of the other large masses. They also seem to be rare in the interior, judging from the low frequency they are met with on the very few cut sections.

The smaller meteorite fragments are irregular, prismatic or angular, and they usually have coatings, 1 cm thick, of rust and caliche. They have mostly been found by systematic excavations.

The reconstructions of the origina body is difficult, but it may be assumed that it was an extended plate about 0.5–1 m thick, rather than a spheroid. Compare Chupaderos. Sections show that the masses were composed of large austenite crystals ranging in size from 5–50 cm. The numerous angular silicate inclusions appear to be concentrated in the grain boundaries along with some troilite and schreibersite. It is not surprising that a tabular mass of this structure should breaks up into a large number of fragments when decelerating the atmosphere; most of the smaller individuals found will represent only one austenite crystal or a fragment of a crystal.

[above] 255-gram complete slice of Campo del Cielo

The individual, large austenite crystals of which some show twins, transformed upon cooling to a coarse Widmanstatten structure of bulky, short kamacite lamellae with an average width of 3 mm. Later grain growth of the ferrite eliminated many of the straight lamellae and created scalloped, equiaxial grains 5–25 mm across. It is also evident from the sections that, before the homogeneous transformation to Widmanstatten lamellae took place in the interior of the austenite grains, a considerable quantity of ferrite had started to grow from many nuclei upon grain boundaries, upon the silicate inclusions, and upon the very few large schreibersite inclusions, forming cellular irregular rims 5–20 mm wide. The resulting structure looks rather confusing to say the least, and the Widmanstatten pattern may pass entirely unrecognized unless large sections are available.

Considering now a single, original austenite grain it is seen that taenite and plessite occur only as minor scattered inclusions, e.g., as 3 x 1 mm degenerated comb plessite, or as still smaller pearlitic and speroidized fields. The plessite development is thus typical of the carbon-containing group I meteorite, but the are percentage is low — about 1%. In a few of th epearlitic or spheroidized plessite fields, minute carbide roses occur, as intricate intergrowths of hexonite, kamacite, taenite and schreibersite.

Neumann bands are well developed and profuse. The hardness of the kamacite phase is 185 +/- 15. Near-surface areas of many specimens show severely distorted structures with bent Neumann bands, lenticular deformation bands and brecciated and boudinaged inclusions. In these zones, which are mainly due to the atmospheric disruption, the hardness increases to 240 and above; several cold-worked kamacite areas wit the very high hardness of 280 +/- 20 were noted. Similar intensive cold defomation occurs in other shower-producing meteorites, too, see, e.g. Gibeon, Sikhote-Alin and Cape York.

Schreibersite is uncommon. Apart from the 0.1–0.05 mm wide veinlets in the former austenite grain boundaries and scattered 3 x 2 or 1 mm skeleton crystals it may be found at 0.5 mm wide rims around the silicate-troilite-graphite complexes and also as 20–50 µ wide grain boundary precipitates. Rhabdites 5–20 µ thick are ubiquitous and often displaced their own thickness along the Neumann bands. Fine phosphide precipitates, 1 µ cross, are common on the numerous subgrain boundaries of the ferrite.

Troilite is not common in the usual nodule form. It does, however, occur in varying amounts together with the silicate-graphite aggregates. These irregular masses occur as black angular inclusions and cover 4–6% by area of polished sections. They are apparently composed of silicates, graphite and troilite in varying amounts, but an estimated average frequency is 1:1:1. The troilite part is normally a polycrystalline aggregate of 5–100 µ grains of which the finest grains will be situated close to the schreibersite, while the largest are nearest the graphite. In the troilite subangluar fragments of sphalerite are dispersed. Graphite occurs as up to 10 mm patches which, in crossed Nicols, display the typical "horsetail" extinction. Numerous 1–5 µ troilite fragments are dispersed through the graphite. Subhedral crystals, 200–400 µ across, of olivine, enstatite and oligoclase are embedded irregularly in the graphite. Again 5–25 µ troilite droplets are found embedded in the silicates. It is most likely that the complex mixtures were produced by preatmospheric shocks that shattered and partially melted a previously more ordered arrangement of the inclusions.

The silicate-graphite-troilite complexes are frequently surrounded by 0.5 mm schreibersite and 0.1–0.2 mm cohenite rims, which are little affected by the shocks. Cohenite is further present as a few isolated 0.5 mm grains with small inclusions of schreibersite, but the typical cm(2) patches of cohenite crystals centrally in the µ -lamellae were not observed in any section. Cliftonite, i.e., graphite precipitated from solid solution with cubic morphology )Brett & Higgins 1967; Buchwald & Wasson 1968), occurs as scattered 100–200 µ crystals in ferrite, normally quite close to the silicate-graphite complexes. Chromite and daubreelite were reported by Bunch & Cassidy (1968) but they are evidently not too common and were not observed in this study.

Campo del Cielo is a polycrystalline, coarse octahedrite with a significant amout of silicate-graphite-troilite inclusions. It has the structure and mineral assemblage typical of the low-nickel end of the group I irons, and it is particularly closely related to Linwood. It is alsorelated to Hope, Sardis, Yardymly and Seelasgen. The characteristic angular silicate inclusions of El Taco appear to be duplicated only in Linwood and Netschaevo. The latter however, does not resemble Campo del Cielo in its metallic structure and in its chemical composition. Campo del Cielo displays Neumann bands and shock structures from some preatmospheric event. Several structural elements, such as the necking and cold-deformation visible on many specimens are, however, of much later date and are due to the violent deceleration and breakup in the atmosphere. While many specimens are corroded, some, e.g. El Taco, are in a surprisingly good state of preservation, displaying fusion crust over many square centimeters. It appears that the large individuals fell as separate masses, having had sufficiently long trajectories in the atmosphere to develop regmaglypts on a large scale. Similar cases are met with in, e.g., Henbury, Wabar and Sikhote-Alin.

 
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