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Met Name

Met Type

Thin Sections

1938 gram full slice.   L6

TKW 104.8 kg. Fall not observed. Found 1954, Colfax County, New Mexico, US.



Steve writes:

Photos

1: Impact Melt Single Lithology Full Slice
2: Impact Melt Single Lithology Full Slice (angled)
3: Vug Enlargements
4: Neophot Photo Collage
5: Impact Melt Dual-Lithology Full Slice
6: Impact Melt Dual-Lithology Full Slice (angled)
7: Impact Melt Vein
8: Vug Metal Rotations

Chico’s story began when a large impact event shocked and melted portions of the L chondrite parent body¹, ejecting a stone that some 500 million years later would be discovered in the northeastern plains of Colfax County in New Mexico. In early January of 1954, Charles Langley came across the meteorite almost completely buried in the rocky soil of his ranch (which, because of its proximity to the nearby Chico post office, is how the Chico meteorite was named). Two months later Langley had the 231 pound, 11 x 15 x 16 inch stone excavated and brought to his ranch house where he broke off a piece and sent it off for analysis. Unfortunately the broken piece was actually caliche and consequently completely lacking in nickel-iron; that, combined with Chico’s exposed vesicles and obvious melt that gave it an appearance resembling common volcanic rock, resulted in the meteorite initially being considered terrestrial.

Lincoln LaPaz, the founder of the University of New Mexico’s Institute of Meteoritics, acquired the stone shortly afterwards and recognized it as a meteorite. Initial studies were done at the Institute of Meteoritics, but several years later it was sent to NASA where Chico was halved using an experimental water cannon (see Edwin Thompson’s 8/8/2012 MPOD submission). Then to further sample Chico’s interior, seven blind core holes were drilled at various places partway into one of the halves and nine more partially into the other, leaving two stones that were described as outwardly resembling Swiss cheese. UNM eventually got back both halves and Chico – with some internal areas still exhibiting undisturbed chondrules – was initially classified as an ordinary L6 chondrite; later it was properly recognized as an L6 impact melt breccia, though the Meteoritical Bulletin Database’s classification was never subsequently updated.

Photos 1 and 2 depict a full slice with only the impact melt lithology (the polish is quite glossy, so pictures were taken at two angles to help better show its characteristics). This part of the meteorite contains vugs ranging in size from nearly microscopic to a full two centimeters across. Some are partially or wholly filled with bright sulfide inclusions (they appear falsely copper-colored in Photo 1), others with metal, and still others with both. A few even show apparent accretion veins through them. Other vugs exhibit stalactite-type silicate formations or, in some cases, similarly shaped metal growth. Still others appear to be coated with druze-like features that terrestrially are only formed in the presence of water. Photo 3 provides magnified views of multiple vugs in varying sizes that exhibit several of the aforementioned characteristics. And several years ago Tom Phillips imaged portions of the accreted minerals at 345x using his Zeiss Neophot inverted reflecting microscope. With Tom’s permission, four of his images were used to create the collage of Photo 4 (Tom took these with cross-polarized light to control glare without significantly altering the natural colors).

Chico exhibits several interesting textures attesting to its violent history of being struck, melted, and then cooled again. Some of its metal appears as microscopic blebs that give areas of the stone a metallic sheen. The melted mineral portion of the meteorite runs through the stone as twisting bands of glassy rock. The slice in Photos 5 and 6 demonstrates both of Chico’s distinct lithologies: a brecciated L6 and the impact melt where the original rock material was plasticized and began to flow along a boundary of differential cooling. The lighter-colored portions in this melt area denote regions where the metal began to differentiate from the stone matrix, while the darker portions did not melt and the rock and metal are still mixed. In Photo 7, vugs in one lithology are seen to have moved along with the melt on one of bifurcated veins – a feature quite common in terrestrial impact melts and lava flows. The fracture in the uppermost melt band and the S-shaped fissure at the right edge in Photo 7 are two of many accretion veins throughout the meteorite. Straight fissures are also present that rise perpendicular to the flow direction and may have been vents for escaping gas. Some of these characteristics have been described as resembling crater fills seen here on Earth, suggesting Chico may possibly represent a similar deposit on its parent asteroid².

Vugs are plentiful in Chico’s melt. Many are circular, though some have been squeezed into elongated shapes. Several appear preferentially lined with metal. A superficial explanation is that the meteoroid was rotating and centrifugal force deposited the still liquefied metal onto the vugs’ edges farthest from the axis of rotation. In that scenario one would expect the deposits to be oriented the same way, yet while the left group in Photo 8 is more generally aligned, the right group is decidedly not. The latter has an obvious swirl in its melt flow, but changes in direction don’t fully explain either group’s radically different orientations across vugs in very close proximity to each other. Another theory suggests metal was deposited as a consequence of gas pressure within the vugs, which depending on a number of assumptions could better explain the observed variations. Still another suggests the metal cooled at different times, so relative changes in the direction of centrifugal force from a wobbling body would cause the nickel-iron to deposit at inconsistent angles. Personally I wonder whether a cross-sectional view of the metal in a single slice through multiple non-coplanar vugs might unfairly represent some vugs’ actual areas of highest metal concentration (which is the basis in Photo 8 for deciding each arrow’s direction); 3D mapping of the metal within full vugs might paint a more consistent picture.

[As an interesting aside, the orientations of the sulfides in vugs of an Antarctic impact melt breccia appear to suggest an influence from the gravity of its LL chondrite parent³. Chico has a number of vugs showing similar preferential growth, which begs the question as to whether a similar study would point to a gravitational influence from its L chondrite parent?]

Slices of Chico became publicly available when Edwin Thompson of E.T. Meteorites was able to acquire each of the original halves in different UNM trades (one 23 years ago and the other 16 years later). Because the meteorite was largely rectangular in shape, most full slices were similarly sized. Only a few were successfully cut from the limited gaps between opposing core holes, while the rest exhibited these exploratory artifacts (an example of the latter is the dual-lithology slice on display at the American Museum of Natural History⁴). But during cutting the deep internal fractures like those seen in Photos 1 and 2 caused many slices to crack, resulting in still relatively large broken pieces that, because of accretion material on the fractured edges, look very much like smaller versions of complete slices. Accordingly, while actual full slices are extremely rare, some very nice smaller pieces of Chico occasionally become available for collectors that enjoy the unusual and varied characteristics of this meteorite.


¹ journals.uair.arizona.edu/index.php/maps/article/view/14566/14536

² www.meteorite-times.com/Back_Links/2002/July/Meteorite_of_Month.htm

³ onlinelibrary.wiley.com/doi/10.1111/j.1945-5100.2011.01187.x/full

⁴ www.amnh.org/exhibitions/permanent-exhibitions