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Rio Negro meteorite, Rio Negro, Paraná, Brazil

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Latitude & Longitude (WGS84): 26° 6' South , 49° 48' West
Latitude & Longitude (decimal): -26.1, -49.8

Ordinary chondrite, black, xenolithic (L4)
Fall, 21 September 1934; 1310 g, single stone

During the night the Rio Negro meteorite fell with accompanying loud sounds on a small farm near Rio Negro. Distinct chondrules of various sizes and textures are quite prominent in a dark and sometimes glassy matrix. Mineralogically, the meteorite consists primarily of somewhat variable olivine (~40 vol%) and frequently twinned Ca-poor clinopyroxene (~25 vol%). Minor amounts of kamacite, taenite, plagioclase, troilite, and two varieties of chromite are also present. Diopside and nepheline are present only in accessory. Bulk iron contents (Fe 21.4 wt%) and mean olivine (Fa~25) and pyroxene (Fs~19) composition are consistent with an L4 classification. A small lithic clast (5 mm) with quite variable silicates, including Fe-poor Forsterite (sensu strictu), adds some additional variety to the overall geochemical and mineralogical inventory. Glassy and darkened regions are indicative of preterrestrial shock apparently due to one or more serious impact events.

Rio Negro is a gas-rich meteorite and a CRE age of ~21 Ma was reported by Bhandari et al., (1995)

The L (relatively low in total iron) chondrites are a large group of ordinary chondrites and representing ~40% of the properly classified and witnessed meteorite falls. The L4 type is a small subset accounting for < 10% of the L group (23 documented L4 or L4-6 falls were listed in June 2016 at the Meteoritical Bulletin Database). Rio Negro is the largest of the three Brazilian L4 falls. Santa Barbara, an 1873 fall, and Porangaba, a 2015 fall, are the others.

Nota bene: The descriptions of glass and silicate variability provided by Gomes & Keil (1980) appear to be somewhat at variance with the L4 classification currently provided at the Meteoritical Bulletin Database. Disentangling the effects of ancient metamorphism from shock-induced heating is not always a routine matter when working with small samples of a heterogeneous object. The possibility of a borderline L3/4 petrologic grade may merit further investigation.

Mineral List

7 valid minerals.

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Gomes, C.B. & Keil, K. (1980) Brazilian Stony Meteorites: University of New Mexico Press: Albuquerque. pp. 162.

Bhandari, N., Castagnoli, G.C., Bonino, G. & Suthar, K. M. (1995). Solar Flare Tracks in Rio Negro (L 4) Chondrite. 24th International Cosmic Ray Conference, Vol. 4, p.1181-1183. (Jan1995).

Grady, M.M (2000). Catalogue of Meteorites (5/e). Cambridge University Press: Cambridge; New York; Oakleigh; Madrid; Cape Town. 689 pages.

Dunn, T.L., Cressy, G., McSween Jr, H.Y. & McCoy, T.J. (2010) Analysis of ordinary chondrites using powder X-ray diffraction: 1. Modal mineral abundances. Meteoritics & Planetary Science 45(1):123-134. (Jan 2010).

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