MORRO DO ROCIO
BRAZIL - SC
Anomalous Equilibrated Ordinary Chondrite H5 because it is rich in silica, which is very rare to be found in chondrites.
Macroscopically, in broken or sawed surfaces, MdR is a rather homogeneous microbreccia with only a possible suggestion of a light-dark brecciated structure. It has a medium gray color with fairly frequent but not very distinct chondrules, which break with the matrix. Thus it meets the criteria for the classic definition of "gray chondrites". The polished surfaces show a normal abundance of metal, for H, hight iron, common type chondrites. microscopically, in thin sections, MdR resembles many H5 chondrites. The usual types of chondrules occur barred olivine, radiating pyroxene, (and/or olivine) and porphyritic, some with "skeletal" or probable corroded olivines. One chondrules contains free silica, apparently tridymita, in small spherules with diopside. The overall texture of MdR exhibit the integration of chondrules and coarsely-crystalline matrix. The texture integration, the ocorrence of orthorrombic low Ca pyroxene only and the devitrified chondrule mesostasis glasses place MdR in the petrographic grade 5 of Van Schmus & Wood (1967). The feldspar component occurs mainly as microcrystalline aggregates in these mesostasis. Only a few clear intersticial feldspar grains were found.
The main silicates, as usual, are olivine and pyroxene in approximately equal amounts. The latter are mostly orthopyroxene, sometimes rimmed by diopside. Accessory silicate phases are plagioclase, SiO2 (tridymite?), SiO2-rich and SiO2-poor glasses and rare K-feldspar. In addition, chromite and another Cr-bearing spinel are present. Olivines have essentialy constant composition Fa17.8 +/- 0.3. Pyroxenes show somewhat larger variations than do the olivines, but still have nearly constant composition with a few striking exceptions. Na and K can vary in different spots of the same chondrule mesostasis, although the majority of analyses cluster around the mean oligoclase composition of H chondrite feldspar.
The polished surfaces show an abundance of FeNi metal, which is characteristic for chondrites of the chemical H group, in addition to the chemical analyses that confirm this classification. The integration of the texture, the occurrence of only low Ca orthorhombic pyroxene and the mesostatic glass and the devitrified chondrules place Morro do Rocio in petrographic type 5 by Van Schmus & Wood (1967). The feldspar component occurs mainly as microcrystalline aggregates in these mesostasis. Only a few light grains of interstitial feldspar were found. More details about the meteorite can be found in the article that was the source, with access to the following link: http://articles.adsabs.harvard.edu//full/1985Metic..20..467F/0000471.000.html. Source: Fredriksson & Wlotzka (1985).
Fedriksson & Wlotzka (1985)
Little is known about the history of Morro do Rocio, except the location where it was found, which is Santa Catarina, on the same island of São Francisco do Sul where the famous Santa Catharina meteorite was found and practically totally sold as if it were a mine of nickel. Because of this fact and the name of the meteorite being from the same location as Morro do Rocio, where some samples of the famous meteorite were found, it gets to be confused as a synonym for Santa Catarina and even appeared in the British Museum Catalog as such. It is also known that it was known before 1928. If it were not two totally different meteorites, Santa Catarina being an ataxite iron meteorite and Morro do Rocio a chondrite stony meteorite, one could even suppose it is the same meteorite. It is not known how three samples of these meteorites, weighing a total of 359g, ended up in Belgrade. It is only known that a small sample of 9.5g was purchased from the Huss collection by the extinct Max-Planck-Institute of Mainz and was an almost complete sample covered with melting crust. Source: Fredriksson & Wlotzka (1985).
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