Electron microscopy study of the iron meteorite Santa Catharina

Abstract

Abstract— Characterization of the microstructural features of the metal of the Santa Catharina meteorite was performed using a variety of electron optical techniques. Sample USNM#6293 is chemically homogeneous on the micron scale and has a Ni content of 28.2 wt.%. Its microstructure is similar to that of the Twin City ataxite and contains clear taenite II, i.e., fcc taenite with domains of tetrataenite, < 10 nm in size. Sample USNM#3043 is a more typical Santa Catharina specimen with dark and light regions as observed with the light optical microscope. The dark regions are inhomogeneous and contain 45–50 wt.% Ni and 7–12 wt.% O. The light regions are homogeneous and contain 35 wt.% Ni and no detectable oxygen. The microstructure is that of cloudy zone, i.e., islands of tetrataenite, ∼20 nm in size, in a honeycomb matrix. The honeycomb phase contains Ni rich oxide in the dark regions and contains metal, fcc taenite, in the light regions. The original metal structure of USNM#3043 is cloudy zone which formed during cooling into the low temperature miscibility gap of the Fe‐Ni phase diagram. The dark regions were developed from the metal by selective corrosion of the honeycomb structure, transforming it into Ni containing oxides, possibly non‐stoichiometric Fe2NiO4 while retaining the tetrataenite islands. Using the results of this study, many of the existing discrepancies concerning the microstructure of Santa Catharina can be explained.

Microstructures of metal grains in ordinary chondrites: Implications for their thermal histories

“…It probably contributes to stabilize the low-Ni phase in the fcc structure, as expected by Albertsen et al (1980) and Rancourt and Scorzelli (1995). It should also be mentioned that afcc taenite honeycomb phase has been also observed by Zhang et al (1990) in the Santa Catharina iron meteorite. The Ni content was not measured directly but the composition was estimated as ap-15% Ni.…”

A Mössbauer effect study of the Soledade meteorite

“…Iron is appearing in the austhenitic phase and alloyed with nickel. An analysis of the Mössbauer spectra at room temperature indicates that the Fe-Ni phase is homogeneously distributed in the matrix, although variations in the composition between different regions are observed.Since the early studies of the microstructure and chemical composition of meteorites the formation of magnetic phases has attracted the attention of metallurgists [1][2][3][4][5][6][7][8][9][10][11]. Most of the metallic specimens presented high contents of nickel and iron as major constituents, and thus the Fe-Ni alloys formed under such special conditions have been the subject of several investigations with a variety of experimental techniques.…”

Barbianello: An ungrouped nickel‐rich iron meteorite found in Italy

“…Barbianello meteorite is characterized by the presence of distinct dark and light regions similar to those described in high-Ni Santa Catharina iron meteorite, namely in sample USNM #3043 (Zhang et al 1990). However, while their structures are remarkably similar, chemical composition of dark and light regions in Barbianello and Santa Catharina differ significantly.…”

“…In Santa Catharina, light regions contain 35 wt% Ni and no detectable oxygen, while dark regions contain Ni 45-50 wt% and oxygen 7-12 wt%. In Barbianello, light regions contain Ni 27.7 wt%, a chemical composition that is similar to the lower-Ni Santa Catharina (e.g., sample USNM #6293; Zhang et al 1990), and dark regions contain Ni 40-58 wt% with a sum of measured major elements ranging from 79 to 96 wt%. If all the missing element were oxygen (not measured by EMPA, but semi-quantified by EDS analyses) it would range between 21 and 4 wt%.…”

“…Dark regions are inhomogeneous with Ni 40-58 wt% and up to 20% oxygen (calculated). The dark regions show compositional trends and textural relationships indicating that, unlike in Santa Catharina, dark regions do not simply represent secondary structures formed by terrestrial alteration superimposed to the light regions (Zhang et al 1990), but light and dark regions have significant, primary, bulk compositional differences.…”