ABSTRACT: We studied five new Antartic achondrites, MacAlpine Hills (MAC) 88177, Yamato (Y)74357, Y75274, Y791491 and Elephant Moraine (EET)84302 by mineralogical techniques to gain a better understanding of the mineral assemblages of a group of meteorites with an affinity to Lodran (stony-iron meteorite) and their formation processes. This group is being called lodranites. These meteorites contain major coarse-grained orthopyroxene (Opx) and olivine as in Lodran and variable amounts of FeNi metal and troilite etc. MAC88177 has more augite and less FeNi than Lodran; Y74357 has more olivine and contains minor augite; Y791491 contains in addition plagioclase. EET84302 has an Acapulco-like chondritic mineral assembladge and is enriched in FeNi metal and plagioclase, but one part is enriched in Opx and chromite. The EET84302 and MAC88177 Opx crystals have dusty cores as in Acapulco. EET84302 and Y75274 are more Mg-rich than other members of the lodranite group, and Y74357 is intermediate. Since these meteorites all have coarse-grained textures, similar major mineral assemblages, variable amounts of augite, plagioclase, FeNi metal, chromite and olivine, we suggest that they are related and are linked to a parent body with modified chondritic compositions. The variability of the abundances of these minerals are in line with a proposed model of the surface mineral assemblages of the S asteroids. The mineral assemblages can best be explained by differing degrees of loss or movements of lower temperature partial melts and recrystallization, and reduction. A portion of EET84302 rich in metal and plagioclase may represent a type of component removed from the lodranite group meteorites. Y791058 and Caddo County, which were studied for comparison, are plagioclase-rich silicate inclusions in IAB iron meteorites and may have been derived by similar process but in a different body.
ABSTRACT: Y74123 is an olivine-rich, relative unshocked ureilite and contains more
interstitial pigeonitic materials than do ureilites which have been
reported previously. These materials were studied by optical microscope,
electron microprobe, SEM, high-resolution TEM, and analytical TEM to
gain a better understanding of their nature and origin. The Y790981,
with shock partial melts, has also been examined by the same techniques.
Bulk chemical compositions of the interstitial materials in Y74123 are
pyroxene-like and have higher CaO and Al2O3 contents than the large
pigeonite and olivine core. Interstitial materials at olivine-pigeonite
grain boundaries are richer in CaO and Al2O3 than those at
olivine-olivine grain boundaries. TEM observations of the interstitial
material of Y74123 show that it consists of alternating pigeonite-augite
lamellae more than 3.5-micron thick on (001). This texture suggests that
the rim material had already crystallized before the parent body
breakup. The shock-produced glassy veins in Y790981 cut through the rim
materials. These observations are consistent with the idea that the
interstitial materials in this ureilite are a mixture of residual
liquids of high Ca melts and shock-produced partial melts of olivine and
pigeonite. This mixture accumulated along the grain boundaries and some
of it is trapped within grains.
Meteoritics 08/1991; 26:195-201.
ABSTRACT: A transmission electron microscopic (TEM) study was conducted to investigate the stress-induced transformation mechanisms of pigeonites from two achondrites. In pigeonites from the Allan Hills 77257 ureilite, abundant lamellae of orthopyroxene-pigeonite intergrowth accompanied by minor amounts of blebby augite precipitates were observed. In pigeonites from the Juvinas eucrite, some stacking faults running through the (001) augite lamellae were observed although most of the stacking faults terminated at the host-lamella interface. High resolution TEM observation shows the fringe offset produced by the stacking fault in the Juvinas pigeonites. The magnitude of the mismatch parallel to  is approximately c/4, which is related to the formation of the partial edge dislocation near the host-lamella interface. The partial dislocation is imaged as an edge dislocation with an (002) extra-plane. Three partial dislocation mechanisms with distinct displacement vectors are proposed in order to explain the stress-induced transformation textures observed in the achondritic pyroxenes. Further study will be required to determine unequivocally the mechanism of atomic displacements during stress-induced transformation. However, regardless of the actual mechanism of transformation, it is not difficult to convert orthopyroxene to clinopyroxene or vice versa by this transformation from the structural point of view. Low-Ca pyroxenes are deformed without a stress-induced transformation by slip at high temperature and geologically reasonable strain rates. However, meteoritic pyroxenes can be deformed by stress-induced transformation even at high temperature because preterrestrial impact processes will produce a deformation condition with high strain rates. Deformation at high strain rates and high temperature is very important when we interpret the microtexture of meteoritic pyroxenes.
Physics and Chemistry of Minerals 01/1988; 15(3):252-259. · 1.73 Impact Factor
Meteoritics 11/1983; 18:358.
Meteoritics 11/1983; 18:403.
Meteoritics 11/1981; 16:390.
Meteoritics 11/1981; 16:362.
ABSTRACT: Polished thin sections of lunar meteorites EET87521 and Y793274,
possibly derived from a mare region of the moon, have been analyzed for
the mineral chemistries and textures. Basaltic components in both
objects contain strongly zoned Fe-Ca-rich pyroxenes like those found in
the VLT basalts. A mesostasis portion found in a basaltic clast of
EET87521 consists of ilmenite, minor Cr-Ti spinel mantled by fayalite
Fo10-prime, and fine-grained mixtures of hedenbergite and fayalite in
contact with a silica mineral. Mg-rich materials are rare in EET87521
but abundant in Y793274. The latter contains the most Mg-rich clast
found in lunar samples with orthopyroxene and ilemnite, a gabbroic clast
with finely exsolved pigeonite, olivine, and plagioclase, and pyroxene
clasts intermediate between those in the basaltic and the Mg-rich clast
types. A pyroxene fragment in EET87521 shows exsolution lamellae of
augite about 300 nm wide, which is consistent with mare basalt origin.
ABSTRACT: The new lunar meteorite MAC88105 has been studied by mineral-chemical techniques and was compared with the Yamato lunar meteorites. Clast types and pyroxene compositions of MAC88105 indicate that evidence for a pairing with any known lunar meteorites is unlikely. Clast-laden vitric breccia and comminuted mineral fragments in glassy matrix are common components as in other lunar meteorites, but granulitic clasts are not as common as in other lunar meteorites. The large and common granulite-like meta-igneous clasts still preserve lath-shaped plagioclase crystals with fine-grained olivine and pyroxenes in the interstices. Pyroxenes with mg number = Mg × 100/(Mg + Fe) between 70-48 mol% are common, but more Mg-rich pyroxenes and plutonic pyroxenes from nonmare pristine crustal rocks are rare. One basaltic clast contains pyroxenes with zoning trends more Mg-rich than the above mg range and than those of the VLT basalts common in lunar meteorites. MAC88105 preserves true glass in the matrix as observed by a transmission electron microscope (TEM). The preservation of glassy materials indicates that MAC88105 had a metamorphic annealing history different from other lunar meteorites.
Geochimica et Cosmochimica Acta.