Rapid methods of determining cooling rates of iron and stony iron meteorites

Two rapid and simple methods have been developed for determining the approximate cooling rates of iron and stony-iron meteorites in which kamacite formed by diffusion-controlled growth along planar fronts. The first method requires only measurements of the mean kamacite bandwidth and the bulk nickel content. The second method requires the determination of the nickel composition near the taenite-kamacite interface with an electron microprobe.     

Ungrouped iron meteorites in antarctica: origin of anomalously high abundance

Eighty-five percent of the iron meteorites collected outside Antarctica are assigned to 13 compositionaily and structurally defined groups; the remaining 15 percent are ungrouped. Of the 31 iron meteorites recovered from Antarctica, 39 percent are ungrouped. This major difference in the two sets is almost certainly not a stochastic variation, a latitudinal effect, or an effect associated with differences in terrestrial ages. It seems to be related to the median mass of Antarctic irons, which is about 1/100 that of non-Antarctic irons. During impacts on asteroids, smaller fragments tend to be ejected into space at higher velocities than larger fragments, and, on average, small meteoroids have undergone more changes in orbital velocity than large ones. As a result, the set of asteroids that contributes small meteoroids to Earth-crossing orbits is larger than the set that contributes large meteoroids. Most small iron meteorites may escape from the asteroid belt as a result of impact-induced changes in velocity that reduce their perihelia to values less than the aphelion of Mars.     

Stone meteorites: time of fall and origin

The fact that twice as many chondritic meteorites are observed falling in the afternoon as in the morning is not believed to be primarily of social origin, but to be a dynamic effect. Monte Carlo calculations show that the observed afternoon excess is not compatible with a lunar or Apollo asteroidal origin. Compatibility appears to require a source having an aphelion near Jupiter, such as could be provided conceivably by the Hilda or Trojan families of asteroids, or by short-period comets.     

Cliftonite in meteorites: a proposed origin

Cliftonite, a polycrystalline aggregate of graphite with cubic morphology, is known in ten meteorites. Some workers have considered it to be a pseudomorph after diamond, and have used the proposed diamond ancestry as evidence of a meteoritic parent body of at least lunar dimensions.We have synthesized cliftonite in Fe-Ni-C alloys in vacuumn, as a product of decomposition of cohenite [(Fe, Ni)(3)C]. We therefore suggest that a high pressure origin is unnecessary for meteorites which contain cliftonite, and that these meteorites were formed at low pressures. This concluision is in agreement with other recent evidence.     

Cohenite in meteorites: a proposed origin

Cohenite [(Fe, Ni)(3)C] is found almost exclusively in meteorites containing from 6 to 8 percent nickel (by weight). On the basis of iron-nickel-carbon phase diagrams at 1 atmosphere and of kinetic data, the occurrence of cohenite within this narrow composition range as a low-pressure metastable phase and the nonoccurrence of cohenite in meteorites outside the range 6 to 8 percent nickel can be explained. Cohenite formed in meteorites containing less than 6 to 8 percent nickel decomposed to metal and graphite during cooling; it cannot form in meteorites containing more than about 8 percent. The presence of cohenite in meteorites cannot be used as an indicator of pressure of formation. However, the absence of cohenite in meteorites containing the assemblage, metal plus graphite, requires low pressures during cooling.     

Potassium: argon dating of iron meteorites

The potassium:argon age of the metal phase of Weekeroo Station iron meteorite, determined by neutronactivation analysis, is about 10(10) years; it is similar to ages previously measured for other iron meteorites, but distinctly disagrees with a strontium: rubidium age of 4.7 X 10(9) years measured by other workers on silicate inclusions in this meteorite.     

X-ray diffraction study of minerals from shocked iron meteorites

Diffraction analysis of minerals from iron meteorites indicates a pronounced shock-induced alteration in the minerals' crystallographic character. The extent of alteration seems to be dependent on the degree of shock and can therefore serve as a measure of shock intensity. The changes appear to be due to the minerals' direct recrystallization during passage of the shock wave.     

Gas-rich meteorites: possible evidence for origin on a regolith

The asymmetry of irradiation features of grains in the Kapoeta and Fayetteville meteorites suggests irradiation on a regolith before meteorite formation. Chondrules and broken grains require approximately 10(4) years of irradiation time between formation or fracturing and compaction into the meteorite. Shock erasure of tracks from irradiated Kapoeta feldspars requires a severe shock event during or after meteorite formation.     

Equilibration temperatures of iron and magnesium in chondritic meteorites

The distribution coefficients for Fe(++) and Mg(++) were calculated from new microprobe analyses of coexisting olivine, orthopyroxene, and calcic pyroxene in chondritic meteorites. Interpretation of the data shows that (i) the equilibration temperatures were of the order of 850 degrees C, and (ii) the olivineorthopyroxene partition does not reflect ideal behavior. This equilibration temperature is much lower than previous estimates.     

Mossbauer investigation of shocked and unshocked iron meteorites and fayalite

M?ssbauer spectra of several iron meteorites have been measured by a resonant scattering technique rather than by the conventional transmission method, thereby eliminating the necessity for the preparation of thin samples. No significant differences were observed in the spectra of specimens of mechanically deformed, shocked, and unshocked iron meteorites, nor in the absorption spectra of artificially shocked and unshocked fayalite.     

Carbynes in meteorites: detection, low-temperature origin, and implications for interstellar molecules

Carbon from the Allende meteorite is not graphite but carbyne (triply bonded elemental carbon), inasmuch as on heating to 250 degrees to 330 degrees C it releases mainly triply bonded fragments: -(C identical withC)(n),- with n = 1 to 5, and -(C identical withC)(n)-CN, with n = 1 to 3. Although carbynes have been known to form only by condensation of carbon vapor above 2600 K or by explosive shock of > 600 kilobars, it is found that they also form metastably by the reaction 2CO --> CO(2) + C (solid) at 300 degrees to 400 degrees C in the presence of a chromite catalyst. Such low-temperature formation by surface catalysis may be the dominant source of carbynes on the earth and in meteorites, and a major source of interstellar carbynes and cyanopolyacetylenes.     

Chondrules in apollo 14 samples: implications for the origin of chondritic meteorites

Chondrules have been observed in several breccia samples returned by the Apollo 14 mission. These lunar chondrules are believed to have formed during a large impact event, perhaps the one that formed the Imbrian Basin. This suggests that some meteoritic chondrules are also formed by impact processes such as crystallization after shock melting and abrasion and diffusion in base-surge and fall-back deposits generated by impacts on planetary surfaces.     

Interstellar polycyclic aromatic hydrocarbons and carbon in interplanetary dust particles and meteorites

Both interplanetary dust particles (IDP(s)) and meteorites may contain material that is similar to polycyclic aromatic hydrocarbons(PAH(s)). The Raman spectra of IDP(s) and meteorites show features that are similar in position and relative strength to interstellar infrared emission features that have been attributed to vibrational transitions in free, molecular-sized PAH(s). The Raman spectra of some IDP(s) also show red photoluminescence that is similar to the excess red emission seen in some astronomical objects and that has also been attributed to PAH(s) and hydrogenated amorphous carbon. Moreover, a part of the carbonaceous phase in IDP(s) and meteorites contains deuterium to hydrogen ratios that are greater than those for terrestrial samples. Deuterium enrichment is expected in small free PAH(s) that are exposed to ultraviolet radiation in the interstellar medium. Taken together, these observations suggest that some of the carbonaceous material in IDP(s) and meteorites may have been produced in circumstellar dust shells and only slightly modified in interstellar space.     

Potassium feldspar in weekeroo station, kodaikanal, and colomera iron meteorites

Sodium plagioclase and small amounts of potassium feldspar are common constituents of silicate inclusions in the Weekeroo Station and Colomera iron meteorites; flamboyant x-ray antiperthite is unique to Kodaikanal silicate inclusions. Enrichment of potassium, sodium, silicon, and aluminum in these inclusions indicates a higher degree of chemical differentiation than in other meteorites.     

根表铁、锰氧化物胶膜对水稻铁、锰和磷、锌营养的影响

采用400目尼龙绢网制作的网袋进行土－砂联合培养的盆栽试验，研究了不同铁、锰肥用量条件下2个水稻品种（杂交稻汕优149和常规稻93－14）根表铁、锰氧化物胶膜数量及其对水稻铁、锰和磷、锌营养的影响。结果表明，锰胶膜中铁胶膜居主要地位，而锰胶膜较少（占胶膜总量的2．32％－7．54％），施用铁、锰肥明显增加铁、锰胶膜数量。胶膜上富集的磷量与铁胶膜数量呈正相关，而与锰胶膜无明显关系；胶膜上锌的富集量与铁、锰胶膜数量均无明显关系。铁、锰胶膜可以减少Fe^2 ,Mn^2 特别是Fe^2 的过量吸收，使水稻免受伤害；杂交稻汕优149根系氧化力较强，根表迎胶膜相对较厚，地上部锰吸收量低于常规稻93－14。试验还发现本试验条件下，根表铁、锰胶膜对水稻磷吸收起抑制作用，而对锌的吸收没有明显的作用。     

Stanfieldite: a new phosphate mineral from stony-iron meteorites

A new mineral, stanfieldite, Ca(4)(Mg,Fe)(5)(PO(4))(6), has been found in the Estherville mesosiderite and several pallasites: Santa Rosalia, Albin, Finmarken, Imilac, Mount Vernon, and Newport. The atom ratio Mg:Fe of this mineral varies from 1.5 in Estherville to a constant ratio of about 15 in the pallasites. X-ray, optical, and chemical data for the mineral resemble those for the only intermediate compound in the system Mg(3)(PO(4))(2)-Ca(3)(PO(4))(2).     

Osmium-187/osmium-186 in manganese nodules and the cretaceous-tertiary boundary

As a result of the radioactive decay of rhenium-187 (4.6 x 10(10) years) the osmium-187/osmium-186 ratio changes in planetary systems as a function of time and the rhenium-187/osmium-186 ratio. For a value of the rhenium-187/osmium-186 ratio of about 3.2, typical of meteorites and the earth's mantle, the present-day osmium-187/osmium-186 ratio is about 1. The earth's continental crust has an estimated rhenium-187/osmium-186 ratio of about 400, so that for a mean age of the continent of 2 x 10(9) years, a present-day osmium-187/osmium-186 ratio of about 10 is expected. Marine manganese nodules show values (6 to 8.4) compatible with this expectation if allowance for a 25 percent mantle osmium supply to the oceans is allowed. The Cretaceous-Tertiary boundary iridium-rich layer in the marine section at Stevns Klint, Denmark, yields an osmium-187/osmium-186 ratio of 1.65, and the one in a continental section in the Raton Basin, Colorado, is 1.29. The simplest explanation is that these represent osmium imprints of predominantly meteoritic origin.     

Burial rates, growth rates, and size distributions of deep-sea manganese nodules

The size distributions of deep-sea manganese nodules are consistent with a simple model of uniform growth at a few millimeters per million years and with the same probability of burial for all nodules, regardless of size. The model suggests that most nodules spend 1 million years or less on the sea floor.     

Hematite nanospheres of possible colloidal origin from a precambrian banded iron formation

Exceptionally small spheres (nanospheres) of hematite (diameters between 120 and 200 nanometers) occur in the Marra Mamba Iron Formation of the Hamersley Basin, Australia. The nanospheres are clustered into small aggregates and may have formed by structural ordering and dehydration of colloidal iron hydroxide particles. Individual spheres consist of numerous thin, curved hematite platelets surrounding a central void that is approximately half the diamter of the sphere; this texture suggests that they formed by a volume reduction of the original colloidal particles by approximately 12.5%. The occurrence of hematite nanospheres supports the hypothesis that some ofthe iron was deposited colloidally during the development ofbanded iron formations, approximately 2.5 billion years ago.