POTENTIOSTATIC STUDY OF IRON METEORITE CORROSION

A potentiostat was used to study the electrolytic corrosion of iron meteorites in a neutral solution. Low current densities were chosen so that the observed potentials would more closely approximate the theoretical Nernst values. Iron, nickel, and cobalt ions, the products of corrosion, were soluble in the electrolyte solution, and were determined after each electrolysis by atomic absorption spectrophotometry. Kamacite and taenite dissolved as individual phases, with kamacite dissolving preferentially. Cobalt dissolved along with iron and nickel from each phase. There is a direct relationship between nickel content and the potential at which a meteorite first starts to dissolve; the higher the nickel content, the more resistant the meteorite is to corrosion. None of the six meteorites observed started to dissolve at a lower potential than pure iron, nor at a higher potential than pure nickel     

THE COMPOSITION OF IRON METEORITES: A STUDY BY FACTOR ANALYSIS

A factor analysis has been performed on nickel and trace element data for iron meteorites. The technique shows that the present distribution of these elements is the result of three processes. These can be identified from the elements involved:

• 1 Ga, Ge, Sb and Zn (condensation and accretion).
• 2 Ni, Pd, Co and Cu (oxidation and sulphuration).
• 3 Ir, Au, As, Re, Pt, Os, Ru and Cr (an igneous event).

The distribution of Mo, however, is not readily explicable in terms of these processes. Within the groups IAB and IIAB only one process is required for all elements, but in groups IIIAB and IVA the situation for Ga, Ge and Sb is more complex.     

TWO NEW IRON METEORITES FROM WESTERN AUSTRALIA

Two new iron meteorites from Western Australia are described: Cosmo Newberry — a 2.156 kg meteorite of unusual spiky shape, and Gnowangerup — a 33.6 kg pear-shaped meteorite. X-ray fluorescence spectrometry shows that Cosmo Newberry can be classified in Group IIA, whilst Gnowangerup is a member of Group IIIAB. Neither iron can be associated with any other Western Australian meteorite.     

THE DERRICK PEAK,ANTARCTICA, IRON METEORITES

Sixteen iron meteorite specimens, Derrick Peak A78001 through DRPA78016, were recovered from the slopes of Derrick Peak, Antarctica, in 1978. Recovery from a relatively small area combined with their strikingly similar appearances suggest they represent a single fall. The metallographic structure of the two specimens that have been cut is dominated by regions of swathing kamacite enclosing coarse schreibersite and schreibersite-troilite inclusions. Patches of coarsest octahedrite Widmanstätten structure are interspersed. Specimens DRPA78008 and DRPA78009 have been analyzed: 6.64, 6.59 wt % Ni; 0.51, 0.46 wt % Co; 0.35, 0.34 wt % P. The specimens are coarsest octahedrites of chemical group IIB.     

DETERMINATION OF Ni,Ga, AND Ge IN IRON METEORITES BY X-RAY FLUORESCENCE ANALYSIS

Although neutron activation analysis for trace Ga and Ge is more sensitive and possibly more accurate, X-ray fluorescence analysis is quicker, uses readily available equipment, and is non-destructive. It is shown that Ga, Ge, and Ni can be determined by X-ray fluorescence on metallographic polished mounts sufficiently accurately for classification according to Wasson's chemical groups. Results are given for 45 irons, including some not previously classified     

IRON METEORITES OF GROUP IA AND IIA: STUDY OF PHOSPHIDE MORPHOLOGY ENHANCED BY DRY CI2 ETCHING

A method is described for etching iron meteorites by heating at 350 °C in dry chlorine gas. Iron is volatilized from low-nickel iron phases, e.g. kamacite, and the residual non-volatile chlorides are removed by solution. The study of the morphological character of the exposed phosphides on 16 specimens from Group IA and IIA showed no distinctive habit restricted to either group.     

THE MINERALOGY OF METEORITES

During the past decade the number of minerals recognized in meteorites has doubled, from about 40 in 1962 to over 80 in 1972. The great expansion in our knowledge can be largely ascribed to the introduction of the electron-beam microprobe as a research tool, enabling the quantitative analysis of microscopic grains in polished sections. While most of these discoveries are of minerals present in minute amounts, their identification has elucidated many aspects of meteorite formation. Of particular interest are five phosphate minerals, three of them unknown in terrestrial rocks; a chromium nitride and a silicon oxynitride; lonsdaleite and chaoite, new polymorphs of carbon; ringwoodite and majorite, the spinel and garnet analogs of olivine and pyroxene respectively; a number of calcium- and aluminum-rich silicates in the Allende meteorite, a Type III carbonaceous chondrite which fell in 1969; and several alkali-rich silicates found as inclusions in iron meteorites. Knowledge of the compositional range of the common minerals olivine, pyroxene, and plagioclase has also been greatly increased by recent researches     

INTEROBJECT STRUCTURE OF UNGROUPED IRON METEORITES AS REVEALED BY ADVANCED CLUSTERING: METHODS AND CHEMICAL FEATURES

Using the MASLOC clustering strategy, 61 ungrouped iron meteorites are probed for physico-chemical clusters. The existing classification of 13 resolved chemical groups is used as an internal coherence reference. Results include tentative establishment of a new group: ID (7 members), with chemical features similar to those of the non-magnetic groups IAB and IIICD. Six additional pairings, all with magmatic chemical signatures, are also recognized.     

THERMOLUMINESCENCE AND THE TERRESTRIAL AGE OF METEORITES

The use of thermoluminescence (TL) to determine the terrestrial age of meteorites is investigated. It is found that meteorites can be divided into two groups. One group, in which members lose their low temperature TL rather rapidly (the “low retentivity” group), may be dated up to about 100 years after fall, although with little accuracy. The other (the “high” group) is more retentive, and may still be dated several hundred years after fall. A meteorite of unknown date of fall may be assigned to the high or low group by laboratory determination of the rate of decay of the low temperature TL. Weathering coats the grains with limonite and lowers the intensity of the TL. The percentage reduction is constant for various intensities, but the peak height ratio is changed. Therefore, for weathered specimens, a method which examines the decrease in the intensity of a single peak is preferred to one which depends upon peak height ratios: this is made possible by artificially irradiating the meteorites. The following terrestrial ages for finds were obtained: Plainview 225–300 years; Dimmitt 280–330 years; Calliham 350–400 years. Bluff, Etter, Potter, Shields and Wellman (c) proved to be too old to be dated by our methods (≥ 500 years). None of the low group finds available to us proved to be young enough to be dated precisely. Terrestrial ages indicate an extremely low efficiency of recovery (≤ 1%) for meteorites that are not seen to fall. Artificially irradiating the meteorites also revealed the fact that 9 of our 19 meteorites were saturated with respect to thermoluminescence when they entered the atmosphere, and therefore that a technique based on this phenomenon would not be applicable to such specimens to obtain their cosmic ray exposure age.     

MAGNETISM OF METEORITES: A REVIEW OF RUSSIAN STUDIES

We present here a summary review of the work of Russian scientists, primarily Gus'kova and Pochtarev, on the magnetism of meteorites. They have measured the initial natural remanent magnetization and the magnetic susceptibility in more than 900 meteorite samples from collections throughout the Soviet Union. More sophisticated studies, involving both thermal and alternating field demagnetization experiments, were also conducted on a few samples. Meteorites almost invariably retain evidence of ancient magnetic fields in their pre-terrestrial history     

CHEMICAL COMPOSITION AND CLASSIFICATION OF 19 YAMATO METEORITES

The elements Na, Mg, Al, Si, S, K, Ca, (V), Cr, Mn, Fe, Co and Ni have been determined in 19 Yamato meteorites by spark source mass spectrometry. For comparison the chondrites Allan Hills 7603, Mt Baldr (b) and Holbrook and the achondrites Johnstown, Pasamonte and Stannern also have been analyzed by the same method. By virtue of their chemical composition the Yamato meteorites 74002 and 74144 prove to be ordinary chondrites of type L; 74001, 74103, 74155 and 74156 are ordinary chondrites of type H; 74662 is a carbonaceous chondrite; Yamato 74010, 74011, 74016, 74037, 74097, 74125 and 74136 are diogenites; Yamato 7308(1) is a howardite; and Yamato 74450 is a eucrite. This agrees with earlier classifications based on petrological and mineralogical arguments (Nagata, 1978; Motylewski, 1978). For the chondrites Yamato 74002, 74106, 74144 and the diogenite 74125, however, no previous classifications could be found in the literature. In a Mg-Al diagram the eucrites, the howardites, the diogenites and the ureilites fall into characteristic fields. This enabled not only the classification of the Yamato achondrites investigated in this paper but also confirmed the previous identification of Yamato 74123 as a ureilite (Hintenberger et al., 1978). A very high chromium content is characteristic of some Yamato diogenites, especially Yamato 74037 (3.4%). Chromium and vanadium are positively correlated in the achondrites investigated.     

A CENSUS OF THE METEORITES OF ROOSEVELT COUNTY,NEW MEXICO

In the five years from June, 1967, to June, 1972, a total of 99 meteorites were found in Roosevelt County, New Mexico and in adjoining Curry County. Of this number, 74 were found by one man. The finds include two achondrites, one pallasite, one carbonaceous chondrite, and 95 chondrites. They appear to represent more than 50 separate meteorite falls. The finding of a large number of meteorites in a small area provides data for an estimate of the probable quantity and average size of the meteorite specimens reaching the earth. The problems involved in allocating a total of 17 available local place names among more than 50 meteorite falls are discussed.     

INFRARED (JHK) PHOTOMETRY OF METEORITES AND ASTEROIDS

We present JHK colors observed for ten asteroids and synthesized JHK colors for seven meteorite groups, samples of iron and nickel metal, pyroxene, olivine, feldspar, a lunar anorthite and some terrestrial mineral samples. Pronounced differences are apparent between the chondritic and achondritic meteorite classes; the chondritic classes show less subdued trends in J-H color which reflect their metamorphic grade We find small but significant differences between the JHK colors of the predominant C and S classes of asteroids. All JHK colors of asteroids observed here fall within the limited domain defined by the various chondritic and iron-rich meteorites but are strikingly different from those of most achondritic meteorites