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Artificial Weathering of the Ordinary Chondrite Allegan

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Artificial Weathering of the Ordinary Chondrite Allegan American Mineralogist, Volume 82, pages II87-l 197, 1997 Artificial weatheringof the ordinary chondriteAllegan: Implications for the presenceof Cl as a structuralcomponent in akagan6ite P.A. Br,lNnrt'x S.P. Knllnyr2 F.J. Bnnnvrt J.M. Cnuoc^lNra aNo C.T. P[r-ntcBnr 'PlanetaryScience Research Institute, Open University,Walton Hall, Milton KeynesMK7 6AA, U.K. '?Departmentof Earth Sciences,Open University, Walton Hall, Milton Keynes MK7 6A,{, U K. rDepartmentof Chemistry, Open University, Walton Hall, Milton Keynes MK7 6AA, U.K. 'School of Physics,University of New SouthWales, PO. Box 1, Kensington,NSW 2003,Australia Ansrucr A sample of an ordinary chondrite fall, Allegan, was shown by sTFeMdssbauer spec- troscopy to be unweathered.Using aerateddeionized water at 25" and 0 'C, and in one case with dissolved salts added, an attempt was made to simulate meteorite weathering processesin the hot and cold desert environments where these samplesaccumulate. The progress of artificial oxidation was monitored by s?FeMcissbauer spectroscopy, and the final products were analyzedby neutron activation. The results confirm weathering mech- anisms proposed for naturally weathered meteorites and suggest that temperatureis the major factor controlling the stability of the observedoxide mineral assemblage.Akagan6ite was observed as a major oxidation product in the experimentally weatheredsample. This finding is interestingbecause it is widely acceptedthat B-FeOOH requiresCl for stability: The sample weatheredin deionized water showed only fface amounts of Cl in both the fresh and weathered meteorite. We suggestthat both Cl- and OH may be effective in filling tunnel sites and that a complete solid solution is possible between theseend-mem- bers. In addition, a calculatedformation age of 4.59 + 0.05 Ga and a cosmic-ray exposure age of 5.7-6.2 Ma, was derived from neutron activation analysesof the weatheredsample. These ages are consistentwith independentmeasurements made on samplesof fresh Al- legan, although small sample weights mean that the results should be ffeated with caution. However, the similar agesobtained in the weatheredsample indicates that a wider spectrum of cosmochemicalanalyses is possible with weatheredmeteorites than previously thought and that congruent dissolution occurs during meteorite weathering. INrnonuctIoN ering on various aspectsof meteoritecosmochemistry and To improve our understandingof weatheringprocesses also differentiate genuine preterrestrial alteration from in meteorites,we carried out artificial weathering exper- oxidation that occurred on Earth. iments in a controlled environment using samplesof the To this end, three experiments were conducted. One Allegan H5 ordinary chondrite (Allegan County, Michi- experiment to simulate weathering in the Antarctic envi- gan). Such studies potentially provide a basis for under- ronment, and two shorter experimentsto study weather- standing the mechanismsof weathering of chondrites in ing at the higher temperaturestypical of a hot desert the field (Bland et al. 1995, 1996a). Apart from stones environment. observed to fall, such as Allegan, and occasional finds, In addition, previous experimentsdesigned to simulate most meteorites in recent years have been found in two meteorite weathering do not appear to have adequately broad geophysicalenvironments: hot desert regions such reproducedoxidation in the field and indicate that Fe-Ni as the Sahara and Nullarbor Plain in Australia and the is relatively resistant to weathering (Fisher and Burns cold desert of Antarctica. Numerous rare and scientifi- 1992a).This anomalousresult suggestedthat a more in- cally valuable specimenswere recovered from these ar- tensive study might be appropiate. eas. However, the great majority of sampleshave expe- The importance of Cl in the weathering of meteorites rienced extensive weathering during their terrestrial could also be tested in our experiments. Buchwald and residency (in general this is much less in Antarctic me- Clarke (1989) suggestedthat in Antarctic meteoritesak- teorites). An improved understanding of weathering aganlite precipitatesnear the reaction front around metal mechanismswould help determine the effect of weath- grains, incorporating Cl ions into ion-exchange sites, where they are retained and made available for further * Presentaddress: Department of Earthand PlanetarySci- ences,Western Australian Museum, Francis Street. Perth, West- corrosive action or are flushed from the system. Cl is ernAustralia 6000. thought to be extracted from pore water by the elecffo- ooo3-oo4xt97I | | | 2-11 87505. 00 1187 1188 BLAND ET AL.: WEATHERING OF ORDINARY CHONDRITE chemical nature of the corrosion of Fe-Ni; it also may be fractometer over 2e : 5-30". Phaseswere detemined by essentialfor the formation of akagan6ite(Post and Buch- comparing the sampleswith data from the CD-ROM da- wald 1991). To test whether Cl is required in meteorite tabase of the Joint Committee for Powder Diffraction weathering,aerated deionized water was chosento weath- Standardsfile. Although XRD is suitable for determining er the meteorite sample in the experiment conducted at the crystalline phasesin a sample(in general,where those low temperatureand also in one of the higher-temperature phasesare >5 wtTo), iron oxides and oxyhydroxides are experiments.The other experiment, designedto simulate frequently too poorly crystalline to be easily resolvable. hot deserl weathering, used water in which NaCl had Given the small sample sizes in our study, concentrating been dissolved (approximately 0.2 molar solution) as a weatheringproducts in the form of a mineral separatewas means of providing Cl ions. In this way various factors not practical. Therefore, in this study, we use XRD to contributing to the mechanism of meteorite weathering support an oxide mineralogical determination by s?Fe (temperature,drainage, dissolved ions) could be evaluat- Mcissbauerspectroscopy. ed and their imporlance assessed. Miissbauer spectroscopy Saupl,n PREPARATIoNAND EXPERIMENTAL Samples (0.3 g) were sandwiched between adhesive PROCEDURE tape (about 1.25 cm') in a lead holder. The sTFeM6ss- Weathering experiments bauer spectra were recorded at 298 K and 77 K with a Samplesof meteorite were ground under acetonein an microprocessor-controlledMcissbauer spectrometer using agate moftar and pestle. Samples (4.0 g each) were then a sTCo-Rhsource. The drive velocity was calibrated with immersed in 20 mL aeratedwater: ice-water in the case the same sourceand a metallic iron foil. Average record- of the experiment designedto simulate Antarctic weath- ing time was 3 d per sample.The Mdssbauerspectra were (the ering sample was held in a laboratory refrigerator) fitted with a constrained non-linear least squaresf,tting and water at room temperaturefor the hot desert weath- program of Lorentzian functions. ering experiments. The hot-deserl sample immersed in Mrissbauer spectra of bulk meteorites are reasonably deionized water was dried under heat lamps every 24 h complex, as the number of Fe-containing components, to mimic periodic wetting and drying in the hot deserr particularly in weatheredsamples, is large. As such there environment. Aliquots of other samples were also peri- is a compromise between including enough components odically removed and dried. The sample immersedin salt in a fit to approximatereasonably the composition of the water (0.2 molar solution) was rinsed several times in sample, while being aware that in adding new compo- deionized water before drying completely to prevent a nents the Mdssbauerparameters of an individual absorp- build-up of NaCl by evaporation. Samplesfrom all three tion are increasingly poorly determined (Dollase 1975). experimentswere removed periodically and analyzedus- In our analysis, while absorptions in individual spectra ing sTFeMcissbauer spectroscopy. are discussed,it is felt that only by comparing spectra The use of a finely ground starting material in our ex- recorded at different velocity ranges and temperatures, periment inevitably leads to a loss of important structural and in some casesmineral separatesof the same sample, relationships(e.g., metal and sulphide) within the sample is it possible to build a picture of the oxide minerals in and a destruction of porosity, both of which have a bear- the weatheredmeteorite. ing on the types of alteration processesobserved. Varia- Iron oxide and oxyhydroxide weathering products fall tions in grain size in this type of study and the associated into two broad groups that may be easily distinguishedin variation in the available mineral surface area also have Mrjssbauer spectra: paramagnetic(two line, doublet ab- a significant effect on reactivity and dissolution rates sorptions) and magnetically ordered (six line, sextet ab- (Hochella and Banfield 1995), as does the water-rock ra- sorptions). Common ferric phases that show a doublet tio. In addition, while hot desert meteorites experience (and similar Mcissbauerparameters, in the region 6 : wetting and drying through occasionalstonns, this clearly 0.3-0.4 mms r, A : 0.5-0.8 mms-r) at 298 K include does not occur on a regular 24 h cycle. Thesefactors need lepidocrocite, small-particle goethite, akagan6ite,and fer- to be consideredwhen interpreting our experimental re- rihydrite. Magnetically orderedferric phasesat 298 K in- sults. One point in favor of this experimentalapproach is clude maghemite (H : 491 kG), magnetite (two the large body of data on the natural weathering of or- sextets at H : 491 458
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