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Elements M Meteoritic Minerals SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES NUMBER 3 Brian Mason Minor and Trace and A. L. Graham pi t Elements m Meteoritic Minerals SMITHSONIAN INSTITUTION PRESS CITY OF WASHINGTON 1970 ABSTRACT Mason, Brian and A. L. Graham. Minor and Trace Elements in Meteoritic Minerals. Smithsonian Contributions to the Earth Sciences, 3:1—17. 1970.—Nickel-iron, troilite, olivine, pyroxenes, plagioclase, chromite, and phosphate minerals (chlor- apatite and/or merrillite) have been separated from a number of meteorites (Modoc, St. Severin, Winona, Haraiya, Marjalahti, Springwater, Johnstown, Mt. Egerton, Soroti) and analyzed for minor and trace elements with the spark-source mass spectrometer. The elements Ni, Go, Ge, As, Ru, Rh, Pd, Sn, Sb, W, Re, Os, Ir, Pt, and Au are concentrated in nickel-iron: Se and Ag in troilite; Th, U, and the lanthanides in the phosphate minerals and in diopside; Eu, Sr, Ba, Rb, and Gs in plagioclase. Molybdenum and tellurium are concentrated in nickel-iron and troilite. The elements Ti, Sc, V, Cu, Zn, Mn, and Ga are distributed over several coexisting minerals. Official publication date is handstamped in a limited number of initial copies and is recorded in the Institution's annual report, Smithsonian Year. UNITED STATES GOVERNMENT PRINTING OFFICE WASHINGTON : 1970 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price 30 cents (paper cover) Brian Mason Minor and Trace and A. L. Graham Elements in Meteoritic Minerals Introduction Vilcsek and Wanke (1965) and Honda and Shima (1967) ; however, study of their results indicates that During the past decade a very large amount of data this selective solution is seldom completely quantitative. has been accumulated on the abundances of minor and Moss et al. (1967) have developed a selective decom- trace elements in meteorites, and adequate informa- position procedure, based on heating a meteorite tion is now available on abundance levels of practically sample in a stream of dry chlorine, which enables all elements for the different classes of meteorite. The measurement of a number of elements in metal, sulfide, distribution of these elements, however, among the in- and silicate phases. dividual phases (minerals) of the meteorites is less well The introduction of the electron-beam microprobe known. This can be ascribed in part to the unavail- has made it possible to measure the concentration of ability of large enough samples of many meteorites many elements in the individual minerals in a polished (especially of the rarer classes) for mineral separation, section of a meteorite, without the necessity of sepa- and in part to the difficulty of making clean separations rating the individual minerals by physical or chemical of the fine-grained minerals in many meteorites, par- techniques. However, the lower detection limit of the ticularly the chondrites. The chondrites are, of course, microprobe is about 100 ppm, whereas comparatively of special interest, since they represent the least frac- few elements are present in the common minerals of tionated group of meteorites, and contain many minor meteorites above that level. and trace elements in amounts comparable to solar The spark source mass spectrometer appears to be abundances. an excellent tool with which to attack this problem. It Some significant information has been obtained by is extremely versatile, being capable of detecting all the analysis of magnetic (e.g., metallic) and nonmag- elements from boron to uranium at concentrations of netic (e.g., sulfide plus silicate) fractions of chon- the order of 0.1 ppm, and considerably less for some dritic meteorites (see, for example, Onishi and elements. Sample requirements are comparatively Sandell (1956), Wardani (1957), Shima (1964), small: 100 mg is a convenient amount, but good results Fouche and Smales (1967). This procedure, however, can be obtained on samples of 10-20 mg. This is very is useful only for distinguishing the extremely sidero- advantageous for chondritic meteorites, in which many phile elements, and provides no information on the dis- elements are concentrated in phases such as phosphates tribution of the other elements within the individual (apatite and/or merrillite) and chromite, present in minerals. Refined techniques of selective solution of amounts of less than 1 percent of the bulk meteorite. the different phases have been applied to chondrites by Brian Mason, Department of Mineral Sciences, National Mu- Experimental seum of Natural History, Smithsonian Institution, Washing- ton, D.C. 20560. A. L. Graham, Department of Geophysics MINERAL SEPARATION.—As originally planned, the and Geochemistry, The Australian National University, Can- principal objective of this research was to obtain com- berra, Australian Capital Territory, 2600. prehensive data on the distribution of minor and trace 1 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES elements among the minerals of the common chon- The portion with D>3.32 was further fractionated drites. Preliminary experiments showed that to obtain using Clerici solution of controlled density. Hyper- adequate samples of accessory minerals such as chro- sthene concentrated in the 3.32-3.45 fraction, olivine mite and the phosphates it would be necessary to start in the 3.50-3.60 fraction; chromite and troilite formed with about 100 g of the meteorite. This limited the se- the fraction with D>4.0. These fractions were fur- lection to chondrites available in considerable amounts. ther processed with the isodynamic separator, yielding Since terrestrial weathering would be a complicating reasonably pure separates of hypersthene, olivine, factor, only fresh falls were considered. Ultimately two troilite, and chromite. The hypersthene and chromite were selected from the meteorite collection of the fractions were further treated with hot HC1 to remove Smithsonian Institution, Modoc (NMNH 360), a any adhering olivine and troilite. hypersthene chondrite belonging to the L6 group of The 100-200 mesh magnetic fraction, consisting of Van Schmus and Wood (1967), and St. Severin metal particles with adhering silicates, was placed in (NMNH 2608), a hypersthene chondrite (amphoter- a beaker of acetone and agitated repeatedly in an ite subclass) belonging to the LL6 group. General de- ultrasonic cleaner. The silicates were separated from scriptions of these meteorites, with bulk analyses, have the metal by magnetic separation (Modoc), and by been published: Modoc (Mason and Wiik, 1967) ; St. centrifugation in Clerici solution (St. Severin). Severin (Jarosewich and Mason, 1969). The purity of these mineral separates was evaluated A crust-free piece weighing about 100 g was cut from by microscopic examination. The Modoc fractions were each of these meteorites. The pieces were broken into uniformly purer than those from St. Severin, this being smaller fragments which were carefully crushed in a conditioned by the somewhat coarser crystallinity of hardened steel mortar until all the material passed Modoc. Notes on the individual minerals follow. through a 10-mesh sieve. Further crushing in a large Modoc olivine: about 99 percent pure; impurity agate mortar reduced all the material to —100 mesh, mainly pyroxene; some grains contain minute opaque except for coarse metal particles, which were removed inclusions, probably chromite; slight limonite staining with a hand magnet. We found the optimum size for on many grains. mineral separation was the —200 +325 mesh fraction, Modoc hypersthene: about 98 percent pure; im- except for the metal, most of which was coarser than purities are tiny inclusions of plagioclase and admixed 200 mesh, and for which the —100 +200 fraction was grains of diopside. used. Modoc feldspar: about 99 percent pure; principal The —200 +325 fraction was sedimented in a impurity is pyroxene, as tiny inclusions; a few grains large beaker with distilled water to remove dust. Metal- show a small amount of opaque dust, probably lic particles were removed with a magnet from the chromite. dried fraction, after which this fraction was separated Modoc troilite: about 98 percent pure, impurities into two portions by centrifuging in pure CH2I2 are olivine and pyroxene. (D = 3.32). The denser portion consisted largely of Modoc chromite: about 99 percent pure; impurity olivine and pyroxene, with minor amounts of troilite is mainly pyroxene. and chromite; the less dense portion contained the Modoc metal: about 95 percent pure; principal plagioclase and phosphate minerals; both portions still impurities are olivine and pyroxene. contained a considerable quantity of composite grains. Modoc phosphate: about 99 percent pure; impuri- Pure plagioclase was obtained by extracting a frac- ties are tiny inclusions of olivine, pyroxene, and plagioclase. tion with D<2.70 (centrifugation in CH2I2-acetone mixture), purification of this fraction with the Frantz The impurities in the St. Severin separates were Isodynamic Separator, and boiling the final separate essentially the same as the corresponding Modoc sepa- in HC1 to remove any adhering acid-soluble minerals. rates, but the estimated purity was lower, viz, olivine, The phosphate minerals were concentrated in the 97 percent; hypersthene, 95 percent; feldspar, 96 per- fraction with D = 3.08-3.20, followed by passage cent; troilite, 94 percent; chromite, 98 percent; metal, through the isodynamic separator at full power, the 90 percent; phosphate, 98 percent. phosphate being entirely nonmagnetic. Only small In order to provide
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