The following native metals have been identified in the Muskox intrusion: native iron, native nickel–iron (awaruite), native cobalt–iron (wairauite), and native copper. Mineral distributions and textures indicate that the native metals formed more or less contemporaneously, during the period of serpentinization of the host dunites and related rocks.Conditions during serpentinization must have been more reducing in the central and lower parts of the layered series than in the margins and upper parts of the intrusion. This is indicated by the fact that most native metals are abundant in the central regions and are essentially lacking elsewhere, even in strongly serpentinized zones. This zoning suggests that reducing conditions may have been generated internally, possibly as a result of the serpentinization process itself. The composition of the primary olivine of forsterite80–88 together with the presence of abundant secondary magnetite in equivalent serpentinites indicates that a redox reaction, olivine + water = serpentine + magnetite + hydrogen, contributed to the development of a progressively more reducing, or hydrogen-rich, fluid phase.Natural phase relations indicate that each native metal formed primarily in situ as a result of the decomposition of specific earlier formed minerals that had become unstable in the reducing environment. Native iron appears to have been formed by the reduction of magnetite; awaruite by the reduction of pentlandite; wairauite by the reduction of an unknown phase, possibly cobalt pentlandite or cobaltian pyrite; and native copper by the reduction of chalcopyrite. The feasibility of most of these reactions was confirmed by experimental studies carried out in systems open to moist hydrogen.
No abstract
Starting with the structure of (low) albite published by Taylor, Darbyshire & Strtmz in 1934, the crystal structures of a low albite and of a high albite, both nearly pure NaA1SiaO s, have been refined by means of a series of Fo and (Fo--Fc) Fourier projections parallel to all three axes.The mean bond lengths within the four non-equivalent tetrahedra are in low albite, 1-743, 1.590, 1.636 and 1.616 /i~, and in high albite 1.653, 1-639, 1.643 and 1.647 /~, with a standard deviation 0.02 A. It is concluded that in low albite the first site Sil(0) contains nearly all the A1 and that in high albite the A1 and Si atoms are randomly distributed throughout the four tetrahedral sites. The dimorphism of soda felspar is thus due primarily to differences in the degree of A1-Si ordering.Four features of the Na atom were noted: (1) In both albites the temperature factor of this atom is much greater than that implied in the f curve of Bragg & West. (2) The temperature factor is greater in high albite than in low. (3) In low albite this atom behaves as though it had an anisotropic thermal vibration with a maximum amplitude nearly along y equivalent to an atomic separation of ~ 0-1 ~. (4) In high albite the effect is similar but much more intense, equivalent to a separation of N 0.6 /~. A possible interpretation is that the Na atom occupies at random through the structures one or other of two positions, within the same large cavity, separated by N 0.1 /~ or ---0.6 A nearly along y. On this view, when the A1-Si atoms are disordered as in high albite the cavity available for Na is effectively larger than that atom can fill, whereas when the A1-Si atoms are largely ordered, as in low albite, the cavity is small enough to nearly enclose the Na atom.On the assumption that maximum stability at room temperature corresponds to local balance of electrostatic charges throughout the structure, a detailed discussion of the two albites and of sanidine and intermediate microcline leads to a number of unexpected and important conclusions: (1) In low-temperature felspars the most stable structure is not necessarily, as is generally assumed, one in which the A1-Si atoms are completely ordered. (2) The most stable potassium felspar is not 'maximum' microcline but a monoclinic (C2/m) orthoclase with A1 partially ordered into one half of the tetrahedra. (3) Intermediate microcline has an unstable charge distribution and all microclines lie outside the normal stability range of the potassium felspars, which runs from disordered sanidine to partially ordered orthoclase. (4) Most microclines probably form as the exsolution product of an alkali felspar which cooled from fairly high temperatures and which, at those high temperatures, contained a sufficient proportion of Na to confer partial ordering and the associated triclinic symmetry on the tetrahedral framework. General introductionSodium felspar (NaA1SiaOs) may exist in one of two well established modifications. The ordinary lowtemperature form, called variously low-temperature * Progress reports...
Three grains of almandine-rich garnet isolated from lunar sample 12021 have the composition Alm(70.7)Gro(25.0)Sp(2.7)Pyr(1.6), with cell edge of 11.624 angstroms and refractive index of 1.81. The garnet probably formed late in the crystallization sequence.
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