DISTINÇÃO ENTRE TEXTURA SPINIFEX E ARRANJOS DE OLlVINA METAMÓRFICA
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Abstract:
Elongated blades of olivine in a talc-carbonate (or sulphide) matrix are formed by metamorphic processes from serpentinites, and have been misidentified as igneous spinifex texrures. The two types may be distinguished texturally and chemically. Metamorphic olivines show straight borders and the crystals overlap in poiquiloblastic habit, whereas volcanic olivines are often skeletal and crystals do not overlap. Whenever crystals are not entirely serpentinized, those of metamorphic origin will show negative elongation , and igneous crystals will show positive elongation. Fo contem is strongly controlled by the Fe/Mg ratio of the rock. Samples with elongated metamorphic olivines in a talc matrix contain approximately 40-43%MgO, as opposed to spinifex textured igneous samples which show less than 33%MgO. Ti0 2 is much lower in olivíne-talc metamorphic rocks.Keywords:
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Two types of tourmaline with igneous and metamorphic origin in metamorphic rocks of north Golpayegan, Isfahan province
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Summary Lodran is a unique meteorite consisting of roughly equal amounts of metal, olivine, and pyroxene with minor amounts of sulphide, chromite, phosphide, chrome-diopside, and a new phase with a composition close to (K,Na)AlSi 5 O 12 . Zähringer reported planetary-type rare gases in both the metal and silicates, suggesting a primitive nature. The pyroxene composition is Fs 13.8 with little variation. Olivine composition averages Fa 12.6 , but varies at least ±20 % both among grains and zoned within single grains; only the Fe-rich olivine is in equilibrium with the pyroxene. The metal probably cooled rapidly (700 K/Myr) at high temperatures and slower (30 K/Myr) at lower temperatures. Two compositional populations of chromite are found. A model for the formation of Lodran includes three steps: Formation of large olivine, pyroxene, and metal grains, with the trapping of small olivine inclusions in pyroxene and pyroxene in olivine. Equilibration and recrystallization of olivine, pyroxene, and metal, loss of alkalis and Ca; this probably occurred in a parent-body setting. And incorporation of reducing materials and mild reheating sufficient to produce the zoning in the olivine but not enough to re-equilibrate the pyroxene. Phase compositions and rare-gas concentrations in ureilites are similar to those in Lodran. In some respects Lodran appears to be a metal-rich ureilite, but the higher Fe/(Fe+Mg) ratios in the latter (Fa 21 olivine) suggest origin on separate parent bodies. The Harvard University meteorite is a mesosiderite and not closely related to Lodran.
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