Pyroxene exsolution microstructures in garnet from the Almklovdalen peridotite, SW Norway
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Peridotite
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Amphibole
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Abstract Amphibole in the lower parts of the Lilloise layered intrusion occurs interstitially and as a replacement of pyroxene; in the upper rocks it is a major cumulus phase. There is an overall trend of increasing Fe/(Fe + Mg) with height. Coupled substitutions which effect the variation in composition of the amphiboles are chiefly Na,K( A )+Al( T ) for □ A +Si T ) and Ti+Al( T ) for Fe 3+ ( C )+ Si( T ). There is considerable variation in composition both on the specimen scale and within individual grains. This variation, plus scatter found in plots of the coupled substitutions, is partly attributed to many of the amphiboles having replaced pyroxene and also to the effects of magmatic-hydrothermal fluids.
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A wide variety of intergrowth microstructures have been observed with high-resolution transmission electron microscopy in eight different pyroxene specimens that have been partially or wholly altered to other minerals. The product phases ofthe alteration reactions include amphibole, clinojimthompsonite, chain-width disordered pyribole, and several sheet silicates. Textural considerations indicate that there are a number of different paths for pyroxene hydration reactions. These include the simple paths pyroxene --+ amphibole, pyroxene -+ clinojimthompsonite, and pyroxene -+ sheet silicate, as well as more complicated, stepwise paths, such as pyroxene -> amphibole -+ sheet silicate and pyroxene ---> clinojimthompsonite -+ sheet silicate. In some cases, multiple reaction paths are observed in the same specimen. The microstructures indicate that in addition to multiple paths for reaction, there may be multiple mechanisms by which a specific reaction may occur. For example, replacement of pyroxene by amphibole or other hydrous pyriboles can take place either by the nucleation and growth of narrow lamellae of the product mineral, or by a bulk replacement mechanism along a broad reaction front. Replacement of pyroxene by amphibole takes place in such a way that multiple nucleation events may result in several diferent types of out-of-phase boundaries in the product amphibole. The distinction between exsolution and alteration reaction as a mechanism for the formation of narrow amphibole lamellae in pyroxenes is chemical, rather than structural. The determination of which mechanism has operated must therefore be based on chemical and textural arguments. It is concluded that all cases of pyroxene replacement by amphibole that have been reported are at least consistent with an alteration mechanism, while the textures occurring in some specimens are clearly inconsistent with an exsolution mechanism. With presently available data it is not possible to identify the physical and chemical conditions that lead to specific types of reaction behavior. Failure to recognize the presence of finely intergrown hydrous pyriboles in pyroxene could lead to significant errors in the application of geochemical techniques relying on cation partitioning, such as geothermometric and geobarometric methods utilizing pyroxene chemistrv.
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Amphibole
Aegirine
Pyroxene
Carbonatite
Metasomatism
Glaucophane
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Peridotite
Pyroxene
Enstatite
Pyrope
Ultramafic rock
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From the microcosmic view,this paper tried to find out the features about how pyroxene and amphibole crystallized,their optical properties,their alteration and identification marks.The research provided a relatively credible evidence for appraising pyroxene and amphibole accurately under the microscope.
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The Journal of the Japanese Association of Mineralogists Petrologists and Economic Geologists (1980)
Studies on the intergrowth and the structural relation between pyroxene and amphibole are summerized.Morphorogical intergrowth of amphibole in pyroxene has been discribed by many investigators from igneous and metamorphic rocks. Clinoamphibole overgrows on clinopyroxen e having a common b-axis to that of the pyroxene. In this case, a-and c-axes of clinoamphibole and pyroxene, respectively, are nearly or exactly parallel to each other, when the crystallographic axes of clinoamphibole and clinopyroxene are chosen in such a way that the space groups of clinoamphibole and clinopyroxene are I2/m and C2/c, respectively. Clinoamphibole structure (I2/m) is related to clinopyroxene structure (C2/c) by a displacement of I/2 (a+c) along (010) plane.Oxyhornblende are occasionally made up of two different phases, clinoa mphibole and clinopyroxene arrenged as lamellae parallel to (010) in submicrodomains. The structural relationship between clinoamphibole and clinopyroxene as above is comfirmed in this cases.Experimental studies on the thermal change of Ca-amphibole concluded that Caamphibole transforms into oxyhornblende composed of clinoamphibole and clinopyroxene phases.Amphibole lamellae have been observed in pyroxenes from basic plutonic rocks and peridotite. A kind of planar defects parallel to (010) in clinopyroxene, related to the formation of clinoamphibole lamellae, are found. From the chemical characteristics and textural relation, these amphibole lamellae are considered to have been formed by exsolution from pyroxene. A possibility of finite solid solution between amphibole and pyroxen is pointed out.
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Amphibole is a common hydrous mineral in mantle rocks. To better understand the processes leading to the formation of amphibole-bearing peridotites and pyroxenites in mantle rocks, we have undertaken an experimental study reacting lherzolite with hydrous basaltic melts in Au-Pd capsules using the reaction couple method. Two melts were examined, a basaltic andesite and a basalt, each containing 4 wt% of water. The experiments were run at 1200°C and 1 GPa for 3 or 12 h, and then cooled to 880°C and 0.8 GPa over 49 h. The reaction at 1200°C and 1 GPa produced a melt-bearing orthopyroxenite-dunite sequence. The cooling stimulates crystallization of orthopyroxene, clinopyroxene, amphibole, and plagioclase, leading to the formation of an amphibole-bearing gabbronorite–orthopyroxenite–peridotite sequence. Compositional variations of minerals in the experiments are controlled by temperature, pressure, and reacting melt composition. Texture, mineralogy, and mineral compositional variation trends obtained from the experiments are similar to those from mantle xenoliths and peridotite massif from the field including amphibole-bearing peridotites and amphibole-bearing pyroxenite and amphibolite that are spatially associated with peridotites, underscoring the importance of hydrous melt-peridotite reaction in the formation of these amphibole-bearing rocks in the upper mantle. Amphiboles in some field samples have distinct textual and mineralogical features and their compositional variation trends are different from that defined by the melt-peridotite reaction experiments. These amphiboles are either crystallized from the host magma that entrained the xenoliths or product of hydrothermal alterations at shallow depths.
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Xenolith
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Amphibole
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