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1.
作者选择成分不同、产状各异的四个斜方辉石样品,分别沿其[100]方向进行了电子衍射分析。通过对实验结果的认真分析和充分论证,作者断定它们的空间群均为P2_1ca,而非Pbca。由于实验样品在成分和产状上均具有广泛的代表性,因此可以推断,在常温常压下,自然界的斜方辉石之空间群均为P2_1ca。 相似文献
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斜方辉石的反演畴结构对其衍射效应的影响 总被引：1，自引：0，他引：1
斜方辉石内部存在着众多的相邻间呈反演关系的(100)晶畴。这些晶畴的存在使斜方辉石的K为奇数的okl衍射之强度明显减弱,且参与衍射的晶畴的数目越多,其减弱的程度也就越大。这一现象在一定程度上掩盖了斜方辉石P2_(1ca)的真实对称性。与X射线衍射相比,利用电子衍射来研究斜方辉石的空间群类型具有明显的优越性。 相似文献
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甘肃金川Ⅱ号岩体辉石化学特征及其地质意义 总被引：3，自引：0，他引：3
金川Ⅱ号岩体主要由二辉橄榄岩和硫化物橄榄岩构成,单斜辉石和斜方辉石为岩相中主要造岩矿物,单斜辉石含量一般高于斜方辉石.矿物间的结构关系表明主要造岩矿物的结晶顺序为:橄榄石-斜方辉石-单斜辉石-斜长石.单斜辉石的TiO2,Al2O3,Cr2O3和Na2O质量分数依次为:0.29%～1.26%、3.17%～3.59%、0.86%～1.14%、0.29%～0.57%,斜方辉石的TiO2,Al2O3,Cr2O3和Na2O质量分数普遍低于单斜辉石,依次为:0.16%～0.40%、1.85%～2.38%、0.48%～0.68%、0.03%～0.10%.元素间的相关关系显示辉石的类质同像置换方式为:M1Fe2 2 Si=M1Ti 2IV Al,M1Fe2 Si=M1Fe3 IVAl,Fe3 和Ti进入矿物晶格置换Fe2 来平衡Al置换Si产生的正电价差.两种辉石较低的TiO2,Al2O3和Na2O质量分数显示出母岩浆为拉斑玄武岩浆的特征.二辉石温度计和单辉石温度计算表明斜方辉石的结晶温度下限和单斜辉石结晶温度上限大致在1146℃～1166℃之间,单斜辉石的结晶一直持续到1032℃或者更低.金川Ⅱ号岩体单斜辉石演化明显表现出向透辉石端员偏离,暗示岩浆在结晶分异过程中,可能与富钙质围岩发生了同化混染作用. 相似文献
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硅酸盐辉石的晶体化学(上) 总被引：1，自引：0，他引：1
导论辉石的一般结构早在1928年就已经知道了,那时Warren和Bragg指出C2/c空间群的透辉石结构是以在平行(100)的层中存在的四面体单链为特征的。几年以后,Warren和Modell(1930)分析出了斜方晶系Pbca空间群的紫苏辉石结构,这是由于注意到它的晶胞参数与透辉石的晶胞参数除了a轴加倍外非常一致之故。单斜P2_1/c型结构曾在1950年由Ito预测过,他假定Pbca斜方辉石是一种单斜辉石的“空间群双晶”,认为斜方辉石结构是由单斜辉石的单位晶胞通过平行于(100)的b滑移面的作用有规律地、双晶式地重复而产生的。 相似文献
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本文对西藏丁青玻安岩类深成岩的辉石和新喀里多尼亚玻安岩的斜顽辉石进行了矿物学工作及高分辨电镜观察。丁青斜方辉石堆晶岩辉石的超微结构包括:单斜辉石的(100)双晶、斜方辉石a/4平移畴结构及斜方辉石—单斜辉石相转变。玻安岩斜顽辉石典型超微结构是(100)双晶。由矿物的碎裂、错动以及斜方辉石—单斜辉石相变中只出现偶数倍单斜辉石等表明:矿物结晶及成岩过程中曾受应力作用、后者可能与岩石所处的构造位置有关。由辉石成分计算的地质温度表明,丁青堆晶斜方辉石岩的辉石是在高温下晶出、岩石固结温度是1013～1111℃,当它们在～1000℃下稳定时,斜方辉石出溶了单斜辉石片晶。将玻安岩斜顽辉石加热至1100℃后淬冷及按120℃/h速度冷却均未得到纯的斜顽辉石,可能斜顽辉石是原顽火辉石在应力环境中高温淬冷所成。 相似文献
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在斜顽辉石加温产物中观察到1．35和2．25um宽度堆垛，前者属单斜晶系。这些（2n＋1）×0．45um宽度堆垛可以由辉石的Si－O链错排引起。按单斜辉石或斜方辉石的Si（A）、Si（B）链结构数据所计算的1．35um堆垛，多属单斜晶系，晶胞参数范围是a＝1．357～1．420，b＝0．886，c＝0．516nm，β＝96．1°～108．3°；当斜方辉石的Si－O链作Si（A）Si（B）St（B′）排列时，1．35um堆垛应为斜方晶系。计算的2．25um堆垛的晶胞参数是a＝2．255，b＝0．886，c＝0．516nm，β＝73．8°。 相似文献
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P2_1ca空间群的斜方辉石晶体,其可能的拓扑结构中包含了对称上不等同的4种四面体单链和4种八面体配位位置。八面体层按++--的顺序重复排布。四面体链的旋转方式可有16种不同的组合,但其中有12种违反Thompson的均势法则;另4种两两等价,构成以a轴为极轴的两对异极反取向关系的结构对。它们的O链与S链之比分别为3:1和1:3。从晶体化学角度来看,最可能的结构是3:1的结构。此结构中对称上和拓扑学上均不等同的两列Ⅰ形单元的构成为: ·O+O·O—O··O+S·O—S· 或 +O·S-O·S+ +O·O-O·O+两种M_2位置的构型分别为P·P和P·N。互为异极反取向关系的结构对之间可形成以(100)为双晶面的补足双晶,其双晶界面正好等于一个倒转界面。此外,当Mg:Fe原子比为1:1或3:1时,都有可能出现有序结构。 相似文献
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南美洲南部的Pali Aike火山岩区第四纪碱性玄武岩中普遍发育含石榴石的斜方辉石岩包体。这种斜方辉石岩既作为独立的捕掳体存在又以细脉的形式穿插于橄榄岩捕掳体中。斜方辉石岩普遍含富Ti矿物,并且次生斜方辉石含橄榄石和单斜辉石残晶。与含石榴石橄榄岩中的斜方辉石相比,这种次生的斜方辉石以高TiO2、中等含量的Al2O3以及低Mg#为特征,表明它是在一种高度分异演化的富Ti熔体交代作用下通过消耗橄榄石和单斜辉石方式形成的。斜方辉石岩全岩的Co、Ni略低,Cr和铂族元素(PGE)含量与地幔橄榄岩相当,表明这些元素在交代作用过程中相对稳定,而交代介质带入的组分以碱质(K2O+Na2O)、Ti、Si、Al和S为主。交代的斜方辉石在现代活动岛弧和古克拉通的地幔橄榄岩捕掳体中多有报道。与这些环境中地幔样品的斜方辉石相比,PaliAike地区的次生斜方辉石含有相对高的Ti和Al,以及相对低的Mg。高Ti低Mg属性反映了交代介质可能来源于下伏的软流圈地幔并且经历了高度的分异和演化过程。Pali Aike地区所见到的这种交代斜方辉石和斜方辉石岩在其他被上涌软流圈影响的陆下岩石圈地幔中可能普遍存在。这些研究对了解中国华北-东北中生代以来的岩石圈地幔减薄机制有着重要的借鉴意义。 相似文献
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In the Laouni region (Central Hoggar, Algeria), retrogression of high-grade orthopyroxene–cordierite-bearing rocks led to the crystallization of orthoamphibole and garnet, and at a later stage of chlorite, from the original paragenesis. Calculated phase diagrams show that this retrogression occurred at about 3 kbar with the simplest model involving hydration at 650–700° C and at around 500° C, with the rocks experiencing a H2 O less than 1, except possibly in the last stages of chlorite crystallization. As the other rock types occurring in the same area as the orthopyroxene–cordierite rocks display similar features, it is concluded that regional hydration occurred, presumably related to the release of fluids during the crystallization of the Pan-African granitic and mafic magmas that are widespread in the Laouni area. 相似文献
12.
Collin Nkono Olivier Fmnias Annick Lesne Jean‐Claude Mercier Fadimatou Yamgouot Ngounouno Daniel Demaiffe 《Geological Journal》2016,51(5):748-759
As rock textures reflect the physical conditions and the mechanisms of formation of the rocks, new approaches are used for improving texture analyses, both qualitatively and quantitatively. Pioneer work has recently boosted interest in fractal analysis for quantifying and correlating patterns. Fractal‐like patterns relate to a degree of multiscale organization, and fractal dimensions (FD) and their potential variations can be used to infer the physical conditions of rock formation at various scales of observation. Here, we characterize quantitatively the shape and distribution of orthopyroxene grains in ultramafic xenoliths in terms of FD and their relation with temperature of equilibration. Fractal analysis has been applied to several populations of mantle xenoliths: 7 xenoliths collected in the vicinity of Pico Santa Isabel on Bioko Island, an alkaline basaltic volcano in oceanic domain (Gulf of Guinea, Equatorial Atlantic), 9 samples from Sangilen, in the Agardag alkaline lamprophyre dyke (Russia), and 11 samples form Śnieżnik (Lutynia, Poland), in the continental domain. Fractal analysis has been conducted to characterize the degree of complexity of the petrographic textures: it is indeed known that large values of FD are associated to more complex textures. The correlation here observed between the orthopyroxene fractal dimension and the temperature of equilibration suggests that FD captures a significant textural feature directly related to the temperature (i.e. generated by a temperature‐controlled process). The significant difference in the FD–T correlation observed for the continental and oceanic mantle domains suggests that the mechanical and rheological behaviour is distinct in the oceanic and continental lithospheres. These first promising results should be confirmed by analysing other mantle suites of rocks in different geodynamic settings. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
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Greywacke occurs in most regionally metamorphosed orogenic terranes, with depositional ages from Archean to recent. It is commonly the dominant siliciclastic rock type, many times more abundant than pelite. Using calculated pseudosections in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O system, the partial melting of metagreywacke is investigated using several natural protolith compositions that reflect the main observed compositional variations. At conditions appropriate for regional metamorphism at mid‐crustal depths (6–8 kbar), high‐T subsolidus assemblages are dominated by quartz, plagioclase and biotite with minor garnet, orthoamphibole, sillimanite, muscovite and/or K‐feldspar (±Fe–Ti oxides). Modelled solidus temperatures are dependent on bulk composition and vary from 640 to 690 °C. Assuming minimal melting at the H2O‐saturated solidus, initial prograde anatexis at temperatures up to ～800 °C is characterized by very low melt productivity. Significant melt production in commonly occurring (intermediate) metagreywacke compositions is controlled by the breakdown of biotite and production of orthopyroxene (±K‐feldspar) across multivariant fields until biotite is exhausted at 850–900 °C. Assuming some melt is retained in the source, then at temperatures beyond that of biotite stability, melt production occurs via the consumption of plagioclase, quartz and any remaining K‐feldspar as the melt becomes progressively more Ca‐rich and H2O‐undersaturated. Melt productivity with increasing temperature across the melting interval in metagreywacke is generally gradational when compared to metapelite, which is characterized by more step‐like melt production. Comparison of the calculated phase relations with experimental data shows good consistency once the latter are considered in terms of the variance of the equilibria involved. Calculations on the presumed protolith compositions of residual granulite facies metagreywacke from the Archean Ashuanipi subprovince (Quebec) show good agreement with observed phase relations. The degree of melt production and subsequent melt loss is consistent with the previously inferred petrogenesis based on geochemical mass balance. The results show that, for temperatures above 850 °C, metagreywacke is sufficiently fertile to produce large volumes of melt, the separation from source and ascent of which may result in large‐scale crustal differentiation if metagreywacke is abundant. 相似文献
14.
在温度为1030～1400℃，压力为0．4～2．0GPa条件下，在自然体系中进行了包含橄榄石-叙方辉石-尖晶石-（熔体）－（斜长石）矿物组合的平衡实验。探讨了斜方辉石-尖晶石间的Al-Cr交换平衡。通过对这平衡的研究，得到Al-Cr在尖晶石中的混合参数WAl-CrSp49541±5000J／mol；它较简单体系中所得WAl-CrSp要大许多，可能是由于自然体系中有Ti4+和Fe3+等混入尖晶石结构中，或是文献中对WAl-CrOpx-0和WMg-AlOpx=0的假设错误所致。实验结果还发现在一定温度下，斜方辉石中Cr的含量随平衡共存尖晶石中Cr的增大而成抛物线状增大。第一次实验验证了Nagata（1983）在自然岩石中的观察。 相似文献
15.
A. A. Graphchikov 《Journal of Earth System Science》1992,101(2):191-199
The compositions of orthopyroxene, associated with magnetite and quartz in the system FeO-MgO-MnO-SiO2 were determined experimentally at temperatures of 700, 750, 800°C and pressures ranging between 3 and 5 kbar. On the basis
of data obtained the value of the Guggenheim parameter of orthopyroxene solid solution was calculated: AFe. Mn = 2400 ±500 cal. For rocks rich in MnO, a modified version of orthopyroxene-magnetite-quartz oxygen barometer is suggested. 相似文献
16.
Exsolution textures in orthopyroxene in aluminous granulites as indicators of UHT metamorphism: New evidence from the Eastern Ghats Belt, India 总被引：1，自引：0，他引：1
Highly aluminous orthopyroxene, coexisting with sapphirine, cordierite, sillimanite, quartz and garnet in various combinations, constitute granoblastic mosaic peak metamorphic assemblages in aluminous granulites from three localities in the Eastern Ghats Belt, India. Orthopyroxene contains four types of intergrowths: (a) involving sapphirine with or without cordierite, (b) involving spinel, but without sapphirine, (c) involving cordierite, but without sapphirine and spinel, and (d) involving garnet, without sapphirine, spinel or cordierite. On the basis of textural and compositional data, origin of the intergrowths is ascribed to breakdown of Mg-Tschermak component, locally also involving Fe- and Ti-Tschermak. An attempt is made to compute the “pre-breakdown” compositions of orthopyroxene by image analysis, which shows maximum Al2O3 content of 13.4 wt.% in the pristine orthopyroxene. Geothermometry, phase equilibria consideration and application of existing experimental data on alumina solubility in orthopyroxene coexisting with sapphirine and quartz, collectively indicate extreme thermal conditions of metamorphism (> 1000 °C) for the studied assemblages. This re-affirms the notion that Al2O3 solubility in orthopyroxene is the most powerful indicator of UHT metamorphism (Harley, S.L., 2004. Extending our understanding of ultrahigh temperature crustal metamorphism. J. Mineral. Petrol. Sci. 99, 140–158). The intergrowths are considered to have formed due to cooling from the thermal peak spanning a temperature range of approximately 150 °C. Appearance of diverse types of intergrowths is probably related to subtle differences in bulk composition, particularly Fe:Mg ratios. 相似文献
17.
V. I. FONAREV S. M. PILUGIN K. A. SAVKO M. A. NOVIKOVA 《Journal of Metamorphic Geology》2006,24(2):135-151
Archaean banded iron formation (BIF) of the Voronezh Crystalline Massif (VCM) contains coexisting clino‐ and orthopyroxenes with exsolution textures. The pyroxene in the VCM BIF is found in two generations, with only the first generation containing such textures. Clinopyroxene contains large (up to 5–10 μm) (0 0 1) orthopyroxene (Opx1L) lamellae in a host clinopyroxene (Cpx1H). This host Cpx, in turn, exsolves into thin (～1 μm) (1 0 0) lamellae of orthopyroxene (Opx2L) and clinopyroxene (Cpx2H). Orthopyroxene exhibits similar exsolution textures with large (up to 50 μm) (0 0 1) clinopyroxene (Cpx1L) lamellae developed in a host orthopyroxene (Opx1H). This clinopyroxene Cpx1L shows further exsolution of thin (1 0 0) Opx2LL lamellae and clinopyroxene (Cpx2LH). Point microprobe analysis, raster‐mode microprobe analysis, and microprobe element mapping of mineral grains with a large number of point analysis were used to determine the composition of the exsolution products and the primary chemistry of the coexisting clinopyroxene (CaO = 14.86–17.26 wt%) and pigeonite (CaO = 4.45–6.23 wt%). These pyroxenes crystallized during the peak of metamorphism, and application of the Lindsley geothermometer suggested that they formed at extremely high temperatures of about 1000 °C. Primary very dense CO2‐rich fluid inclusions (ρ = 1.152 g cm?3, Th = ?49.2 °C) were discovered for the first time in these BIF. With these data, the metamorphic pressure was estimated as 10–11 kbar (depth 36–40 km). Such ultrahigh temperature–high pressure (UHT–HP) conditions for the regional metamorphism of the Precambrian BIF have previously been reported only for Archaean meta‐ironstone from the Napier Complex (Enderby Land, Antarctica). They give an insight into the peak metamorphic conditions of the BIF of the VCM, their burial under thickened continental crust during this period of Earth evolution and suggest a more complicated multistage metamorphic and tectono‐thermal history for the region than has previously been postulated. 相似文献
18.
斜方顽火辉石-正铁辉石(MgSiO_3-FeSiO_3)中Fe~(2+)及Mg~(2+)在M_1和M_2两种非等效位置上进行非理想混合。研究表明,辉石的等压热容(C_p)强烈依赖于组成,呈非线性关系,可表示为: 低温时热容的非理想混合效应显著,随着温度的升高,等温热容曲线(空间热容曲面)从一个极大值点(峰脊)分解为两个极大值点(峰脊),且逐渐趋向于理想混合热容直线(或平面)。 相似文献
19.
在两个斜方辉石样品中观察到G.P.区亚稳相。内蒙古暗色辉长岩古铜辉石成分是En_(77)Fs_(18·6)Wo_(4·4),其高分辨象显示了密集的单层G.P.区;玄武岩中变辉长岩包体古铜辉石成分是En_(72·2)Fs_(25·7)Wo_(2·1),具双层G.P.区。该包体与具多层G.P.区的月岩橄长岩相似,后期冷却极快。在地体火成岩体结晶和冷却过程中易出现单层G.P.区,而岩体的大小,冷却的快慢可能影响G.P.区的密度。对N-14样品不同放大倍数的高分辨象G.P.区线密度和面密度的统计值同样具代表性。文中设想了G.P.区出溶的假二元系相图。 相似文献
20.
R. J. M. Taylor C. Clark S. L. Harley A. R. C. Kylander‐Clark B. R. Hacker P. D. Kinny 《Journal of Metamorphic Geology》2017,35(7):759-775
The use of rare earth element (REE) partition coefficients is an increasingly common tool in metamorphic studies, linking the growth or modification of accessory mineral geochronometers to the bulk silicate mineral assemblage. The most commonly used mineral pair for the study of high‐grade metamorphic rocks is zircon and garnet. The link from U–Pb ages provided by zircon to the P–T information recorded by garnet can be interpreted in relation to experimental data. The simplistic approach of taking the average REE abundances for zircon and garnet and comparing them directly to experimentally derived partition coefficients is imperfect, in that it cannot represent the complexity of a natural rock system. This study describes a method that uses all the zircon analyses from a sample, and compares them to different garnet compositions in the same rock. Using the most important REE values, it is possible to define zircon–garnet equilibrium using an array rather than an average. The array plot describes partitioning between zircon and garnet using DYb and DYb/DGd as the defining features of the relationship. This approach provides far more sensitivity to mineral reactions and diffusional processes, enabling a more detailed interpretation of metamorphic history of the sample. 相似文献