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1.
Kanenori Suwa 《Lithos》1979,12(2):99-107
Twinning patterns and petrofabrics of plagioclases are examined in three specimens of anorthosite from the Bushveld Complex, the Quebec Massif, and the Fiskenaesset Complex. Their plagioclases have petrographical characteristics exhibiting their different petrogeneses.In an anorthosite from the Bushveld Complex, plagioclase grains are twinned after the albite-Carlsbad, pericline, albite and Carlsbad laws. Frequency percentage of the albite-Carlsbad and Carlsbad laws reaches 43% Plagioclase grains in the adcumulate layers are developed with their composition plane (010) subparallel to the cumulate plane, whereas those in the heteradcumulate layers are developed with their composition plane (010) subperpendicular to the cumulate plane.In an equigranular anorthosite from the Quebec Massif, plagioclase grains are polysynthetically twinned after the albite and pericline laws with rare examples of the albite-Carlsbad and Carlsbad laws. Frequency percentage of the latter two laws is only 1% together. Some regularities are recognized in the petrofabrics of c-axis and (010) plane.In a calcic anorthosite from the Fiskenaesset Complex, plagioclase grains are polysynthetically twinned, exclusively according to the pericline law or a combination of pericline and albite laws. The pericline law is predominant and reaches 64% and this twinning pattern cleaarly differs from that of the former two anorthosites. 相似文献
2.
S. Mizutani 《Mathematical Geosciences》1975,7(4):335-348
An extinction-angle distribution was computed for randomly oriented grains of plagioclase feldspar with a known An-composition. Expressed by a histogram with eighteen columns, the results are shown by an 18×19 data array for nineteen types of low-temperature plagioclase. In a mixture of grains with various An-compositions, the extinction-angle distribution is given by a linear combination of the obtained data matrix with the frequencies of An-composition. If we have a sample containing randomly oriented grains of plagioclase feldspars, a frequency distribution of those with An-composition can be calculated after measuring their extinction angles. By taking into account the constraints of inequalities, the calculation is treated with the dual method of linear programming. The procedure provides for a simpler and more expedient method for determining the frequency distribution of An-composition. This method was tested by applying it to a model sample containing randomly oriented plagioclase grains with known An-composition frequencies, whose extinction-angle distribution was determined by the Monte Carlo method. An example of application of this method to Permian sandstones from central Japan is presented. 相似文献
3.
斜长石组构的显微测量方法及应用 总被引：1，自引：0，他引：1
介绍了目前国外用费氏台测量斜长石组构的三种方法：（１）ＢＰＷ图解法;（２）（０１０）,（００１）双面交点法;（３）（０１０）消光角法;并分析了其适用范围及各自的优缺点。最后总结了确定斜长石组构的应用意义;（１）确定滑移系;（２）判断剪切方向;（３）计算地震波速。 相似文献
4.
K. S. Scheidl A. -K. Schaeffer E. Petrishcheva G. Habler F. D. Fischer J. Schreuer R. Abart 《Physics and Chemistry of Minerals》2014,41(1):1-16
Fracturing in alkali feldspar during Na+–K+ cation exchange with a NaCl–KCl salt melt was studied experimentally. Due to a marked composition dependence of the lattice parameters of alkali feldspar, any composition gradient arising from cation exchange causes coherency stress. If this stress exceeds a critical level fracturing occurs. Experiments were performed on potassium-rich gem-quality alkali feldspars with polished (010) and (001) surfaces. When the feldspar was shifted toward more sodium-rich compositions over more than about 10 mole %, a system of parallel cracks with regular crack spacing formed. The cracks have a general (h0l) orientation and do not correspond to any of the feldspar cleavages. The cracks are rather oriented (sub)-perpendicular to the direction of maximum tensile stress. The critical stress needed to initiate fracturing is about 325 MPa. The critical stress intensity factor for the propagation of mode I cracks, K Ic, is estimated as 2.30–2.72 MPa m1/2 (73–86 MPa mm1/2) from a systematic relation between characteristic crack spacing and coherency stress. An orientation mismatch of 18° between the crack normal and the direction of maximum tensile stress is ascribed to the anisotropy of the longitudinal elastic stiffness which has pronounced maxima in the crack plane and a minimum in the direction of the crack normal. 相似文献
5.
Michael E. Fleet 《Physics and Chemistry of Minerals》1981,7(2):64-70
The orientation of the plane of modulation of the e-plagioclase superstructure appears to be defined by minimization of interface energy during spinodal-like unmixing of preexisting high plagioclase. Computations with strain-free high-temperature lattice parameters show that the plane of minimization of interface energy for the unmixing of anorthite from calcic plagioclase lies in the zone of s vectors [34, 8, 22] (Ab cell) and is rotated about 15° beyond the orientation of the An 75 superstructure. In contrast, the corresponding plane for unmixing of high albite from sodic plagioclase is rotated about 15° beyond the orientation of the An 25 superstructure. The orientation of individual intermediate plagioclase superstructures is the resultant of the proportional constraints acting on anorthiteplagioclase and albite-sodic plagioclase interfaces. The modulation period (S=1/‖s‖) at any composition is approximately equal to the modulation translation distance parallel to [?21, 39, ?55] resolved onto the modulation direction. The structural consequences of the present study are consistent with the essential features of the crystal structure model of Kitamura and Morimoto. However, the e-plagioclase superstructure is considered to represent a category of unmixing phenomena, rather than atomistic ordering, and it seems inappropriate to attempt to describe it with a composition-related supercell. 相似文献
6.
《International Geology Review》2012,54(2):100-105
Different forms of high-temperature alkaline autometasofnatism in granitoids of certain tin bearing Trnansifs in the upper course basin of the Kolyma River are considered.' The relative depths of massif, deVelopment have been established from the degree of erosional shearing, higher in massifs enclosed in anticlinal structures. In massifs develoried at the greatest depth the reciprocal replacement of feldspars is accompanied by the destruction of the crystalline lattice of the replaced mineral. In plagioclase formed in place of K-Na-feldspar, myrmekite intergrowths are common; their amount and even their appearance itself depend chiefly on the degree of difference between the crystalline lattice orientation of the replaced and replacing feldspars. As the depth of massif development in granitoids decreases, metasomatic replacement with preservation of the replaced feldspar crystalline lattice becomes more frequent in massifs of the least depth of development; such a kind of alkaline autometasomatism is most widespread. In granitoid massifs of all depths, the formation of metasomatic K-Na-feldspar occurs before the development of metasomatic acid plagioclase varieties. From this it becomes possible to establish two stages of alkaline autometasomatism: an earlier potassium and later sodium stage. In chessboard plagioclasestwins develop according to the (010) law with a rhoMbic section as twinning seam; they intergrow also along the (010)-plane, the perpendicular raised to the latter almost coinciding with (010) and making the impression of albite twinning. Besides in chessboard albites the (001)-law is common (twinning seam (010) with an additional intergrowth along the rhombic section; the perpendicular raised to the latter showing several degrees of deviation from (001)). All of the tin showings associated with the massifs under consideration spatially coincide with the rock zone where the feldspar replacement takes place with the crystalline lattices preserved. Therefore, in massifs of the greatest depths of development the tin showings are-located at the exocontacts; and as the depth of development decreases they move towards the central parts of the massifs. — Author's English Summary 相似文献
7.
W. Johannes 《Contributions to Mineralogy and Petrology》1979,68(2):221-230
The ternary feldspar system KAlSi3O8 - NaAlSi3O8 - CaAl2Si2O8 was reinvestigated at 650 ° C and 800 ° C (P H2O = 1 kb) using mixtures of crystalline plagioclases and alkali feldspars as starting materials. The compositions of plagioclases and alkali feldspars of the run products were determined by X-ray means. The Or-content of the feldspar phases was determined by measuring the position of the (201) X-ray peak of the unexchanged feldspars, whereas the An-content was determined by measuring the same X-ray peak of the K-exchanged feldspars. The reaction rate of a reaction leading to a more An-rich plagioclase (type II reaction) is much faster than a reaction producing a more Ab-rich plagioclase (type I). In a type II reaction run times of approximately 20 days are needed to reach new constant plagioclase and alkali feldspar compositions at 650 ° C, and 10 days are needed to reach constant compositions at 800 ° C. In a reaction of type I only the outer zone of the plagioclases reacts to more Abrich compositions. A diffuse zone with a wide range of compositions was observed in 650 ° C runs. Equilibrium could not be reached in these experiments within 45 days. At 800 ° C a new zone having a specific composition develops in 42 days. This new zone is believed to be in equilibrium with the coexisting alkali feldspar. The depth of reaction is calculated as 0.03 μm after 42 days (800 ° C, P f= 1 kb). The reaction between the two feldspar phases could be reversed at 800 ° C. The following compositions are considered to represent equilibrium data at 800 ° C and P t = 1 kb: An 43 Ab 51 Or 6 coexisting with Or 79 Ab 20 An 1, and An 40 Ab 54 Or 6 coexisting with Or 75 Ab 24 An 1. Recent data obtained with gels of ternary feldspar composition as starting materials do not agree with the results presented in this paper. Gels obviously crystallize spontaneously forming coexisting feldspars of non - equilibrium composition - alkali feldspars too rich in Ab and plagioclases too rich in An. 相似文献
8.
An antiperthite feldspar (composition of the main part An27.2 Ab69.2Or3.6) has been studied by x-rays and transmission electron microscopy. Complex twinning and exsolution on very fine scale are described for the first time for this compositional range. Evidence is given for a distinct intermediate region between the plagioclase and the potash feldspar. The formation of the crystal probably involves partial replacement, at least two step exsolution, and transformation of monoclinic plagioclase to triclinic plagioclase. 相似文献
9.
A new method to determine stress directions using the preferential orientation of plagioclase mechanical twins has been applied to high-temperature mylonitic rocks from the Além Paraíba shear zone, Ribeira fold belt, southeastern Brazil. We have measured the lattice-preferred orientation of plagioclase grains and calculated the orientation of the stress axes possible for the observed twin orientations. The maximum compressive stress direction (σ1), determined for all studied samples, is a function of the mechanical twin orientations of a number of distinct plagioclase populations. The σ1 direction is generally subperpendicular to the (010) plane. The statistical treatment for most of the plagioclase grains examined for each sample shows that σ1 is almost perpendicular to the foliation plane, suggesting a significant coaxial component in the deformation process of these rocks. 相似文献
10.
松辽盆地白垩系砂岩长石碎屑的钠长石化作用 总被引：12，自引：0，他引：12
松辽盆地白垩系砂岩以长石岩屑砂岩和岩屑长石砂岩为特征。砂岩中发育长石碎屑、岩屑等不稳定组分，而且随着埋藏深度加深（成岩作用加强）钾长石逐渐减少并最终在2700m以下消失。斜长石碎屑中钠长石组分逐渐增多，钙长石组分逐渐减少，最终形成纯钠端元的钠长石。在成岩过程中长石碎屑的钠长石化主要有3种方式：①由离子交代作用导致长石碎屑的钠长石化；②长石碎屑边缘钠长石次生生长；③与长石碎屑溶解伴生的新生钠长石作用。结合热动力学平衡原理分析，斜长石的钠长石化基本不受成岩温度和压力的制约，而钾长石的钠长石化需要较高的成岩温度和压力作用才能进行。因此，斜长石的钠长石化可见于成岩早期，而钾长石的钠长石化只发生于成岩晚期。 相似文献
11.
I. Parsons 《Geological Journal》1972,8(1):71-82
The normative mineralogy of the Loch Borralan, Loch Ailsh and Loch Loyal leucosyenites is compared to the System Ab – Or – An - Qz. At Borralan, the upper quartz-syenites of Cnoc-na-Sroine plot within the plagioclase volume, the lower perthosites within the alkali feldspar volume. This mineralogy cannot have been attained by in situ crystal fractionation. The Ailsh rocks plot within the plagioclase volume, the Loyal syenites within the alkali feldspar volume. The three complexes cannot have been derived from a single magma body at depth, and are not precisely coeval. 相似文献
12.
Clinopyroxene and plagioclase (andesine) microlites in an obsidian flow from Glass Mountain (NE California, USA) display strong alignment. Synchrotron X-ray diffraction, coupled with Rietveld analysis, was used to quantify crystallographic-preferred orientation (CPO). Clinopyroxene, with a rod-shaped morphology, shows a strong alignment of [001] in the flow direction and (010) aligned parallel to the inferred flow plane. Andesine, with a platy morphology, displays an alignment of (010) platelets in the flow plane. Some pole densities exceed 90 multiples of random distribution. Applying a model of rigid ellipsoidal inclusions in a viscous matrix, the local pure shear strains are between 2 and 3. 相似文献
13.
Evan R. Phillips 《Lithos》1974,7(3):181-194
Myrmekite, first detected by Michel-Lévy in 1875 and named by Sederholm in 1899, is an intergrowth between vermicular quartz and (sodic) plagioclase situated next to potash feldspar. In felsic plutonic rocks it occurs as: rims bordering granular plagioclase, intergranular blebs set between adjacent microperthite crystals, lobes associated with muscovite in deformed alkali feldspar megacrysts or as bulbous growths at their margins, and rims on plagioclase inclusions held within orthocalse megacrysts. A literature review based largely on papers published in the past quarter century shows that hypotheses for myrmekite genesis fall mainly into five categories: simultaneous or direct crystallization, replacement of potash feldspar by plagioclase, replacement of plagioclase by potash feldspar, solid-state exsolution, and recrystallizing quartz involved with blastic plagioclase. 相似文献
14.
Dislocations in intermediate plagioclase feldspars, which were deformed under granulite facies conditions, have been analysed. The study reveals extensive ductile deformation by intracrystalline slip and by twinning. Six out of the seven possible Burgers vectors were identified: \(b = \left[ {001} \right],\tfrac{1}{2}\left[ {110} \right],\tfrac{1}{2}\left[ {1\bar 10} \right],\left[ {101} \right],\tfrac{1}{2}\left[ {112} \right]and\tfrac{1}{2}\left[ {1\bar 12} \right]\) . Most, perhaps all, dislocations are dissociated by up to 200 Å. The microstructure is dominated by [001] screw dislocations, most of which appear to be dissociated in (010). The dominant slip system appears to be (010) [001]. Large grain-to-grain variations in the density of free dislocations indicate that the plastic strain in individual grains depended upon the Schmid factor for (010) [001]. The microstructure suggests that the rate-controlling step for high-temperature creep of plagioclase is cross-slip of extended [001] screw dislocations. The rheological contrast between feldspar and quartz is partly due to a difference in stacking fault energy. 相似文献
15.
Myrmekite, as defined here, is the microscopic intergrowth between vermicular quartz and modestly anorthitic plagioclase (calcic albite-oligoclase), intimately associated with potassium feldspar in plutonic rocks of granitic composition. Hypotheses previously invoked in explanation of myrmekite include: (1) direct crystallization; (2) replacement; (3) exsolution. The occurrence of myrmekite in paragneisses and its absence in rocks devold of discrete grains of potassium feldspar challenge those hypotheses based on direct crystallization or replacement. However, several lines of evidence indicate that myrmekite may in fact originate in response to kinetic effects associated with the exsolution of calcic alkali feldspar into discrete potassium feldspar and plagioclase phases. Exsolution of potassium feldspar system projected from [AlSi2O8] involves the exchange CaAlK-1Si-1, in which the AlSi-1 tetrahedral couple is resistant to intracrystalline diffusion. By contrast, diffusion of octahedral K proceeds relatively easily where it remains uncoupled to the tetrahedral exchange. We suggest here that where the ternary feldspar system is open to excess silica, the exchange reaction that produces potassium feldspar in the ternary plane is aided by the net-transfer reaction K+Si=Orthoclase, leaving behind indigenous Si that reports as modal quartz in the evolving plagioclase as the CaAl component is concomitantly incorporated in this same phase. Thus silica is pumped into the reaction volume from a silica reservoir, a process that enhances redistribution of both Si and Al through the exsolving ternary feldspar. 相似文献
16.
文象花岗岩具有特殊文象结构,研究其三维拓扑结构和形成过程有助于了解花岗质岩石的结晶作用.以北京周口店房山岩体和湖北罗田蕙兰山的文象花岗岩为研究对象,综合利用光学显微镜、扫描电镜、电子探针和电子背散射衍射等技术方法,对岩石矿物组成、结晶学取向和拓扑结构进行了系统研究.结果表明:(1) 文象花岗岩的矿物组成与其形成地质环境有关,石英和长石的含量变化范围很大,其中石英含量通常在20%~45%,但是相同地区同期形成的文象花岗岩具有相对稳定的矿物组成;(2) 长石作为寄主矿物通常呈半自形－自形粗大晶体,可以是碱性长石或斜长石,其端元组分以钾长石和钠长石为主,低温下常分解为条纹长石;(3) 石英在长石寄主矿物中规则穿插生长,在三维空间通常呈近似平行板状、长条状/柱状或非连通枝杈状,并只在特定岩石断面形似象形文字;(4) 正交偏光显微镜下,石英可以具有多种消光位,但是通常在一定范围内同时消光;(5) 石英普遍发育道芬双晶,偶见日本双晶;(6) 条纹长石中钾长石与钠长石对应(100)、(010)、(001) 面和[001]轴近似平行;(7) 多数石英颗粒与寄主长石之间具有密切结晶学取向关系,即石英[1123]轴近似平行长石c[001]轴.该研究证实文象花岗岩是石英和长石同时生长的结果,而长石作为寄主矿物影响并控制着石英的成核与生长方向. 相似文献
17.
Shock-effects on the K-Ar system of plagioclase feldspar and the age of anorthosite inclusions from North-Eastern Minnesota 总被引：1，自引：0，他引：1
Artificial shock pressures up to 52.5 GPa have no influence on the K-Ar system of plagioclase feldspar. The 40Ar-39Ar analysis of feldspar (labradorite An67 from anorthosite of North-Eastern Minnesota) shocked up to 45 GPa—in vacuo, to prevent massive entrapment of atmospheric argon-shows that the age spectra and the argon diffusion properties remain unaltered. Similar feldspar samples (labradorite An51 from Nain, Labrador), shocked in air up to 52.5 GPa and dated by the conventional K-Ar method, also yield the same age as unshocked samples but with a higher atmospheric argon contribution. The Minnesota anorthosite has an 40Ar-39Ar age of 1075 ± 10 m.y. No information on a possible previous history of the anorthosite became apparent. 相似文献
18.
Li Zhang 《Geochimica et cosmochimica acta》2009,73(10):2832-4139
The more rapid dissolution of Ca-rich feldspars relative to Na, K-rich feldspars has been attributed to the preferential leaching of Al deep within the feldspar structure. Evidence from surface microanalysis (e.g., Hellmann et al., 2003), however, shows that preferential dissolution of Al is confined to the top layers of the feldspar lattice and that the amorphous surface layer most likely results from precipitation versus dissolution. It is thus critical to examine the extent of preferential Al removal. Here we present a theoretical study of plagioclase dissolution behavior using parameterized Monte Carlo simulations. Two different dissolution mechanisms, a mechanism involving preferential leaching of Al and an interfacial dissolution-reprecipitation mechanism, are tested using compositions representing the entire plagioclase solid solution series. Our modeling results indicate that under the control of the preferential Al leaching mechanism, the influence of (Al, Si) disorder on the dissolution rate is significant. At a fixed composition, an increase in the degree of (Al, Si) disorder yields an increased dissolution rate, with an 8-fold increase in dissolution rate observed for highly disordered albite (An0) compared to low albite. Increasing anorthite content tends to decrease the variation in the dissolution rate due to disorder. The difference in the dissolution rate of 293 tested oligoclase configurations with a composition of An20 is 3-fold, and the difference is reduced to 2-fold among 107 andesine configurations of An30. Furthermore, feldspar configurations with completely disordered (Al, Si) distributions yield a consistent log-linear dependence of dissolution rate on the anorthite content (An), while other feldspar configurations with modest degrees of (Al, Si) disorder exhibit rates less than this trend. In contrast, when Al removal is confined to the top surface layers, a variety of feldspar configurations with different (Al, Si) disorder but a single fixed composition have similar dissolution rates; and the dissolution rate of Ca-rich feldspars departs positively from its log-linear relationship with anorthite content. This departure occurs around An80 and is in good agreement with previous experimental studies. Subsequent modeling results of aluminum inhibition, ΔG dependence, and formation of altered surface layers in the framework of the interfacial dissolution-reprecipitation mechanism are all comparable with experimental investigations, and these results suggest that an interfacial dissolution-reprecipitation mechanism governs the dissolution of plagioclase feldspars. 相似文献
19.
Determination of Fe3+ and Fe2+ concentrations in feldspar by optical absorption and EPR spectroscopy
Ferrous and ferric iron concentrations in feldspars with low total iron content (<0.32 wt% total Fe) were determined from optical and electron paramagnetic resonance (EPR) spectra to better than ±15 percent of the amount present. Optical spectra indicate that Fe2+ occupies two distorted M-sites in plagioclases of intermediate structural state. The linear dependence of the Fe2+/Fe total ratio on An content demonstrates that Fe2+ substitutes for Ca (not Na) so that the number of Ca-sites is a principal factor in iron partitioning in plagioclase. EPR powder spectra show that the number of sites for Fe3+ depends on structural state rather than on plagioclase chemistry. The observed linear correspondence of EPR double-integrated intensities with optical peak areas shows that all Fe3+ is tetrahedrally coordinated in both plagioclase and disordered potassium feldspar. Microcline perthites show, in addition to tetrahedral Fe3+, a signal due to axially coordinated ferric iron, which we associate with formation of hematite inclusions. 相似文献
20.
Paul Henderson 《Geochimica et cosmochimica acta》1984,48(2):415-416
Evidence that REE have been metamorphically redistributed between plagioclase feldspar and mafic minerals, as suggested by Weaveret al. (1981) is not forthcoming from the six Fiskenaesset rocks used in the study. REE patterns of the separated feldspars and of two rocks containing small modal amounts of mafic minerals are consistent with a light REE-enriched magma during the formation of the Fiskenaesset Complex. 相似文献