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1.
S. G. Viladkar 《Petrology》2010,18(5):544-554
Pseudoleucite occurs as large megacrysts (giant phenocrysts) in tinguaite at Ghori which is located in the Panwad-Kawant carbonatite-alkalic complex, India. In thin sections the pseudoleucite crystals show mainly an oriented intergrowth of nepheline and orthoclase with additional minerals phases such as white mica, analcime, sodalite and cancrinite. From mineralogy and geochemistry it is inferred that the scheme involving subsolidus breakdown of leucite and subsequent reaction with Na-rich fluids can satisfactorily explain origin of pseudoleucite crystals in tinguaites.  相似文献   

2.
方沸石响岩是一种罕见的碱性火山岩.采用电子探针、粉晶X射线衍射、扫描电镜、拉曼光谱等研究了青藏高原当雄地区方沸石响岩的主要造岩矿物种属、共生关系和结晶顺序.研究表明, 斑晶由方沸石和长石组成.方沸石为岩浆结晶晚期形成的原生矿物; 长石均发育“次生边”结构, 中央相为斜长石, 边缘相为碱性长石.基质由碱性长石、次透辉石、钛磁铁矿和褐铁矿组成.原生矿物的结晶顺序是: 斑晶长石的中央相→斑晶方沸石+斑晶长石边缘相富钾长石→基质长石→次透辉石→钛磁铁矿和褐铁矿.利用方沸石-熔体平衡估算出方沸石结晶时岩浆的温度和压力条件分别为600~640 ℃和(5~13) ×108 Pa, 考虑到青藏高原当时已形成巨厚地壳, 认为岩浆房存在于地壳深部.   相似文献   

3.
Analcime (NaAlSi2O6.H2O) forms a characteristic authigenic phase in Andean alluvium of northern Chile. Three stratigraphic units ranging from the lower Cretaceous to the Miocene were studied in order to investigate the presence of analcime in diagenetic assemblages of different ages. The units were deposited in forearc, foreland/back-arc, and intra-arc basinal settings. Characteristics of other sedimentary analcime occurrences were examined in order to define conditions favourable for analcime precipitation. Zeolite zonation and evidence for the original presence of volcanic glass, common to many sedimentary analcime occurrences, are absent in the studied units. Analcime precipitation requires a high Na+/H+ ratio, relatively low Si4+/Al3+ activity and low H2O activity. Diagenetic examination of the three stratigraphic units revealed early quartz and feldspar overgrowths, mechanical clay infiltration, and dissolution of labile rock and mineral fragments, particularly intermediate plagioclase (An30–80). Conditions for these processes are favoured by oxidizing groundwater at near neutral pH. Later diagenesis involved the precipitation of analcime, non-ferroan calcite, quartz, and authigenic clays as the main cementing phases. These formed due to the concentration of oxidizing groundwater and resulted from its continued reaction with detrital feldspars and rock fragments of andesitic and dacitic composition. Analcime precipitation in Andean alluvium resulted from the evolution of saline, alkaline brines under an arid, evaporative climate in closed hydrographic basins with circulating groundwater systems.  相似文献   

4.
Q. GALL  R. HYDE† 《Sedimentology》1989,36(5):875-887
In the Deer Lake Basin of western Newfoundland, Canada, analcime has been found within fine-grained, siliciclastic and carbonate, lake and lake-margin sediments of the Carboniferous Rocky Brook Formation. Analcime is the only zeolite observed in these unmetamorphosed and non-volcanogenic rocks. Microscopic analcime occurs as vug-filling, limpid, isotropic to anisotropic crystals. Microprobe analyses show this type of analcime to be lower in silica than most analcime in sedimentary rocks. A second textural type of finer-grained submicroscopic analcime can be detected from X-ray diffraction spectra of Rocky Brook Formation mudstones, where it is associated with phyllosilicates (illite, interstratified chlorite/smectite, smectite, chlorite), other silicates (quartz, feldspar) and carbonates (dolomite, calcite). Results of this study suggest that the analcime in the Rocky Brook Formation formed either by direct lakewater/porewater precipitation or by reaction of these waters with one or more clay mineral types or plagioclase.  相似文献   

5.
The Brown Leucitic Tuff (BLT) is a poorly to strongly lithified compositionally zoned pyroclastic-flow deposit with a minimum volume of 3 to 5 km3. It erupted from Roccamonfina Volcano about 385000 years ago, after formation of the summit caldera. Individual flow units are grouped into three facies (white, brown, and orange) which primarily differ in pumice color, lithic content, and matrix cementation. Pumices from the BLT range from phonolitic leucite-tephrites to leucite-trachytes (7.0 to 2.2 wt% CaO), covering over half of the total spectrum of High-K Series magmas known from Roman Region volcanoes. White-facies units dominate in lower stratigraphic levels and their pumices have the lowest CaO contents, indicating a general trend toward more basic compositions as the eruption evolved. At higher stratigraphic levels, however, orange- and brown-facies units are interbedded with other whitefacies units, indicating reversals in the dominant compositional progression.BLT pumices have crystal contents of 9.9 to 0.6 vol%, with green salite>plagioclase>sanidine>biotite>titanomagnetite>analcime (after leucite)>apatite>pyrrhotite. In most samples, plagioclase (An85–95) and sanidine (Or75–90) have much lower Na2O contents than usually found in coexisting feldspars, yet these are interpreted as equilibrium pairs. Primary leucite has been almost completely replaced by analcime. All samples also contain xenocrysts of colorless diopside and forsteritic olivine (Fo83–92). Recurrent alternations from colorless diopside to green salite are present in single clinopyroxene crystals and appear to reflect a complex history of magma mixing.Whole-rock BLT pumice compositions conform closely to High-K Series lavas from Roccamonfina for all elements except Na2O and K2O. The former is relatively enriched and the latter relatively depleted in mafic BLT pumices with >5.6% CaO; these differences reflect strong analcimization of abundant groundmass leucite crystals in these pumices. Otherwise, major and trace element data support fractionation of observed minerals in generating the compositional diversity among BLT pumices. Mineral assemblages and compositions of cumulate monzonite and syenite nodules carried to the surface during the BLT eruption correspond closely to the fractionated phases predicted by least-squares modeling.  相似文献   

6.
The Ghansura Rhyolite Dome of the Bathani volcano-sedimentary sequence in eastern India originated from a subvolcanic felsic magma chamber that was intruded by volatile-rich basaltic magma during its evolution leading to the formation of a porphyritic andesite. The porphyritic andesite consists of rapakivi feldspars, which are characterized by phenocrysts of alkali feldspar mantled by plagioclase rims. Results presented in this work suggest that intimate mixing of the mafic and felsic magmas produced a homogeneous hybrid magma of intermediate composition. The mixing of the hot volatile-rich mafic magma with the relatively colder felsic magma halted undercooling in the subvolcanic felsic system and produced a hybrid magma rich in volatiles. Under such conditions, selective crystals in the hybrid magma underwent textural coarsening or Ostwald ripening. Rapid crystallization of anhydrous phases, like feldspars, increased the melt water content in the hybrid magma. Eventually, volatile saturation in the hybrid magma was reached that led to the sudden release of volatiles. The sudden release of volatiles or devolatilization event led to resorption of alkali feldspar phenocrysts and stabilizing plagioclase, some of which precipitated around the resorbed phenocrysts to produce rapakivi feldspars.  相似文献   

7.
杨喆  钟大康  张硕  郭强  路昭 《地球科学》2018,43(10):3733-3748
二连盆地白音查干凹陷下白垩统暗色沉积物中发育由白色颗粒构成的同沉积纹层、条带,其矿物组合中包含大量成因不明的钠沸石和方沸石.在薄片鉴定和电子探针（EMPA）分析的基础上,通过场发射扫描电镜（FE-SEM）和电感耦合等离子体质谱仪（ICP-MS）法对两种沸石矿物的形貌、微结构、共生矿物组合和微量元素成分进行分析.实验结果表明,钠沸石和方沸石自形程度较高,其中钠沸石为半自形-自形的棱柱状,长轴方向长度最大可达600 μm,集合体为纤维状或块状;方沸石呈自形的四角三八面体,粒径变化较大,介于10~300 μm.钠沸石和方沸石既能独立生长,也能互相交代,与重晶石、含铁白云石、菱镁矿和菱铁矿以及立方体黄铁矿等矿物紧密共生.与地壳及同区和同层位湖相泥岩微量元素配分曲线比较,含沸石岩石的全岩微量元素富集Pb、Sb、Tl、W、Mo、Bi,亏损V、Cr、Co、Ni,暗示了中高温含深源物质的碱性热液流体的加入.在综合分析沸石矿物形态结构、共生矿物组合及微量元素特征的基础上,认为在异常热流驱动下,中高温热液流体沿着断裂等运移通道喷出或溢流至湖底沉积物表面,与冷的湖水混合直接结晶沉淀形成方沸石、钠沸石和其他共生矿物,属于热水沉积成因.   相似文献   

8.
The Reid's Mistake Formation represents the stratigraphic interval that separates the Lower and Upper Pilot Coals within the Boolaroo Sub‐Group of the Newcastle Coal Measures. At Swansea Head it consists mainly of intercalated cherts and claystones with minor amounts of sandstone. The cherts, which frequently have a vitroclastic texture, are composed of chalcedony and analcime with or without feldspars and clay whereas the claystones consist of chalcedony and mixtures of mixed‐layer clay minerals. The intergradation between these rock types indicates that the entire sequence was derived from a common source, either acid volcanics of the New England fold belt or, more likely, contemporaneous ash showers. The detritus apparently accumulated in a flood basin environment where the climate was arid giving rise to saline conditions and the breakdown of the volcanic materials to clay minerals and chalcedony. During protracted periods of desiccation however, sodium ions were concentrated and the pH rose accordingly, sufficient to promote conversion of the clay minerals in the uppermost layers to analcime.  相似文献   

9.
In ternary feldspars of essentially one phase, calcium content has a dominant influence on the optic axial angle. In such feldspars and also in binary feldspars from solvsbergite rocks, variations of cooling histories do not significantly affect the optic axial angle. In ternary feldspars which are unmixed into two or three prominent phases, Al/Si ordering has an important effect on the 2V value. A recent suggestion of several writers that in feldspars the alkali structural site may be partially occupied by (H3O)+ ions is applied as a possible way to explain a correlation observed between petrographical features of the rocks and the optic axial angles of their feldspar phenocrysts.  相似文献   

10.
R. W. RENAUT 《Sedimentology》1993,40(2):271-301
Late Quaternary fluviolacustrine siltstones, mudstones and claystones (Loboi Silts) on the northern margins of the saline, alkaline Lake Bogoria in the Kenya Rift Valley contain up to c. 40% authigenic analcime and minor natrolite. The zeolitic sediments are reddish brown and up to 1 m thick. The amount of analcime increases upward in the profile, but decreases with distance from the lake. The altered sediments show many pedogenic features including zeolitic root mats, rootmarks, concretions and carbonate rhizoliths. Residual patches of calcrete locally cap the zeolitic rocks. The profile is interpreted as an exhumed palaeosol and land surface on the former margins of the lake. The analcime occurs as submicroscopic (0–5–2–5 μ.m) subhedral and euhedral crystals, which have an average Si/A 1 ratio of 2–33 (as determined by X-ray microanalysis) or 2–18 (d-value of 639 analcime peak). The analcime formed in lake marginal sediments (soils) by reaction of silicate detritus with Na2CO3 rich pore waters concentrated close to the land surface by evaporative pumping and evapotranspiration. Poorly ordered clay minerals were probably the main reactants. Authigenic illite may have been a by-product of the reactions. Chemical analyses suggest that pore waters supplied some of Na+, and possibly K+ and SiO2. The associated calcrete and rhizoliths were formed during or shortly after the main period of zeolitic alteration. The Ca2+ may have originated from infiltrating dilute runoff and groundwater. Authigenic smectite was precipitated in open porosity following analcime formation. The zeolitic alteration at Lake Bogoria provides a relatively recent analogue for lake marginal zeolites found in many ancient saline, alkaline lake sediments.  相似文献   

11.
Summary A suite of lithics (ejecta) collected from the latest erupted pyroclastic products of the Alban Hills volcano (Central Italy) has been studied to determine their mineralogical composition and to investigate their genesis. The ejecta commonly have granular texture and consist of coarse-grained crystals often associated with a fine- to medium-grained matrix. The mineralogical composition is variable and consists of both typical igneous minerals and contact metamorphic phases. Garnet, clinopyroxene K-feldspar are almost ubiquitous, whereas leucite, wollastonite, sodalite-group minerals, phlogopite, nepheline and phillipsite are present in most of the ejecta; minor and accessory phases include cuspidine, amphibole, pyrrhotite, magnetite, apatite, uranpyrochlore, sphene, kalsilite, and melilite; anorthite, zircon and fluorine-bearing Ca, Zr silicate phases, larnite, and baryte are found sporadically. Ca, REE, Th silicophosphates occur in many samples generally disseminated along interstices and fractures of main minerals. Calcite is present as discrete crystals sometimes enclosed in other minerals, as granules in the fine-grained matrix and as late microcrystalline veins. It shows high oxygen and low carbon isotope ratios with δ18O = + 17.96 to + 27.19, and δ13C = −4.74 to −19.57. Clinopyroxene ranges from diopside to compositions strongly enriched with both Ca-Tschermak’s and esseneite components. Feldspars are generally potassic even though Ba and Sr are found in significant concentrations in some samples. K-feldspars from wollastonite-bearing ejecta are often rimmed with elongated felty crystals identified by X-ray diffraction analysis as leucite. These feldspars show a depletion in Si, and enrichment in Al and K from core to rim. Significant compositional variations are also shown by various other phases such as nepheline, apatite, Ca, REE, Th silicophosphate. The occurrence of igneous and contact metamorphic minerals, as well as the chemical variations of clinopyroxenes and feldspars in the investigated ejecta reveal complex genetic processes related to the interaction between potassic magma and wall rocks. The Ca-rich composition of most phases points to a carbonate nature for the wall rocks. Textural evidence suggests that coarse-grained rocks formed at the margin of the magma chamber were invaded by a late, volatile rich potassic liquid which crystallized as a fine-grained matrix and produced disaggregation and reaction of early formed minerals. Fluid phases percolating through the rocks generated infiltration metasomatism and deposited some uncommon phases rich in Ca, REE, Th, U, which are found along cracks and at the margins of early crystallized minerals. Overall, the all spectrum of the minerals found in this study are also typical of carbonatitic rocks. Their presence in the Alban Hills ejecta demonstrates that their genesis can be related to interaction between ultrapotassic melts and carbonate wall rocks, in addition to precipitation from carbonatitic melts. Received February 20, 2001; revised version accepted September 23, 2001  相似文献   

12.
Turbidity is an almost universal feature of alkali feldspars in plutonic rocks and has been investigated by us in alkali feldspars from the Klokken syenite using SEM and TEM. It is caused by the presence of myriads of tubular micro-inclusions, either fluid-filled micropores or sites of previous fluid inclusions, and is associated with coarsening of microperthite and development of sub-grains. Micropores are abundant in coarsened areas, in which porosities may reach 4.5%, but are almost absent from uncoarsened, pristine braind-microperthite areas. The coarsening is patchy, and involves a scale increase of up to 103 without change in the composition of the phases, low albite and low microcline, or in the bulk composition of the crystal. It occurs abruptly along an irregular front within individual crystals, which retain their original shapes. The coherent braid microperthite gives way across the front to an irregular semi-coherent film perthite over a few m and then to a highly coarsened irregular patch perthite containing numerous small sub-grains on scales of a few hundred nm, in both phases. The coarsening and micropore formation occured at a T400°–450° C and it is inferred to have been driven by the release of coherent strain energy, low-angle grain-boundary migration being favoured by a fluid. The patchy nature of the coarsening and the absence of a relationship with initial grain boundaries suggest that the fluid was of local origin, possibly arising in part through exsolution of water from the feldspar. The sub-grain texture and microporosity modify profoundly the permeability of the rock, and greatly enhance the subsequent reactivity of the feldspars.  相似文献   

13.
The Square Top intrusion, near Nundle, New South Wales, revealsin its mineralogy and chemistry the transition of analcime-olivinetheralite to analcime tinguaite. Chemical, X-ray, and opticaldata are presented on 2 plagioclases, 9 alkali feldspars, 2nephelines and 3 analcimes, from various members of the differentiationsequence. With differentiation, plagio-clase becomes enrichedin Ab and, to a lesser extent, in Or. The coexisting alkalifeldspars initially reveal enrichment in Ab and trend from sanidineto lime anorthoclase; subsequently in the tinguaites, the sanidinesbecome enriched in Or. Nephelines in the lower theralites havehighly sodic compositions and become slightly enriched in Kin the later differentiates. Replacement of Si by NaAl in theanalcimes decreases with progressive differentiation. The rapidcooling of this relatively small intrusion assisted fractionalcrystallization, and resulted in the preservation of disorderedplagioclase and alkali feldspars. The Square Top feldspar crystallizationtrends are compared with trends presented by Tuttle and Bowen(1958). The location of the feldspar boundary curve in the Ab–An–Orsystem and some general relationships in the undersaturatedportion of the Ne–Ks–Qz system, including the heteromorphicfelsic assemblages developed from the lowest temperature liquids,are also discussed.  相似文献   

14.
The petrology and mineralogy of three lavas from the Lupata Gorge, Mocambique, containing primary euhedral analcime phenocrysts, up to 1.5 cm in diameter, as well as potassium feldspar and nepheline phenocrysts, are described. Electron microprobe analyses of these phases and a whole rock analysis of the blairmorite are given. Reference to published and unpublished experimental work indicates that these rocks must have been generated at water pressures between about 5 and 13 kbars, implying depths of origin of between 20 and 50 km. Very rapid transport to the surface and quenching is implied. It is suggested that these indications of the considerable depth of origin of these rocks, taken together with the absence of associated intermediate and basic rocks, lends credence to the hypothesis of D. K. Bailey that the voluminous associations of salic igneous rocks found in parts of the African continent, unaccompanied by associated basic rocks, are explicable in terms of partial melting, under high water pressures, of the lower part of the crust.  相似文献   

15.
Summary Spectacular large (3 cm) euhedral trapezohedra composed of single crystals of analcime or aggregates of pumpellyite occur in three stratigraphically adjacent phonolitic lavas from Aghda. The trapezohedra contain concentrically arranged inclusions of plagioclase, pyroxene, titanomagnetite and apatite which are fresh in the analcime but extensively altered in the pumpellyite. The analcime crystals are remarkably homogeneous in composition and are interpreted as having formed by ion-exchange pseudomorphous replacement of primary leucite. In the lower phonolite, however, the analcime became unstable and was replaced by pumpellyite which has a wide compositional range, reflecting variable input of Fe, Mg, Ca and Al from the precursor analcime and alteration of inclusions. The occurrence of analcime and pumpellyite at Aghda is indicative of zeolite facies conditions with low fCO 2and H2O activity and probably high Pfluid.
Große pseudomorphe Trapezoeder von Analcim im Pumpellyit nach Leuzit, Aghda-Gebiet Zentraliran
Zusammenfassung Ungewöhnlich große idiomorphe Trapezoeder, die aus einzelnen Kristallen von Analcim oder aus Aggregaten von Pumpellyit bestehen, kommen in drei stratigraphisch benachbarten phonolitischen Lagen von Aghda vor. Die Trapezoeder enthalten konzentrisch verteilte Einschlüsse von Plagioklas, Pyroxen, Titanomagnetit und Apatit, die im Analcim frisch, im Pumpellyit jedoch intensiv umgewandelt sind. Die Analcim-Kristalle sind von bemerkenswerter Homogenität in ihrer Zusammensetzung und werden auf pseudomorphe Verdrängung von primärem Leucit durch Ionenaustausch zurück-geführt. Im unteren Phonolit ist der Analcim jedoch instabil geworden, und wurde durch Pumpellyit ersetzt, der ein weites Spektrum von Zusammensetzungen erkennen läßt und auf variierende Zufuhr von Fe, Mg, Ca, and Al vom Vorläuferanalcim und auf Umwandlung von Einschlüssen hinweist. Das Vorkommen von Analcim und Pumpellyit bei Aghda ist ein Hinweis auf Zeolithfazies-Bedingungen mit niedrigen fCO 2 und H2O und hohem Pfluid.

With 5 Figures  相似文献   

16.
《Chemical Geology》1999,153(1-4):81-91
Cathodoluminescence (CL) investigations of igneous, metamorphic and sedimentary feldspars indicate that rare earth elements (REE)-activated CL in feldspars is more common than previously assumed. Hot-cathode CL microscopy combined with high-resolution spectrometric analysis of CL emission allow to detect some REE below the detection limits of electron microprobe and proton-induced X-ray emission analysis (PIXE) and reveal variations in the REE distribution within single feldspar crystals. Differently luminescing zones can reflect changes during feldspar crystallization and/or element fluctuations during secondary alteration processes which are not discernible using conventional polarizing microscopy. The results of the study document Eu2+, Sm3+, Dy3+, Tb3+, and Nd3+-activated CL in feldspars of different origin. The influence of the crystal field on shape and position of REE luminescence spectra significantly differs for divalent and trivalent REE ions. Whereas Eu2+ shows a broad band emission (∼420 nm) which is influenced by the local crystal field, trivalent ions of the rare earth show narrow emission lines which reflect the transitions between excited state wave functions lying inside closed electronic shells. The positions of these peaks and the characteristic energies are described for the different REE3+.  相似文献   

17.
Jadeitites, albitites and related rocks from the Motagua Fault Zone, Guatemala   总被引：12，自引：0，他引：12  
Jadeitites from Guatemala are found as weathered blocks in tectonized serpentinite in a 15-km zone north of the Motagua Fault Zone. Rock types found with jadeitite include albitites, albite-mica rocks, omphacite/taramitic amphibole-bearing metabasites, chlorite-actinolite schists, talc-carbonate rocks and antigorite schists. In addition to the predominant jadeitic (Jd93_100) pyroxene, common phases in jadeitite include micas (paragonite and/or phengite ± rarer phlogopite), omphacite, albite, titanite /Pm zircon, apatite and graphite. Conditions of jadeitite formation are 100-400d? C, 5-11 kbar with 0.0 > log10asio2≥= 0.7. Fluid inclusions, coarse textures, vein structures, and rhythmic zoning of pyroxene indicate an aqueuos fluid was involved. Jadeitites are either (1) metasomatic modifications of former felsic-to-pelitic inclusions that have undergone silica depletion plus efficient soda exchange and enrichment, or (2) solution precipitations derived from such a source. The close spatial relationship of faults and shear zones, serpentinites, and jadeitites suggests jadeitites form in a relatively high-P/T setting with substantial flow of sodic fluid in a tectonized zone. Most Guatemalan jadeitites are extensively altered to analcime, albite, taramitic amphibole, (clino)zoisite ± nepheline and preiswerkite. This alteration reflects depressurization /Pm heating to below the jadeite + fluid = analcime reaction at high aNa. With progressive alteration, analcime and nepheline are replaced by albite; the increase in silica content may result from fluid flowing up a tectonized zone reaching saturation with an albite assemblage. Albitite phases, albite, actinolite, zoisite, /Pm chlorite, phengite, K-feldspar and quartz, record conditions of c. 3-8 kbar at T < 400d? C, indicating a clockwise P-T trajectory of the blocks. Barium aluminosilicates—banalsite, celsian, cymrite and hyalophane—are common minor late-stage phases in jadeitites and albite-rich rocks. Barian phengite is common in albite-mica rocks.  相似文献   

18.
Critical aspects of the crystallization dynamics of H2O-saturated melts of a typical granitic composition as well as granitic melts enriched in lithium (8800 ppm) were investigated in real-time experiments using the hydrothermal diamond anvil cell at 480–700 °C and 220–960 MPa. Complete crystallization of the charges was achieved within 5–118 h with average crystal growth rates ranging from 3 to 41 cm/year for quartz and from 18 to 58 cm/year for alkali feldspars, demonstrating that crystals formed from a silicate melt in the presence of a coexisting aqueous phase crystallize rapidly. The combination of substantial nucleation delays, low nucleation densities, and rapid growth rates for quartz and alkali feldspars led to the formation of euhedral megacrysts of these minerals in the vicinity of clusters comprised of much smaller muscovite or α-spodumene crystals. Subsolidus replacement processes initiated during crystal–fluid interactions after the silicate melt was consumed were directly observed in the experiments. The experimental results underscore the important role of water as a medium for the transport of essential elements such as Si, Al, Na, and K from the silicate melt to the newly formed crystals, and provide important insights into the crystallization of miarolitic pegmatites.  相似文献   

19.
为详细探讨含钠长石翡翠的成因机制,笔者选取了若干来自缅甸的含钠长石翡翠,对其进行了详细的岩相学、矿物化学等方面的研究。含钠长石翡翠样品属于豆青种,主要由硬玉、钠长石、方沸石和少量的多硅白云母、钡铝硅酸盐等矿物组成。其中的硬玉发育清晰的环带结构,成分从核部至边缘发生规律性的成分变化。翡翠同时受到两期后期流体活动的改造,第一期以钠长石为代表,第二期以方沸石为代表,流体的改造作用使硬玉呈现碎裂状、碎斑状结构和交代穿孔等结构。结果表明,含钠长石翡翠样品表现出从成岩流体中直接结晶的特点,该流体富集Na、Al、Si、K、Ba以及少量的Ca、Fe、Mg等元素,微量元素则相对富集LREE、HFSE和sr等元素。结合前人的研究结果以及该玉石中的矿物反应关系,笔者推测缅甸翡翠形成的压力和温度范围分别在6-14kbar和300℃-450℃。  相似文献   

20.
Charge contrast imaging in the scanning electron microscope can provide new insights into the scale and composition of alkali feldspar microtextures, and such information helps considerably with the interpretation of their geological histories and results of argon isotope thermochronological analyses. The effectiveness of this technique has been illustrated using potassium-rich alkali feldspars from the Dartmoor granite (UK). These feldspars contain strain-controlled lamellar crypto- and microperthites that are cross-cut by strain-free deuteric microperthites. The constituent albite- and orthoclase-rich phases of both microperthite generations can be readily distinguished by atomic number contrast imaging. The charge contrast results additionally show that sub-micrometre-sized albite ‘platelets’ are commonplace between coarser exsolution lamellae and occur together to make cryptoperthites. Furthermore, charge contrast imaging reveals that the orthoclase-rich feldspar is an intergrowth of two phases, one that is featureless with uniform contrast and another that occurs as cross-cutting veins and grains with the {110} adularia habit. Transmission electron microscopy shows that the featureless feldspar is tweed orthoclase, whereas the veins and euhedral grains are composed of irregular microcline that has formed from orthoclase by ‘unzipping’ during deuteric or hydrothermal alteration. The charge contrast imaging results are especially important in demonstrating that deuteric perthites are far more abundant in alkali feldspars than would be concluded from investigations using conventional microscopy techniques. The unexpected presence of such a high volume of replacement products has significant implications for understanding the origins and geological histories of crustal rocks and the use of alkali feldspars in geo- and thermochronology. Whilst the precise properties of feldspars that generate contrast remain unclear, the similarity between charge contrast images and corresponding cathodoluminescence images of deuteric microperthites indicates that trace element chemistry and possibly also elastic strain within the crystal play a major role.  相似文献   

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