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1.
Summary Fine-grained homogeneous powder samples of thirteen trioctahedral micas, mostly intermediate members of the phlogopite – annite solid solution series, and samples close to the phlogopite, fluor-phlogopite and tetra-ferriphlogopite end members have been examined at the potassium K-edge by X-ray absorption fine structure spectroscopy. The interlayer K+ cation is in a coordination that is certainly lower than 12, in contrast to what is expected from the ideal hexagonal symmetry model of the mica structure, and approaches – but it does not reach – coordination 6, as it should be when the effective ligands are the three nearest outer bridging oxygens of two facing upper and lower tetrahedral sheets. The observed range of coordinations implies that only some of the three inner bridging oxygen atoms in each sheet are involved, thus leading to 6±(1 … 6) effective configurations depending on the composition of the individual mica terms. The effective coordination number was found to vary continuously with composition from 11 to 7 and to be related to the tetrahedral rotation angle (α) according to two different linear relationships for the phlogopite – annite series (Fe2+Mg−1 exchange vector, involving the octahedral sheet only) and the phlogopite – tetra-ferriphlogopite series (Fe3+Al−1 vector, involving the tetrahedral sheet), respectively. Substitutions affecting either the A cation in the interlayer or the X anion in the octahedral sheet also affect the observed trends. In particular, the latter substitution effect is best seen in two near end member phlogopites, where the fluorine to hydroxyl substitution (F− (OH)−−1 exchange vector), which greatly changes the α tetrahedral rotation angle is, reflected in the experimental K XANES spectra by modifying not only the energy but also the intensities of most multiple scattering features.  相似文献   

2.
The surface structure of muscovite was imaged using an atomic force microscope (AFM) in contact mode in water. The following three types of AFM images were observed: (1) those showing clearly the arrangement of hexagonal rings of SiO4 tetrahedra; (2) those showing a hexagonal array of bright spots separated by a distance of about 5.3 Å; and (3) those changing gradually from image (2) to image (1). Image (1) successfully provides information on the tetrahedral tilt and basal surface corrugation that are particularly characteristic of dioctahedral micas. The mean unit cell dimensions for the muscovite surface measured from Image (1) were slightly longer than those of the bulk structure, due to the rehydration of the tetrahedral sheet and/or surface relaxation. Image (2) was made by varying the scan angles, even on the same surface in which Image (1) was obtained. Image (3) has information on a single plane rather than on two or more planes involving steps, kinks and so on. Therefore, what is depicted in Images (2) and (3) is not the arrangement of interlayer K ions but the basal plane of the tetrahedral sheet. Some structural relaxation of the tetrahedral sheet surface was also observed. Gradual expansion and contraction of hexagonal rings were randomly found on the muscovite surface. The surface relaxation results from a tetrahedral rotation and/or tilting after cleaving, since significant variations of both distances and bond angles between adjacent SiO4 tetrahedra on the surface were found.  相似文献   

3.
Summary Ti-bearing phlogopite-biotite is dominant in Ugandan kamafugite-carbonatite effusives and their entrained alkali clinopyroxenite xenoliths. It occurs as xeno/phenocrysts, microphenocrysts and groundmass minerals and also as a major xenolith mineral. Xenocrystic micas in kamafugites and carbonatites are aluminous (> 12 wt% Al2O3), typically contain significant levels of Cr (up to 1.1 wt% Cr2O3), and are Ba-poor. Microphenocryst and groundmass micas in feldspathoidal rocks extend to Al-poor compositions, are depleted in Cr, and are generally enriched in Ba. In general, xenocrystic micas occupy the Al2O3 and TiO2 compositional field of the xenolith mica, and on the basis of Mg#, and high P, T experimental evidence they probably crystallised at mantle pressures. Mica xenocryst Cr contents range from those in Cr-poor megacryst and MARID phlogopite to higher values found in primary and metasomatic phlogopites in kimberlite-hosted peridotite xenoliths. Such Cr contents in Ugandan mica xenocrysts are considered consistent with derivation from carbonate-bearing phlogopite wehrlite and phlogopite-clinopyroxenite mantle. Olivine melilitite xenocryst micas are distinguished by higher Mg# and Cr content than mica in clinopyroxenite xenoliths and mica in Katwe-Kikorongo mixed melilitite-carbonatite tephra. Higher Al2O3 distinguishes Fort Portal carbonatite xenocrysts and some contain high Cr. It is suggested that the genesis of Katwe-Kikorongo olivine melilitite and Fort Portal carbonatite involves a carbonate-bearing phlogopite wehrlite source while the source of the mixed carbonatite-melilitite rocks may be carbonate-bearing phlogopite clinopyroxenite. Received January 24, 2000; revised version accepted September 27, 2001  相似文献   

4.
 Coexisting muscovite and paragonite have been observed in an eclogite from the Sesia–Lanzo Zone (Western Alps, Italy). The P-T conditions of this eclogite reached 570–650 °C and 19–21 kbar and the rocks show several stages of mineral growth during their retrograde path, ranging from the subsequent lower-P eclogite facies to the blueschist facies and then the greenschist facies. Muscovite and paragonite are very common in these rocks and show two texturally different occurrences indicating equilibrium and non-equilibrium states between them. In one mode of occurrence they coexist in equilibrium in the lower-P eclogite facies. In the same rock muscovite ± albite also replaced paragonite during a greenschist-facies overprint, as evidenced by unique across – (001) layer boundaries. The chemical compositions of the lower-P eclogite-facies micas plot astride the muscovite – paragonite solvus, whereas the compositions of the greenschist-facies micas lie outside the solvus and indicate disequilibrium. The TEM observations of the textural relations of the greenschist-facies micas imply structural coherency between paragonite and muscovite along the layers, but there is a sharp discontinuity in the composition of the octahedral and tetrahedral sheets across the phase boundary. We propose that muscovite formed through a dissolution and recrystallization process, since no gradual variations toward the muscovite – paragonite interfaces occur and no intermediate, homogeneous Na-K phase has been observed. Because a solid-state diffusion mechanism is highly unlikely at these low temperatures (300–500 °C), especially with respect to octahedral and tetrahedral sites, it is assumed that H2O plays an important role in this process. The across-layer boundaries are inferred to be characteristic of such non-equilibrium replacement processes. The characterization of these intergrowths is crucial to avoiding erroneous assumptions regarding composition and therefore about the state of equilibrium between both micas, which in turn may lead to misinterpretations of thermometric results. Received: 3 February 1999 / Accepted: 19 October 1999  相似文献   

5.
Pre-ore hydrothermal alteration in an unconformity-type uranium deposit   总被引：1，自引：0，他引：1  
 Hydrothermal alteration associated with the formation of unconformity-type uranium deposits is manifested mainly by intense chloritic replacement of metamorphic mineral assemblages. At the Koongarra deposit (northern Australia), three stages of pre-ore alteration were identified in chloritized metamorphic muscovite using optical microscopy, the electron microprobe, and transmission and analytical electron microscopy (TEM and AEM). During the first stage metamorphic muscovite was altered to a mica with very low interlayer occupancy (K content as low as 0.35 compared to 1 for ideal muscovite), high octahedral cation content (up to 2.3 whereas ideal muscovite has 2) and high Si content. High resolution TEM images show that these changes in composition were accompanied by frequent layer terminations in contrast to the metamorphic muscovite, whereas selected area electron diffraction patterns indicate that the altered muscovite remained a two-layer polytype. The alteration of muscovite appears more extensive in the outer alteration halo and has been almost completely eradicated in the ore zone where chloritization is more complete. During the second stage of the pre-ore alteration the altered muscovite was replaced by Mg-rich serpentine (lizardite) and clinochlore. High resolution TEM images show two 10 A˚ mica layers coalescing to one 14 A˚ chlorite layer. This type of coalescence appears to involve transformation of one dioctahedral layer of the mica structure to one trioctahedral brucite layer in the chlorite structure through dissolution of the tetrahedral sheets and cation influx in the octahedral site. The final stage of the pre-ore alteration resulted in replacement of lizardite and clinochlore by Fe-rich chlorite. Comparison of the alteration observed in metamorphic muscovite with the alteration observed in metamorphic biotite and garnet shows clearly that chloritization of these phases was also a pre-ore alteration event. Pre-ore alteration was principally characterized by the evolution of chlorite from a Mg-rich phase to an Fe-rich one. Received: 29 July 1994/Accepted: 8 January 1995  相似文献   

6.
Micas from magmatic and metamorphic rocks differ from one another in chemical composition and in trace element content. The chemical composition of micas is discussed in relation to their occurrence, paragenesis and sequence of crystallization. On the basis of previous studies of the relationship between the physical properties and the chemical composition of 34 chemically analysed micas, reliable physical methods have been established which permit identification of different mica varieties in the same rock. Structural formulae and trace element content of micas from basic and granitic rocks, as well as from skarns, schists, ortho- and paragneisses are discussed. The relationship between the components of the tetrahedral and octahedral layers and of the interlayer are illustrated as ratios. Poorly differentiated, hybrid and metasomatic rocks often contain more than one variety of mica. Some prophyritic basalts and lamprophyres contain an early phlogopite which is paragenetically related to pyroxene phenocrysts and late biotite which occurs in the groundmass and in the fractures as a result of the crystallization of residual magma. The biotitemuscovite assemblage was observed in granodiorites, quartz-monzonites, schists and gneisses. In the albite-K-feldspar granites, muscovite predominates and the biotite is usually altered. The chemical composition of micas from metamorphic rocks depends on the grade of metamorphism and on the nature of associated minerals. The biotite from paragneisses contains considerable quantities of octahedral alumina. Pre-metamorphic micas show variable deficiencies of the (OH, F) group. The micas are useful minerals in determining the degree of differentiation and subsequent alteration of igneous rocks. The present study was carried out on the basis of 34 recent complete chemical analyses andca 100 X-ray fluorescence analyses. Dedicated to Professor Dr.Carl W. Correns on the occasion of his 70th birthday.  相似文献   

7.
The dependence on the composition of the thermodynamic stability of an illite can be treated in terms of a regular solution site-mixing model. Four end-member micas (muscovite, pyrophyllite, phlogopite, and annite) were mixed to simulate an illite in this study. In the model, random mixing of cations was assumed over each given class of cation sites. Mixing over cation sites between different classes of cation sites was not allowed. The resulting free energy and chemical potential equations contain four site interaction parameters: three for octahedral site interactions and one for interlayer and tetrahedral site interactions. These parameters cannot presently be evaluated because of a lack of experimental data on Fe3+-free illites. The model does imply that the octahedral site interaction parameters must be significantly more positive than the interlayer and tetrahedral site interaction parameter to account for the dominant dioctahedral nature of most natural illites. This constraint is necessary to balance out the increase in stability due to the configurational entropy of an illite having a major trioctahedral component.The model can be extended to cover a wider range of illite compositions by the inclusion of an end-member mica containing Fe3+ ions in the octahedral sites. At present the thermodynamic properties of such an end-member are unknown.  相似文献   

8.
 Dioctahedral 2:1 phyllosilicates with different interlayer charge have been studied theoretically by using transferable empirical interatomic potentials. The crystal structures of pyrophyllite, muscovite, margarite, beidellite, montmorillonite, and different smectites and illites have been simulated. The interatomic potentials were able to reproduce the experimental structure of phyllosilicates with high, medium and low interlayer charge. The calculated structures are in agreement with experiment for the main structural features of the crystal lattice. The effect of the cation substitution in the octahedral and tetrahedral sheets on the structural features has been also studied. Good linear relationships have been found, and the calculated effects are consistent with experimental results. Some unknown structural features of the crystal structures of clays are predicted in this work. Received: 8 March 2000 / Accepted: 19 September 2000  相似文献   

9.
孙世华  于洁 《地质科学》1989,(2):176-189
给出云母化学成分比值FM=6(Fe+2+Mn)/(Fe2++Mn+Mg),用以表示Mg-Fe云母成分变化的基本趋势。根据291个云母的化学成分相对于FM值的异常变化,提出了天然Mg-Fe云母自然分类的新方案:把FM≌1.20(1.05-1.35)、4.80作为区分金云母、黑云母和铁叶云母的界线;FM>0.60的金云母称为铁金云母;以FM=2.70、3.60把黑云母再细分为富镁黑云母、镁铁黑云母和富铁黑云母。Mg-Fe云母自然分类的本质是各成员云母之间白云母置换方式和限度的潜在差异在不同的自然结晶条件下的反映。  相似文献   

10.
Phlogopites in the wolgidites, fitzroyites and cedricites (leucite lamproites) of the West Kimberley area range in composition from titaniferous phlogopite to titaniferous tetraferriphlogopite. Two distinct compositional trends are present. In wolgidites micas are characterized by strong total iron enrichment, moderate Al and Mg depletion and little enrichment in tetrahedral ferric iron. Micas in fitzroyites and cedricites are characterized by strong depletion in Al and strong enrichment in tetrahedral ferric iron at approximately constant Mg contents. Individual centers of fitzroyite intrusion are characterized by distinct mica TiO2 contents. The phlogopite compositional trends at low pressures are very different to those of kimberlite micas. An origin of these lamproites involving differentiation of a kimberlitic magma is considered unlikely.  相似文献   

11.
焦家式金矿中铬铝云母化学成分的标型性   总被引：1，自引：1，他引：1  
本文系统研究了胶东地区金矿中产出的铬铝云母亚族中３个白云母、１６个绢云母和３２个含Ｃｒ云母种属的化学成分特征。提出了含Ｃｒ云母种属结构层内四面体、八面体及层间离子种类和数量在焦家式金矿成矿过程中的指示作用，阐明了白云母、绢云母及含Ｃｒ云母种属之间的演化特征在与焦家式金矿化有关的成岩成矿作用中的标型意义。  相似文献   

12.
This paper gives several new and strong arguments in favour of the possibility of fixation of anhydrous lithium in the interlayer space of trioctahedral potassium micas. From the chemical viewpoint Li+ can replace K+, but is located out of the alkaline cation site; it enters pseudo-octahedral cavities limited by the triangular bases of two aluminous tetrahedra of two consecutive sheets. The solubility limit of Li+ in the interlayer is a function of the AlIV content of the mica. It is given by the relation (Li/Li + K)max = 2[(Al/Si + Al)IV]2. In both micas investigated — phlogopite, KMg3(Si3Al)O10(OH)2, and eastonite, K(Mg2.5Al1.5)(Si2.5Al1.5)O10(OH)2 — there is a remarkable agreement between the calculated values of the solubility limits and those measured by exchange reactions with hydrothermal solutions, at 600°C, 2 kbar. In high-Al micas, the interlayer Li content can be very important, with about one-third of K+ replaced by Li+.

The fixation of Li+ according to this model provokes a strong flattening of the interlayer (strong decrease of the reticular distance d005) and a slight increase of the reticular distance d060. Infra-red (IR) absorption spectrometry shows that vacant K+ sites are created when Li+ enters the interlayer; one observes low-frequency OH stretching bands attributed to OH dipoles lying towards these empty sites. Fixation of Li+ does not provoke any modification of the IR spectra in the region 1200-300 cm−1, indicating that Li+ is really out of the sheet. For both cell dimensions and IR spectra, a comparison is made with “ordinary” lepidolites, having Li+ in the octahedral sheet; it provides a guide for the distinction between the two species of Li-bearing micas.  相似文献   


13.
Two different dry-ground muscovites were placed separately in deionized water saturated with 1 atm partial pressure of carbon dioxide at room temperature and pressure for about 1000 hr. The carbonic acid formed buffered the bulk solution between 4.6 and 5.1 during the whole time of the experiment. K and Na were rapidly released by a reaction with hydrogen ions in the solution, which is usually attributed to a surface exchange reaction. The cation exchange capacity is close to the amount of K and Na released to the solution. However, measurement of surface areas of the micas both before and after the rapid release of K and Na, indicate that only one third of the cation exchange capacity can be attributed to release from the outer surfaces, whereas two thirds must be attributed to removal from within deeper parts of the structure.Approximately 2.5% of total K and Na from the interlayer regions, 1.5% of Mg from the octahedral sheets, and 0.5% of Si from the tetrahedral sheets were released by the end of the experiment, indicating that dissolution was incongruent in both cases. Si showed the slowest total release rate of all the ions. In the later stages (after 400 hr), the release rate of Si becomes a linear function of time. The linear release rate of Si was found to be 2.4 × 10?17 mol/cm2/sec for the ruby mica, and 2.7 × 10?17 mol/cm2/sec for the green mica. The rate-controlling mechanism in the dissolution of muscovite in the later stages is probably related to the rate of destruction of the tetrahedral sheets (Si-O bonds) which in turn is reflected by the rate of release of Si into the solution.  相似文献   

14.
High-pressure structural study of muscovite   总被引：1，自引：0，他引：1  
The compressibility and structural variations of two 2M1 muscovites having compositions (Na0.07K0.90 Ba0.01□0.02)(Al1.84Ti0.04Fe0.07Mg0.04)(Si3.02Al0.98) O10 (OH)2 (7 mole % paragonite) and (Na0.37K0.60□0.03)(Al1.84Ti0.02 Fe0.10Mg0.06)(Si3.03Al0.97) O10(OH)2 (37 mole % paragonite) were determined at pressures between 1 bar and 35 kbar, by single-crystal X-ray diffraction using a Merrill-Bassett diamond anvil cell. Isothermal bulk moduli, setting K′ = 4, were 490 and 540 (± 30) kbar for the Na-poor and Na-rich samples respectively. Both samples show highly anisotropic compressibility patterns, with β a ∶β b ∶β c = 1∶1.15∶3.95 for the Na-poor sample and β a ∶β b ∶β c = 1∶1.19∶3.46 for the Na-rich one. HP structural refinements showed that the different compressibility was largely due to the partial substitution of Na for K in the interlayer region. Moreover, the different compressibility of the tetrahedral and octahedral layers, observed in both micas, increased the a rotation of the tetrahedral layer by about 2° in 28 kbar, as also indicated by the evolution of interlayer cation bond lengths. This increases the repulsion of oxygens of the basal layers and between the high-charged cations of the tetrahedral layer. As a consequence, phengitic substitution, reducing α rotation, would increase the baric stability of mica. Comparison between the HP structures of muscovite and phlogopite indicated the lower compressibility of the latter, mainly due to the greater compressibility of the dioctahedral layer with respect to that of the trioctahedral layer. The HT and HP behaviour of di- and trioctahedral micas showed an anisotropy in the compressional pattern which was markedly greater than that observed in the dilatation pattern. This unexpected result was explained by the different evolution with P and T of alkaliO bond lengths. By combining HP and HT data, a tentative equation of state of muscovite is proposed.  相似文献   

15.
Microprobe analyses are performed on micas (biotite, muscovite and phlogopite) and chlorite from 1.9–1.8 Ga acid K- or Na-rich metavolcanics, cordierit-emica schists and manganiferous rocks from the Hästefält area in central Sweden. The results indicate that Fe-rich biotites and muscovites containing 10 to 25% celadonite and/or pyrophyllite are common in the K- and Na-rich metavolcanics. In the cordierite-mica schists the biotites are Mg-rich and the muscovites contain less than 10% celadonite and/or pyrophyllite. The predominant mica in the manganiferous rocks are phlogopite and less frequent rather pure muscovite. The chlorites show a wide range in composition, but principally those occurring in the K- and Na-rich metavolcanics are brunsvigite and diabantite and those in the cordierite-mica schists and the manganiferous rocks are mainly sheridanite and clinochlore. The chlorites of the manganiferous rocks show enrichment in Mn compared to those in other rock types. In general the compositional variations in the micas and less commonly chlorites are strongly controlled by rock type and fluid chemistry, particularly with respect to the ratio of FeO/(FeO+MgO). Estimates of maximum prograde metamorphic temperature, based on phyllosilicates and co-existing cordierite and garnets, indicate a value of up to 500° C.  相似文献   

16.
The Khaluta carbonatite deposit located in the western Transbaikal region was formed during the Late Mesozoic rifting in the southern framework of the Siberian Craton. Carbonatite is associated with shonkinite and syenite and is accompanied by fenitization. The composition of mica in more than 160 samples of country rocks, carbonatites, silicate rocks, and fenites was studied. The Fe3+ and Fe2+ contents, as well as oxygen isotopic composition, were determined. The Mg and Fe contents increase, whereas the Ti and Al contents decrease in micas when passing from silicate rocks and fenites to carbonatites. Micas from carbonatites are depleted in Al, enriched in Fe3+, and distinguished by high Si and F contents. According to our calculations, in some cases Al replaces Si in the tetrahedral site instead of replacement of Fe3+ as is characteristic of tetraferriphlogopite. Formally, the mica from carbonatites falls within the tetraferriphlogopite field, but typical inverse pleochroism is not always observable. The δ18O values of micas from carbonatite, shonkinite, syenite, and fenite are similar to those of mantle-derived silicate minerals. The δ18O values in the minerals coexisting with phlogopite testify to their isotopic equilibrium and make it possible to calculate the crystallization temperature of carbonatite.  相似文献   

17.
 K-richterite/phlogopite-bearing peridotite xenoliths and MARID inclusions have been found in Late Cretaceous (67±0.2 million years) monchiquites and an olivine nephelinite from North Eastern Morocco. It is the first evidence of MARID rocks and K-richterite/phlogopite-bearing peridotites outside the kimberlitic context. In the hydrous xenoliths, textural features suggest that K-richterite, phlogopite and Al-poor diopside are replacement minerals. K-richterites contain 2–5 wt% FeO, 0.1–1.5 wt% TiO2 and <0.5 wt% Cr2O3. Micas contain 5.4–7.4 wt% FeO and 0.3–2.2 wt% TiO2, with Cr and Ni contents <0.2 wt%. Diopsides are Al-poor (<0.2 wt% Al2O3) and contain 0.1–0.2 wt% TiO2, 0.9–1.1 wt% Na2O and 1.3–1.7 wt% Cr2O3. Compared to known K-richterites and micas from metasomatised peridotite nodules (PKP types), the Moroccan minerals are more Fe rich, K-richterites have higher Ti and micas less Cr and Ni. They are thus closer to MARID than to PKP minerals. K-richterites and mica from the MARID inclusions show typical characteristics, e.g. high FeO (4.3–4.7 wt% in richterite and 7.2 wt% in mica), low NiO and/or Cr2O3 and the incomplete filling of the tetrahedral site by Si+Al. Ion probe D/H analyses of amphiboles and micas from both xenolith types give high δD values ranging from –8 to –73, with large variations within single grains (up to 50‰). Both the D-enrichment and the δD variations are inherited from the mantle. The similar chemical composition and δD values of K-richterite/phlogopite from the hydrous peridotites and MARID minerals suggest a genetic link between the two types of xenoliths. The conditions required for producing MARIDs and K-richterite/phlogopite-bearing peridotites may thus exist in contexts other than stable cratonic settings. MARID rocks and the associated metasomites may result from a hydrous fluid interaction with a peridotite, the metasomatic agent being characterised by a high K and low Al signature and a high δD value. A D-rich source is involved in the metasomatic event producing the hydrous minerals, and the scatter observed in the δD values suggests a mixing between this source and another one with typical upper mantle D/H composition. As indicated by the low δD (–74) values of micas from the host lava, metasomatism predated and is unrelated to the alkaline volcanism. Received: 9 March 1995 / Accepted: 4 April 1996  相似文献   

18.
Coexisting white micas and plagioclase were studied by electronmicroprobe (EMP), and transmission and analytical electron microscopy(TEM—AEM) in greenschist- to amphibolite-grade metabauxitesfrom Naxos. The TEM—AEM studies indicate that sub-micronscale (0.01–1.0 µm thick) semicoherent intergrowthsof margarite, paragonite and muscovite are common up to loweramphibolite conditions. If unrecognized, such small-scale micainterlayering can easily lead to incorrect interpretation ofEMP data. Muscovite and paragonite in M2 greenschist-grade Naxosrocks are mainly relics of an earlier high-pressure metamorphism(M1). Owing to the medium-pressure M2 event, margante occursin middle greenschist-grade metabauxites and gradually is replacedby plagioclase + corundum in amphibolite-grade metabauxites.The margarite displays minor IVAl3 VI(Fe3+, Al) Si-3 VI--1 andconsiderable (Na, K) SiCa-1Al-1 substitution, resulting in upto 44 mol% paragonite and 6 mol % muscovite in solution. Thecompositional variation of muscovite is mainly described byVI(Fe2+, Mg) Si VI Al-1VI Al-1 and VI(Fe3+Al-1) exchanges, thelatter becoming dominant at amphibolite grade, Muscovite issignificantly richer in Fe than margarite or paragonite. Ca—Na—Kpartitioning data indicate that margarite commonly has a significantlyhigher Na/(Na+ K+Ca) value than coexisting muscovite or plagioclase.Exceptions are found in several greenschist-grade rocks, inwhich M1-formed mussovite may have failed to equilibrate withM2 margarite. The sluggishness of K-rich micas to recrystallizeand adjust composidonally to changing P-T conditions is alsoreflected in the results of mus-covite-paragonite solvus thermometry.Chemical data for Ca—Na micas from this study and literaturedata indicate that naturally coexisting margarite—paragonitepairs display considerably less mutual solubility than suggestedby experimental work. The variable and irregular Na partitioningbetween margarite and muscovite as observed in many metamorphicrocks could largely be related to opposing effects of pressureon Na solubility in margarite and paragonite and/or non-equilibriumbetween micas. KEY WORDS: Ca—Na—K mica; margarite; metabauxite; Naxos; sub-micron-scale mica interlayering  相似文献   

19.
 Cation partitioning data for coexisting muscovite and biotite are shown to be useful indicators of relative interlayer bond length/strength in these minerals. These data therefore provide a useful crystal-chemical perspective on relative mass-transfer kinetics of radiogenic isotopes, and account for the observation that biotite is generally less retentive of 40Ar and 87Sr than coexisting muscovite. Partitioning behavior of trace elements underscores three reasons why overall interlayer bonding in biotite is weaker than in muscovite. First, the preferences of large (Rb, Cs)+ in biotite and of small La3+ and Na+ in muscovite indicate a relatively spacious interlayer volume in biotite (suggesting a longer mean K−O bond). Second, the preference of interlayer vacancies in biotite (with some/all possibly H2O/H3O+-filled) suggests that its adjacent 2:1 sheets are connected by fewer interlayer bonds per unit cell than those of muscovite. Third, the relative exclusion of large Ba2+ from biotite despite its large interlayer sites is attributed to O−H bonds pointing into the interlayer cavity sub-normal to (001); (K+, Ba2+)-H+ repulsion thereby induced by the bare proton both destabilizes Ba2+ and weakens K−O bonds. In contrast, muscovite offers a more favorable electrostatic environment for Ba2+ substitution since its O−H bonds are directed into the vacant M 1 octahedral site sub-parallel to (001). This hypothesis is supported by the observation that progressive F(OH)−1 exchange enhances Ba2+ partitioning into biotite/phlogopite relative to coexisting muscovite. These crystal-chemical differences between biotite and muscovite are mirrored in calculated values of “ionic porosity”, Z i , defined here as the percentage of their interlayer unit-cell volume not occupied by ions. A monitor of ionic packing density and geometry, Z i is inversely correlated with K−O bond strength, which appears to be the rate-determining “kinetic common denominator” for a variety of processes affecting micas – including those responsible for loss of radiogenic isotopes in biotite and muscovite. Accordingly, the relatively longer/weaker K−O bonds of biotite are envisioned as being more easily stretched (during volume diffusion) or broken (during recrystallization or retrograde alteration). This in turn accounts for common observations of enhanced radiogenic Ar/Sr loss and younger 40Ar/39Ar and Rb/Sr ages in natural biotite (high Z i ) relative to coexisting muscovite (lower Z i ). Significantly, this pattern may arise irrespective of isotopic loss mechanism (diffusion or recrystallization, etc.), and it follows that any age discordance observed between muscovite and biotite cannot be ascribed uniquely to one mechanism or the other without appropriate field, petrographic, and petrologic constraints. Extension of this partitioning/porosity-based synthesis leads to prediction of corollary age-retentivity-composition effects among chemically diverse trioctahedral and dioctahedral micas, which are best field tested in terranes that cooled slowly under dry, static conditions. Pressure effects on argon retention are also inferred from the porosity model. Received: 9 February 1995 / Accepted: 8 September 1995  相似文献   

20.
Experiments have been conducted in a peralkaline Ti-KNCMASH system representative of MARID-type bulk compositions to delimit the stability field of K-richterite in a Ti-rich hydrous mantle assemblage, to assess the compositional variation of amphibole and coexisting phases as a function of P and T, and to characterise the composition of partial melts derived from the hydrous assemblage. K-richterite is stable in experiments from 0.5 to 8.0 GPa coexisting with phlogopite, clinopyroxene and a Ti-phase (titanite, rutile or rutile + perovskite). At 8.0 GPa, garnet appears as an additional phase. The upper T stability limit of K-richterite is 1200–1250 °C at 4.0 GPa and 1300–1400 °C at 8.0 GPa. In the presence of phlogopite, K-richterite shows a systematic increase in K with increasing P to 1.03 pfu (per formula unit) at 8.0 GPa/1100 °C. In the absence of phlogopite, K-richterite attains a maximum of 1.14 K pfu at 8.0 GPa/1200 °C. Titanium in both amphibole and mica decreases continuously towards high P with a nearly constant partitioning while Ti in clinopyroxene remains more or less constant. In all experiments below 6.0 GPa ΣSi + Al in K-richterite is less than 8.0 when normalised to 23 oxygens+stoichiometric OH. Rutiles in the Ti-KNCMASH system are characterised by minor Al and Mg contents that show a systematic variation in concentration with P(T) and the coexisting assemblage. Partial melts produced in the Ti-KNCMASH system are extremely peralkaline [(K2O+Na2O)/Al2O3 = 1.7–3.7], Si-poor (40–45 wt% SiO2), and Ti-rich (5.6–9.2 wt% TiO2) and are very similar to certain Ti-rich lamproite glasses. At 4.0 GPa, the solidus is thought to coincide with the K-richterite-out reaction, the first melt is saturated in a phlogopite-rutile-lherzolite assemblage. Both phlogopite and rutile disappear ca. 150 °C above the solidus. At 8.0 GPa, the solidus must be located at T≤1400 °C. At this temperature, a melt is in equilibrium with a garnet- rutile-lherzolite assemblage. As opposed to 4.0 GPa, phlogopite does not buffer the melt composition at 8.0 GPa. The experimental results suggest that partial melting of MARID-type assemblages at pressures ≥4.0 GPa can generate Si-poor and partly ultrapotassic melts similar in composition to that of olivine lamproites. Received: 23 December 1996 / Accepted: 20 March 1997  相似文献   

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