首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 31 毫秒
1.
2.
Semi‐pelitic rocks ranging in grade from the prehnite–pumpellyite to the greenschist facies from south‐eastern Otago, New Zealand, have been investigated in order to evaluate the reactions leading to formation and breakdown of stilpnomelane. Detrital grains of mica and chlorite along with fine‐grained authigenic illite and chlorite occur in lower‐grade rocks with compactional fabric parallel to bedding. At higher grades, detrital grains have undergone dissolution, and metamorphic phyllosilicates have crystallized with preferred orientation (sub)parallel to bedding, leading to slaty cleavage. Stilpnomelane is found in metapelites of the pumpellyite–actinolite facies and the chlorite zone of the greenschist facies, but only rarely in the biotite zone of the greenschist facies. Illite or phengite is ubiquitous, whereas chlorite occurs only rarely with stilpnomelane upgrade of the pumpellyite‐out isograd. Chemical and textural relationships suggest that stilpnomelane formed from chlorite, phengite, quartz, K‐feldspar and iron oxides. Stilpnomelane was produced by grain‐boundary replacement of chlorite and by precipitation from solution, overprinting earlier textures. Some relict 14 Å chlorite layers are observed by TEM to be in the process of transforming to 12 Å stilpnomelane layers. The AEM analyses show that Fe is strongly partitioned over Mg into stilpnomelane relative to chlorite (KD≈2.5) and into chlorite relative to phengite (KD≈1.9). Modified A′FM diagrams, projected from the measured phengite composition rather than from ideal KAl3Si3O10(OH)2, are used to elucidate reactions among chlorite, stilpnomelane, phengite and biotite. In addition to pressure, temperature and bulk rock composition, the stilpnomelane‐in isograd is controlled by variations in K, Fe3+/Fe2+, O/OH and H2O contents, and the locus of the isograd is expected to vary in rocks of different oxidation states and permeabilities. Biotite, quartz and less phengitic muscovite form from stilpnomelane, chlorite and phengite in the biotite zone. Projection of bulk rock compositions from phengite, NaAlO2, SiO2 and H2O reveals that they lie close to the polyhedra defined by the A′FM minerals and albite. Other extended A′FM diagrams, such as one projected from phengite, NaAlO2, CaAl2O4, SiO2 and H2O, may prove useful in the evaluation of other low‐grade assemblages.  相似文献   

3.
The solubility of Tio2 in phlogopites has been experimentally determined in the system K2Mg6Al2Si6O20(OH)4-K2Mg4TiAl2Si6O20(OH)4-K2Mg5TiAl4Si4O20(OH)4 between 825–1300°C and 10–30 kbar under vapour absent conditions. Starting compositions lie along the join K2Mg6Al2Si6O20(OH)4-K2Mg4.5TiAl3Si5O20(OH)4 which represents a combination of the Mg[VI]2Si[IV] = Ti[VI]2Al[VI] and 2Mg[VI] = Ti[VI][VI] substitution mechanisms for Ti in phlogopites. The results of the experiments indicate a systematic increase in solubility of Ti with increasing temperature and decreasing pressure for given bulk Tio2 content. Under isobaric conditions high temperature Ti-saturated phlogopite breaks down to Ti-deficient phlogopite + rutile + vapour. Mass balance calculations suggest that the vapour phase may contain K2O dissolved in H2O and that the reaction is controlled by the vapour phase. Analyses of phlogopites coexisting with rutile and vapour can be represented in terms of the end-member components phlogopite [K2Mg6Al2Si6O20(OH)4], eastonite [K2Mg5Al4Si5O20(OH)4], an octahedral site deficient Ti-phlogopite (Ti-OSD) of composition K2(Mg4Ti□)Al2Si6)O20(OH)4, and Ti-eastonite [K2Mg5TiAl4Si4O20(OH)4]. With decreasing amounts of Ti in these phlogopites there is a decrease in the Ti-eastonite component and increase in the eastonite component.The general equation for the breakdown of Ti-phlogopite solid solution to Ti-free phlogopite + rutile + vapour is: 14 Ti-eastonite + 7 Ti-OSD ? 16 eastonite + 3 phlogopite + 21 rutile + 4 H2O + 2 K2O. Lack of knowledge of H2O and K2O activities in the vapour phase does not permit evaluation of thermodynamic constants for this reaction. The Ti solubility in phlogopites and hence its potential as a geothermobarometer under lower crustal to upper mantle conditions is likely controlled by common mantle minerals such as forsterite.  相似文献   

4.
Polarized infrared (IR) spectroscopy of olivine crystals from Zabargad, Red Sea shows the existence of four pleochroic absorption bands at 3,590, 3,570, 3,520 and 3,230 cm?1, and of one non pleochroic band at 3,400 cm?1. The bands are assigned to OH stretching frequencies. Transmission electron microscopy (TEM) shows no oriented intergrowths in this olivine; it is concluded that OH is structural. On the basis of the pleochroic scheme of the absorption spectra it is proposed that [□O(OH)3] and [□O2(OH)2] tetrahedra occur as structural elements, assuming that the vacancies are on Si sites. If M2 site vacancies were assumed [SiO3(OH)] and [SiO2(OH)2] tetrahedra occur as structural elements.  相似文献   

5.
Subsolidus phase relations for a K-doped lherzolite are investigated in the model system K2O–Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O at 1.5–6.0 GPa and 680–1,000°C. Phlogopite is ubiquitous and coexists with Ca-amphibole up to 3.2 GPa and 900°C. High-pressure phlogopites show a peculiar mineral chemistry dependent on pressure: e.g., at 5.5 GPa and 680°C, excess of Si (up to 3.4 apfu) coupled with deficiency in Al (as low as 0.58 apfu) and K + Na (as low as 0.97 apfu), suggest a significant amount of a talc/10 Å phase component ([v]XIISi1K?1Al ?1 IV , where [v]XII is interlayer vacancy). Mixed layering or solid solution relations between high-pressure phlogopites and the 10 Å phase, Mg3Si4O10(OH)2 nH2O, are envisaged. Phlogopite modal abundance, derived by weighted least squares, is maximum at high-pressure and relative low-temperature conditions and therefore along the slab–mantle interface (10.3 ± 0.7 wt.%, at 4.8 GPa, 680°C). In phlogopite-bearing systems, Ca-amphibole breaks down between 2.5 and 3.0 GPa, and 1,000°C, through the water conservative reaction 5(pa + 0.2 KNa?1) + 17en + 15phl = (10di + 4jd) + 5py + 12fo + 20(phl + 0.2 talc), governed by bulk composition and pressure-dependent variations of K/OH in K-bearing phases and as a result, it does not necessarily imply a release of fluid.  相似文献   

6.
7.
Polarized Raman spectra were collected for single crystal buergerite (NaFe3Al6(BO3)3Si6O18(O0.92(OH)0.08)3F) from room temperature to near 1,375°C. Vibrational assignments to features in the room temperature spectra were determined by lattice dynamics calculations, where internal BO3 motions dominate modes near 1,300 cm−1, internal SiO4 displacements dominate modes between 900 and 1,200 cm−1, while less localized displacements within the isolated Si6O18 ring mix with motions within Na, Fe, Al, F, and BO3 environments for fundamental modes below 780 cm−1. At elevated temperatures, most buergerite Raman features broaden and shift to lower frequencies up to 900°C. Above this temperature, the lattice mode peaks evolve into broad bands, while OH stretch modes near 3,550 cm−1 disappear. According to Raman spectroscopy, X-ray diffraction, differential thermal analysis, and scanning electron microscopy, buergerite undergoes a complex transition that starts near 700°C and extends over a 310°C interval, where initially, Al and Fe probably become disordered within the Y- and Z-sites, and most F and all OH are later liberated. A reversible crystal-to-amorphous transition is seen by Raman for buergerite fragments heated as high as 930°C. Buergerite becomes permanently altered when heated to temperatures greater than 930°C; after cooling to room temperature, these altered fragments are comprised of mullite and Fe-oxide crystals suspended in an amorphous borosilicate matrix.  相似文献   

8.
 中国东部花岗岩类141个Mg-Fe云母的化学成分将近90%的变化属于八面体层内的类质同象置换,置换矢量Mg 1Fe+2和Fe-3+2(R+3)-2组成了天然黑云母平面,大约80%的变化应当解释为基本置换8Mg 1Fe+2+Fe-3+2(R+3)2.这些是Mg-Fe云母在广泛的自然条件下表现出来的最主要的晶体化学关系。文中还提出了置换矢量的长度、分量和以及电价和三个参数,用以识别矿物化学成分变化的类质同象置换特征。  相似文献   

9.
10.
11.
The molar volume of glaucophane [Na2Mg3Al2Si8O22(OH)2] has been determined in this study by correcting synthetic glaucophane-rich amphiboles made in the system Na2O–MgO–Al2O3–SiO2–H2O for very small deviations from ideal glaucophane composition using recent volume data on key amphibole components. The derived unit-cell volume for end-member glaucophane is 862.7±1.6 Å3, which gives a molar volume of 259.8±0.5 cm3/mol and a calculated density of 3.016±0.006 g/cm3. This value has been corroborated through an essentially independent method by correcting the volumes of natural sodic amphiboles reported in the literature for non-glaucophane components, particularly including calcium-rich components, to yield a value of 861.2±1.9 Å3. The unit-cell volume derived from the synthetic amphiboles, which is considered here to be more reliable, is somewhat smaller than that reported previously in the literature. A thermal expansion (αV) at 298 K of 1.88±0.06×10?5/K was derived from unit-cell volumes measured in the range of 25–500°C for a synthetic glaucophane sample, which is noticeably smaller than previously reported.  相似文献   

12.
13.
The solubility and incorporation mechanisms of hydrogen in synthetic stishovite as a function of Al2O3 content have been investigated. Mechanisms for H incorporation in stishovite are more complex than previously thought. Most H in stishovite is incorporated via the Smyth et al. (Am Mineral 80:454–456, 1995) model, where H docks close to one of the shared O–O edges, giving rise to an OH stretching band in infrared (IR) spectra at 3,111–3,117 cm−1. However, careful examination of IR spectra from Al-stishovite reveals the presence of an additional OH band at 3,157–3,170 cm−1. All H is present on one site, with interstitial H both coupled to Al3+ substitutional defects on adjacent octahedral (Si4+) sites, and decoupled from other defects, giving rise to two distinct absorption bands. Trends in IR data as a function of composition are consistent with a change in Al incorporation mechanism in stishovite, with Al3+ substitution for Si4+ charge-balanced by oxygen vacancies at low bulk Al2O3 contents, and coupled substitution of Al3+ onto octahedral (Si4+) and interstitial sites at high bulk Al2O3 contents. Trends in OH stretching frequencies as a function of Al2O3 content suggest that any such change in Al incorporation mechanism could alter the effect that Al incorporation has on the compressibility of stishovite, as noted by Ono et al. (Am Mineral 87:1486–1489, 2002).  相似文献   

14.
15.
16.
Aqualite, a new eudialyte-group mineral from hydrothermally altered peralkaline pegmatites of the Inagli alkaline pluton (Sakha-Yakutia, Russia) is described in this paper. Natrolite, microcline, eckermanite, aegirine, batisite, innelite, lorezenite, thorite, and galena are associated minerals. Aqualite occurs as isometric crystals up to 3-cm across. The color is pale pink, with a white streak and vitreous luster. The mineral is transparent. The fracture is conchoidal. The mineral is brittle; no cleavage or parting is observed. The Mohs’ hardness is 4 to 5. The density is 2.58(2) g/cm3 (measured by the volumetric method) and 2.66 g/cm3 (calculated). Aqualite is optically uniaxial (+), α = 1.569(1) and β = 1.571(1). The mineral is pleochroic from colorless to pale pink on X and pink on Y, α < β. Aqualite is weakly fluorescent with a dull yellow color under ultraviolet light. The mineral is stable in 50% HCl and HNO3 at room temperature. Weight loss after ignition at 500°C is 9.8%. Aqualite is monoclinic, and the space group is R3. The unit-cell dimensions are a = 14.078(3) Å, c = 31.24(1) Å, V = 5362 Å3, and Z = 3. The strongest reflections in the X-ray powder pattern [d, Å (I)(hkl)] are: 4.39(100)(2005), 2.987(100)(315), 2.850(79)(404), 10.50(44)(003), 6.63(43)(104), 7.06(42)(110), 3.624(41)(027), and 11.43(39)(101). The chemical composition (electron microprobe, H2O determined with the Penfield method) is as follows (wt %): 2.91 Na2O, 1.93 K2O, 11.14 CaO, 1.75 SrO, 2.41 BaO, 0.56 FeO, 0.30 MnO, 0.17 La2O3, 0.54 Ce2O3, 0.36 Nd2O3, 0.34 Al2O3, 52.70 SiO2, 12.33 ZrO2, O.78 TiO2, 0.15 Nb2O5; 1.50 Cl, 9.93 H2O,-O=Cl2 0.34; where the total is 99.46. The empirical formula calculated on the basis of Si + Zr + Ti + Al + Nb = 29 apfu is as follows: [(H3O)7.94Na2.74K1.20Sr0.49Ba0.46Fe0.23Mn0.12]Σ13.18(Ca5.79REE0.19)Σ5.98 (Zr2.92Ti0.08)Σ3.0(Si25.57Ti0.21Al0.19Nb0.03)S26.0[O66.46(OH)5.54]Σ72.0 [(OH)2.77Cl1.23]Σ4.0. The simplified formula is (H3O)8(Na,K,Sr)5Ca6Zr3Si26O66(OH)9Cl. Aqualite differs from typical eudialyte by the extremely low contents of Na and Fe, with more than 50% Na being replaced with the (H3O)+ group. The presence of oxonium ions is confirmed by IR spectroscopic and X-ray single-crystal diffraction analysis. The mineral is compared with five structurally studied high-oxonium analogues from alkaline plutons of other regions. All of these minerals were formed at a relatively low temperature through the ion-exchange transformation of “protoeudialytes”; the successor minerals inherited the principal structural and compositional features of the precursor minerals. The name aqualite is derived from the Latin aqua in reference to its specific chemical composition. The type material of aqualite is deposited in the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow.  相似文献   

17.
A series of alumina-free micas was synthesized hydrothermally in the potassium-poor portion of the system K2O-MgO-SiO2-H2O. One end member of this series has the composition KMg2.5[Si4O10](OH)2, which, because of its octahedral occupancy, is intermediate between the dioctahedral and trioctahedral micas.From this end member a series of mica solid solutions extends towards more Mg-rich compositions. Single phase micas were obtained along the substitution line 2Mg for Si which appears to involve incorporation of part of the Mg in tetrahedral sites. It leads to a theoretical end member with a structural formula KMg3[Si3.5Mg0.5O10](OH)2. Solid solutions containing up to 75 mole % of this theoretical end member could be synthesized. The observed densities, water contents, and a one-dimensional Fourier synthesis are consistent with the assumed substitution.At 1 kb fluid pressure and 620° C the Si-rich end member KMg2.5[Si4O10](OH)2 decomposes to a more Mg-rich mica, the roedderite phase K2Mg5Si12O30, liquid, and H2O-rich vapor. With increasing Mg-content the thermal stability of the mica solid solutions increases up to 860°C at a composition of about K2O·6.2MgO·7.4SiO2·2H2O, i.e. KMg2.8[Si3.7Mg0.3O10](OH)2. This mica disintegrates directly into forsterite + liquid + H2O-rich vapor. The mica phase richest in Mg with a composition of about K2O·6.5MgO·7.25SiO2·2H2O, i.e. KMg2.875 [Si3.625Mg0.375O10](OH)2, breaks down at 765° C into forsterite, a more Si-rich mica, liquid, and H2O-rich vapor.This binary series of alumina-free micas forms a complete series of ternary solid solutions with normal phlogopite, KMg3[Si3AlO10](OH)2. Analyses of some natural phlogopites showing Si in excess of 3.0 (up to 3.18) per formula unit can be explained through this ternary miscibility range.  相似文献   

18.
Si+4 Content of natural phengites   总被引:2,自引:0,他引:2  
The chemical compositions of white micas separated from adjacent rocks of glaucophane and greenschist facies are compared with respect to their Si+4 content. The micas are predominantly phengitic, i.e. between muscovite, K[Al2Si3AlO10(OH)2] and celadonite, K[(R+2R+3)Si4O10(OH)2] in composition. Constancy of Si content in micas coming from rocks of different bulk chemical composition but closely similar physical conditions indicates that the silica content of a potassic dioctahedral mica can be used to indicate the pressure and temperature conditions of its formation. This conclusion is in part based upon previous experimental data obtained for synthetic phengites.  相似文献   

19.
Wadeite-type K2Si4O9 was synthesized with a cubic press at 5.4 GPa and 900 °C for 3 h. Its unit-cell parameters were measured by in situ high-T powder X-ray diffraction up to 600 °C at ambient P. The TV data were fitted with a polynomial expression for the volumetric thermal expansion coefficient (αT = a 0 + a 1 T), yielding a 0 = 2.47(21) × 10?5 K?1 and a 1 = 1.45(36) × 10?8 K?2. Compression experiments at ambient T were conducted up to 10.40 GPa with a diamond-anvil cell combined with synchrotron X-ray radiation. A second-order Birch–Murnaghan equation of state was used to fit the PV data, yielding K T = 97(3) GPa and V 0 = 360.55(9) Å3. These newly determined thermal expansion data and compression data were used to thermodynamically calculate the PT curves of the following reactions: 2 sanidine (KAlSi3O8) = wadeite (K2Si4O9) + kyanite (Al2SiO5) + coesite (SiO2) and wadeite (K2Si4O9) + kyanite (Al2SiO5) + coesite/stishovite (SiO2) = 2 hollandite (KAlSi3O8). The calculated phase boundaries are generally consistent with previous experimental determinations.  相似文献   

20.
Biachellaite, a new mineral species of the cancrinite group, has been found in a volcanic ejecta in the Biachella Valley, Sacrofano Caldera, Latium region, Italy, as colorless isometric hexagonal bipyramidal-pinacoidal crystals up to 1 cm in size overgrowing the walls of cavities in a rock sample composed of sanidine, diopside, andradite, leucite and hauyne. The mineral is brittle, with perfect cleavage parallel to {10$ \bar 1 $ \bar 1 0} and imperfect cleavage or parting (?) parallel to {0001}. The Mohs hardness is 5. Dmeas = 2.51(1) g/cm3 (by equilibration with heavy liquids). The densities calculated from single-crystal X-ray data and from X-ray powder data are 2.515 g/cm3 and 2.520 g/cm3, respectively. The IR spectrum demonstrates the presence of SO42−, H2O, and absence of CO32−. Biachellaite is uniaxial, positive, ω = 1.512(1), ɛ = 1.514(1). The weight loss on ignition (vacuum, 800°C, 1 h) is 1.6(1)%. The chemical composition determined by electron microprobe is as follows, wt %: 10.06 Na2O, 5.85 K2O, 12.13 CaO, 26.17 Al2O3, 31.46 SiO2, 12.71 SO3, 0.45 Cl, 1.6 H2O (by TG data), −0.10 −O=Cl2, total is 100.33. The empirical formula (Z = 15) is (Na3.76Ca2.50K1.44)Σ7.70(Si6.06Al5.94O24)(SO4)1.84Cl0.15(OH)0.43 · 0.81H2O. The simplified formula is as follows: (Na,Ca,K)8(Si6Al6O24)(SO4)2(OH)0.5 · H2O. Biachellaite is trigonal, space group P3, a =12.913(1), c = 79.605(5) ?; V = 11495(1) ?3. The crystal structure of biachellaite is characterized by the 30-layer stacking sequence (ABCABCACACBACBACBCACBACBACBABC). The tetrahedral framework contains three types of channels composed of cages of four varieties: cancrinite, sodalite, bystrite (losod) and liottite. The strongest lines of the X-ray powder diffraction pattern [d, ? (I, %) (hkl)] are as follows: 11.07 (19) (100, 101), 6.45 (18) (110, 111), 3.720 (100) (2.1.10, 300, 301, 2.0.16, 302), 3.576 (18) (1.0.21, 2.0.17, 306), 3.300 (47) (1.0.23, 2.1.15), 3.220 (16) (2.1.16, 222). The type material of biachellaite has been deposited at the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia, registration number 3642/1.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号