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1.
Solid solution in vesuvianite is elucidated by examining chemical trends and cation abundances in 22 microprobe analyses of samples from the Big Maria Mountains, southeastern California. Two recent structure refinements indicate 50 filled cation sites per formula, providing the basis for data normalization. Previous optical absorption and Mössbauer studies help clarify site occupancies. Stoichiometric abundances of Si and Ca + Na indicate 18 and 19 per formula, filling all 4- and 8-fold sites respectively. The four 6-fold A-sites are filled with Al. The solid solution occurs mainly within the eight 6-fold AlFe-sites (Al, Mg, Fe2+, Fe3+, Ti) and one 5-fold B-site (Mg, Fe2+, Fe3+). Chemical trends and crystal chemical constraints delineate eight independent substitutions.An extensive solid solution in the elements Mg, Fe, Al, and Ti suggests considerable potential as a petrogenetic indicator. In order to treat equilibria involving vesuvianite thermodynamically, a reference composition must be chosen and activity-composition relations modeled. For a reference composition, Mg-vesuvianite (Fe, Ti, Na-free) was chosen because of its chemical simplicity, but problems in ascertaining its stoichiometry have led previous workers to propose at least six different formulas. In this study, its formula is determined from the microprobe analyses by applying exchange vectors to substitute components of pure Mg-vesuvianite for Fe and Ti. This yields Ca19Mg2Al11-Si18 O69(OH)9, with AlFe-sites=MgAl7, and B-sites=Mg. Subdivision of the AlFe-sites into at least two distinct sites is suggested by observed chemical trends which are explanable only when different substitutions are considered to operate within different AlFe-sites.A thermodynamic mole fraction is formulated for Mg-vesuvianite based on an ideal mixing-on-sites solution model. A method is provided for estimating the distribution of Fe between the AlFe- and B-sites. Thermodynamic mole fractions calculated using Fe site distributions estimated from microprobe data yield results similar to those calulated using Fe site distributions determined from Mössbauer analysis.  相似文献   

2.
Hydrogrossular replacement of plagioclase in basaltic rocks enclosed in serpentinite, and relationships between hydrogrossular, pumpellyite, vesuvianite are described. Rogingitic rocks are dominated by mixed layer chlorite-smectite, chlorite, pumpellyite, hydrogrossular and vesuvianite. In these rocks pumpellyite attains maximal Mg contents, and chemical analysis shows extreme removal of Na2O, K2O, TiO2 and SiO2 from the basalts, and increase in the Fe3+/Fe2+ ratio. Leaching during serpentinization by extremely alkaline solutions dominated by Ca-Mg(OH) may explain removal of components, but the oxidation may suggest that at an earlier stage dykes may have acted as input aquifers.  相似文献   

3.
Twenty-two hornblendes separated from amphibolites and granulites of the Grenville Orogen of Ontario have been quantitatively analyzed for major and minor elements by electron microprobe, for FeO/Fe2O3 by wet chemistry, and for H2O by manometric measurement as H2. Hornblende formulae were calculated on the basis of 24O+OH+Cl+F. Most samples are magnesio-hornblendes, ferroan pargasitic hornblendes and ferroan hastingsitic hornblendes, with weight fractions of Fe3+/(Fe2++Fe3+) ranging from 0.15 to 0.50. An oxy-amphibole component of 0–25 mol%, with an average value of 17 mol%, is obtained for these complete analyses. When compared with structural formulae determined solely from microprobe data, normalization based on 13=Si+Ti+Al+Fe+Mn+Mg cations provides the best approximation to hornblende formulae calculated from the complete analyses. Less satisfactory agreement is obtained from a normalization scheme based on 15=Si+Ti+Al+Fe+Mn+Mg+Ca, while worst agreement is obtained from normalization to 23 oxygens assuming all Fe is Fe2+. No normalization scheme based on microprobe data alone consistently replicates the measured FeO, Fe2O3, and H2O; accurate determination of these values requires complete chemical analysies. Ionic solution models previously have been proposed to evaluate the activity of Ca2Mg5Si8 O22(OH)2(a Trem) in hornblende for use in equilibria that constrain the activity of H2O (a H 2O) in igneous and metamorphic rocks. Application of ionic models to typical hornblendes produces low a Trem (usually<0.01), consequetly yielding extremely low a H 2O. If an oxy-amphibole component is present, the calculated a Trem and H2O is further reduced. An oxy-amphibole component of 25% reduces the calculated H2O activity and that of any hydroxyl-amphibole component by 50% below that calculated with simplified assumptions regarding X OH in the hydroxyl site (i.e., X OH=1, or X OH=1–X Cl–X f). Thus, methods of amphibole normalizations appear to have a substantial effect on calculated amphibole and H2O activites. Before quantitative hornblende thermobarometry can be calibrated and applied, the amounts of FeO, Fe2O3 and H2O must be measured in order to fully characterize hornblende solid solutions.Contribution No. 478 from the Mineralogical Laboratory, University of Michigan  相似文献   

4.
Occurences of stable vesuvianite-epidote paragenesis within greenschist facies of metamorphism are described. It is suggested that this paragenesis gives way to grossularite-diopside (plus vesuvianiteor epidote) assemblage in the almandine-amphibolite facies. This interpretation conforms best to the results of petrographic observations and also satisfies the topological requirements of the CaO-(Mg, Fe2+, Mn)O-(Al, Fe3+)2O3-SiO2H2O-CO2 system, to which these assemblages belong. Contrary to the ideas in vogue, it is suggested that vesuvianite can very well originate within the greenschist facies, whereas the appearence of grossularite and diopside (consequent on which vesuvianite and epidote becomesmutually incompatible) ushers the almandine-amphibolite facies in.  相似文献   

5.
Experiments at high pressures and temperatures were carried out (1) to investigate the crystal-chemical behaviour of Fe4O5–Mg2Fe2O5 solid solutions and (2) to explore the phase relations involving (Mg,Fe)2Fe2O5 (denoted as O5-phase) and Mg–Fe silicates. Multi-anvil experiments were performed at 11–20 GPa and 1100–1600 °C using different starting compositions including two that were Si-bearing. In Si-free experiments the O5-phase coexists with Fe2O3, hp-(Mg,Fe)Fe2O4, (Mg,Fe)3Fe4O9 or an unquenchable phase of different stoichiometry. Si-bearing experiments yielded phase assemblages consisting of the O5-phase together with olivine, wadsleyite or ringwoodite, majoritic garnet or Fe3+-bearing phase B. However, (Mg,Fe)2Fe2O5 does not incorporate Si. Electron microprobe analyses revealed that phase B incorporates significant amounts of Fe2+ and Fe3+ (at least ~?1.0 cations Fe per formula unit). Fe-L2,3-edge energy-loss near-edge structure spectra confirm the presence of ferric iron [Fe3+/Fetot?=?~?0.41(4)] and indicate substitution according to the following charge-balanced exchange: [4]Si4+?+?[6]Mg2+?=?2Fe3+. The ability to accommodate Fe2+ and Fe3+ makes this potential “water-storing” mineral interesting since such substitutions should enlarge its stability field. The thermodynamic properties of Mg2Fe2O5 have been refined, yielding H°1bar,298?=???1981.5 kJ mol??1. Solid solution is complete across the Fe4O5–Mg2Fe2O5 binary. Molar volume decreases essentially linearly with increasing Mg content, consistent with ideal mixing behaviour. The partitioning of Mg and Fe2+ with silicates indicates that (Mg,Fe)2Fe2O5 has a strong preference for Fe2+. Modelling of partitioning with olivine is consistent with the O5-phase exhibiting ideal mixing behaviour. Mg–Fe2+ partitioning between (Mg,Fe)2Fe2O5 and ringwoodite or wadsleyite is influenced by the presence of Fe3+ and OH incorporation in the silicate phases.  相似文献   

6.
A detailed study of the chemical composition and substitutions in calcium tourmalines from a scapolite-bearing rare-metal pegmatite vein from the Sol’bel’der River basin has shown that their species attribution is determined by occupancy of octahedral site Y. The composition of the yellow tourmaline most abundant in the central part of the pegmatite bodyis rather constant and characterized by the ideal formula Ca(Mg2Li)Al6(Si6O18)(BO3)3(OH)3F. Variations in the chemical composition of zonal tourmaline crystals from the contact part of the pegmatite are controlled by abrupt change in the chemical medium during their formation. The yellow cores of these crystals are close in composition to tourmaline from the central part of the pegmatite vein. The Mg content abruptly decreases toward the crystal margin: Mg2+ → Fe2+, 2Mg2+ → Li+ + Al3+, and Mg2+ + OH → Al3+ + O2−. The composition of dark green marginal zones in tourmaline is characterized by the ideal formula Ca(Al1.5Li1.5)Al6(Si6O18)(BO3)3 (OH2O)(F). The results indicate specific formation conditions of pegmatite. The crystallochemical formulas of the studied tourmalines allow us to regard them as new mineral species in the tourmaline group.  相似文献   

7.
Lithian ferrian enstatite with Li2O = 1.39 wt% and Fe2O3 7.54 wt% was synthesised in the (MgO–Li2O–FeO–SiO2–H2O) system at P = 0.3 GPa, T = 1,000°C, fO2 = +2 Pbca, and a = 18.2113(7), b = 8.8172(3), c = 5.2050(2) Å, V = 835.79(9) Å3. The composition of the orthopyroxene was determined combining EMP, LA-ICP-MS and single-crystal XRD analysis, yielding the unit formula M2(Mg0.59Fe 0.21 2+ Li0.20) M1(Mg0.74Fe 0.20 3+ Fe 0.06 2+ ) Si2O6. Structure refinements done on crystals obtained from synthesis runs with variable Mg-content show that the orthopyroxene is virtually constant in composition and hence in structure, whereas coexisting clinopyroxenes occurring both as individual grains or thin rims around the orthopyroxene crystals have variable amounts of Li, Fe3+ and Mg contents. Structure refinement shows that Li is ordered at the M2 site and Fe3+ is ordered at the M1 site of the orthopyroxene, whereas Mg (and Fe2+) distributes over both octahedral sites. The main geometrical variations observed for Li-rich samples are actually due to the presence of Fe3+, which affects significantly the geometry of the M1 site; changes in the geometry of the M2 site due to the lower coordination of Li are likely to affect both the degree and the kinetics of the non-convergent Fe2+-Mg ordering process in octahedral sites.  相似文献   

8.
Ilvaite samples from six different localities in Japan are found to be members of a solid-solution series varying from Ca(Fe2+,Fe3+)2Fe2+(OH)O Si2O7 to approaximately Ca(Fe2+,Fe3+)2Fe 0.5 2+ Mn 0.5 2+ (OH)O Si2O7, and have been studied by Mössbauer spectrometry and magnetic measurements. The variation in intensity of Mössbauer doublets confirms that Mn substitutes for Fe2+ in the M(B) cation site. An temperatures decreasing from 300 K to 4K, an abrupt change in the reciprocal mass magnetic susceptibility, 1/x g, occurs about 120 K; 1/x g depends linearly upon temperature above 120 K. This change, which is characterized by an unusual mode of decrease in 1/x g, has been interpreted based on Mössbauer spectra at 80 K: the spectra of Fe2+ and Fe3+ in the M(A) site show Zeeman splitting, whereas those of Fe2+ in the M(B) site do not show the effect. This Mössbauer evidence suggests that magnetic spins of Fe in M(A) are in an ordered state, very likely of antiparallel coupling, whereas those of Fe in M(B) are randomly oriented, showing that below 120 K ilvaite has two different magnetic states for Fe ions. As there is a line of evidence that the spins of Fe in M(B) would take an ordered state at extremely low temperatures, ilvaite magnetism may be regarded as basically antiferromagnetic. The magnetic spins of Fe in M(A) and M(B) undergo magnetic transitions at different specific temperatures, thus giving as a whole unusual features of magnetism.  相似文献   

9.
Summary Synthesis of Mn-bearing ilvaites, CaFe 2–x 2+ MnxFe3+ [Si2O7/O/(OH)], with 0 x 0.19, have been performed under hydrothermal conditions at 2 and 3 kbars, T = 300 -400°C and at oxygen fugacities defined by the Fe2O3/Fe3O4 - and the Ni/NiO -buffer. As shown by X-ray diffraction, the substitution of Fe2+ by Mn2+ decreases the monoclinic angle and causes a phase transition from monoclinic to orthorhombic at x = 0.19. The Fe-distribution has been determined by Mössbauer spectroscopy.
Synthese und Charakterisierung von Mn-haltigem Ilvait CaFe 2–x 2+ MnxFe3+ [Si2O7/O/(OH)]
Zusammenfassung Mn-haltiger Ilvait CaFe 2–x 2+ Mnx Fe3+ [Si2O7/O/(OH)] wurde unter hydrothermalen Bedingungen bei Drucken von 2 und 3 kbar, Temperaturen zwischen 300 und 400°C und bei Sauerstoff Fugazitäten, die durch Festkörperpuffer (Fe2O3/Fe3O4 und Ni/NiO) kontrolliert wurden, hergestellt. Röntgenbeugungsuntersuchungen zeigen, daß mit steigendem Mn-Einbau der monokline Winkel kleiner wird, und daß bei x = 0.19 ein Phasenübergang von der monoklinen zur orthorhombischen Struktur erfolgt. Die Fe-Verteilung wurde mit Mössbauer-Spektroskopie bestimmt.

With 4 Figures  相似文献   

10.
The partitioning of Fe and Mn between the large M(4) site and the octahedral sites, M(1,2,3) in the amphibole structure has been investigated in two natural manganogrunerites of compositions Ca0.1Mn1.9 Mg1.25Fe2+ 3.56Fe3+ 0.38Si7.81O22(OH)2 and Ca0.24Mn1.57 Mg2.27 Fe2+ 2.76Fe3+ 0.32Si7.84O22(OH)2. The long-range cation distribution in the two samples has been elucidated by in situ neutron powder diffraction revealing that Mn is preferentially ordered onto M(4) ? M(2) >M(1) >M(3) in both samples. Partitioning of Mn from M(4) into the octahedral sites begins at 350 °C, with site exchange energies of ?16.6 kJ mol?1 and ?14.9 kJ mol?1, in samples containing 1.90 and 1.57 Mn apfu, respectively. Mössbauer and infrared spectroscopy have been used to study the samples at room temperature, and Mössbauer data agree well with the diffraction results, confirming that high-temperature cation distributions are retained during cooling. The fine structure in the hydroxyl-stretching region of the IR absorption spectra has been used to discuss qualitatively the site occupancies of the coordinating M(1)M(3)M(1) triplet, linked by O(3). On the basis of such modelling, we conclude that a degree of local clustering is present in both samples.  相似文献   

11.
A revised regular solution-type thermodynamic model for twelve-component silicate liquids in the system SiO2-TiO2-Al2O3-Fe2O3-Cr2O3-FeO-MgO-CaO-Na2O-K2O-P2O5-H2O is calibrated. The model is referenced to previously published standard state thermodynamic properties and is derived from a set of internally consistent thermodynamic models for solid solutions of the igneous rock forming minerals, including: (Mg,Fe2+,Ca)-olivines, (Na,Mg,Fe2+,Ca)M2 (Mg,Fe2+, Ti, Fe3+, Al)M1 (Fe3+, Al,Si)2 TETO6-pyroxenes, (Na,Ca,K)-feldspars, (Mg,Fe2+) (Fe3+, Al, Cr)2O4-(Mg,Fe2+)2 TiO4 spinels and (Fe2+, Mg, Mn2+)TiO3-Fe2O3 rhombohedral oxides. The calibration utilizes over 2,500 experimentally determined compositions of silicate liquids coexisting at known temperatures, pressures and oxygen fugacities with apatite ±feldspar ±leucite ±olivine ±pyroxene ±quartz ±rhombohedral oxides ±spinel ±whitlockite ±water. The model is applicable to natural magmatic compositions (both hydrous and anhydrous), ranging from potash ankaratrites to rhyolites, over the temperature (T) range 900°–1700°C and pressures (P) up to 4 GPa. The model is implemented as a software package (MELTS) which may be used to simulate igneous processes such as (1) equilibrium or fractional crystallization, (2) isothermal, isenthalpic or isochoric assimilation, and (3) degassing of volatiles. Phase equilibria are predicted using the MELTS package by specifying bulk composition of the system and either (1) T and P, (2) enthalpy (H) and P, (3) entropy (S) and P, or (4) T and volume (V). Phase relations in systems open to oxygen are determined by directly specifying the f o 2 or the T-P-f o 2 (or equivalently H-P-f o 2, S-P-f o 2, T-V-f o 2) evolution path. Calculations are performed by constrained minimization of the appropriate thermodynamic potential. Compositions and proportions of solids and liquids in the equilibrium assemblage are computed.  相似文献   

12.
Chemical composition, unit cell parameters, and trace elements of tourmalines from Mesozoic gold-quartz-sulphide and gold-bearing copper-porphyry ore-magmatic systems of the Trans-Baikal area and Mongolia show that they belong to the specific schorl-dravite highly ferruginous oxytourmaline series. They are low in alumina (Al2O3 = 16–33%) and have MgO contents (up to 10%) and Fe2O3 (1%). There is a direct correlation of unit cell parameters (a,c,V) with total iron, which permits composition estimates from X-ray diffraction analyses. As a rule, these tourmalines contain high concentrations of Au, Pb and Cu, which are mainly hosted by inclusions of native gold and ore minerals. The highest As abundances are contained in the tourmalines of the copper-porphyry field.Two trends of isomorphic replacement are related to increasing Fe content of oxyferruginous tourmalines:(1) “Acid leaching” trend (less ferruginous part of the series) Mg + Fe2+ + 4Al + 40 4Fe3+ + 2 + 4(OH,F); and (2) “conjugate deposition” trend Mg + 1.5Fe2+ + 1.5Al + 4(OH,F) 4Fe3+ + 4O.These features distinguish tourmalines from gold-bearing systems from schorl-dravites of tin and rare-metal deposits. They may be used in metallogenic analyses, interpretation of the origin of primary and secondary anomalies, and assessment of the type and zonation of ore fields.  相似文献   

13.
A model is proposed for the thermodynamic properties of multicomponent pyroxenes in the composition space defined by the end-member component CaMgSi2O6 and the exchange components Fe(Mg)-1, TiAl2(MgSi2)-1, Fe3+(Al)-1, Fe3+Al(MgSi)-1, and Mg(Ca)-1. It is formulated for the simplifying assumptions that: (1) a molecular mixing type approximation describes changes in the molar configurational entropy associated with the coupled exchange substitutions TiAl2MgSi2, Fe3+AlMgSi, and Al2MgSi (and their ferroan equivalents), and (2) Fe2+ and Mg2+, and Al3+ and Fe3+ display long-range non-convergent ordering between M2 and octahedral M1 sites, and octahedral M1 and tetrahedral sites, respectively. The molar vibrational Gibbs energy is described by a Taylor expansion of second degree in seven linearly independent composition and ordering variables, which is extended to third degree to account for asymmetry in the mixing of Ca and Mg, and Ca and Fe on the M2 site, and is further modified for the assumption that the standard state properties of Ca end-member components of clinopyroxenes are linearly dependent on the coordination number of Ca2+ on the M2 site. The model is shown to be consistent with miscibility gap feaures of pyroxenes in the system CaMgSi2O6–CaTiAl2O6–CaAl2SiO6. In subsequent papers, the model is calibrated for the simplifying assumptions that: (1) all regular-solution-type parameters are constants independent of temperature, (2) Pbca and C2/c end-members have identical heat capacities and coefficients of thermal expansion and compressibility, and (3) the heat capacities and coefficients of thermal expansion and compressibility are zero for all reciprocal reactions relating Pbca and pigeonite or high-calcium pyroxene C2/c endmember components.  相似文献   

14.
Two pumpellyites with the general formula W 8 X 4 Y 8 Z 12O56-n (OH) n were studied using 57Fe Mössbauer spectroscopic and X-ray Rietveld methods to investigate the relationship between the crystal chemical behavior of iron and structural change. The samples are ferrian pumpellyite-(Al) collected from Mitsu and Kouragahana, Shimane Peninsula, Japan. Rietveld refinements gave Fe(X):Fe(Y) ratios (%) of 41.5(4):58.5(4) for the Mitsu pumpellyite and 46(1):54(1) for the Kouragahana pumpellyite, where Fe(X) and Fe(Y) represent Fe content at the X and Y sites, respectively. The Mössbauer spectra consisted of two Fe2+ and two Fe3+ doublets for the Mitsu pumpellyite, and one Fe2+ and two Fe3+ doublets for the Kouragahana pumpellyite. In terms of the area ratios of the Mössbauer doublets and the Fe(X):Fe(Y) ratios determined by the Rietveld refinements, Fe2+(X):Fe3+(X):Fe3+(Y) ratios are determined to be 22:14:64 for the Mitsu pumpellyite and 27:8:65 for the Kouragahana pumpellyite. By applying the Fe2+:Fe3+-ratio determined by the Mössbauer analysis and the site occupancies of Fe at the X and Y sites given by the Rietveld method together with chemical analysis, the resulting formula of the Mitsu and Kouragahana pumpellyites are established as Ca8(Fe 0.88 2+ Mg0.68Fe 0.77 3+ Al1.66)Σ3.99(Al5.67Fe 2.34 3+ )Σ8.01Si12O42.41(OH)13.59 and Ca8(Mg1.24Fe 0.65 2+ Fe 0.46 3+ Al1.66)Σ4.01(Al6.71Fe 1.29 3+ )Σ8.00Si12O42.14(OH)13.86, respectively. Mean Y–O distances and volumes of the YO6 octahedra increase with increasing mean ionic radii, i.e., the Fe3+→Al substitution at the Y site. However, change of the sizes of XO6 octahedra against the mean ionic radii at the X site is not distinct, and tends to depend on the volume change of the YO6 octahedra. Thus, the geometrical change of the YO6 octahedra with Fe3+→Al substitution at the Y site is essential for the structural changes of pumpellyite. The expansion of the YO6 octahedra by the ionic substitution of Fe3+ for Al causes gradual change of the octahedra to more symmetrical and regular forms.  相似文献   

15.
The effect of CaO, Na2O, and K2O on ferric/ferrous ratio in model multicomponent silicate melts was investigated in the temperature range 1450–1550?°C at 1-atm total pressure in air. It is demonstrated that the addition of these network modifier cations results in an increase of Fe3+/Fe2+ ratio. The influence of network modifier cations on the ferric/ferrous ratio increases in the order Ca?<?Na?<?K. Some old controversial conceptions concerning the effect of potassium on Fe3+/Fe2+ ratio in simple model liquids are critically evaluated. An empirical equation is proposed to predict the ferric/ferrous ratio in SiO2–TiO2–Al2O3–FeO–Fe2O3–MgO–CaO–Na2O–K2O–P2O5 melts at air conditions.  相似文献   

16.
The stability of pumpellyite + actinolite or riebeckite + epidote + hematite (with chlorite, albite, titanite, quartz and H2O in excess) mineral assemblages in LTMP metabasite rocks is strongly dependent on bulk composition. By using a thermodynamic approach (THERMOCALC), the importance of CaO and Fe2O3 bulk contents on the stability of these phases is illustrated using P–T and P–X phase diagrams. This approach allowed P–T conditions of ～4.0 kbar and ～260 °C to be calculated for the growth of pumpellyite + actinolite or riebeckite + epidote + hematite assemblages in rocks containing variable bulk CaO and Fe2O3 contents. These rocks form part of an accretionary wedge that developed along the east Australian margin during the Carboniferous–Triassic New England Orogen. P–T and P–X diagrams show that sodic amphibole, epidote and hematite will grow at these conditions in Fe2O3‐saturated (6.16 wt%) metabasic rocks, whereas actinolite and pumpellyite will be stable in CaO‐rich (10.30 wt%) rocks. With intermediate Fe2O3 (～3.50 wt%) and CaO (～8.30 wt%) contents, sodic amphibole, actinolite and epidote can coexist at these P–T conditions. For Fe2O3‐saturated rocks, compositional isopleths for sodic amphibole (Al3+ and Fe3+ on the M2 site), epidote (Fe3+/Fe3+ + Al3+) and chlorite (Fe2+/Fe2+ + Mg) were calculated to evaluate the efficiency of these cation exchanges as thermobarometers in LTMP metabasic rocks. Based on these calculations, it is shown that Al3+ in sodic amphibole and epidote is an excellent barometer in chlorite, albite, hematite, quartz and titanite buffered assemblages. The effectiveness of these barometers decreases with the breakdown of albite. In higher‐P stability fields where albite is absent, Fe2+‐Mg ratios in chlorite may be dependent on pressure. The Fe3+/Al and Fe2+/Mg ratios in epidote and chlorite are reliable thermometers in actinolite, epidote, chlorite, albite, quartz, hematite and titanite buffered assemblages.  相似文献   

17.
Summary Microprobeanalyses of 44 vesuvianite specimens of different color, morphology, and occurrence indicate the chemical variation of the species. Vesuvianites can be divided into four types, based on chemistry and color; most vesuvianites can be assigned to one of these types with confidence. Type 1 vesuvianites contain 2 atoms of Mg, <0.25 atoms of Ti, and variable Fe, apparently trivalent, in a formula based on 50 non-H cations. They are dark to pale green, white, or pink. Type 2 vesuvianites contain > 2.5 atoms of Mg, variable (divalent?) Fe, and <0.5 atoms of Ti. In color they are yellow, yellow-brown, or yellow-green. Type 3 vesuvianites contain divalent Fe, 0.5 to 1.5 atoms of Ti and close to 18 atoms of Si (full occupancy of tetrahedral sites). They are yellow, brown, red-brown, or black. Type 4 vesuvianites are manganoan or cuprian; the studied samples are from Pajsberg, Sweden, Franklin, New Jersey, and Telemark, Norway. In color they are red-brown, purple, blue (cyprine), or green. Iron and much of the Mn is apparently trivalent.Unlike garnets, which they resemble structurally, vesuvianites contain almost exclusively Ca in the eight-fold sites in the structure. Silicon occupies 95% or more of the tetrahedral sites. Aluminum fills the smaller octahedral site, A. Chemical variation occurs predominantly in the more open, octahedral, general or G site and in the five-coordinated B site. Simple substitutions in G include Fe3+ or Mn3+ for Al3+, and Fe2+, Mn2+, or Zn2+ for Mg2+. Coupled substitutions include TiO = AIOH, MgTi = AlAl and AlAl = MgSi. The B site may contain Cue2+, Fe2+, Fe3+, or A13+. Changes in the amounts of O and OH in two different positions give a range of anion charge from 146 (0670H12) to about 148 (0690H10).
Chemische variation in Vesuvianen
Zusammenfassung Mikrosondenanalysen von 44 Vesuvian-Proben, die in Farbe, Morphologie and Vorkommen unterschiedlich sind, zeigen die chemische Variation der Spezies. Aufgrund von Chemismus und Farbe können Vesuviane in vier Typen eingeteilt werden; die meisten Vesuviane konnen einem dieser Typen klar zugeordnet werden. Vesuviane vom Typ 1 enthalten in einer Formel, die sich auf 50 nicht-H Kationen bezieht, 2 Mg-Atome, < 0,25 Ti-Atome and variables, anscheinend dreiwertiges Fe. Sie sind dunkel- bis blaßgrün, weiß oder rosa. Vesuviane vom Typ 2 enthalten > 2,5 Mg-Atome, variables (zweiwertiges?) Fe and < 0,5 Ti-Atome. Sie sind gelb, gelbbraun oder gelbgrün gefärbt. Vesuviane vom Typ 3 enthalten zweiwertiges Fe, 0,5 bis 1,5 Ti-Atome and beinahe 18 Si-Atome (völlige Besetzung der Tetraederpositionen). Sie sind gelb, braun, rotbraun oder schwarz. Vesuviane vom Typ 4 sind Mangan- oder Kupfer-haltig; die untersuchten Proben sind von Pajsberg, Schweden, Franklin, New Jersey, and Telemark, Norwegen. Sie sind rotbraun, purpur, blau (Cyprin) oder grün gefarbt. Eisen und ein großer Teil des Mangns sind anscheinend dreiwertig.Im Unterschied zu den strukturell ähnlichen Granaten enthalten die Vesuviane auf den acht-koordinierten Positionen der Struktur fast nur Ca. Silizium besetzt 95% oder mehr der Tetraederpositionen. Aluminium füllt die kleinere Oktaederposition A. Chemische Variabilität tritt hauptsächlich in der offeneren, oktaedrischen, allgemeinen oder G Position and in der fiinf-koordinierten B Position auf. Einfache Substitutionen auf G umfassen Fe3+ und Mn3+ fur Al3+, sowie Fe2+, Mn2+ and Zn2+ für Mgt2+. Gekoppelte Substitutionen beinhalten TiO = AlOH, MgTi = AlAl and AlAl = MgSi. Die B Position kann Cu2+, Fe2+, Fe3+ and A13+ enthalten. Wechsel in den Beträgen an O und OH auf zwei unterschiedlichen Positionen gibt für die Anionenladung einen Bereich von 146 (O67OH12) bis etwa 148 (O69OH10).[/p]
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18.
Fifteen samples of (Mg,Fe)SiO3 majorite with varying Fe/Mg composition and one sample of (Mg,Fe)(Si,Al)O3 majorite were synthesized at high pressure and temperature under different conditions of oxygen fugacity using a multianvil press, and examined ex situ using X-ray diffraction and Mössbauer and optical absorption spectroscopy. The relative concentration of Fe3+ increases both with total iron content and increasing oxygen fugacity, but not with Al concentration. Optical absorption spectra indicate the presence of Fe2+–Fe3+ charge transfer, where band intensity increases with increasing Fe3+ concentration. Mössbauer data were used in conjunction with electron microprobe analyses to determine the site distribution of all cations. Both Al and Fe3+ substitute on the octahedral site, and charge balance occurs through the removal of Si. The degree of Mg/Si ordering on the octahedral sites in (Mg,Fe)SiO3 majorite, which affects both the c/a ratio and the unit cell volume, is influenced by the thermal history of the sample. The Fe3+ concentration of (Mg,Fe)(Si,Al)O3 majorite in the mantle will reflect prevailing redox conditions, which are believed to be relatively reducing in the transition zone. Exchange of material across the transition boundary to (Mg,Fe) (Si,Al)O3 perovskite would then require a mechanism to oxidize sufficient iron to satisfy crystal-chemical requirements of the lower-mantle perovskite phase.  相似文献   

19.
Summary Anandite has an approximate formula of Ba(Fe3+, Fe2+)3[Si2(Fe3+, Fe2+, Si)2O10–x(OH)x] (S, Cl) (OH), withx=0–1, and belongs to the 2 O brittle mica group. It is orthorhombic; space groupPnmn;a=5.468(9) Å,b=9.489(18)Å,c=19.963(11) Å;Z=4.The structure was determined from 3dim. Weissenberg-data, starting with an approximate structure in the pseudo space groupCcmm. Least squares refinement resulted inR=0.061 for 409 photometric intensities, andR=0.131 for all 853 observedhkl-reflexions.The iron of the tetrahedral layer is concentrated in one of the two crystallographically different kinds of tetrahedra. The basal oxygen rings of the tetrahedral layer form approximate hexagons and have not the ditrigonal configuration of the common micas. This peculiarity is considered to be a consequence of the size and charge of the barium ion. The role of OH in the common micas is played partly by S2– and Cl– in anandite.
Die Kristallstruktur des 2 O Sprödglimmers Anandit
Zusammenfassung Anandit hat die ungefähre Formel Ba(Fe3+, Fe2+)3[Si2(Fe3+, Fe2+, Si)2O10–x(OH)x] (S, Cl) (OH) mitx=0–1 und gehört zur 2O Sprödglimmergruppe. Er ist rhombisch; RaumgruppePnmn; a=5,468(9) Å,b=9,489(18) Å,c=19,963(11) Å;Z=4.Die Struktur wurde aus Weissenberg-Daten bestimmt, wobei mit einer approximativen Struktur in der PseudoraumpruppeCcmm begonnen wurde. Die Verfeinerung nach der Methode der kleinsten Quadrate führte für 409 photometrierte Reflexe aufR=0,061 und für alle 853 beobachtetenhkl-Reflexe aufR=0,131.Der Eisengehalt der Tetraederschicht ist in einer der beiden kristallographisch verschiedenen Tetraederarten konzentriert. Die basalen Sauerstoffringe der Tetraederschicht bilden annäherungsweise Sechsecke und haben nicht die ditrigonale Konfiguration der gewöhnlichen Glimmer. In Anandit spielen S2– und Cl– teilweise die Rolle der Hydroxylgruppen in den gewöhnlichen Glimmern.

With 4 Figures  相似文献   

20.
Garnet-biotite gneisses, some of which contain sillimanite or hornblende, are widespread within the Otter Lake terrain, a portion of the Grenville Province of the Canadian Shield. The metamorphic grade is upper amphibolite to, locally, lower granulite facies. The atomic ratio Fe2+/(Fe2++ Fe3+) in biotite ranges from 0.79 to 0.89 (ferrous iron determinations in 10 highly pure separates), with a mean of 0.86. Mg and Fe2+ atoms occupy 67–78% of the octahedral sites, the remainder are occupied by Fe3+, Ti, and Al, and some are vacant. Mg/(Mg + Fe2+), denoted X, in the analysed samples ranges from 0.32 to 0.65. Garnet contains 1–24% grossular, 1–12% spessartine and X ranges from 0.07 to 0.34. Compositional variation in biotite and garnet is examined in relation to three mineral equilibria: (I) biotite + sillimanite + quartz = garnet + K-feldspar + H2O; (II) pyrope + annite = almandine + phlogopite; (III) anorthite = grossular + sillimanite + quartz. Measurements of X (biotite) and X (garnet) are used to construct an illustrative model for equilibrium (I) which relates the observed variation in X to a temperature range of 70°C or a range in H2O activity of 0.6; the latter interpretation is preferred. In sillimanite-free gneisses, the distribution of Mg and Fe2+ between garnet (low in Ca and Mn) and biotite is adequately described by a distribution coefficient (KD) of 4.1 (equilibrium II). The observed increase in the distribution coefficient with increasing Ca in garnet is ln KD= 1.3 + 2.5 × 10?2 [Ca] where [Ca] = 100 Ca/(Mg + Fe2++ Mn + Ca). The distribution coefficient is apparently unaffected by the presence of up to 12% spessartine in garnet. In several specimens of garnet-sillimanite-plagioclase gneiss, the Ca contents of garnet and of plagioclase increase in unison, as required by equilibrium (III). The mean pressure calculated from these data (n= 17) is 5.9 kbar, and the 95% confidence limits are ±0.5 kbar.  相似文献   

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