共查询到20条相似文献,搜索用时 15 毫秒
1.
Yoshiharu Nakajima Nobuo Morimoto Masao Kitamura 《Physics and Chemistry of Minerals》1977,1(2):199-212
A modulation function representing the position and density of (Na, Ca) atoms in the superstructure of the e-plagioclase has been derived from the average structures of different plagioclase and a general modulation theory. Based on this function the superstructure of bytownite (An73) has been studied with the single crystal X-ray method. The cell dimensions by Megaw's axes are a=7.946(3)A, b=67.09(2)A, c=12.236(4)A, α=39.03(1)°, β=45.63(1)° and γ=59.63(1)°. Z=18(Na, Ca) Al(Al, Si)Si2O8. The initial phase factor of the modulation function for bytownite has been obtained from the intensity data of the satellite reflections. This modulation function indicates a coherent small-scale alternation of the Na-rich and Ca-rich bands in the superstructure. This superstructure has been refined by applying the albite and anorthite structures to the Na-rich and Ca-rich bands, respectively. The change of the superstructure of the e-plagioclase due to the compositional change has been described based on the movements of the satellites in reciprocal space. The direction of the coherent small-scale intergrowth of the anorthite-like and albite-like bands is perpendicular to the t vector. The thickness of the intergrowth is 1/|t|. Both direction and thickness change regularly from An75 to An25. 相似文献
2.
M. Tribaudino R. J. Angel F. Cámara F. Nestola D. Pasqual I. Margiolaki 《Contributions to Mineralogy and Petrology》2010,160(6):899-908
The volume thermal expansion coefficient and the anisotropy of thermal expansion were determined for nine natural feldspars
with compositions, in terms of albite (NaAlSi3O8, Ab) and anorthite (CaAl2Si2O8, An), of Ab100, An27Ab73, An35Ab65, An46Ab54, An60Ab40, An78Ab22, An89Ab11, An96Ab4 and An100 by high resolution powder diffraction with a synchrotron radiation source. Unit-cell parameters were determined from 124
powder patterns of each sample, collected over the temperature range 298–935 K. The volume thermal expansion coefficient of
the samples determined by a linear fit of V/V
0 = α(T − T
0) varies with composition (X
An in mol %) as:
aV = 2.90( 4 ) ×10 - 5 - 3.0( 2 ) ×10 - 7 *X\textAn + 1.8( 2 ) ×10 - 9 *X\textAn2 \alpha_{V} = 2.90\left( 4 \right) \times 10^{ - 5} - 3.0\left( 2 \right) \times 10^{ - 7} *X_{\text{An}} + 1.8\left( 2 \right) \times 10^{ - 9} *X_{\text{An}}^{2} 相似文献
3.
Optical and transmission electron microscopy have been used to study the microstructures in a series of plagioclase feldspars which had been experimentally deformed in compression. The observations show that deformation takes place by three mechanisms: (1) brittle fracture, (2) twinning and (3) slip due to the generation and motion of dislocations. Optical “deformation lamellae” are shown to be due to bunches of microfractures and to walls of tangled dislocations. Twins and fractures are often intimately associated and dislocations are often generated at fracture steps or voids. Moving dislocations apparently always generate a strip of fault in the slip plane. This, together with structural considerations as well as the visibility of dislocations (under various diffracting conditions) and the orientation of the applied stress, has made it possible to determine the slip systems which have operated in the deformed specimens. 相似文献
4.
S. Mizutani 《Mathematical Geosciences》1975,7(4):335-348
An extinction-angle distribution was computed for randomly oriented grains of plagioclase feldspar with a known An-composition. Expressed by a histogram with eighteen columns, the results are shown by an 18×19 data array for nineteen types of low-temperature plagioclase. In a mixture of grains with various An-compositions, the extinction-angle distribution is given by a linear combination of the obtained data matrix with the frequencies of An-composition. If we have a sample containing randomly oriented grains of plagioclase feldspars, a frequency distribution of those with An-composition can be calculated after measuring their extinction angles. By taking into account the constraints of inequalities, the calculation is treated with the dual method of linear programming. The procedure provides for a simpler and more expedient method for determining the frequency distribution of An-composition. This method was tested by applying it to a model sample containing randomly oriented plagioclase grains with known An-composition frequencies, whose extinction-angle distribution was determined by the Monte Carlo method. An example of application of this method to Permian sandstones from central Japan is presented. 相似文献
5.
The diffraction, n.q.r. and optical data on plagioclase feldspars are used to derive kinetic interpretations of structural changes induced by laboratory heat treatment and by geological processes. For anorthite, the Si, Al configuration is essentially ordered except for unusual transient processes. Cooperation between Ca atoms, and random nucleation, produces a domain texture in the primitive structure which is highly sensitive to temperature. The rapid inversion from the primitive to the body-centered structure is explained by increasingly rapid “rattling” of the Ca ions in the interstices of the semi-flexible alumino-silicate framework. The weakening of “b” reflections at higher temperatures is ascribed to incipient Si, Al disorder associated with irregular vibration of the alumino-silicate framework and the Ca atoms. Quenching phenomena are explained by variation of the domain boundary texture inherited from disorder at high temperature. For albite, the Si, Al configuration changes sluggishly from an ordered to a disordered pattern, and vice versa. Kinetic data are reinterpreted using a model in which the cell dimensions depend on local rather than distant order: the major change in distant order is deduced to occur at 450–600° C. Sodic plagioclase grown at high temperature shows distant disorder of the atoms, but cell dimensions suggest development of strong local order for calcic compositions. Low-entropy plagioclases of intermediate composition show complex intergrowths and domain structures because of kinetic barriers to atomic diffusion. X-ray diffraction data for slowly-cooled specimens are consistent with nucleation of albite- and anorthite-like regions from a high-temperature disordered phase. Electrostatic energy calculations show that Na and Ca atoms, although they face smaller energy barriers for diffusion, cannot form domains until the Si and Al atoms have moved jointly. The Si, Al ordering patterns of low albite and anorthite are topologically incompatible in a continuous framework if oxygen is not to be bonded to two Al. Therefore domains of low-albite and anorthite must be separated by disordered boundaries. For intermediate compositions, An15-An75, domains remain small. The anorthite-like domains probably form at higher temperatures than the albite-like domains. The latter tend to be about the same size for all bulk compositions. The atomic positions are influenced by neighboring atoms. Upon heating rapidly, Si and Al atoms remain in position and provide a memory for reformation of an identical structure upon cooling. The framework changes shape, and some Na, Ca atoms inter-diffuse to yield a quasi-homogeneous structure with a diffraction pattern which qualitatively approaches that of high albite. Upon prolonged heating at high temperature, Si, Al atoms inter-diffuse producing nonquenchable changes to the high-albite structure. At Na-rich bulk compositions, some domains of low albite grow into large lamellae while others remain small in contact with anorthite domains producing alternate lamellae of intermediate structure type; hence the peristerite intergrowth. A similar but opposite process could cause an intergrowth of lamellae of anorthite structure interposed with an intermediate type structure. A unique low plagioclase series is not expected. Plagioclases of intermediate composition trend towards slightly different endproducts depending on the details of the cooling history. Breaks and bends in plots of physical properties, and intergrowths for certain specimens, depend on special compositional, growth and annealing factors. The intergrowth responsible for iridescence of intermediate plagioclase is ascribed to Na, K segregation prior to development of the complex domain structure. Prolonged annealing at high temperature in a dry environment is suggested. It is futile to attempt to describe low entropy plagioclases in terms of classical thermodynamics: only a kinetic interpretation based on atomic and sub-microscopic textural factors can be viable. 相似文献
6.
Dislocations in intermediate plagioclase feldspars, which were deformed under granulite facies conditions, have been analysed. The study reveals extensive ductile deformation by intracrystalline slip and by twinning. Six out of the seven possible Burgers vectors were identified: \(b = \left[ {001} \right],\tfrac{1}{2}\left[ {110} \right],\tfrac{1}{2}\left[ {1\bar 10} \right],\left[ {101} \right],\tfrac{1}{2}\left[ {112} \right]and\tfrac{1}{2}\left[ {1\bar 12} \right]\) . Most, perhaps all, dislocations are dissociated by up to 200 Å. The microstructure is dominated by [001] screw dislocations, most of which appear to be dissociated in (010). The dominant slip system appears to be (010) [001]. Large grain-to-grain variations in the density of free dislocations indicate that the plastic strain in individual grains depended upon the Schmid factor for (010) [001]. The microstructure suggests that the rate-controlling step for high-temperature creep of plagioclase is cross-slip of extended [001] screw dislocations. The rheological contrast between feldspar and quartz is partly due to a difference in stacking fault energy. 相似文献
7.
Raman microprobe spectra were made on three post shock, diaplectic plagioclase feldspars. Optical and X-ray diffraction studies indicated that feldspars maintained a partially or totally crystalline state after having passed through the mixed phase zone of Hugoniot response to shock waves (15–38 GPa). The appearance of uniquely glass-type spectra occurs at different shock pressures for each specimen according to its atomic structural arrangement, below 38 GPa for mosaic structured labradorite, near 40 GPa for anorthite and above 50 GPa for the highly ordered low albite. The diaplectic anorthite and labradorite glasses give spectra which indicate the presence of two glass types. Shifts in the band envelope frequencies compared to spectra of fused glass and statically pressure densified glass suggest that these glasses have specific structural arrangements. These differences suggest that the shock and fusion glass-forming processes are not exactly identical. The results from material shocked in the mixed phase region of Hugoniot response show that the phase transitions are effected at different pressures depending upon the feldspar structural type. 相似文献
8.
Taking account of the Cˉ1/Iˉ1 (Al/Si order/disorder) transformation at high temperatures in the albite-anorthite solid solution
leads to a simple model for the mixing properties of the high structural state plagioclase feldspars. The disordered (Cˉ1)
solid solution can be treated as ideal (constant activity coefficient) and, for anorthite-rich compositions, deviations from
ideality can be ascribed to cation ordering. Values of the activity coefficient for anorthite in the Cˉ1 solid solution (γ
An
Cˉ1
) are then controlled by the free energy difference between Cˉ1 and Iˉ1 anorthite at the temperature (T) of interest according
to the relation: ΔˉG
ord
Iˉ1
⇌Cˉ1 =RT ln γ
An
Cˉ1
. If the Iˉ1⇌Cˉ1 transformation in pure anorthite is treated, to a first approximation, as first order and the enthalpy and
entropy of ordering are taken as 3.7±0.6 kcal/mole (extrapolated from calorimetric data) and 1.4–2.2 cal/mole (using an equilibrium
order/disorder temperature for An100 of 2,000–2,250 K), a crude estimate of γ
An
Cˉ1
for all temperatures can be made. The activity coefficient of albite in the Cˉ1 solid solution (γ
Ab
Cˉ1
) can be taken as 1.0. The possible importance of this model lies in its identification of the principal constraints on the
mixing properties rather than in the actual values of γ
An
Cˉ1
and γ
Ab
Cˉ1
obtained. In particular it is recognised that γ
An
Cˉ1
depends critically on ordering in anorthite as well as, at lower temperatures, any ordering in the Cˉ1 solid solution.
A brief review of activity-composition data, from published experiments involving ranges of plagioclase compositions and from
the combined heats of mixing plus Al-avoidance entropy model (Newton et al. 1980), reveals some inconsistencies. The values
of γ
An
Cˉ1
calculated using the approach of Newton et al. (1980), although consistent with Orville's (1972) ion exchange data, are slightly
lower than values derived from experiments by Windom and Boettcher (1976) and Goldsmith (1982) or from ion-exchange experiments
of Kotel'nikov et al. (1981). Based on the Cˉ1/Iˉ1 transformation model, values of γ
An
Cˉ1
<1.0 are unlikely.
Discrepancies between the experimental data sets are attributed to incomplete (non-equilibrium) Al/Si order attained during
the experiments. It is suggested that the choice of activity coefficients remains somewhat subjective. The development of
accurate mixing models would be greatly assisted by better thermodynamic data for ordering in pure anorthite and by more thorough
characterisation of the state of order in plagioclase crystals used for phase equilibrium experiments. 相似文献
9.
The replacement of plagioclase feldspars by albite: observations from hydrothermal experiments 总被引:7,自引:3,他引:4
Jörn Hövelmann Andrew Putnis Thorsten Geisler Burkhard C. Schmidt Ute Golla-Schindler 《Contributions to Mineralogy and Petrology》2010,159(1):43-59
Oligoclase and labradorite crystals have been experimentally replaced by albite in an aqueous sodium silicate solution at
600°C and 2 kbars. The replacement is pseudomorphic and is characterised by a sharp chemical interface which progresses through
the feldspar while preserving the crystallographic orientation. Reaction rims of albite, up to 50 μm thick, can be readily
achieved within 14 days. Re-equilibration of plagioclase in an 18O-enriched sodium- and silica-bearing solution results in oxygen isotope redistribution within the feldspar framework structure.
The observed characteristics of the reaction products are similar to naturally albitised plagioclase and are indicative of
an interface-coupled dissolution–reprecipitation mechanism. Chemical analyses demonstrate that the albitisation is accompanied
by the mobilisation of major, minor and trace elements also including elements such as Al and Ti which are commonly regarded
as immobile during hydrothermal alteration. The results contribute to developing our understanding of the close association
between large-scale albitisation and secondary ore mineralisation which is common in nature. 相似文献
10.
The NMR spectra of 27Al and 23Na and the EPR spectra of Fe3+ and Mn2+ have been studied in a range of plagioclase feldspars, of intermediate composition. Many of these crystals have previously been investigated by TEM methods (McLaren and Marshall, 1974). The broad, central 27Al signals observed in labradorites have been interpreted in terms of an overall order/disorder of the Al/Si distribution. Spin Hamiltonian parameters have been determined for the Fe3+ spectra observed in labradorites and bytownites; these spectra were similar and yielded evidence of disordering of the Al/Si distribution. The severe limitations imposed by the variety of sites, the complex microstructures and the disordering, limit the structural information available from the resonance methods. 相似文献
11.
William P. Leeman 《Geochimica et cosmochimica acta》1979,43(1):171-175
Pb contents were determined by isotope dilution in separated glass, sanidine, and plagioclase from 18 rocks ranging in composition from basalt to rhyolite. These data indicate that Pb is partitioned into silicate melt relative to plagioclase, but is equally distributed between melt and sanidine. Plagioclase/glass distribution coefficients increase from 0.1 to 0.7 in going from basalt to rhyolite. This relationship suggests that the distribution coefficient is dependent upon bulk composition, temperature, or both. Sanidine/glass distribution coefficients are close to unity in rocks ranging in composition from quartz latite to rhyolite. The variation in Pb contents in a natural magma series from Craters of the Moon National Monument, Idaho, indicates that minerals (olivine, plagioclase, magnetite, apatite and clinopyroxene) fractionated from these magmas all have very low crystal/liquid distribution coefficients for Pb. 相似文献
12.
Sense of shear in high-temperature movement zones from the fabric asymmetry of plagioclase feldspars
《Journal of Structural Geology》1988,10(1):73-81
A new method for determining the sense of shear in plagioclase-bearing tectonites from the (010) orientation of plagioclase feldspar is presented. The method is based on the asymmetry of the (010) plane with respect to the structural frame (foliation and lineation) and the dominant activity of the (010) slip plane in the high-temperature plasticity of plagioclase feldspar. Using examples from the Zabargad gneisses (Red Sea) the method is applied to plagioclases of An25–An45 and compared with other methods of shear-sense determination (quartz c-axis fabrics and microstructural criteria). 相似文献
13.
P. Tropper C. E. Manning E. J. Essene L.-S. Kao 《Contributions to Mineralogy and Petrology》2000,139(2):146-162
Sodic amphiboles in high pressure and ultra-high pressure (UHP) metamorphic rocks are complex solid solutions in the system Na2O–MgO–Al2O3–SiO2–H2O (NMASH) whose compositions vary with pressure and temperature. We conducted piston-cylinder experiments at 20–30?kbar and 700–800?°C to investigate the stability and compositional variations of sodic amphiboles, based on the reaction glaucophane=2jadeite+talc, by using the starting assemblage of natural glaucophane, talc and quartz, with synthetic jadeite. A close approach to equilibrium was achieved by performing compositional reversals, by evaluating compositional changes with time, and by suppressing the formation of Na-phyllosilicates. STEM observations show that the abundance of wide-chain structures in the synthetic amphiboles is low. An important feature of sodic amphibole in the NMASH system is that the assemblage jadeite–talc?±?quartz does not fix its composition at glaucophane. This is because other amphibole species such as cummingtonite (Cm), nyböite (Nyb), Al–Na-cummingtonite (Al–Na-Cm) and sodium anthophyllite (Na-Anth) are also buffered via the model reactions: 3cummingtonite?+?4quartz?+?4H2O=7talc, nyböite?+?3quartz=3jadeite?+?talc, 3Al–Na-cummingtonite + 11quartz + 2H2O=6jadeite + 5talc, and 3 sodium anthophyllite?+?13quartz?+?4H2O=3 jadeite + 7talc. We observed that at all pressures and temperatures investigated, the compositions of newly grown amphiboles deviate significantly from stoichiometric glaucophane due to varying substitutions of AlIV for Si, Mg on the M(4) site, and Na on the A-site. The deviation can be described chiefly by two compositional vectors: [NaAAlIV]<=>[□ASi] (edenite) toward nyböite, and [Na(M4)AlVI]<=>[Mg(M4)MgVI] toward cummingtonite. The extent of nyböite and cummingtonite substitution increases with temperature and decreases with pressure in the experiments. Similar compositional variations occur in sodic amphiboles from UHP rocks. The experimentally calibrated compositional changes therefore may prove useful for thermobarometric applications. 相似文献
14.
Ten specimens of plagioclase feldspars (An100 to An25) have been examined by transmission electron microscopy using dark field and direct lattice imaging. The specimens are classified into two groups: (i) those that have been quenched from the melt (synthetic An100, An95, An66 and An50) and (ii) those that have been cooled relatively slowly (An94, An77, An75, An52, An32 and An26). The observed contrast is interpreted in detail using the two-beam dynamical theory of electron diffraction contrast. The fault vectors of the antiphase boundaries (APBs) observed in An100 and An77 are determined. Direct resolution of the sublattice and superlattice in specimens of intermediate composition (An75 to An25) indicates that the superlattice is not a regular array of out-of-step faults and is probably due to a simple sinusoidal distortion of the structure. Only three specimens (An100, An77 and An50) exhibited a single structural type. All the remaining specimens consisted of intergrowths of two structural types. The structural type present in any particular specimen depends upon its An-content and thermal history. The rate of transformation from any structural type to a lower-temperature structural type appears to decrease with decreasing An-content. 相似文献
15.
The magma mixing origin of mantled feldspars 总被引:25,自引:1,他引:25
M. J. Hibbard 《Contributions to Mineralogy and Petrology》1981,76(2):158-170
The key to mantled feldspar genesis is epitaxial nucleation of plagioclase on K-feldspar or K-feldspar on plagioclase. Once this nucleation takes place there is a relatively straightforward process of crystal growth yielding rapakivi and antirapikivi textures. The most common mantling is plagioclase on K-feldspar which occurs in both volcanic and plutonic environments. In the volcanic environment the morphology of the plagioclase overgrowth typically is dendritic, though in subvolcanic and shallow plutonic environments dendritic growth is followed by a more or less continuous non-cellular shell of plagioclase. In the plutonic environment, early stages of plagioclase overgrowth also tend to be dendritic, although with coarser-grained characteristics. Dendritic morphology is thus a common denominator in rapakivi genesis. Since growth of dendritic plagioclase is clearly related to marked undercooling in silicate melt systems its occurrence in many volcanic rocks is to be expected. Equivalent quenching in the plutonic environment requires a cooling mechanism independent of conductive heat transfer to wallrock and also independent of effective cooling related to sudden loss of volatile phases that could only occur late in the crystallization of most magmas and therefore after much dendritic plagioclase had already formed. Internal quenching of portions of magma systems must occur if mafic magma is abruptly mixed with felsic magma. Such magma mixing yields a heterogeneous system at first, one that is in a drastic state of disequilibrium and tending to force nucleation of one feldspar type on the surface of another resulting in epitaxial crystallization of dendritic plagioclase on K-feldspar. Mantling of one feldspar type by another during magma mixing is paralleled by dendritic growth zones in coexisting plagioclase crystals.Mantling textures occur in hybrid rocks of magma mixing origin. Some of the hybrid rocks are fine-grained, mafic-rich, and may contain phenocrysts of quartz, plagioclase, and K-feldspar. They occur as rounded inclusions in calc-alkaline granites and granodiorites. The host plutons themselves commonly have mantled feldspars or at least plagioclase with the unusual zoning characteristics commonly accompanying rapakivi texture. Magma-mixing tends to occur in batches so that hybrid crystal-melt systems, the calc-alkaline granitic plutons, become intrusive into earlier hybrid crystal-melt systems, represented by the mafic-rich inclusions. 相似文献
16.
文章利用野外试验设施分别把地下水埋深控制在1.0,1.4,1.8,2.2和2.5m,研究在不同地下水埋深作用下苏打盐渍土土壤水分平衡和水分动态变化。同时,跟踪在地下水位不断波动条件下田间土壤水分的动态变化。结果表明:地下水埋深在1.0~2.5m时,苏打盐渍土土壤水和地下水的转化关系非常微弱,仅在长期干旱的条件下发生微量地下水毛细上升量;降雨和蒸散作用对苏打盐渍土体积含水量的影响深度不超过40cm,埋深大于1.0m的潜水对0~80cm深度苏打盐渍土体积含水量变化没有明显影响。 相似文献
17.
Michael A. Carpenter 《Physics and Chemistry of Minerals》1986,13(2):119-139
Approximately 125 hydrothermal annealing experiments have been carried out in an attempt to bracket the stability fields of different ordered structures within the plagioclase feldspar solid solution. Natural crystals were used for the experiments and were subjected to temperatures of ~650°C to ~1,000°C for times of up to 370 days at \(P_{{\text{H}}_{\text{2}} {\text{O}}} \) =600 bars, or \(P_{{\text{H}}_{\text{2}} {\text{O}}} \) =1,200 bars. The structural states of both parent and product materials were characterised by electron diffraction, with special attention being paid to the nature of type e and type b reflections (at h+k=(2n+1), l=(2n+1) positions). Structural changes of the type C \(\bar 1\) → I \(\bar 1\) , C \(\bar 1\) → “e” structure, I \(\bar 1\) → “e” and “e” structure → I \(\bar 1\) have been followed. There are marked differences between the ordering behaviour of crystals with compositions on either side of the C \(\bar 1\) ? I \(\bar 1\) transition line. In the composition range ~ An50 to ~ An70 the e structure appears to have a true field of stability relative to I \(\bar 1\) ordering, and a transformation of the type I \(\bar 1\) ? e has been reversed. It is suggested that the e structure is the more stable ordered state at temperatures of ~ 800°C and below. For compositions more albite-rich than ~ An50 the upper temperature limit for long range e ordering is lower than ~ 750°C, and there is no evidence for any I \(\bar 1\) ordering. The evidence for a true stability field for “e” plagioclase, which is also consistent with calorimetric data, necessitates reanalysis both of the ordering behaviour of plagioclase crystals in nature and of the equilibrium phase diagram for the albite-anorthite system. Igneous crystals with compositions of ~ An65, for example, probably follow a sequence of structural states C \(\bar 1\) → I \(\bar 1\) → e during cooling. The peristerite, Bøggild and Huttenlocher miscibility gaps are clearly associated with breaks in the albite, e and I \(\bar 1\) ordering behaviour but their exact topologies will depend on the thermodynamic character of the order/disorder transformations. 相似文献
18.
Michael A. Carpenter 《Contributions to Mineralogy and Petrology》1980,71(3):289-300
The free energy curves for simple binary solid solutions with limited miscibility or atomic ordering have been combined to
predict the phase relations and exsolution mechanisms for a system in which both ordering and exsolution are possible. The
nature of the ordering process affects which exsolution mechanisms may be used. If the ordering is second (or higher) order
in character then continuous mechanisms predominate and a ‘conditional spinodal’ (Alien and Cahn, 1976) can be described which
operates between ordered and disordered end members. For a first order case, the ordered phase can only precipitate a disordered
phase by nucleation and growth.
Microstructures in omphacites observed by transmission electron microscopy include exsolution lamellae and antiphase domains
and the relations between them in selected specimens have been used to interpret the exsolution mechanisms which operated
under geological conditions. It appears that most omphacites undergo cation ordering, and then remain homogeneous or exsolve
a disordered pyroxene by spinodal decomposition. The predominance of continuous mechanisms has been used to indicate that
the C2/c→P2/n transformation may be second (or higher) order in character.
A possible phase diagram for jadeite-augite is presented. It is based on the idea that there should be limited miscibility
between the disordered end members at low temperatures and that the cation ordering at intermediate compositions (omphacite)
is superimposed on a solvus. It is adequate to explain many of the observed microstructures and fits with petrographic evidence
of broad two phase fields between impure jadeite and omphacite and between omphacite and sodic augite. The effect of adding
acmite is analogous to increasing temperature so that the phase relations for jadeite-acmite-augite can also be predicted. 相似文献
19.
The orientation dependence of the luminescence of a well-characterized plagioclase crystal at room temperature and 40 K is reported. A beam of H + ions was used to provide the excitation. Ion beam luminescence provides emissions effectively from the bulk of the material, and therefore minimizes the contribution to the luminescence from atypical regions. The intensity of the luminescence is strongly orientation-dependent. The intensity and photon energy, particularly of the red/infrared and yellow emission bands, vary significantly. We interpreted this as resulting from Fe 3+ and Mn 2+ activator ions, respectively, on crystallographic sites with low point symmetry. An emission at 860 nm was also significantly orientation-dependent. The blue luminescence showed the least variability. At room temperature, a 350 nm near-UV emission was noted, whereas at 40 K, emissions were at 240, 260, 300 and 340 nm. UV emissions may result from Na + diffusion along interfaces within the plagioclase, notably albite-law (010) twins. This variability has significant consequences for the use of single-crystal quantitative luminescence techniques. We have also studied the dependence of the peak intensities and profiles during prolonged ion beam bombardment with heavier (He +) ions. Broadening of the red-infrared emission is interpreted as reflecting growing amorphization of the sample. 相似文献
20.
ABSTRACT There is no significant difference in the diffusion profiles across albite-adularia bicrystals that were simultaneously deformed at a strain rate of 10-6S-1 and those from hydrostatic experiments at the same conditions (1500 MPa and 1000°C for 156 h). This indicates that the bulk alkali diffusion rate, which is the sum of lattice diffusion (D, 1) and dislocation pipe diffusion (Dp), is not significantly enhanced by dislocations at these conditions, and that the maximum value for the ratio of Dp/D1 is about 105. This is equal to the value previously reported for‘oxygen’diffusion in albite. If this ratio is independent of temperature, the contribution of either static (pre-deformed) or moving (syn-deformed) dislocations to the bulk diffusion rate of alkalis is probably minor at all metamorphic conditions. For Al and Si diffusion the ratio of Dp/D1 may be larger if D1 is lower. Thus a significant contribution from dislocations to bulk diffusion cannot be ruled out, especially during simultaneous deformation. 相似文献
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