共查询到20条相似文献，搜索用时 687 毫秒
1.
Titania, TiO 2, precipitation in natural blue sapphire (Fe, Ti: -Al 2O 3) has been investigated using high resolution and analytical transmission electron microscopy. The structure and habit of the TiO 2 precipitate depends on both the Ti 4+ concentration and the temperature at which the precipitate formed. Tetragonal TiO 2 (Rutile) grows at 1350° C but at 1150° C an orthorhombic non-equilibrium TiO 2 polymorph precipitates. Both TiO 2 polymorphs nucleate in the (0001)s plane as lens shaped discs twinned along their diameter. The crystallographic alignment of each type of TiO 2 precipitate with respect to the -Al 2O 3 host matrix provides a high degree of structural coherency with minimal lattice mismatch. Electron diffraction analysis established the following precipitate/host orientation relationships: tetragonal TiO 2: {011} r {11
07B;100} r(0001)s and 01
r10
0s twinned along the (011) r planeand orthorhombic TiO 2: {021}{11
0}s, {100}(0001)s and 0
2 10
0s twinned along the (021) plane. 相似文献
2.
The thermodynamic stability constants for the hydrolysis and formation of mercury (Hg 2+) chloride complexes
have been used to calculate the activity coefficients for Hg(OH)
n
(2–n)+ and HgCl
n
(2–n)+ complexes using the Pitzer specific interaction model. These values have been used to determine the Pitzer parameters for
the hydroxide and chloro complexes and C
ML). The values of and have been determined for the neutral complexes (Hg(OH) 2 and HgCl 2). The resultant parameters yield calculated values for the measured values of log to ±0.01 from I = 0.1 to 3 m at 25°C. Since the activity coefficients of and are in reasonable agreement with the values for Pb(II), we have estimated the effect of temperature on the chloride constants
for Hg(II) from 0 to 300°C and I = 0–6 m using the Pitzer parameters for complexes. The resulting parameters can be used to examine the speciation of Hg(II) with Cl − in natural waters over a wide range of conditions. 相似文献
3.
Two metamorphic isograds cut across graphitic schist near Pecos Baldy, New Mexico. The southern isograd marks the first coexistence of staurolite with biotite, whereas the northern isograd marks the first coexistence of andalusite with biotite. The isograds do not record changes in temperature or pressure. Instead, they record a regional gradient in the composition of the metamorphic fluid phase. Ortega Quartzite, which contains primary hematite, lies immediately north of the graphitic schist. Mineral compositions within the schist change gradually toward the quartzite, reflecting gradients in
and
. The chemical potential gradients, locally as high as 72 cal/m in
and 9 cal/m in
, controlled the positions of the two mapped isograds. The staurolite-biotite isograd records where
fell below 0.80, at
near 10 –23 bars; the andalusite-biotite isograd records where
fell below 0.25, at
near 10 –22 bars. Dehydration and oxidation were coupled by graphite-fluid equilibrium.The chemical potential gradients apparently formed during metamorphism, as graphite in schist reacted with hematite in quartzite. Local oxidation of graphite formed CO 2 which triggered dehydration reactions along the schistquartzite contact. This process created a C-O-H fluid which infiltrated into overlying rocks. Upward infiltration, local fluid-rock equilibration and additional infiltration proceeded until the composition of the infiltrating fluid evolved to that in equilibrium with the infiltrated rock. This point occurs very close to the staurolite-biotite isograd. Pelitic rocks structurally above this isograd show no petrographic evidence of infiltration, even though calculations indicate that volumetric fluid/rock ratios may have exceeded 15 and thin, rare calc-silicate beds show extensive K-metasomatism and quartz veining. 相似文献
4.
Fractionation of yttrium (Y) and the rare earth elements (REEs) begins in riverine systems and continues in estuaries and the ocean. Models of yttrium and rare earth (YREE) distributions in seawater must therefore consider the fractionation of these elements in both marine and riverine systems. In this work we develop a coupled riverine/marine fractionation model for dissolved rare earths and yttrium, and apply this model to calculations of marine YREE fractionation for a simple two-box (riverine/marine) geochemical system. Shale-normalized YREE concentrations in seawater can be expressed in terms of fractionation factors (
ij
) appropriate to riverine environments (
) and seawater (
):
where
and
are input-normalized total metal concentrations in seawater and
is the ratio of total dissolved Y in riverwater before
and after
commencement of riverine metal scavenging processes. The fractionation factors (
ij
) are calculated relative to the reference element, yttrium, and reflect a balance between solution and surface complexation of the rare earths and yttrium. 相似文献
5.
Observations in the North Sea Basin max indicate significant overpressure in sediments over horst blocks but not over grabens at the same submudline depth. The purpose is to show that over a horst, of top width W, with grabens on either side of top widths G 1 and G 2, respectively, the equivalent mud density. r can be estimated approximately from the simple equation. |
相似文献
6.
To investigate the point defect chemistry and the kinetic properties of manganese olivine Mn 2SiO 4, electrical conductivity ( ) of single crystals was measured along either the [100] or the [010] direction. The experiments were carried out at temperatures T=850–1200 °C and oxygen fugacities
atm under both Mn oxide ( MO) buffered and MnSiO 3 ( MS) buffered conditions. Under the same thermodynamic conditions, charge transport along [100] is 2.5–3.0 times faster than along [010]. At high oxygen fugacities, the electrical conductivity of samples buffered against MS is 1.6 times larger than that of samples buffered against MO; while at low oxygen fugacities, the electrical conductivity is nearly identical for the two buffer cases. The dependencies of electrical conductivity on oxygen fugacity and temperature are essentially the same for conduction along the [100] and [010] directions, as well as for samples coexisting with a solid-state buffer of either MO or MS. Hence, it is proposed that the same conduction mechanisms operate for samples of either orientation in contact with either solid-state buffer.The electrical conductivity data lie on concave upward curves on a log-log plot of vs
, giving rise to two
regimes with different oxygen fugacity exponents. In the low-
regime
, the
exponent, m, is 0, the MnSiO 3-activity exponent, q, is 0, and the activation energy, Q, is 45 kJ/mol. In the high
regime
10^{ - 7} {\text{atm}}} \right)$$
" align="middle" border="0">
, m=1/6, q=1/4–1/3, and Q=45 and 200 kJ/mol for T<1100 °c=" and="> T>1100 °C, respectively. 相似文献
7.
The partitioning of Cr and Al between coexisting spinel and clinopyroxene and the dependence of spinel-cpxgarnet equilibria on Cr/Al ratio have been investigated by a combination of phase equilibrium experiments, high temperature solution calorimetry and thermodynamic calculations.The exchange equilibrium:
has a measured enthalpy change for pure phases of –2,100±500 cal at 970 K and 1 atm. Experimental reversals of Cr-Al partitioning between the spinel and clinopyroxene phases yield the following partitioning relationship:
where X
i
j
refers to atomic fraction of i in the octahedral sites of phase j. The compositional dependence of partitioning implies that Al-Cr mixing in spinel is nonideal with, on the symmetrical model, a W
Cr-Al
Sp
of 2,700±500 cal/gm. atom. In contrast, aluminum-chromium mixing in clinopyroxene is close to ideal.The measured stability field of knorringite (Mg 3Cr 2Si 2O 12) and mixing properties of garnet have been used in conjunction with our experimental data to calculate the influence of Cr/Al ratio on the important reaction: orthopyroxene+clinopyroxene+spinel=olivine+garnetThe stability field of spinel lherzolite increases by about 2.8 Kb for every increase of 0.1 in Cr/(Cr+Al) ratio up to Cr/(Cr+Al) of 0.7. The calculated stabilization is in very good agreement with the experimental results of O'Neill (1981). The partitioning relationships are such that, at the low ratios of Cr/Al (0.07) of primitive lherzolite, clinopyroxene buffers spinel composition and sharpens the spinelgarnet reaction interval from 10 Kb (little or no clinopyroxene) down to about 2 Kb in pyroxene-rich pyrolite. 相似文献
8.
Multivariate statistical analyses have been extensively applied to geochemical measurements to analyze and aid interpretation of the data. Estimation of the covariance matrix of multivariate observations is the first task in multivariate analysis. However, geochemical data for the rare elements, especially Ag, Au, and platinum-group elements, usually contain observations the below detection limits. In particular, Instrumental Neutron Activation Analysis (INAA) for the rare elements produces multilevel and possibly extremely high detection limits depending on the sample weight. Traditionally, in applying multivariate analysis to such incomplete data, the observations below detection limits are first substituted, for example, each observation below the detection limit is replaced by a certain percentage of that limit, and then the standard statistical computer packages or techniques are used to obtain the analysis of the data. If a number of samples with observations below detection limits is small, or the detection limits are relatively near zero, the results may be reasonable and most geological interpretations or conclusions are probably valid. In this paper, a new method is proposed to estimate the covariance matrix from a dataset containing observations below multilevel detection limits by using the marginal maximum likelihood estimation (MMLE) method. For each pair of variables, say Y and Z whose observations containing below detection limits, the proposed method consists of three steps: (i) for each variable separately obtaining the marginal MLE for the means and the variances,
,
,
, and
for Y and Z: (ii) defining new variables by
and
and letting A= C+ D and B= C– D, and obtaining MLE for variances,
and
for A and B; (iii) estimating the correlation coefficient YZ by
and the covariance
YZ
by
. The procedure is illustrated by using a precious metal geochemical data set from the Fox River Sill, Manitoba, Canada. 相似文献
9.
Spatial factor analysis (SFA) is a multivariate method that determines linear combinations of variables with maximum autocorrelation at a given lag. This is achieved by deriving estimates of auto-/cross-correlations of the variables and calculating the corresponding eigenvectors of the covariance quotient matrix. A two-point spatial factor analysis model derives factors by the formation of transition matrix U comparing auto-/cross-correlations at lag 0, R
0, with those at a specified lag d, R
d, expressed as U
d= R
0
–1
R d. The matrix U
d can be decomposed into its spectral components which represent the spatial factors. The technique has been extended to include three points of reference. Spatial factors can be derived from the relationship: |
相似文献
10.
The impact of land-use change on the quality of groundwater in the Xiaotjiang watershed, China was assessed for the period
1982–2004. Groundwater samples were collected from 30 monitoring points across the watershed, and were representative of the
various changes, determined by remote sensing and geographical information systems. The results indicate that 610 km 2 (60% of the total watershed area) were subject to land-use change during the period. The most important changes were the
conversion of 135 km 2 of forested land to cultivated land, and 211 km 2 of unused land to cultivated land. The main impact was ascribed to diffuse pollution from fertilizers applied to newly cultivated
land, and from building development. Overall the groundwater pH value was significantly increased, as were the concentrations
of ions , , , , and Cl − in groundwater whilst the concentrations of Ca 2+ and declined. More precisely, in the regions where forested land and unused land were converted into cultivated land, the pH
value and the concentrations of Mg 2+, , , , , Cl − increased whilst the concentrations of Ca 2+ and declined. However in the region where cultivated land was converted into construction land, the pH value and the concentrations
of Ca 2+, Mg 2+, , , , , , Cl − increased.
Résumé L’impact des changements de l’utilisation du territoire sur la qualité de l’eau souterraine dans le bassin versant de Xiaojiang,
en Chine, a été évalué de 1982 à 2004. Des échantillons d’eau souterraine ont été récoltés à partir de 30 points d’observation
éparpillés sur le bassin, représentant les divers changements déterminés par télédétection et système d’information géographique.
Les résultats indiquent que 610 km2 (soit 60% de la surface du bassin) ont été sujets à des modifications de l’utilisation du territoire sur cette période. Les
changements les plus importants furent la conversion de 135 km2 de forêt et 211 km2 de terres inutilisées en terres cultivées. Le principal impact est attribué à la pollution diffuse des engrais utilisés en
agriculture et pour les batiments. De manière générale le pH de l’eau souterraine a augmenté significativement, ainsi que
les concentrations des ions , , , , et Cl−, tandis que les concentration en Ca2+ et ont diminué. Plus précisément dans les régions transformées en terres cultivées, la valeur du pH et les concentrations en
Mg2+, , , , , Cl− ont augmenté tandis que les concentrations en Ca2+ et ont diminué. Toutefois dans les régions cultivées converties en zones de construction, le pH et les concentrations en Ca2+, Mg2+, , , , , , Cl− ont augmenté.
Resumen El impacto del cambio en uso de la tierra en la calidad del agua en la cuenca Xiaojiang, China fue evaluado para el periodo
1982–2004. Muestras de agua subterránea fueron tomadas de 30 puntos de monitoreo a través de la cuenca, y fueron representativas
de los múltiples cambios, determinados por sensores remotos y sistemas de información geográfica. Los resultados indican que
610 km2 (60% del área total de la cuenca) estaban sujetos a cambios de uso de la tierra durante el periodo estudiado. Los cambios
más importantes fueron la conversión de 135 km2 de bosques a tierra cultivada, y 211 km2 de tierra sin uso (ociosa) a tierra cultivada. El impacto principal fue causado por contaminación difusa de fertilizantes
aplicados a la tierra recientemente cultivada, y a desarrollo de construcciones. En general el pH en agua subterránea creció
significantemente, al igual que las concentraciones de los iones , , , , y Cl− en agua subterránea mientras que las concentraciones de Ca2+ y decrecieron. Mas precisamente, en las regiones donde bosque y tierra ociosa fueron convertidas en tierra cultivada, el valor
de pH y las concentraciones de Mg2+, , , , , Cl− crecieron mientras las concentraciones de Ca2+ y decrecieron. Sin embargo en la región donde tierra cultivada fue convertida en construcciones, el valor de pH y las concentraciones
de Ca2+, Mg2+, , , , , , Cl− crecieron.
相似文献
11.
The equilibrium partitioning of Fe 2+ and Mg between olivine and liquid along a liquid line of descent has been determined for a calc-alkaline system, ranging in composition from picritic to andesitic. Experiments were conducted between 1000–1450° C and 1 bar to 30 kbar. Within the compositional range investigated
and
, the compositional dependence of the Fe 2+ and Mg partitioning is a function of the Mg-content of the liquid. The Mg-content of the liquid correlates strongly with temperature. The variation of the Fe 2+ and Mg partitioning were therefore evaluated individualy as functions of composition and temperature alone. The composition dependence of the cation-partitioning coefficients (Kd) is given by the following two equations: |
相似文献
12.
The volume of fluid and amount of heat involved in a portion of a metamorphic event around three synmetamorphic granitic stocks has been quantitatively estimated using mineral composition and modal data from carbonate rocks. Values of volumetric fluid-rock ratios range, with respect to a reference zoisite isograd, from 0.001 to 0.434. Amounts of heat involved range from –25 to 134 cal/cm 3 rock. Contours of constant fluid-rock ratio and of constant amount of heat are generally concentric about the granitic stocks indicating that the stocks are foci of high heat and fluid fluxes during metamorphism. In addition, contours of fluid-rock ratios and amount of heat outline NE-SW-trending channelways of high fluid and heat fluxes that alternate with regions of lower fluid and heat fluxes. The NE-SW-trending vertical bedding and schistosity in the area — of premetamorphic origin — probably constrained fluid and heat transfer to occur preferentially in NE-SW directions. Large values of heat involved in metamorphism are strongly correlated with large fluid-rock ratios, suggesting that fluids are an important carrier of heat during metamorphism. Configurations of mapped isograds in the area mimic the patterns of contours of constant fluid-rock ratio and of heat content, indicating that configurations of isograds may contain useful information about regional patterns of heat and fluid transfer during metamorphism.Notation
T
Last temperature recorded by metacarbonate rocks (°C)
-
P
Lithostatic pressure (bars)
-
Pi
Partial pressure of component i (bars)
-
of last fluid in equilibrium with carbonate rocks during metamorphism
-
R
1.987 cal/bar-degree
-
K
s
Activity constant for an assemblage of solid mineral phases
- In
Natural logarithm
-
c
v
Volumetric heat capacity (cal/cm 3-degree)
-
Q
Heat added to or subtracted from a rock during metamorphism in the zoisite zone (kcal/100 cm 3 rock; cal/cm 3 rock)
-
Q{ibrxn}
Heat added to or subtracted from a rock due to mineral reactions during metamorphism in the zoisite zone (kcal/100 cm 3 rock; cal/cm 3 rock)
- Std. Dev.
Standard Deviation
-
Average
of fluid in equilibrium with carbonate rocks during their metamorphism in the zoisite zone
-
of fluid in equilibrium with carbonate rocks at the zoisite isograd
-
T
Temperature at the zoisite isograd (°C)
-
X
i,j
Mole fraction of component i in phase j
-
H
i
Molar enthalpy of reaction i at 0 bars pressure
-
¯V
i
Change of molar volume due to reaction ii
- i
Measure of progress of reaction i
-
V
Change in rock volume due to fluid-rock reactions
-
iV
Initial rock volume before metamorphism within the zoisite zone
-
¯V
s,i
Change in molar volume of solid minerals due to reaction i
Component notation an
CaAl 2Si 2O 8
Phase notation Pl
Plagioclase
- Am
Amphibole
- Cc
Calcite
- Qz
Quartz
- Di
Diopside
- Zo
Zoisite
- Ga
Garnet
- Bi
Biotite
- Kf
Microcline
- Mu
Muscovite 相似文献
13.
The volume variation with pressure of seven intermediate plagioclase feldspars has been determined by high-pressure single-crystal X-ray diffraction. The bulk moduli of plagioclases for a 3rd-order Birch-Murnaghan EoS can be described by the following pair of equations:
with
for plagioclase with X
An
<20 and
for X
An
>35. These parameters can also be used in a Murnaghan EoS to describe the volume variation of plagioclase feldspars up to pressures of 3 GPa. For a Murnaghan EoS with
, the values of the bulk moduli can be described by a single equation,
, with a small loss in the accuracy of the predicted volumes up to pressures of 3 GPa.Editorial responsibility: T.L. GroveAn erratum to this article can be found at 相似文献
14.
To investigate high-temperature creep and kinetic decomposition of nickel orthosilicate (Ni 2SiO 4), aggregates containing 3 vol% amorphous SiO 2 have been deformed in uniaxial compression at a total pressure of one atomsphere. Twenty-three samples with grain sizes (d) from 9 to 30 m were deformed at temperatures (T) from 1573 to 1813 K, differential stresses () from 3 to 20 MPa, and oxygen fugacities ( f
o
2) from 10 -1 to 10 5 Pa. At temperatures up to 1773 K, the steady-state creep rate () can be described by the flow law |
相似文献
15.
Diffusion rates of 18O tracer in quartz ( c, 1 Kb H 2O) and Amelia albite ( 001, 2 Kb H 2O) have been measured, using Secondary Ion Mass Spectrometry (SIMS). A new technique involving hydrothermal deposition of labelled materials has removed the possibility of pressure solution-reprecipitation processes adversely affecting the experiments. Reported diffusion constants are: -quartz ( c),
, Q=98±7 KJ mol –1 (600–825° C, 1 Kb); Amelia albite ( 001),
, Q=85±7 KJ mol –1, (400–600° C, 2 Kb). Measured quartz 18O diffusivities decrease discontinuously at the - transition, reflecting strong structural influences. The reported albite data agree with previously recorded studies, but -quartz data indicate significantly lower activation energies. Possible causes of this discrepancy, and some geological consequences, are noted. 相似文献
17.
Summary The crystallography of roscherite is more complicated than previously thought. Single crystal X-ray work on material from Foote Mine (California) gave triclinic symmetry. The unit cell corresponding to the one adopted for monoclinic roscherite has a=15.921, b=11.965, c=6.741 Å, =91°04, =94°21, =89°59 1/2, space group
. The least-squares refinement of the structure using 2380 non zero reflections with anisotropic temperature factors resulted in a conventional reliability factor R=0.060.The X-ray study indicates the formula
while that proposed for monoclinic roscherite is
The atomic arrangements of both varieties of roscherite are very similar. The lowering of symmetry is caused by the segregation of the trivalent cations into only half of the sites of a monoclinic point position. Crystallochemical considerations suggest that the symmetry of roscherite does not depend on the kind of trivalent cations occupying the 6-coordinated position, but rather by the ratio between trivalent and divalent metal ions.
Die Kristallstruktur eines triklinen Roscherites Zusammenfassung Die Kristallographie des Roscherites ist komplizierter als man bisher annahm. Einkristall-Röntgenuntersuchungen an Material von Foote Mine (Kalifornien) ergaben trikline Symmetrie. Die Elementarzelle, welche der für monoklinen Roscherit angenommenen entspricht, hata=15,921,b=11,965,c=6,741 Å, =91°04, =94°21, =89°59 1/2, Raumgruppe
. Die Verfeinerung der Struktur mit der Methode der kleinsten Quadrate ergab unter Verwendung anisotroper Temperaturfaktoren für 2380 beobachtete Reflexe einen konventionellen ZuverlässigkeitsindexR=0,060.Die Röntgenuntersuchung weist auf die Formel
, während für monoklinen Roscherit
vorgeschlagen wurde. Die Atomanordnungen beider Abarten des Roscherites sind sehr ähnlich. die Symmetrieerniedrigung wird dadurch hervorgerufen, daß die dreiwertigen Kationen nur die Hälfte der Positionen einer monoklinen Punktlage besetzen. Kristallchemische Überlegungen weisen darauf hin, daß die Symmetrie nicht von der Art der dreiwertigen Kationen, welche eine 6-koordinierte Punktlage besetzen, abhängt, sondern vielmehr von dem Mengenverhältnis zwischen 3-wertigen und 2-wertigen Metallionen.
With 1 Figure 相似文献
18.
The diffusion of water in a peralkaline and a peraluminous rhyolitic melt was investigated at temperatures of 714–1,493 K
and pressures of 100 and 500 MPa. At temperatures below 923 K dehydration experiments were performed on glasses containing
about 2 wt% H 2O
t
in cold seal pressure vessels. At high temperatures diffusion couples of water-poor (<0.5 wt% H 2O
t
) and water-rich (~2 wt% H 2O
t
) melts were run in an internally heated gas pressure vessel. Argon was the pressure medium in both cases. Concentration profiles
of hydrous species (OH groups and H 2O molecules) were measured along the diffusion direction using near-infrared (NIR) microspectroscopy. The bulk water diffusivity
() was derived from profiles of total water () using a modified Boltzmann-Matano method as well as using fittings assuming a functional relationship between and Both methods consistently indicate that is proportional to in this range of water contents for both bulk compositions, in agreement with previous work on metaluminous rhyolite. The
water diffusivity in the peraluminous melts agrees very well with data for metaluminous rhyolites implying that an excess
of Al 2O 3 with respect to alkalis does not affect water diffusion. On the other hand, water diffusion is faster by roughly a factor
of two in the peralkaline melt compared to the metaluminous melt. The following expression for the water diffusivity in the
peralkaline rhyolite as a function of temperature and pressure was obtained by least-squares fitting: where is the water diffusivity at 1 wt% H 2O
t
in m 2/s, T is the temperature in K and P is the pressure in MPa. The above equation reproduces the experimental data (14 runs in total) with a standard fit error
of 0.15 log units. It can be employed to model degassing of peralkaline melts at water contents up to 2 wt%. 相似文献
19.
The interdependence of the Fe(Mg) –1 (e.g., FeO-MgO in silicate melt; CaFeSi 2O 6-CaMgSi 2O 6 in pyroxene) and TiAl 2(MgSi 2) –1 exchange reactions between silicate melts and coexisting Ca-pyroxene has been examined. High-calcium clinopyroxenes were grown in 1 atmosphere melting and crystallization experiments on rock powders spanning the composition range tholeiite to melilitite (1,092 2+Mg2+ exchange and
suggest that at given values of
extent of Fe(Mg)–1 substitution is strongly coupled with the TiAl2(MgSi2)–1 substitution in pyroxenes near the five-component space CaMg(Si2O6-CaFe(Si)2O6-CaTi(Al)2O6-CaFe(Al,Si)2O6-CaAl(Al,Si)2O6. The inferred stabilization of Ti in iron-rich relative to magnesium pyroxene is consistent with the operation of Fe2+Ti4+ intervalence charge transfer interactions (e.g., Rossman 1980) and observations on zoning in natural titanaugites (e.g., Tracy and Robinson 1977). Although the rims of some pyroxenes grown in some melting experiments exhibit prominent zoning in TiAl2(MgSi2)–1, the average values of
inferred from the compositions of these pyroxenes, together with those of the relatively homogeneous pyroxenes produced in crystallization experiments, exhibit a 11 correlation with values of
derived from the solution model of Ghiorso et al. (1983) with a standard error of 750 calories. The Ti contents of Ca-rich pyroxenes crystallizing from a wide range of natural silicate liquids can therefore be predicted. 相似文献
20.
The assemblages rutile-hematite, hematite, hematite-magnetite, hematite-ilmenite-magnetite, and ilmenite-magnetite occur in sillimanite- and kyanite-grade quartzites exposed in western New Hampshire. Different assemblages are found in interlayered sedimentary beds of single outcrops. Magnetites are nearly pure Fe 3O 4 and contain trace amounts of Al, Si, Ti, V, Cr, Mn, and Ni. Magnetites in contact with hematites contain up to 0.4 weight % MnO, but magnetites in contact with ilmenites containing up to 2.3 weight % MnO have no detectable Mn. Ilmenite is enriched in Mn relative to coexisting hematite, and hematite is so enriched with respect to magnetite. Systematic partitioning of elements between oxide minerals and absence of crossing tie lines suggest that the minerals attained chemical equilibrium during regional metamorphism. None of the assemblages are divariant because of the presence of components in addition to FeO, Fe 2O 3, and TiO 2; therefore, none of them constitute oxygen buffers. Nevertheless, gradients in
between adjacent sedimentary beds can be measured using variations of oxide mineral composition in trivariant and quadrivariant phase assemblages. Oxygen behaved as an initial value component or inert component during regional metamorphism. It is likely that the
gradients are due to differences in bulk composition of sedimentary beds at the time of deposition. 相似文献
|
|
|
|
|
