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1.
I. J. Richards Theodore C. Labotka Robert T. Gregory 《Contributions to Mineralogy and Petrology》1996,123(2):202-221
Late Proterozoic to Cambrian carbonate rocks from Lone Mountain, west central Nevada, record multiple post-depositional events
including: (1) diagenesis, (2) Mesozoic regional metamorphism, (3) Late Cretaceous contact metamorphism, related to the emplacement
of the Lone Mountain granitic pluton and (4) Tertiary hydrothermal alteration associated with extension, uplift and intrusion
of silicic porphyry and lamprophyre dikes. Essentially pure calcite and dolomite marbles have stable isotopic compositions
that can be divided into two groups, one with positive δ13C values from+3.1 to +1.4 ‰ (PDB) and high δ18O values from +21.5 to +15.8 ‰ (SMOW), and the other with negative δ13C values from –3.3 to –3.6‰ and low δ18O values from +16.9 to +11.1‰. Marbles also contain minor amounts of quartz, muscovite and phlogopite. Brown and blue luminescent,
clear, smooth textured quartz grains from orange luminescent calcite marbles have high δ18O values from +23.9 to +18.1‰, while brown luminescent, opaque, rough textured quartz grains from red luminescent dolomite
marbles typically have low δ18O values from +2.0 to +9.3‰. The δ18O values of muscovite and phlogopite from marbles are typical of micas in metamorphic rocks, with values between +10.4 and
+14.4‰, whereas mica δD values are very depleted, varying from −102 to −156‰. No significant lowering of the δ18O values of Lone Mountain carbonates is inferred to have occurred during metamorphism as a result of devolatilization reactions
because of the essentially pure nature of the marbles. Bright luminescence along the edges of fractures, quartz cements and
quartz overgrowths in dolomite marbles, low δD values of micas, negative δ13C values and low δ18O values of calcite and dolomite, and depleted δ18O values of quartz from dolomite marbles all indicate that meteoric fluids interacted with Lone Mountain marbles during the
Tertiary. Partial oxygen isotopic exchange between calcite and low 18O meteoric fluids lowered the δ18O values of calcite, resulting in uniform quartz-calcite fractionations that define an apparent pseudoisotherm. These quartz-calcite
fractionations significantly underestimate both the temperature of metamorphism and the temperature of post-metamorphic alteration.
Partial oxygen isotopic exchange between quartz and meteoric fluids also resulted in 18O depletion of quartz from dolomite marbles. This partial exchange was facilitated by an increase in the surface area of the
quartz as a result of its dissolution by meteoric fluids. The negative δ13C values in carbonates result from the oxidation of organic material by meteoric fluids following metamorphism. Stable isotopic
data from Lone Mountain marbles are consistent with the extensive circulation of meteoric hydrothermal fluids throughout western
Nevada in Tertiary time.
Received: 1 February 1994/Accepted: 12 September 1995 相似文献
2.
Fe and Al oxides distribution in some ultisols and inceptisols of southeastern Nigeria in relation to soil total phosphorus 总被引:3,自引:1,他引:2
Ten highly weathered soils in southeastern Nigeria were sampled from their typical A and B horizons for analyses. The objectives
were to determine the different forms of Fe and Al oxides in the soils and relating their occurrence to phosphate availability
and retention in the soils. The soils are deep and often physically degraded but are well drained and coarse in the particle
size distribution. They are mostly dominated by kaolinite in their mineralogy with very high values of SiO2. The soils are acidic with low soil organic carbon (SOC) contents. The elements in the exchange complex are also low thus
reflecting in the low CEC of the soil. Available phosphorus (P) in the soils are generally low while total P ranged from 157
to 982 mg kg−1 with an overall average of 422 mg kg−1. Total Fe in the soil is highest and their order represented as follows: Fet > Fed > Feox ≥ Fep. The pyrophosphate extractable Fe was always higher in the top soil than in the subsoil and was attributed to the fact that
these forms of Fe are associated with organic matter which is more abundant in topsoil than in subsoil. Like in Fe forms,
the order of Al occurrence could generally be presented as; Alt > Ald > Alox > Alp. More Fe and Al oxides in the soils are strongly crystalline while a small quantity is poorly crystalline Fe forms. The amorphous
forms of both Fe and Al are very low in the soils when compared with the crystalline forms. The oxides that show very strong
affinity to total P are Fed–Feox, Fed, Ald, Fet, Feox and Alox/Ald. To overcome this problem of P retention in the soil, we recommend constant liming of these soils to neutralize them, application
of organic matter and of high dosage of phosphate fertilizer to the soils. 相似文献
3.
The effect of distribution of iron-oxyhydroxide grain coatings on the transport of bacterial cells in porous media 总被引:3,自引:0,他引:3
E. P. Knapp Janet S. Herman George M. Hornberger Aaron L. Mills 《Environmental Geology》1998,33(4):243-248
Among the demonstrated processes influencing the transport of bacteria through aquifers, the deposition of cells on mineral
surfaces is one of the most important. For example, understanding the transport of introduced bacteria through aquifers is
essential to designing some in situ bioremediation schemes. The impact of the presence and distribution of Fe(III)-oxyhydroxide-coated
sand grains on bacterial transport through porous media was evaluated in column experiments in which bacteria (short rods;
1.2 μm length) were eluted through columns of quartz sand (0.5–0.6 mm in diameter) for several conditions of chemical heterogeneity
of mineral substrate. Fe(III)-oxyhydroxide-coated sand was present as 10% of the mass, and it was arranged in three treatments:
(1) homogeneously distributed, and present as a discrete layer (2) at the top and (3) at the bottom of 14-cm-long sand columns.
A pulse input of 108 cells ml–1 was introduced in an artificial groundwater solution flowing at 14 cm h–1 through the column, and eluted cells were counted. Peak breakthrough occurred at 1.0 pore volume. A large proportion of cells
were retained; 14.7–15.8% of the cells were recovered after three pore volumes of solution had eluted through clean quartz
sand, and only 2.1–4.0% were recovered from the Fe(III)-oxyhydroxide-coated sand mixtures. The three physical arrangements
of the chemical heterogeneity resulted in essentially the same breakthrough of cells, indicating that the spatial distribution
of iron coating does not affect the transport of bacteria. The results of the column transport experiments, which mimic hydrogeological
conditions encountered in field problems, are consistent with our mechanistic understanding of bacterial sorption.
Received: 10 April 1996 · Accepted: 17 February 1997 相似文献
4.
Implications from inclusions in topaz for greisenisation and mineralisation in the Hensbarrow topaz granite, Cornwall, England 总被引:3,自引:0,他引:3
B. J. Williamson C. J. Stanley J. J. Wilkinson 《Contributions to Mineralogy and Petrology》1997,127(1-2):119-128
Textural and geochemical studies of inclusions in topaz from greisens in the Hensbarrow topaz granite stock (St. Austell,
Cornwall) are used to constrain the composition of fluids responsible for late stage greisening and mineralisation. The topaz
contains an abundant and varied suite of inclusions including aqueous liquid + vapour (L + V), quartz, zinnwaldite, albite,
K-feldspar, muscovite, ilmenorutile, apatite, columbite, zircon, varlamoffite [(Sn, Fe)(O, OH)2] and qitianlingite [(Fe+2,Mn+2)2(Nb,Ta)2W+6O10]. Primary L + V inclusions in topaz show relatively high T
h (mainly 300 to >500 °C) and a narrow range of salinities (23–30 wt % NaCl equivalent) compared with those in greisen quartz
(150–450 °C, 0–50 wt % NaCl equivalent). Textures indicate that topaz formed earlier than quartz and the fluid inclusion data
are interpreted as indicating a cooling of the hydrothermal fluids during greisenisation, mixing with meteoric waters and
a decrease in pressure causing intermittent boiling. The presence of early-formed albite and K-feldspar as inclusions in the
topaz is likely to indicate that the greisen-forming fluid became progressively more acid during greisenisation. The most
distinctive inclusions in the topaz are wisp- and bleb-shaped quartz, < 50 μm in size, which show textural characteristics
indicating former high degrees of plasticity. They often have multiple shrinkage bubbles at their margins rich in Sn, Fe,
Mn, S and Cl and, more rarely, contain euhedral albite, K-feldspar, stannite or pyrrhotite crystals up to 40 μm in size. The
quartz inclusions show similar morphologies to inclusions in topaz from quartz-topaz rocks elsewhere which have been interpreted
as trapped “silicate melt”. Their compositions are, however, very different to those expected for late stage topaz-normative
granitic melts. From their textural and chemical characteristics they are interpreted as representing crystallised silica
colloid, probably trapped as a hydro gel during greisenisation. There is also evidence for the colloidal origin of inclusions
of varlamoffite in the topaz. These occurrences offer the first reported evidence in natural systems for the formation of
colloids in high temperature hydrothermal fluids. Their high ore carrying potential is suggested by the presence of varlamoffite
and the occurrence of stannite, pyrrhotite and SnCl within the quartz inclusions.
Received: 9 April 1996 / Accepted: 12 November 1996 相似文献
5.
Stichtite, a rare (14 known localities worldwide) hydrated carbonate-hydroxide of Mg and Cr with ideal formula Mg6Cr2 (OH)16 CO3 · 4H2O, occurs exclusively in Cr-rich serpentinites of ophiolites or greenstone belts. Physical properties (hardness = 1.5–2, specific
gravity = 2.16–2.2, perfect basal [0001] cleavage, grain size commonly < 100 μm) resemble talc, but the mineral has an attractive
purple to lilac color; chemical analyses demonstrate it to be a non-silicate. Stichtite generally occurs as irregular to rounded
masses (< 1 cm – 30 cm across) and as veinlets (< 1 mm – > 2 cm wide) within serpentinite. Macroscopic and microscopic textures,
such as crosscutting veinlets and stringers, demonstrate that stichtite formation invariably post-dated serpentinization.
In some specimens stichtite surrounds relict grains of Cr-rich spinel; in others stichtite has completely replaced euhedral
or subhedral chromites. Chemical analyses of stichtites reveal substantial substitution of Al and Fe3+ for Cr in specimens from many localities, reflecting a possible compositional continuum between stichtite and rhombohedral
polymorphs hydrotalcite (Mg6Al2 (OH)16 CO3 · 4H2O) and pyroaurite (Mg6Fe2 (OH)16 CO3 · 4H2O). We report the first electron microprobe analyses of stichtites from seven localities, and summarize all available published
chemical data. Stichtites very likely inherited part of their trivalent cation chemistry from precursor Cr-rich spinels, but
stichtite growth apparently post-dated characteristic “ferritchromit” alteration, as demonstrated by the depletion of Al and
enrichment in Fe3+ in stichtite relative to primary chromite core compositions. Stichtite appears to form by reaction between serpentine and
altered chromite, during addition of substantial fluid, either as separate H2O and CO2 phases, or as a mixed volatile phase. Such reactions must involve removal of substantial SiO2, possibly by transport and remote deposition of silica by throughgoing aqueous and carbonic fluid.
Received: 4 April 1996 / Accepted: 16 September 1996 相似文献
6.
The Malanjkhand copper ( + molybdenum) deposit, India: mineralization from a low-temperature ore-fluid of granitoid affiliation 总被引:2,自引:0,他引:2
Copper and subordinate molybdenum mineralization at Malanjkhand occurs within a fracture-controlled quartz-reef enclosed
in a pink granitoid body surrounded by grey-granitoids constituting the regional matrix. Sulfide-bearing stringers, pegmatites
with only quartz + microcline and sulfide disseminations, all within the pink-granitoid, represent other minor modes of occurrences.
Despite this diversity in mode of occurrence, the mineralogy of ores is quite consistent and conform to a common paragenetic
sequence comprising an early `ferrous' stage of precipitation of magnetite (I) and pyrite (I) and, the main-stage chalcopyrite
mineralization with minor sphalerite, pyrite (II), magnetite (II), molybdenite and hematite. Both stages witnessed continuous
precipitation of quartz ± microcline ± (chlorite, biotite and epidote). The enclosing pink-granitoid and the regional grey-granitoids
display alteration features such as saussuritization of plagioclase, breakdown of hornblende and chloritization of biotite
on a regional scale, indicating interaction with a pervasive fluid. Quartz and microcline precipitation mostly restricted
within the pink granitoid, postdates this alteration. Four types of primary inclusions were encountered in quartz from ore samples: (1) type-I – aqueous-biphase(L + V) inclusions, the commonest variety in all ore types; (2) type-II – aqueous-carbonic(Laq + Lcarb ± Vcarb); (3) type-III – pure-carbonic(Lcarb ± Vcarb) – type-II and III being restricted to stringer and pegmatitic ores, and (4) rare polyphase (Laq + Vaq + calcite/gypsum) inclusions. Quartz in granitoids contain primary type-I inclusions only. Type-I inclusions from ore samples furnish a temperature range (after a rough pressure
correction to the T
H
-maxima of 140–180 °C) of 150–275 °C and a moderately low salinity of 4–12 wt.% NaCl equivalent. This is inferred to represent
the signature of the major component (F2) of the ore fluid. A few type-I inclusions of higher T
M
(up to 380 °C) and low salinity and density represent the other (F1) identifiable component of the ore fluid present in low
proportion. The T
H
-maxima and the total range in salinity of type-I inclusions in quartz from granitoids are strikingly similar to those from the ore samples. Composition of syn-ore chlorites furnished a temperature range of 185–327
°C, which conforms to the fluid inclusion microthermometric data. Pressure estimates using standard fluid inclusion geobarometric
methods, vary from 550 to 1790 bar in the stringer ores. Observed temperature-salinity/density relationships are best explained
by a two-stage evolution model of the ore fluid: the first stage witnessed mixing of the two components, F1 and F2 in unequal
proportion, bringing about mineralization. The second stage of evolution was marked by the separation of a carbonic component
on continued sulfide precipitation and attendant increase in salinity of the fluid. The F1 component emerged as a distinct,
heated and (CO2 + S)-charged entity due to steam-heating and contamination of the early-ingressed F2 fluid at the fracture zone. The pervasive
fluid phase in the surrounding granitoids contributed the F2 component.
Received: (10 August 1994), 15 August 1995 / Accepted: 12 January 1996 相似文献
7.
8.
A palygorskite unit was discovered in a road cut of undifferentiated Tertiary limestone between the villages of El Pariso
and San Roman (18°49.309N, 88°37.861W) in the southeastern Yucatán Peninsula, Mexico. This is the southern most locality of
a clay-rich sedimentary unit reported in the literature for the Tertiary carbonates of the Yucatán Peninsula. This occurrence
indicates a much wider range of palygorskite-rich clay deposition than previously recognized. The lithology is 99% clay and
1% sand to silt size diagenetic quartz grains. The clay consists of approximately 85% palygorskite, 15% montmorillonite and
trace amounts of titanium oxides. EDS analyses on palygorskite are largely consistent with sedimentary palygorskites from
other coastal marine settings, however palygorskite has a low total Fe content (average = 0.40 wt% expressed as Fe2O3) compared to many other sedimentary palygorskites. Montmorillonite chemical compositions are typical and compared to the
palygorskite have substantially higher Fe2O3 concentrations (average = 3.90 wt%). The low percentage of coarse grains in the lithology combined with a high proportion
of palygorskite and lack of detrimental trace minerals suggest the deposit is of industrial grade; however, it has limited
reserves (6,000 m3). The unit could be potentially used in a wide array of environmental applications which are needed in the region including
liners for landfills and constructed wetlands. The unit is in a geographic location which would serve the expanding economy
of the region. This resource has the potential to have great impact on the quality of the local environment and the economy
of a region under great environmental threat. 相似文献
9.
Adsorption of arsenic by natural aquifer material in the San Antonio-El Triunfo mining area, Baja California, Mexico 总被引:1,自引:0,他引:1
Several experiments of arsenic (As) adsorption by aquifer material of the San Antonio-El Triunfo (SA-ET) mining area were
conducted to test the feasibility of this material acting as a natural control for As concentrations in groundwater. This
aquifer material is mineralogically complex, composed of quartz, feldspar, calcite, chlorite, illite, and magnetite/hematite.
The total iron content (Fe2O3) in the fine fraction is ∼12%, whereas Fe2O3 in the coarse fraction is <10 wt%. The experimental percent total As adsorbed vs. pH curves obtained match the topology of
total As adsorbed onto iron oxi-hydroxides surface (arsenate + arsenite; high adsorption at low pH, low adsorption at high
pH). A maximum of about 80% adsorbed in the experiments suggests the presence of arsenite in the experimental solutions. The
experimental adsorption isotherm at pH 7 indicates saturation of surface sites at high solute concentrations. Surface titration
of the aquifer material indicates a point of zero charge (PZC) for the adsorbent of about 8 to 8.5 (PZC for iron oxyhydroxides
=7.9–8.2). Comparison between experimental and modeled results (using the MICROQL and MINTEQA2 geochemical modeling and speciation
computer programs) suggests that As is being adsorbed mostly by oxyhydroxides surfaces in the natural environment. Based on
an estimated retardation factor (R), the travel time of the As plume from the SA-ET area to La Paz and Los Planes is about 700 to 5000 years.
Received: 17 March 1997 · Accepted: 8 September 1997 相似文献
10.
Sivajini Gilchrist Alexander E. Gates Matthew Gorring Evert Jan Elzinga 《Environmental Earth Sciences》2011,63(5):1029-1041
Organic material in metal contaminated soils around an abandoned magnetite mine–smelter complex in the critical Highlands
watershed protects the groundwater and surface water from contamination. Metals in these waters were consistently below local
and national water standards. Two groups of soil types cover the area: (1) Group A disturbed metal-rich soils, and (2) Group
B undisturbed organic soils. Chromium and nickel were more elevated than other metals with Cr more widespread than Ni. In
Group A, Cr correlated strongly with sesquioxides in the lower horizons (Fe2O3: r = 0.74, p < 0.025; Al2O3: r = 0.92, p < 0.005). In Group B, Cr correlated strongly (r = 0.96, p < 0.005) with soil organic matter (SOM) in the O-horizons. Ni–Cr (Group A: 52 and 70% in O- and lower horizons, respectively;
Group B: ~100% in both horizons) and V–Cr correlations (78% only in Group A lower horizons) suggest similar retention mechanisms
for these elements. Average soil
\textpH\textCaCl2 {\text{pH}}_{{{\text{CaCl}}_{2} }} for both groups ranged between 3.65 and 5.91, suggesting that soil acidity is determined by organic acids and solubility
of Al3+ releasing H+ ions. SOM and sesquioxides contribute significantly to creating naturally occurring filtration systems, removing metals,
and protecting water quality. High Ca, Fe, and Ti in Group A soils suggest slag and ash were mixed into the soils. Some low-Cr
sources include magnetite, slag, and ash (100, 100 and 200 mg/kg, respectively). Constant ZrO
2
:TiO
2
ratios in the lower soils indicate soil formation from breakdown of underlying tailing rocks, contributing Cr to these layers. 相似文献
11.
Shulin Liu Tao Wang Guangting Chen Jian Guo Xian Xue Shaoxiu Ma 《Environmental Geology》2008,53(6):1225-1233
The characteristics of sand and dust movement over different sandy grasslands in China’s Otindag Sandy Land were explored
based on field observations and laboratory analyses. Threshold wind speeds (the speed required to initiate sand movement)
at a height of 2 m above the ground were estimated in the field for different surface types. Threshold wind speed above shifting
dunes in the study area is about 4.6 m s−1 at this height. This value was smaller than values observed above other surfaces, resulting in a greater risk of blowing
sand above these dunes. Differences in sand transport rates (STR) as a function of the severity of desertification resulted
primarily from differences in surface vegetation cover and secondarily from the soil’s grain-size distribution. STR increased
exponentially with increasing near-bed wind velocity. Under the same wind conditions, STR increased with increasing severity
of desertification: from 0.08 g cm−2 min−1 above semi-fixed dunes to 8 g cm−2 min−1 above semi-shifting dunes and 25 g cm−2 min−1 above shifting dunes. Vegetation’s affect on STR was clearly large. Different components of sand and dust were trapped over
different lands: mostly sand grains but little dust were trapped above shifting dunes, but much dust was collected over semi-shifting
and semi-fixed dunes. Human disturbance is likely to produce dust even from fixed dunes as a result of trampling by animals
and vehicle travel. In addition, spring rainfall decreased the risk of sand and dust movement by accelerating germination
of plants and the formation of a soil crust. 相似文献
12.
The microstructure of hematite-ilmenite exsolution intergrowth of a natural titanohematite crystal from a granulite has been
investigated in a transmission electron microscope equipped with an energy filter. Special emphasis is on quantitative compositional
mapping at the nanometre scale using electron spectroscopic imaging, as well as mapping the Fe3+ and Fe2+ valence distribution in the intergrowth. Quantitative point analyses by energy-dispersive X-ray analysis have been compared
with results from electron energy-loss spectroscopy and element-distribution mapping. The results indicate that the coexisting
compositions of the two phases (Ilm88Hem12 and Ilm16Hem84) are independent of the size of the exsolution. The application of quantitative mapping to determining diffusion profiles
around precipitates is demonstrated.
Received: 30 March 2000 / Accepted: 7 September 2000 相似文献
13.
The environmental conditions prevailing in the Chicam-Toctina drainage system (approx. 138 km2 in Córdoba, Argentina) are considered representative of a number of catchments in Argentina's Sierras Pampeanas Range. Two
groups of ions reflect the sources of dissolved species in the catchment: a) a group (Cl–, SO2–
4, and Na+) which recognizes natural and anthropogenic sources, and which exhibits significant correlations with N–
3 and NO–
2, and b) another group of components (Ca2+, Mg2+, and HCO–
3) which is clearly controlled by carbonate rocks and their waste rock products. In the headwaters, stockpiled marble quarry
mining wastes provide a more open system to CO2 gaseous exchange than the outcropping rocks, thus promoting the increase of carbonate dissolution (up to 4.88 g km–2 s–1 during the rainy season). This specific yield was 20% higher than the amount estimated for an area with fewer extended mining
activities. The dissolved load delivered by the upper reaches is subjected in the lower drainage area to various processes,
mainly controlled by the presence of the city of Alta Gracia (approx. 40,000 inhabitants). In the dry season, due to nutrient
inputs supplied by the city, photosynthetic activity plays a major role controlling stream pH. Hence, the high values of calcite
saturation indexes and the increase of CaCO3 concentration in bed sediments can be explained by calcite precipitation. Such a process could be accompanied by the coprecipitation
on calcium carbonate of low solubility heavy metal carbonates.
Received : 17 January 1997 · Accepted : 31 March 1997 相似文献
14.
The relative stabilities of orthozoisite, Ca2Al3[O|OH|Si2O7|SiO4], space group Pnma, and the monoclinic polymorph, clinozoisite, space group P21/m, have been investigated using calculations based on density functional theory. It is found that orthozoisite is more stable
than clinozoisite by about 1 kJ mol−1 at zero pressure in the athermal limit. The bulk moduli of the two polymorphs have been calculated to be Bortho=117.5(1.7) GPa and Bclino=136(4) GPa.
Received: 20 March 2000 / Accepted: 26 February 2001 相似文献
15.
Stability of aggregates of some weathered soils in south-eastern Nigeria in relation to their geochemical properties 总被引:1,自引:0,他引:1
The stability of some highly weathered soils of the tropics is controlled by their organo-mineral substances. Highly weathered soils from 10 different locations were sampled from their A and B horizons to determine their aggregate stability. The objective of the study was to determine the aggregate stability of the soils and their relationships with geochemical constituents. The major geochemical elements of the soils are quartz and kaolinite, SiO2, Al2O3 and Fe2O3, while the dithionite extractable Fe and Al was greater than their corresponding oxalate and pyrophosphate forms. The mean-weight diameter from dried aggregates (MWDd) and their corresponding wet mean-weight diameter (MWDw) were related significantly (r = 0.64*). The dithionite extracted Al and Fe or the crystalline forms of these elements were outstanding in the stability of the aggregates. However, this did not diminish the influence of SOC reduced to third order level in the stability of the soils. The influence of SOC in these soils, however, indirectly manifested on the role of Fep and Alp in the aggregation of these soils. The crystalline Fe and Al sesquioxides were very prominent in the aggregation and stability of these soils. 相似文献
16.
The Samborombon Bay wetland is located on the west margin of the Rio de la Plata estuary, in the Province of Buenos Aires,
Argentina. This paper analyses the geological, geomorphologic, soil and vegetation characteristics of the southernmost sector
of this wetland and their influence on surface water and groundwater. The study area presents three hydrologic units: coastal
dunes, sand sheets and coastal plain. Coastal dunes and sand sheets are recharge zones of high permeability with well-drained,
non-saline soils, and a few surface water flows. Changes in the water table are related to rainfall. Groundwater in coastal
dunes is Ca–Mg–HCO3 to Na–HCO3, and of low salinity (590 mg/l). Groundwater in sand sheets is mainly Na–HCO3 with a salinity of about 1,020 mg/l. The coastal plain exhibits medium to low permeability sediments, with submerged saline
soils poorly drained. Groundwater is Na–Cl with a mean salinity of 16,502 mg/l. A surface hydrological network develops in
the coastal plain. Surface water levels near the shoreline are affected by tidal fluctuations; far from the shoreline water
accumulates because of poor drainage. Both sectors have Na–Cl water, but the former is more saline. Human intervention and
sea level rise may affect the wetland severely. 相似文献
17.
The evolution of columnar fiber texture was studied in wollastonite reaction rims synthesized by the reaction calcite + quartz=wollastonite
+ CO2. Experiments were performed at 850 to 950 °C at 100 MPa in dry CO2 and were evaluated by scanning and transmission electron microscopy. Rim growth rates are interpreted as controlled by the
diffusion of the SiO2 component through the rims from the quartz–wollastonite to the wollastonite–calcite interface. The temperature dependence
of rim growth rates yields an apparent activation energy of 314 ± 53 kJ mol−1. The columnar fibrous wollastonite crystallizes at the quartz–wollastonite interface and comprises the largest parts of the
rims. Ultimately, at the growth front strain contrast centers are present in the quartz. The strained volume extends about
200 nm into the quartz grains. We suggest that this might signify deformation of the quartz lattice due to wollastonite crystallization.
Wollastonite fiber thickness was measured from TEM images along traverses that represent intermediate positions of the growth
front during the experiments. The average thickness is in the 100–200 nm range. Fiber thickness increases with increasing
growth temperature. At a given temperature, the thickness of the fibers at the growth front slightly decreases with time,
i.e., the number of fiber tips per unit area in the growth front increases. The decrease of the fiber thickness is well fitted
by a parabolic rate law.
The generation of the columnar fiber texture is interpreted as an effect of induced stresses at the growth front, resulting
from the volume increase due to the local reaction. This volume increase forces SiO2 to diffuse along the growth front to the grain boundaries between the wollastonite fibers. These serve as fast diffusion
pathways through the rims. The fiber thickness monitors the diffusion distances in the growth front and thus the height of
the induced stress gradients. Since interface reactions are usually associated with volume changes, growth rates of reaction
rims and zones in coronas are not only controlled by the diffusive mobility of the components but also by the volume restraints
on the interface reactions.
Received: 19 July 2002 / Accepted: 14 February 2003 相似文献
18.
Anurag Sharma 《Contributions to Mineralogy and Petrology》1996,125(2-3):263-275
The stability of pargasite in the presence of excess quartz has been determined in the range of 0.5–6.0 kbar and 500–950 °C
in the system Na2O– CaO–MgO–Al2O3–SiO2–H2O, using synthetic minerals. The experimental results from this study indicate the presence of two distinct mineral assemblage
regions: (1) a high temperature supersolidus region containing tremolitic amphibole+melt+quartz; (b) a low temperature subsolidus region consisting of Al-rich amphibole+plagioclase+enstatite+quartz. Compositional reversals have been determined for the
following three equilibria:
(a) 2 pargasite+9 quartz=tremolite+4 plagioclase (An50)+1.5 enstatite+H2O, (b) 2 pargasite+10 quartz=tremolite+4 plagioclase (An50)+talc, and (c) pargasite+diopside+5 quartz=tremolite+2 plagioclase (An50). These experiments indicate a continuous change of amphibole composition from pargasite to tremolite with increasing temperature,
and an opposite effect with increasing pressure. The third equilibria is used to constrain a site-mixing model for the pargasitic
amphiboles, which favor a single-coupled NaA-AlT1 site mixing. The thermochemical data for pargasite estimated from the reversal data of the three equilibrium reactions is
estimated as for ΔG
0
f
,Pg=−12022.11±5.2 kJ mole-1, and S
0
Pg=591.7 ±7.9 JK-1 mole-1.
Received: 31 July 1995/Accepted: 3 June 1996 相似文献
19.
Epithermal mineralization and ore controls of the Shasta Au-Ag deposit, Toodoggone District, British Columbia, Canada 总被引:1,自引:0,他引:1
The Shasta gold-silver deposit, British Columbia, Canada, is an adularia-sericite-type epithermal deposit in which deposition
of precious metals coincided with the transition of quartz- to calcite-dominant gangue. Mineralization is associated with
stockwork-breccia zones in potassically altered dacitic lapilli tuffs and flows, and consists of pyrite, sphalerite, chalcopyrite,
galena, acanthite, electrum and native silver. Pre- and post-ore veins consist solely of quartz and calcite, respectively.
Fluid inclusion microthermometry indicates that ore minerals were deposited between 280 ° and 225 °C, from a relatively dilute
hydrothermal fluid (˜1.5 wt.% NaCl equivalent). Abundant vapor-rich inclusions in ore-stage calcite are consistent with boiling.
Oxygen and hydrogen isotopic data (δ18Ofluid = −1.5 to −4.1‰; δDfluid = −148 to −171‰) suggest that the fluid had a meteoric origin, but was 18O-enriched by interaction with volcanic wallrocks. Initial (˜280 °C) fluid pH and log f O2 conditions are estimated at 5.3 to 6.0, and −32.5 to −33 bar, respectively; during ore deposition, the fluid became more
alkaline and oxidizing. Ore deposition at Shasta is attributed to localization of meteoric hydrothermal fluids by extensional
faults; mineralization was controlled by boiling in response to hydraulic brecciation. Calcite and base metal sulfides precipitated
due to the increase in pH that accompanied boiling, and the associated decrease in H2S concentration led to precipitation of gold and silver.
Received: 23 February 1995 / Accepted: 16 April 1996 相似文献
20.
The spinel solid solution was found to exist in the whole range between Fe3O4 and γ-Fe2SiO4 at over 10 GPa. The resistivity of Fe3−
x
Si
x
O4 (0.0<x<0.288) was measured in the temperature range of 80∼300 K by the AC impedance method. Electron hopping between Fe3+ and Fe2+ in the octahedral site of iron-rich phases gives a large electric conductivity at room temperature. The activation energy
of the electron hopping becomes larger with increasing γ-Fe2SiO4 component. A nonlinear change in electric conductivity is not simply caused by the statistical probability of Fe3+–Fe2+ electron hopping with increasing the total Si content. This is probably because a large number of Si4+ ions occupies the octahedral site and the adjacent Fe2+ keeping the local electric neutrality around Si4+ makes a cluster, which generates a local deformation by Si substitution.
The temperature dependence of the conductivity of solid solutions indicates the Verwey transition temperature, which decreases
from 124(±2) K at x=0 (Fe3O4) to 102(±5) K at x=0.288, and the electric conductivity gap at the transition temperature decreases with Si4+ substitution.
Received: 15 March 2000 / Accepted: 4 September 2000 相似文献