It is argued that if everything is necessarily what it is, thengiven the equivalence pa= (x)(x=ap), it follows that whateverhappens or is the case, had to happen or had to be the case.It is argued ...that if everything is necessarily what it is, then given the equivalence p a = (x)(x = a p), it follows that whatever happens or is the case, had to happen or had to be the case.
The spatial and temporal changes in element and mineral concentrations in regolith profiles in a chronosequence developed on marine terraces along coastal California are interpreted in terms of ...chemical weathering rates and processes. In regoliths up to 15
m deep and 226
kyrs old, quartz-normalized mass transfer coefficients indicate non-stoichiometric preferential release of Sr
>
Ca
>
Na from plagioclase along with lesser amounts of K, Rb and Ba derived from K-feldspar. Smectite weathering results in the loss of Mg and concurrent incorporation of Al and Fe into secondary kaolinite and Fe-oxides in shallow argillic horizons. Elemental losses from weathering of the Santa Cruz terraces fall within the range of those for other marine terraces along the Pacific Coast of North America.
Residual amounts of plagioclase and K-feldspar decrease with terrace depth and increasing age. The gradient of the weathering profile
b
s is defined by the ratio of the weathering rate,
R to the velocity at which the profile penetrates into the protolith. A spreadsheet calculator further refines profile geometries, demonstrating that the non-linear regions at low residual feldspar concentrations at shallow depth are dominated by exponential changes in mineral surface-to-volume ratios and at high residual feldspar concentrations, at greater depth, by the approach to thermodynamic saturation. These parameters are of secondary importance to the fluid flux
q
h, which in thermodynamically saturated pore water, controls the weathering velocity and mineral losses from the profiles. Long-term fluid fluxes required to reproduce the feldspar weathering profiles are in agreement with contemporary values based on solute Cl balances (
q
h
=
0.025–0.17
m
yr
−1).
During saturation-controlled and solute-limited weathering, the greater loss of plagioclase relative to K-feldspar is dependent on the large difference in their respective solubilities instead of the small difference between their respective reaction kinetics. The steady-state weathering rate under such conditions is defined as
R
=
q
h
·
m
sol
M
total
·
1
S
v
·
b
s
·
The product of
q
h and the ratio of solubilized to solid state feldspar (
m
sat/
M
total) define the weathering velocity. The weathering gradient
b
s reflects the kinetic rate of reaction where
S
v is the volumetric surface area of the residual feldspar. Both this rate expression and the spreadsheet calculations produce similar plagioclase weathering rates (
R
=
5–14
×
10
−16
mol
m
−2
s
−1) which agree with those reported for other environments of comparable climate and age. Weathering-dependent concentration profiles are commonly described in literature. The present paper provides methods by which these data can yield a more fundamental understanding of the weathering processes involved.
The spatial and temporal changes in hydrology and pore water elemental and
87Sr/
86Sr compositions are used to determine contemporary weathering rates in a 65- to 226-kyr-old soil chronosequence ...formed from granitic sediments deposited on marine terraces along coastal California. Soil moisture, tension and saturation exhibit large seasonal variations in shallow soils in response to a Mediterranean climate. These climate effects are dampened in underlying argillic horizons that progressively developed in older soils, and reached steady-state conditions in unsaturated horizons extending to depths in excess of 15
m. Hydraulic fluxes (
q
h
), based on Cl mass balances, vary from 0.06 to 0.22
m
yr
−1, resulting in fluid residence times in the terraces of 10–24
yrs.
As expected for a coastal environment, the order of cation abundances in soil pore waters is comparable to sea water, i.e., Na
>
Mg
>
Ca
>
K
>
Sr, while the anion sequence Cl
>
NO
3
>
HCO
3
>
SO
4 reflects modifying effects of nutrient cycling in the grassland vegetation. Net Cl-corrected solute Na, K and Si increase with depth, denoting inputs from feldspar weathering. Solute
87Sr/
86Sr ratios exhibit progressive mixing of sea water-dominated precipitation with inputs from less radiogenic plagioclase. While net Sr and Ca concentrations are anomalously high in shallow soils due to biological cycling, they decline with depth to low and/or negative net concentrations. Ca/Mg, Sr/Mg and
87Sr/
86Sr solute and exchange ratios are similar in all the terraces, denoting active exchange equilibration with selectivities close to unity for both detrital smectite and secondary kaolinite. Large differences in the magnitudes of the pore waters and exchange reservoirs result in short-term buffering of the solute Ca, Sr, and Mg. Such buffering over geologic time scales can not be sustained due to declining inputs from residual plagioclase and smectite, implying periodic resetting of the exchange reservoir such as by past vegetational changes and/or climate.
Pore waters approach thermodynamic saturation with respect to albite at depth in the younger terraces, indicating that weathering rates ultimately become transport-limited and dependent on hydrologic flux. Contemporary rates
R
solute
are estimated from linear Na and Si pore weathering gradients
b
solute
such that
R
solute
=
q
h
b
solute
β
S
v
where
S
v
is the volumetric surface area and
β is the stoichiometric coefficient. Plagioclase weathering rates (0.38–2.8
×
10
−15
mol
m
−2
s
−1) are comparable to those based on
87Sr/
86Sr mass balances and solid-state Na and Ca gradients using analogous gradient approximations. In addition, contemporary solute gradients, under transport-limited conditions, approximate long-term solid-state gradients when normalized against the mass of protolith plagioclase and its corresponding aqueous solubility. The multi-faceted weathering analysis presented in this paper is perhaps the most comprehensive yet applied to a single field study. Within uncertainties of the methods used, present day weathering rates, based on solute characterizations, are comparable to average long-term past rates as evidenced by soil profiles.
We show that the thesis that identity is necessary is equivalent to the thesis that everything is necessarily what it is. Hence the challenges facing either, faces them both.
The influence of layer charge and charge distribution of dioctahedral smectites on the rheological and swelling properties of bentonites is examined. Layer charge and charge distribution were ...determined by XRD using the LayerCharge program Christidis, G.E., Eberl, D.D., 2003. Determination of layer charge characteristics of smectites. Clays Clay Miner. 51, 644–655.. The rheological properties were determined, after sodium exchange using the optimum amount of Na
2CO
3, from free swelling tests. Rheological properties were determined using 6.42% suspensions according to industrial practice. In smectites with layer charges of −
0.425 to −
0.470 per half formula unit (phfu), layer charge is inversely correlated with free swelling, viscosity, gel strength, yield strength and thixotropic behaviour. In these smectites, the rheological properties are directly associated with the proportion of low charge layers. By contrast, in low charge and high charge smectites there is no systematic relation between layer charge or the proportion of low charge layers and rheological properties. However, low charge smectites yield more viscous suspensions and swell more than high charge smectites. The rheological properties of bentonites also are affected by the proportion of tetrahedral charge (i.e. beidellitic charge), by the existence of fine-grained minerals having clay size, such as opal-CT and to a lesser degree by the ionic strength and the pH of the suspension. A new method for classification of smectites according to the layer charge based on the XRD characteristics of smecites is proposed, that also is consistent with variations in rheological properties. In this classification scheme the term smectites with intermediate layer charge is proposed.
We show that Kripke’s argument for the necessity of identity statements relating objects a and b by their rigid designators demands an additional significant premise.
Naturally weathered feldspar surfaces in the Jurassic Navajo Sandstone at Black Mesa, Arizona, was characterized with high-resolution transmission and analytical electron microscope (HRTEM-AEM) and ...field emission gun scanning electron microscope (FEG-SEM). Here, we report the first HRTEM observation of a 10-nm thick amorphous layer on naturally weathered K-feldspar in currently slightly alkaline groundwater. The amorphous layer is probably deficient in K and enriched in Si. In addition to the amorphous layer, the feldspar surfaces are also partially coated with tightly adhered kaolin platelets. Outside of the kaolin coatings, feldspar grains are covered with a continuous 3–5
μm thick layer of authigenic smectite, which also coats quartz and other sediment grains. Authigenic K-feldspar overgrowth and etch pits were also found on feldspar grains. These characteristics of the aged feldspar surfaces accentuate the differences in reactivity between the freshly ground feldspar powders used in laboratory experiments and feldspar grains in natural systems, and may partially contribute to the commonly observed apparent laboratory-field dissolution rate discrepancy. At Black Mesa, feldspars in the Navajo Sandstone are dissolving at ∼10
5 times slower than laboratory rate at comparable temperature and pH under far from equilibrium condition. The tightly adhered kaolin platelets reduce the feldspar reactive surface area, and the authigenic K-feldspar overgrowth reduces the feldspar reactivity. However, the continuous smectite coating layer does not appear to constitute a diffusion barrier. The exact role of the amorphous layer on feldspar dissolution kinetics depends on the origin of the layer (leached layer versus re-precipitated silica), which is uncertain at present. However, the nanometer thin layer can be detected only with HRTEM, and thus our study raises the possibility of its wide occurrence in geological systems. Rate laws and proposed mechanisms should consider the possibility of this amorphous layer on feldspar surface.
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