Aeolian Sand Grains by fjzhangweiyun

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									In the name of God
Shiraz University
College of Agriculture
Department of Desert Region Management

Geomorphology Lab.                       Aeolian Sand Grains
      Aeolian sand grains has a specific characteristics. Of these characteristics that
make them separable from other kind of sands are their size sorting, and their shape
sorting. The size sorting can be determined by drawing the soil particle size
distribution curve through sieve analysis. The shape sorting can be determined by the
parameters such as roundness, sphericity, the surface texture, the matt or glossy
surface of the grains and so on.
      In order to find these characteristics of a sand grains in a sample, the sand grains
must be seen in larger scale under binocular microscope. To have a better illustration
of the sample under the microscope the sand grains must be prepared at first. And
then the shape sorting for them can be determined.

Preparing the sample:
-Sieve the soil sample and separate the soil grain to mesh number of 30 to 40, 40 to50
and 50 to 60 intervals. And tag the three samples as 40, 50 and 60 respectively.
-Wash the samples, by distilled water and dry it in oven for about several hours.
-Put about 5 grams of each samples in a separate Petri dish so that the grains cover the
container bottom with thickness of one grain.
-Determine the parameters given below for each sample.
It must be noted that:
-The Quartz grains are only studied in each sample. All the quartz grains in a field of
view were counted each time to avoid selection.
- In each analysis at least one hundred grains must be counted.

Selection of field of view:
The quartz grains are the main grains. Move the container horizontally finely, until a
layer with thickness of one grain covers the bottom of the dish.
-Put it under the binocular and select a magnifying degree of about 30-40, so that the
number of quartz grains in your field of view is about 100.
-Now you have to save the field of view in your mind and find each of the following
parameters in 100 grain in your field of view and record the determined parameter in
a table. By doing this at the end you can determined that how many percent of the
grains has a specific characteristics.
- May be for a certain characteristics the magnifying degree of microscope must be
changed to make the features clearer, but after determining that characteristics the
magnifying degree must be reset to its original value.

The parameters and characteristics:

Grain size:
      The sand grain size is its largest diameter (D, in the
figure). It can be measured by using the graduated reticules
of the binocular microscope. You can put a ruler under the
microscope and determine the length of each reticule unit
in a certain magnification. Then by using this scale
determine the size of quartz grains in the field of view.
Sphericity:
      The sphericity is the nearness of a shape of the grain
to that of a perfect sphere. In order to determine the
sphericity of a grain, the largest diameter of the grain (D)
and the diameter perpendicular to it (L) are measured. The
L/D shows the sphericity of a grain.
     The sphericity of a grain also can be determined by
measuring the area of a grain relative to the area of a circle
by which it is enclosed.


Roundness:
Roundness is the overall smoothness of the grain. On
the Powers index, roundness is based on dividing the
average of the radii of the corners of the grain image
by the radius of the maximum inscribed circle
(Wadell,1935). In an Aeolian sample it is expected
that by decreasing the grain size it becomes less
rounded (Carroll,1939). Test it by comparing the
roundness and the size of quartz grains in your field
of view. Or by comparing the average roundness in
three samples (40, 50 and 60). To find simply the
roundness of the grains under binocular, the following
chart can be used.
Is the average roundness of each sample related to its
average size?




Chart for visual estimation of roundness according to
Powers (1953) and Shepard (1963). Classes A = very
angular (class intervals 0.12–0.17, geometric mean
0.14), B = angular (class intervals 0.17–0.25,
geometric mean 0.21), C = sub-angular (class
intervals 0.25–0.35, geometric mean 0.30), D = sub-
rounded (class intervals 0.35–0.49, geometric mean
0.41), E = rounded (class intervals 0.49–0.70,
geometric mean 0.59) and F = well rounded (class
intervals 0.70–1.00, geometric mean 0.84).
glossy/matt or shiny/matt surface texture:
The quartz grains were divided in two groups: glossy and matt particles. The Aeolian
sands have usually matt surface texture and the fluvial or alluvial sands have glossy
surface texture. In the field of view determine the matt or shiny luster for the quartz
grains.

It can be prepared a table in which the percent of roundness classes and Matt or glossy
texture of the sample be shown in percents.

Sample A%          B%        C%       D%        E%        F%        Matt%    Glossy%
name




Mineralogy:
Percent of quartz, coloured minerals, and oxides or opaque minerals can be
determined by using the following chart.




How much is the percent of different kind of, Quartz, colored and opaque minerals in
each samples (40, 50 and 60)?
Is the size of grains of each sample related to the kind of mineral?
In a sample is the roundness of the grains related to the type of minerals?
Maturity of the aeolian sand is proportional to its quartz content (Folk, 1968) and the
roundness of the quartz particles.
The surface texture and features:
The surface features encountered were classified according to Krinsley and
Doornkamp (1973) and Al-Saleh and Khalaf (1982). The features present of the
quartz surface textures were: 1. Precipitated upturned silica plates, 2. Conchoidal
fracture, 3. Mechanical V-forms, 4. Straight or slightly curved grooves, 5. Dish-
shaped concavities, 6. Deep surface solution, 7. Flat cleavage face, 8. Cleavage planes
(semi-parallel lines), 9. Adhering particles.




         Upturned silica plates on quartz grain surface. Sample 3/top/100cm grain 2. Magnification
                                    of the original grain surface is 465 x.
Upturned silica planes:
Upturned silica planes are mainly associated with aeolian environment, but are also
indicative of source material or loess. Grains with upturned silica are derived from
weathered granites. The proportion of upturned silica plates is indicative of aeolian
transportation. These plates appear as more or less parallel ridges ranging in width
from 0.5 to 10  m and have been interpreted to result from breakage of quartz along
cleavage planes in the quartz lattice (Pye and Tsoar, 1990). Experimental evidence
shows that the spacing and size of upturned plates might be boardly related to wind
energy (Krinsley and Wellendorf, 1980). Apart from Aeolian environments, the
presence of upturned silica plates has been connected to weathered granite, a source
material of many aeolian sediments.

Dish-shaped concavities:
      Another surface texture feature that can be associated with aeolian environment
is dish-shaped concavities. Dish-shaped concavities are also linked to aeolian
transportation. Unlike other surface features, these are indicative only for aeolian
abrasion. These concavities are believed to develop as a result of direct impacts
rather than glancing blows during saltation (Pye and Tsoar, 1990). This indicates quite
high wind velocities.



Conchoidal fracture:
On the contrary, conchoidal fracture, flat cleavage face and cleavage-planes are
indicative of glacial environment. Sub-aqueous and glacial/sub-aqueous environments
are characterized with conchoidal fracture, mechanical V-forms, straight or slightly
curved grooves and flat cleavage face (Krinsley and Doornkamp, 1973).
      Conchoidal fractures (Fig. below) and cleavage planes are the most abundant
features of this kind in the quartz grain surface textures of the studied samples.
Conchoidal fractures develop typically in quartz particles in the glacial grinding
process and collisions during transportation.




             Conchoidal fracture on quartz grain surface. Sample 3/lee side/100 cm, grain 2.
                          Magnification of the original grain surface is 465 x.

Frosting of quartz grains:
      The large grain seen in the following figure are frosted as well as rounded. The
frosted appearance , typical of many Aeolian sands, appears under the normal optical
microscope to result frm light diffraction associated with small pits on the surface of
the grain. Kuenen and Perdok (1962) claim that the frosting is not the result of pits
made by the impact on one grain against another. They suggest that it is caused by
micro-chemical attack of the quartz surface associated , perhaps, with desert dews
(Glennie, 1970).
     V-shaped depressions:
     Sub-aqueous and glacial/sub-aqueous environments are characterized with
mechanical V-forms, straight or slightly curved grooves and flat cleavage face
(Krinsley and Doornkamp, 1973, Friedman and Sanders, 1978).




The most part of this laboratory instruction is derived form:
Donner J., V. Salonen, (2004)" Dune Stratigraphy as an Indicator of Holocene
     ClimaticChange and Human Impact in Northern Lapland, Finland " Annales
     Academiae         Scientiarum      Fennicae       Geologica-Geographica 166,
     SUOMALAINEN TIEDEAKATEMIA HELSINKI.
Friedman G.M. and J.E. Sanders, (1978) " Principles of Sedomentology" John Wiley
     & Sons, Inc.
Glennie, K.W., (1970) " Desert sedimentary environments" Elsevier Publishing
     Company, Developments in sedimentology 14. (Code in Mollasadra Library,
     College of Sciences, Shiraz University: 551.37 G55)

								
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