Geoscience Reference
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sound by shearing (see Chap. 10 ). Grain shape also affects
the fluid-dynamic properties, such that the drag and thus the
saltation threshold depend on the shape. Generally, how-
ever, the shape effects on threshold are small compared with
the influence of particle density, and the range of angles of
repose encountered in nature is actually quite small.
2.2
Compositional Considerations
As discussed in the introduction to this section, sand grains
consist of a wide variety of compositions. It is worth dis-
cussing, however, why we so often think of sand on Earth as
referring to the mineral quartz. The abundance of quartz in
sand on Earth is not a direct indicator of the quantity of
silica in crustal rocks, and in particular the granitic rocks
that dominate the continental crust (as opposed to the
basaltic ocean crust). Rather, it is the result of the strong
resistance of quartz to chemical weathering, at least com-
pared with most other common rock-forming minerals. The
combination of field observations of rock weathering pro-
cesses and chemical experimentation on the stability of
common minerals leads to a ranking of mineral in the order
of their stability under terrestrial weathering conditions
(Press and Siever 1974, p. 208; Table 2.2 ), with quartz by
far the most stable common mineral, and olivine the least
stable There are nonetheless two locations on Earth—
Hawaii (e.g., Tirsch et al. 2012) and the Galapagos (both
sites of basaltic volcanism)—where olivine-rich grains are
present in such abundance as to form perceptibly green sand
beaches. These two beaches (Iceland may also have some
similar beaches) amount to perhaps a millionth of the sur-
face area covered by predominantly quartz sands on Earth.
Among other exotic sand compositions, there are reportedly
a few garnet sand dunes in Namibia, and wave-ground steel
particles are abundant on some beaches in Normandy.
It is the chemical instability of the silicate minerals
which dominate igneous rocks on Earth that leads to the
formation of clays, which are a major component of all of
the different types of soils present around our planet.
Extreme weathering conditions, such as the torrential rain-
fall that is common in equatorial jungles, can even leach
away the quartz, which then leads to the formation of lat-
eritic soils that are mined for their enhanced concentrations
of aluminum and iron oxides. Thus, with increasing length
or intensity of exposure to weathering (particularly by
water), the mineral abundances within sand on Earth stea-
dily evolves as the least stable materials progressively dis-
appear, which also results in the steadily increasing relative
abundance of the most stable mineral, quartz.
Measurements of composition, as well as of the size and
shape of the sand, have been major tools in tracing the
origins
Fig. 2.5 MER microscopic imager view of basaltic sand (Spirit at El
Dorado—see Chap. 12 ) showing individual grains; view is *3cm
across. The annular depression is from the contact plate of an APXS
instrument (see Chap. 17 )
Haakon Wadell devised a simple method whereby the
two-dimensional view of grains derived from photographs
taken through microscopes could be systematically mea-
sured, which also provided a good way to distinguish
between shape and roundness (Siever 1998, p. 52). Wadell
defined shape as the ratio of the cross-sectional area of the
grain to the smallest circumscribed circle, and roundness as
the ratio of the average radius of curvature circles inscribed
within corners on the grain to the radius of the maximum
inscribed circle within the entire grain. These ratios led to
both shape and roundness ranging from 0 to 1, where both a
shape and a roundness of 1 would result from a perfect
sphere. Rather than always having to carry out detailed
measurements for each individual grain, calibrated silhou-
ettes of measured grains allowed researchers to estimate the
numerical value of both parameters quickly through visual
inspection.
These shape parameters are important first in giving a
quantitative, or at least consistent, basis for comparing
different samples. Is one rounder than another? And what
does this mean about how far it has been transported? For
example, the sands of the Sahara, which have swirled
around in North Africa for ages, are notably fine and round.
The shape influences how grains interact, defining the
friction coefficient and thus the angle of repose. This is
something that has been measured on Mars by making little
piles of sand using the sampling arm of the Viking lander,
for example. Grain shape is also important in generating
of
sands
in
Earth's
deserts.
This
approach
 
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