Sand - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 21

Fig. 2.6 A schematic of shape
descriptors for sand grains

Table 2.3 Densities of bulk sand-forming materials. Porous materi-
als (e.g., snowflakes, or ooids made of thin shells of calcite) can have
much lower effective densities (see Chap. 4 ) . The composition of
Titan's sands is not exactly known, but simple alkanes and paraffins at
Titan surface temperatures (94 K) have densities below 1000 kg/m 3 .
PAHs (polycyclic aromatic hydrocarbons) like Pyrene and Phenan-
threne have densities rather higher than this, so Titan's sand density is
somewhat uncertain at present
Material

Table 2.2 Stability of some common minerals under weathering

conditions at Earth's surface
Mineral Product of weathering

Most stable Fe-oxides

Al-oxides

Quartz

Density (kg/m 3 ) Clay minerals

Muscovite
K-feldspar (orthoclase)
Biotite
Na-feldspar (albite)
Amphibole
Pyroxene
Ca-feldspar (anorthite)

Ice 900

Gypsum 2300

Quartz 2600

Calcite 2700

Basalt 3000

Olivine 3300

Garnet 3100-4000

Least stable Olivine

Magnetite 5180

Hydrocarbons *800

PAHs *1300

dominates, for example, the formidable study of the Rub'
Al Khali by Helga Besler (Besler 2008), and the studies of
the US and other deserts by Daniel Muhs (e.g., Muhs 2004,
notes that, on the basis of the quartz vs feldspar abundance,
the Algodones dunes were not derived from Whitewater
river sands).
The terrestrial weathering sequence is not necessarily
followed on the surface of other planets, where environ-
mental conditions are very non-Earthlike, particularly with
regard to the abundance and stability of liquid water. For
example, spectroscopic data from Mars have revealed very
little quartz on the Red Planet (Bandfield 2002; Bibring
et al. 2005; Smith and Bandfield 2012), let alone in the
abundant sand deposits present in many places on Mars,

while at the same time the dark sand dunes in the Nili Patera
region show an olivine signature (Mangold et al. 2007);
both results are precisely the opposite of what would be
expected from our terrestrial weathering experience. Rover
investigations of dark sand in the Columbia Hills area show
that the grains are predominantly comprised of fragments of
basalt (Fig. A, Sullivan et al. 2008), but the Martian dunes
in some areas have been shown to include olivine and
pyroxene (Tirsch 2008) and even gypsum (Fishbaugh et al.
2007; Feldman et al. 2008; Calvin et al. 2009). Also,
whereas terrestrial sand evolution may generally be