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as a gel or directly as amorphous silica under the influ-
ence of evaporation or possibly organic complexation (cf.
Krauskopf, 1957; Watchman, 1992; Perry et al. , 2006).
Full details of the processes driving silica mobilisation
and precipitation are given in the section concerning sil-
crete (Section 8.6.4 above).
A range of mechanisms are required to explain the for-
mation of the various types of silica glaze. Type I and II
glazes probably developed through the direct precipita-
tion of silica gels. The other varieties require additional
processes to mobilise and precipitate aluminium and iron.
Dorn (2009) suggests that the presence of abundant alu-
minium within Type III, IV and VI glazes could be due
to the incorporation of soluble aluminium silicate com-
plexes (Al(OSi(OH) 3 ) 2 + ) released during the weathering
of phyllosilicate minerals. Al-Si complexes can be mo-
bilised by gentle wetting events and ultimately bond to an
existing silica glaze (Zorin et al. , 1992). The presence of
enhanced iron contents in Type III and V glazes is more
easily explained by the strong adherence of Fe released
by weathering to silica through Fe-O-Si bonds (Dove and
Rimstidt, 1994).
Iron films are widespread in desert and other environ-
ments and occur in three varieties (Dorn, 1998). Type I
iron films consist of homogeneous iron with few other
constituents. Type II iron films include additional Si and
Al, but iron remains the dominant component. In Type
III films, iron constitutes less than a third by weight of
the coating but is present in sufficient quantities to give a
strong red/orange colour. The mobilisation and fixation of
iron may involve abiotic processes, with the oxidation of
Fe 2 + to Fe 3 + occurring rapidly above pH 5. The formation
of chemical Fe-O-Si bonds and interactions with inter-
stratified clays may be key to the development of Type II
and III films respectively (Dorn, 2009).
limits their preservation at the land surface to regions
where aridity has persisted. This can provide valuable in-
formation on the ages of deserts if the crusts can be dated
(Dan et al. , 1982; Reheis, 1987; Watson, 1988). One line
of evidence for the early initiation of the Atacama Desert,
for example, is the existence of a gypsum crust preserved
beneath an ignimbrite dated to around 9.5 million years
(Hartley and May, 1998). The high solubility of gypsum
implies that aridity has prevailed since that time. As both
phreatic and pedogenic forms of halite and gypsum crust
usually accrete within a host material, assessing the age of
incorporated artefacts and other datable substances may
not be representative of the true ages (Watson, 1988).
Furthermore, gypsum and halite crusts form in several
different ways, so environmental interpretations must pro-
ceed carefully. Isotopic studies of gypsum crusts have
provided insights into their origins (Carlisle et al. , 1978;
Sofer, 1978; Drake, Eckardt and White, 2004) and the
sources of their constituent materials (Chivas et al. , 1991;
Eckardt and Spiro, 1999; Rech, Quade and Hart, 2003).
Most reported occurrences of halite and gypsum crusts
in the stratigraphic record have been interpreted as lacus-
trine, lagoonal or sabkha evaporites (Schreiber, 1986). In
part, this reflects a lack of clear criteria for identifying
pedogenic evaporite facies.
Valuable information can be obtained by coring mod-
ern playa floors to recover sequences of interbedded
evaporites and clastic sediments (Smith, 1979; Smith
and Bischoff, 1997). Microfossil, particle-size and min-
eralogical assessments, plus analyses of the chemistry
and isotopic signatures of salts, pore water and fluid
inclusions, are all routinely used to elucidate lacustrine
palaeoenvironments (e.g. Spencer, Eugster and Jones,
1985; Teller and Last, 1990). Fluid inclusions within halite
crystals can yield homogenisation temperatures for the
time when halite precipitation took place (Roberts and
Spencer, 1995; Lowenstein et al. , 1999; Lowenstein and
Brennan, 2001). Direct temperature estimates for evap-
orite sequences may also be provided by amino acid
palaeothermometry, which produces diagenetic tempera-
ture information for broad depositional phases (Kaufman,
2003).
The presence of a pedogenic calcrete in the landscape
or geological record is an indicator that semi-arid or arid
conditions prevailed at the time of its formation. Calcretes
have been identified in rocks from every continent and dat-
ing from the Proterozoic to the present (see Alonso-Zarza,
2003; Wright, 2007). Careful identification of the type of
calcrete is needed before any palaeoenvironmental infer-
ences are drawn; some calcretes have been interpreted as
altered reef carbonates or cold climate carbonate accumu-
8.8 Palaeoenvironmental significance
of crusts
Desert crusts and rock coatings are of value as palaeoenvi-
ronmental indicators only if their mode of origin, age and
the environmental conditions during their formation are
correctly interpreted. Ideally, environments where crusts
are forming today should be used as analogues; this is
feasible for calcrete, gypsum and halite crusts and some
rock coatings, but not for silcrete, where the majority of
documented examples are relict features.
Sodium nitrate, gypsum and halite crusts can be valu-
able palaeoclimatic indicators because they attest to ex-
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