Desert crusts and rock coatings - Arid Zone Geomorphology - page 129

tion. Nitratite is also hygroscopic (it absorbs atmospheric
moisture) and deliquescent (it dissolves in that moisture).
When relative humidity exceeds a critical level (73.8 %
at 25 ◦ C for pure sodium nitrate; see Tereschenko and
Malyutin, 1985), water condenses on and dissolves min-
eral surfaces; when the relative humidity falls back be-
low that level, sodium nitrate will recrystallise. Ericksen
(1981, 1983) recognised two main types of nitrate de-
posit: regolith cement (or alluvial caliche , not to be con-
fused with the term caliche used to describe calcrete
in some regions) and impregnated bedrock (or bedrock
caliche) . A typical alluvial caliche profile contains sev-
eral zones. At the top is a 10-30 cm thick chuca zone, a
powdery to poorly cemented leached surface layer con-
taining clastic fragments, some gypsum and anhydrite,
and salts such as bloedite, humberstonite and thenardite.
Beneath this is a moderate to well cemented 0.5-2.0 m
thick costra zone. Below this is a well cemented 1.0-3.0
m thick caliche zone, containing layers of white nitrate-
rich deposits referred to as caliche blanco . The caliche
zone grades downwards into weakly cemented ( conjelo )
or uncemented ( coba ) regolith. Bedrock caliche consists
of layers of nitrate minerals that have forced open rock
fissures during crystallisation. These may be several tens
of cm thick and, in advanced cases of replacement, may
so disrupt the bedrock that it consists of minor rock frag-
ments surrounded by dominant caliche blanco.

pH

Rainfall

Figure 8.2 Schematic representation of the relationship be-
tween the formation of selected duricrusts and prevailing envi-
ronmental conditions (after Summerfield, 1991). Note that in
many instances local factors such as topography and drainage
may outweigh broad climatic controls, particularly in the case
of nonpedogenic crusts.

8.2.2 Micromorphology, chemistry and mode
of formation

Claridge and Campbell, 1968; Ericksen, Hosterman and
St Amand, 1988; Bohlke, Ericksen and Revesz, 1997;
Graham et al. , 2008). Most is known about sodium nitrate
deposits from the Atacama; for further information about
similar crusts elsewhere, see the recent review by Goudie
and Heslop (2007).
High-grade sodium nitrate deposits in the Atacama oc-
cur along a 700 by 30 km strip inland of the Coastal
Range between 19 ◦ 30 S and 26 ◦ S (Ericksen, 1981). Most
deposits are found below 2000 m a.s.l. but some may ex-
tend up to 4000 m a.s.l. They reach greatest thicknesses
on the lower slopes of hills and piedmont plains but also
occur in a range of topographic settings from hilltops to
the centres of valleys. Many salars (see Chapter 15) in the
Atacama contain thick nitrate crusts, including the Pampa
Blanca, Pampa Lina, Salar del Carmen and Salar de La-
gunas. Nitrate deposits occur on and within a wide range
of rock and sediment types, and there appears to be little
lithological control on their different structural types and
mineral assemblages (Goudie and Heslop, 2007).
The high solubility of sodium nitrate in water means
that large volumes of the salt can be precipitated within

With the exception of some near-surface veins in bedrock
(Ericksen and Mrose, 1972) and some caliche blanco ,the
matrix of sodium nitrate deposits rarely consists of pure
nitratite. Ericksen, Hosterman and St Amand (1988), for
example, report deposits from the Atacama that contain
anhydrite, borax, darapskite, glauberite, gypsum, halite,
tincalconite and trona in addition to nitratite. The most
important of these impurities is halite, which may exceed
the percentage of nitrate present (Penrose, 1910).
Relatively few accounts of the micromorphology of
sodium nitrate sequences have been published. Pueyo,
Chong and Vega (1998) describe a range of microscale
features in deposits from the Antofagasta region of Chile.
Here, sodium nitrate deposits contain silt and other detrital
material cemented by a mix of salt crystals of different
compositions. Layers of relatively pure sulfate minerals
(bloedite and humberstonite) may be interspersed with
veins of purer nitrate (Ericksen and Mrose, 1972). Small
chalcedony nodules may also be present. Due to the high