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rock-forming minerals produced by igneous rock formation. These include quartz,
plagioclase, K-feldspar, amphibole, biotite, and muscovite. Metamorphic rocks
contribute many of these same minerals plus chlorite. Common dust minerals
from noncrystalline rocks include calcite and dolomite from carbonate terrain and
evaporite minerals, such as gypsum and halite. In contrast, many clay-sized particles
are phyllosilicates or layered aluminosilicate minerals, such as mica, kaolinite,
smectite, vermiculite, and chlorite, and occasionally non-phyllosilicate clays such
as palygorskite or sepiolite.
With the exception of micas, other phyllosilicate clay minerals, such as kaolinite,
smectite, vermiculite, and many chlorites, as well as Al-dominated minerals such as
gibbsite and boehmite, form at low temperatures at or near the Earth's surface, pri-
marily in soils (Birkeland 1999 ). The process of clay mineral formation is typically
alteration of primary rock-forming minerals, such as feldspars, to phyllosilicate
clay minerals by hydrolysis. Phyllosilicate clay minerals form a very important
component of LRT dust (Glaccum and Prospero 1980 ; Biscaye et al. 1997 ; Stuut
et al. 2005 ; Scheuvens et al. 2013 ) and identifying dust source areas requires a good
understanding of how clay minerals form in soils and how clay mineralogy changes
with soil geography. Dust sources can sometimes be identified by examination of
clay mineralogy, which we illustrate later.
Finally, studies of dust origins often overlook the process of simple particle
inheritance from sedimentary rocks such as siltstones or shales. Silt is abundant
in the sedimentary rock record. Indeed, Blatt ( 1987 ) estimates that fully half of the
detrital quartz in the world's sedimentary rocks consists of silt-sized particles. In
Australia, for example, siltstones may be the primary source of much of the silt-
sized dust derived from arid basins (McTainsh 1989 ). In the Great Plains of North
America, Aleinikoff et al. ( 1999 , 2008 ) show that sedimentary rock (volcaniclastic
siltstone) is the most important source of silt-sized particles in loess of last-glacial
age, which we discuss in more detail below.
3.3
Methods of Identifying Contemporary Dust Sources
3.3.1
Geomorphic Perspectives on Dust Sources
In a recent study, Bullard et al. ( 2011 ) developed a useful conceptual geomorphic
scheme for dust sources. Recognizing that most dust sources occur in arid regions,
these investigators identified six geomorphic systems (see examples in Fig. 3.1 )that
could be considered as possible dust-emitting sources. These include (1) lakes, (2)
high-relief alluvial systems, (3) low-relief alluvial systems, (4) stony surfaces (not
including those connected to fluvial sources of fine-grained particles), (5) aeolian
systems, and (6) low-emission surfaces (including bedrock surfaces, rocky slopes,
and duricrusted surfaces). Bullard et al. ( 2011 ) further classified these geomorphic
systems into subgroups that demonstrate a range of dust emission potentials.
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