On Composition, Morphology, and Size Distribution of Airborne Mineral Dust - Mineral Dust: A Key Player in the Earth System - page 19

regions (e.g., Taklamakan Desert) to very low carbonate contents in eastern source
regions can be observed (Shen et al. 2009b ). Furthermore, Li et al. ( 2007 ) proposed
that the occurrence of dolomite in Asian dusts is characteristic for specific potential
source areas. Other minerals detected by XRD in minor or trace amounts are, for
example, hematite, gypsum, halite, or amphibole.
The amount of clay minerals in bulk samples varies considerably from trace
amounts to 60 wt% or more. It is suggested that this parameter does not only depend
on the composition of the source sediments but also reflects the conditions during
dust uplift (e.g., wind speed) and transport (e.g., transport distance).
XRD analysis of oriented specimen of the fine fraction (<2 m) offers the
possibility to characterize the occurring clay minerals in more detail. The domi-
nating clay minerals in mineral dust samples are members of the illite, kaolin, and
smectite group, respectively. Other more rarely observed phyllosilicates are chlorite,
palygorskite, and white mica (muscovite). Only sporadically were clay minerals of
the mixed layer type or halloysite detected. As particles of the clay mineral groups
occur (by definition) in approximately the same size class, a significant segregation
during uplift, transport, and deposition is not expected (however, for a more detailed
discussion, see Scheuvens et al. 2013 ). Hence, the ratio between the amounts of
different clay minerals was proposed as a compositional fingerprint of the source
region (Schütz and Sebert 1987 ; Caquineau et al. 2002 ). The illite/kaolinite (I/K)
ratio was repeatedly claimed as a viable source marker. In northern Africa, mineral
dusts with I/K ratios >1.0 are mainly restricted to source areas located in northern
and western Algeria, Morocco, and Western Sahara. A trend with decreasing I/K
ratios to the south can also be seen off the coast of western Africa by numerous
analyses from different ship cruises (Chester et al. 1971 , 1972 ; Chester and Johnson
1971a , b ; Stuut et al. 2005 ). Mineral dust samples from the sub-Saharan (Sahelian)
zone are mainly characterized by I/K ratios <0.5 (Caquineau et al. 2002 ; Formenti
et al. 2008 ; Kandler et al. 2011b ). On average, I/K ratios of eastern Asian dust
samples are higher compared to northern African dust samples (often >5.0). Highest
values are reported from samples from the Taklamakan Desert (Shen et al. 2005 ).
Hence, Asian mineral dust samples that are characterized by elevated carbonate
contents and high illite abundances most probably derive from the westernmost
source region of eastern Asia. In many northern African dust samples, chlorite is
not detected at all or only occurs in trace amounts (see Scheuvens et al. 2013 ;
their Fig. 10), whereas in eastern Asian dust samples, the abundance of chlorite
is mostly greater than the abundance of kaolinite. However, significant overlapping
in the chlorite/kaolinite (C/K) ratios of Asian and African dust samples prevents any
assignment of far-travelled dusts to one of both major source regions solely based on
the C/K ratio. In many dust samples from northern Africa, members of the smectite
group (e.g., montmorillonite) or illite-smectite mixed layers were not detected at all
or only occur in minor and trace amounts. However, mineral dust samples that had
their origin in the region of southern Algeria and northern Mali exhibit significantly
elevated smectite contents (>40 wt% of the fine fraction; Paquet et al. 1984 ;
Skonieczny et al. 2011 ). These high smectite abundances are probably coupled
with intensely weathered basic volcanics of this region (Coudé-Gaussen 1989 ).