Geoscience Reference
In-Depth Information
2.2.1
Mineralogical Data
The mineralogical composition of mineral dust samples is predominantly analyzed
by X-ray diffractometry (XRD) and has been now used for more than 40 years
for the characterization of mineral dust samples and their possible source sedi-
ments. By means of XRD techniques, the crystalline compounds of a sample are
determined, neglecting noncrystalline phases such as opaline diatoms or volcanic
glass fragments which may constitute a significant fraction in some dust samples.
However, XRD has to be viewed as a semiquantitative technique with high detection
limits (>3-5 wt%) and comparatively large analytical errors. The amount of mass
required for analysis (>800 g; Formenti et al.
2011b
) is usually much larger than
the sample mass obtained by short-time impactor sampling and hence, X-ray diffrac-
tometry has been mainly applied to source sediments and to long-term deposited
samples. X-ray diffractometry is used for the mineralogical characterization of non-
oriented bulk samples and for the determination of the fine-grained phases (<2 m;
clay mineral size spectrum) on oriented samples. A summary of results obtained by
XRD techniques on mineral dust samples (aerosols and deposited samples) mainly
from northern Africa and eastern Asia will be provided in the following paragraphs.
Bulk dust samples from all over the Earth are mainly composed of silicates
and carbonates. The major mineralogical phases are the silicates quartz (SiO
2
),
feldspar, and members of the group of phyllosilicates (see below) and the carbonates
calcite (CaCO
3
) and dolomite (CaMg((CO
3
)
2
)). The amount of quartz significantly
varies but mainly falls in a range between 15 and 60 wt% with exceptional high
amounts reaching 80 wt% and more in some deposited samples in northern Africa
(Nigeria: Adedokun et al.
1989
, Libya: O'Hara et al.
2006
). The different feldspar
minerals K-feldspar (KAlSi
3
O
8
) and plagioclase (solid solution series between
albite (NaAlSi
3
O
8
) and anorthite (CaAl
2
Si
2
O
8
)) are present in most mineral dust
samples but rarely exceed 10 wt%. Considering the present data set, neither the
abundance of quartz or feldspar in a dust sample nor any ratio between quartz
and feldspar or between both feldspars are characteristic for a specific source area
and hence they cannot be recommended for any source apportionment studies. On
the other hand, the abundance of the carbonates calcite and dolomite in mineral
dust samples is clearly correlated to the composition of the soils and sediments
in the source area and can be used as a compositional fingerprint in northern
African as well as in eastern Asian dust samples (Formenti et al.
2011b
; Scheuvens
et al.
2013
). In northern Africa, mineral dusts with the highest carbonate contents
(occasionally more than 50 wt%) are reported from the northwestern part of
northern Africa (northern Algeria, Morocco) and are probably associated with
source sediments deposited along the foothills of the Atlas mountains. Mineral
dusts uplifted in this area are mainly transported northwards to the Mediterranean
region and southwestern Europe or to the west towards the Canary Islands. Hence,
it can be expected that mineral dusts sampled in this regions should be generally
rich in carbonates (if no solution or transformation processes have taken place).
In eastern Asia, a general trend with higher carbonate contents in western source
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