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Abstract
Mineralogical and Geotechnical investigations were carried out on the weathered
zones of the Basement rocks of Aswan Area, Egypt. The suitability of the
composition of these zones in industrial applications were tested.
The stratigraphic units recognized in the studied area comprise metamorphic (micaand
hornblende-schists, amphibolites and gneisses) and igneous (minor intrusions,
granites, granodiorite and epidiorites) rocks of Pre-Cambrian age; sandstones and
clays belonging to the Nubian Sandstones Series of Upper Cretaceous age; gravels
and sands and muds of Pleistocene and Recent ages.
The most of weathering profiles developed on granitic rocks include the W2
(slightly weathered rock with discoloration in discontinuity surface), W3
(moderately weathered rock with less than half of the rock decomposed), W4 highly
weathered rock with more than half of the material transformed to a soil) and W5
(completely weathered rock with all the material transformed to a soil but the
original mass structure still largely intact) zones of weathering. The weathering
profiles developed on the metamorphic rocks can be classified into two zones
namely W4 and W5.
The main petrographic alteration features recorded in weathered zones of granites of
the studied area include: (1) fractured and fragmented quartz grains; (2) orthoclase
coarse crystals with turbid surfaces; (3) microcrackes interspersed with clay
minerals throughout the feldspar and mafic minerals; (4) potash-feldspar highly
destroyed to fine grained of silica and clay minerals in the advanced stage of
weathering process; (5) clay-filled vugs replacing plagioclase with progress
weathering ; and (6) fissure systems characterized by inter-, intra-, and transgranular
microcracks in plagioclase.
The identified minerals in weathered materials (regoliths) can be categorized into
two groups clay/phyllosilicate minerals and non clay minerals. The most dominant
clay minerals are kaolinite, mica-illite, smectite, smectite-illite mixed layers and
chlorite whilst the non-clay minerals are represented by quartz as the main mineral
with remarkable amounts of feldspars (microcline, albite, anorthite and orthoclase,
and with traceable amounts of iron oxides (hematite), alunite, anatase and calcite.
Five types of clay minerals were identified throughout the clay fraction of the
studied weathered materials, namely kaolinite, smectite, illite, smectite-illite mixedlayers
and chlorite arranged according to their abundance. The non-clay minerals
recorded include quartz, microcline, anatase, alunite and hematite.
Kaolinite and quartz are the most abundant mineral in less than 2 micron fraction
and the powdered whole samples of saprolite (regolith), SEM-EDX study confirms
that (1) the feldspars (Plagioclase feldspar, K-feldspar), quartz, biotite, and
muscovite are the main primary minerals in the studied regoliths, and (2) The main
secondary minerals are kaolinite, illite, allunite and a smectitic material.
The morphology of particles reveals irregular platelets, well-developed and irregular
flakes, and booklets of kaolinite. The morphological features of some investigated
samples give a good indication for a good crystallinity of kaolinite mineral.
EDX analysis shows Al, and Si with nearly equal amounts and minor amount of K
for kaolinite; K, Al, and Si for illite (or other mica); and Si, Al, K, and Ca for etched
feldspar (orthoclase or plagioclase). EDX spectra of some samples indicate Fe, Al,
Si, Mg and K present which are indicative of altered biotite or either goethite or
hematite forming a thin coating on the kaolinite crystals.
The textural disposition of the secondary minerals indicates important data about the
processes of alteration undergone by the primary minerals.
Mineralogical, chemical and microtextural analysis of the studied kaolin deposits
showed that the progressive weathering of plagioclase and biotite of the parent rocks
are the main source of kaolinite in the studied area. Well-preserved books and
vermiform kaolinite characterize the studied kaolin deposits. The localized
crystallization of kaolinite depending on the distribution of primary minerals
strongly influenced the clay mineralogy of the studied weathering profiles.
Excepting for SiO2, all the major element show a remarkable increase in content
towards the top of these profiles. Trace and rare elements (Ba, Y, Sr, V, Eu, Zn, La,
and Cr) tend to increase gradually upward. On the other hand, Ni, Nd, Sm, Ce, Yb
and Zr show a general decrease towards the top of the profiles. The weathering
indices values (CIA, CIW, PIA, V, WIP, R, SIT and MIA) of regoliths of these
profiles correlated well with their mineralogy (i.e., the content of kaolinte as an
indicator to intensive weathering of the regolith materials).
The pH values of regolith samples show a wide range of values. LL ranges between
51 and 59; PL varies from 29 to 36 and the minimum of PI is 18 and its maximum is
29. The studied regolith can be characterized as medium expansive since the values
of Atterberg limits for all samples fall within the range of medium degree of
expansion. The plasticity charts show that studied regoliths belong mainly to the
high plasticity inorganic clay groups. The general relationship between the clay
content and the Atterberg limits show that the higher the clay content, the higher is
the Atterberg limits. Also, these regoliths show that it is not sufficient to consider
the liquid limit is a function of the smectite content but the other clay minerals need
to be considered as well.
The textural, chemical and mineralogical of whole rock data of the studied regolith
have been evaluated by international standard specifications. The clays of Wadi
Agabat, Wadi Abu Sobeira and West Aswan can be used for the production of cotto
toscano (porous products), whereas the clays of Wadi Allaqi Road fall mainly in the
red stoneware. Regarding the color (red, gray, purple) and the high iron oxide
content, these kaolins cannot be used in fine ceramics. whereas, the clays of West
Aswan regoliths only can be are suitable for paper filling and coating and paint
filling.
KEY WORDS: Granite; Mica schist; Weathering; Regolith; Texture; kaolinte ;
Smectite;Illite; weathering indices; SEM analysis; EDX analysis; Atterberg limits;
Plasticity, ceramic quality; Ceramics; Bricks; Paper coating, Paper filler.