Mineralogical Composition of Limestone Rock and Soil from Jubaila Formation
A.S. Al- Farraj
Jubaila Formation is of the Upper Jurassic and covers the shallow shelf and the Central Arabian intrashelf basin. Unweathered limestone and soil samples were collected from the base of the roadcut (about 10-15 m below the surface of Jubaila Formation). XRD was applied for whole sample and clay fraction of limestone rock and soil samples. Moreover, Differential thermal and thermo gravimetric analysis (DTA, TGA) were carried out with whole sample fraction of limestone rocks and soils. Calcite was recognized as a major mineral of limestone rocks and other samples. Also, quartz and kaolinite were approved with all samples. Soil samples have low content of 2:1 clay minerals and more kaolinite. Clay fraction of samples has kaolinite, illite and illite/smectite. The content of illite and illite/smectite was more with soil samples comparison with other samples. The existence of calcite and absence of other calcium carbonates (such as aragonite, Mg-calcite and nesquehonite, etc.) indicates how old is study area (Jubaila Formation). Transformation of Mg-calcite to calcite, supply Mg ion which is necessary of smectite generated by transformation of illite. Because of, soils strap was stuck between limestone rocks made low leaching and drainage environment which supported generation of illite/smectite mineral.
Received: October 05, 2011;
Accepted: December 21, 2011;
Published: February 22, 2012
Weathering significantly alters the chemical, mineralogical and physical properties
of the rocks and results in different properties. However, the effect of weathering
on the geological and geomechanical properties of limestone rock is not well
known (Tugrul and Zarif, 2000). Limestone composition
is dominantly that CaO in CaCO3 (CO2 content usually not
given). The weathering of sedimentary rocks could proceed as predicted by their
mineralogy. The Molar SiO2/Al2O3 ratio of limestone
is around 9 (Birkeland, 1999). The alterations that would
have to occur during soil formation would produce clay minerals from rocks.
Magnesium ion, comparing with other ions, has the greatest ability to prevent
the growth of calcite. Moreover, the presence of Mg++ favors to formation
of aragonite, a mg-calcite and hydrated carbonate from aqueous solutions at
surface conditions (Heakal et al., 2000). The
hydrates tend to convert to the anhydrous forms (calcite or aragonite) with
time even at low temperatures. In marine environments and some soils, the amount
of Mg substitutions can reach 10-15% mole Mg (Mackenzie
et al., 1983). The solubility of magnesian calcites generally increases
as the percent of Mg substitution increases (Walter and
Morse, 1984). The dry transformation of aragonite to calcite under 100°C,
needs tens of millions of years (Fyfe and Bischoff, 1965).
Three principal processes to account for genesis of clay minerals were distinguished:
(1) inheritance from parent materials, (2) transformation of other clay minerals
and (3) neoformation from soil solution (Millot, 1970).
For example, there are three possible origins for smectite clay. It may be inherited
from shale and hydrothermally altered rocks (Jackson and
Sherman, 1953). Moreover, Reid-Soukup and Ulery (2002)
noticed that, smectites may form as result of weathering transformations of
other 2:1 phyllosilicates, particularly micas. Also, neoformed smectites are
those that precipitate directly from soil or matrix solution. In arid and semi-arid
regions, palygorskite, smectite, chlorite, illite, kaolinite and vermiculite
are the dominant clay minerals (Bouza et al., 2007;
Shadfan et al., 1985).
Many researchers believe that soils forming on or in association with limestone
are derived from limestone rocks. The clay content which originated from limestone,
might be considered as evidence for intensive weathering. Ross
and Hendrick (1945) found that kaolinite is an end-product of clay mineral
development in limestone soils. Murray and Keller (1993)
noticed that most kaolinite results from weathering, by hydrothermal alteration
and as an authigenic sedimentary mineral. Also, Keller and
Frederiekson (1952) noticed that the little of montmorillonite in the Lenoir
limestone may come from slight weathering of the limestone while release of
Mg and Ca may allow these ions to take part in the crystal lattice of hydrous
mica producing montmorillonite. Moreover, in Jordan, smectite/vermiculite interstratified
mineral dominated the clay minerals for limestone-derived soils (Khresat
and Taimeh, 1998).
Illite/smectite is a common mixed-layered mineral. It is a mineral formed of
illite and smectite. Illite/smectite is the most abundant, varied and widespread
of the mixed layered clay minerals in sedimentary rocks and soils (Moore
and Reynolds, 1997). White (1950) concluded that
the transformation of illite into montmorillonite could occur at both high and
low temperatures. Also, it was asserted that this transformation can be produced
by MgCl2. In addition, the conversion of illite to mixed-layered
I/S is apparently continuous around 150°C and discontinuous above 200°C
MATERIALS AND METHODS
Study area and sampling protocol: The study was undertaken in west of
Riyadh city, Saudi Arabia. The study area is located on Jubaila formation at
24.641 N and 46.590 E. It is around 700 m above sea level. According to Al-Malik
(1994), the rainy season normally starts in December and ends in May, with
a peak that occurs in April. The main annual temperature is 24.7°C, while
the main annual rainfall is 83.3 mm. Unweathered Jubaila limestone and soil
were collected from the base of the roadcut (about 10-15 m below the surface)
(Fig. 1). Samples were two limestone rocks (1 and 2), three
unconsolidated rocks (3, 4, 5) and three soil samples (6, 7, 8).
||The location of studied samples showing Cross-section of limestone
rocks with strip of soil (Jubaila Formation)
Physical and chemical analysis: Rock and soil samples were air-dried,
powdered, homogenized under standard conditions using an agate mortar (<2
mm). Soil pH and EC were measured on 1:1 soil/water mixture according to Thomas
(1996) and Rhoades (1996), respectively. Cations and
inions (Ca2+, Mg2+, Na+, K+, HCO3¯,
CO32¯, Cl¯, SO42¯)
were analyzed in those extracted soil solutions (Gupta, 2007).
Mineralogical analyses: XRD was applied for whole sample of limestone
rocks, soils and clay fraction of those samples. A part of each sample (Rock
and soil) was treated chemically prior to particle size fractionation. Soluble
salts and carbonates were removed by using the sodium acetate buffer method
(Kunze and Dixon, 1994). Organic matter was removed by
using H2O2 (Moore and Reynolds, 1997).
Finally, free iron oxides were removed by using dithionite citrate-sodium bicarbonate
(Kunze and Dixon, 1994). After chemical treatment, soil
suspensions were dispersed by a combination of chemical and physical methods
using Na-hexametaphosphate. Subsequently, a 5 min mixing with a standard electrical
mixer was performed (Gee and Bauder, 1994).
All clay fractions of samples were oriented by using the glass slide method
(Moore and Reynolds, 1997). Samples of clay fractions were
saturated by Mg and K. Mg-saturated clay samples were solvated by ethylene glycol
vapors (in a dessicator) over a period of 48 h prior. The K-saturated clay samples
were studied both after air-drying and heating (for 2 h) at 550°C. Samples
were subjected to XRD using CuKα (1.5406 A°) radiation (45 kV, 35 mA)
on a Shimadzu (7000) vertical goniometer in a range of 2° 2θ to 40°
2θ. Moreover, powder samples of limestone rock and soil were subjected
to XRD using same condition in a range of 2° 2θ to 60° 2θ
(Whittig and Allardice, 1994). Mineral identification
was determined using the standard parameters using the computer program ICDD
for calcite and quartz. While, clay minerals were identified as explained by
Moore and Reynolds (1997) and Dixon and
Differential Thermal Analysis (DTA) and Thermo Gravimetric Analysis (TGA) were carried out with whole sample fraction of limestone rocks and soils. Precalcined alumina was used as the inert material. Analyses were carried out by DTG 60H with a heating rate of 20°C min-1 from 25 to 1100°C in N. Weight of clay samples were around 25 mg.
The basic chemical properties of studied soil samples are summarized in Table
1. Figure 2a-c shows the X-ray diffraction
patterns of untreated rock and soil samples. Calcite was recognized by its strong
peaks 012, 104, 006, 110, 113, 202, 024, 018, 116, 121 and 122. It was a major
mineral of limestone rocks and other samples. X-ray diffractogram of the eighth
sample (soil sample) indicates absent of most peaks of calcite. Table
2 shows the intensity of all peaks of minerals of samples. The existence
of calcite and absence of other calcium carbonates (such as aragonite, Mg-calcite
and nesquehonite, etc.) indicates how old is study area (Jubaila Formation).
Because of, calcite and dolomite are described as stable phases; while, aragonite,
Mg-calcite, nesquehonite, lansfordite and artinite are recognized as metastable
(Doner and Grossl, 2002). The transformation of aragonite
to calcite needs tens of millions of years (Fyfe and Bischoff,
1965). Previous studies specified age of Jubaila Formation to be of the
Upper Jurassic which extends from about 161.2±4 to 145.5±4 Mya
(million years ago) (Vaslet et al., 1991).
Moreover, quartz was identified by its characteristic peak of 0.425 nm (100)
and 0.335 nm (101). Other peaks of quartz were appeared with soil samples (6-8).
A small peak could be noticed at ~0.727 nm among all samples which could indicate
kaolinite mineral. In rock limestone (sample 1, 2), the relative peak intensity
of 0.727 nm was smaller than other samples; whereas, the highest intensity of
that peak was with soil samples (6-8).
||X-ray powder diffraction data of studied samples
Finally, soil samples have another peak around 1.468 nm, suggesting the presences
of 2:1 clay minerals (Borchardt, 2002; Dixon
and Schulze, 2002). The small intensity, at ~0.727 and 1.468 nm points to
low content of clay minerals among studied samples.
With soil samples (6-8), the DTA signal exhibits an endothermic peak around
100°C which might be due to the removal of adsorbed water on external surfaces
for clay minerals. These clay minerals could be smectite and/or illite (Borchardt,
2002) or which called expansive phyllosilicates (Reid-Soukup
and Ulery, 2002). On the TGA curves (Fig. 3) a 2.74-4.68%
(with average 3.96%) weight loss was determined associated with that endothermic
peak. Samples (3-5) gave the smaller intensity of the first peak. Whereas, it
was disappeared with limestone rock samples (1, 2) (Fig. 3).
The low intensity or absence of the first peak confirms a lower presence of
expansive phyllosilicates than in soil samples. The patterns of XRD support
this result. Figure 2 shows the small intensity at (1.468
nm) of illite/smectite of soil samples (6-8), while that peak was absence with
||XRD patterns of whole samples fraction (K: Kaolinite; Q: Quartz;
|| DTA and TGA curves of whole samples fraction
An endothermic peak was observed at 520-560°C with soil samples (6-8).
This peak is associated with dehydroxylation of kaolinite. This endothermic
peak was disappeared with limestone rocks and other samples except for sample
(6) which had a small endothermic at 526°C. The dehydroxylation temperatures
were ≤560°C. These temperatures are in the upper range of the usually
reported temperatures of dehydroxylation of soil kaolins. They are often reported
to be around or even below 500°C (Hart et al.,
2002, 2003) while those of reference kaolinites often
are between 500 and 550°C (Hart et al., 2002).
The size of the peak, as well as the peak temperature, is reduced slightly as
the particle size decreases and as the crystallinity decreases. The difference
seems to be greater for the crystallinity factor than for the particle size
(Grim, 1968). The crystallinity of kaolinite has been found
to be associated with pedo-environmental factors of soil. For example, the presence
of interstratified 2:1 minerals and Fe in kaolinite is considered to be responsible
for decreasing kaolinite crystallinity (Singh and Gilkes,
The influence of pH on structural of kaolinites has been reported by Sei
et al. (2006). Thus acid media result in ordered and coarse-grained
particles. While, alkaline media cause disordered and fine grained kaolinites.
|| The changes in the peak's temperature and weight loss due
to the CO3= decomposition of calcite mineral
Smykatz-Kloss (1974) recognized the following classification
to realize the degree of structural order of kaolinite: extremely disordered
kaolinites have (Tendo<530°C), very disordered kaolinites
(530°C<Tendo<555°C), less disordered kaolinites (555°C<Tendo<575°C)
and well ordered kaolinites (Tendo>575°C). From these suggestions,
kaolinite of the clays studied could be described as disordered. The structural
disorder is expected to modify the chemical and physical properties of kaolinite.
Moreover, the differential thermal analysis is sensitive to shape of kaolinite
particles. Spherical particles have lower dehydroxylation temperatures than
hexagonal particles (Huertas et al., 1997). The
ideal form for the well crystallized kaolinite particles is hexagonal. The exothermic
peak of kaolinite (≈1000°C) did not appear in this study. This could
be explained by the presence of even a small amount of iron oxide or hydroxide
which are suggested to modify the temperature exothermic.
Finally, other endothermic peak was observed between 668-854°C. This peak
could be explained by the presence of calcite (Paterson and
Swaffield, 1987). The DTA diagram shows systematic decreases in the peak
area of the calcite decomposition with the increases of weathering of limestone
rock (Fig. 3). This endothermic peak signified the thermal
decomposition of the CO3= present in the Ca-bearing minerals
(Calcite) of each sample. Table 3 illustrates the change in
the peak temperature, the weight loss and the calculation of calcite content
in samples. In general, the order of calcite content follows the same order
of intensity of peak (104).
Clay fraction: As shown in Fig. 4, clay fraction of all samples had reflections at around 0.72 nm (001) and 0.358 nm (002) was not affected by solvation but vanished after heating. In view of these behaviors, the dominant clay mineral of the limestone rocks and soil samples is kaolinite. The d (001) values for kaolinite in the clay fraction varied from 0.718 to 0.728 nm with a median value of 0.721 nm. The range for the (002) d-values was narrow (0.358-0.359 nm) compared with (001) d-values. The intensity ratio of 001/002 ranged from 0.73-0.89, except with sample 1 which had a higher intensity of 002 comparing with 001 reflection. According to the intensity of first peak (001), the limestone rock samples (1 and 2) have the lowest content of kaolinite.
Moreover, there are reflections at around 1.024 nm (001), 0.502 nm (002) and
0.335 nm (003). Those reflections didnt affected by other treatments which
suggested presence of illite mineral. Sample 1 which is one of limestone rocks,
has just the first reflection of illite. Moreover, the intensity of first peak
(001) for both limestone rocks was small comparing with other samples. The low
intensity indicates a low amount of illite minerals in limestone rocks. Thompson
and Ukrainczyk (2002) reported formation of illite in pedogenic and sedimentary
Figure 4 shows that, the glycol-treated gives a strong reflection
at about 5.24° 2θ (1.687 nm) which, in the air-dried condition, shifts
to about 6.14° 2θ (1.44 nm). Moreover, heating treatment at 550°C
resulted in a diffraction pattern similar to that of illite. Because of EG salvation
has caused significant changes in the diffraction pattern of sample 3, smectite
component should be presented.
||XRD analysis of clay fraction of all samples
Furthermore, a reflection is noted at 1.768° 2θ, therefore, the diagnosis
is likely illite/smectite (Moore and Reynolds, 1997). This
suggestion was supported by absence of the (003) and (005) peaks. With exception
of the first, other samples have the same reflections around 1.43-1.47 nm with
which don't shift with EG treatments (Fig. 4). Smectite lost
its ability to expand in the presence of ethylene glycol, as it becomes more
deeply buried by overlying sediments (Thompson and Ukrainczyk,
2002). From above, all samples, except the first, contain illite/smectite
mineral. Some researchers found that a part of smectite has been generated in
an Mg and Si rich basin by the transformation of the illite in a sedimentary
environment (Birkeland, 1999). Moreover, Bernhard
et al. (1999) suggested that in an Si rich environment with moderate
or low leaching combined with low drainage, I/S may occur.
X-ray diffraction and DTA with DTG indicated that, calcite and quartz are the major minerals in all analyzed rock and soil samples. Moreover, low amount of kaolinite was found in all samples; while 2:1 was noticed with soil samples only. Kaolinite was the dominant mineral in clay fraction of all samples. Furthermore, illite and illite/smectite were found with clay fraction. The content of illite and illite/smectite was more with soil samples comparison with other samples. The existence of calcite and absence of other calcium carbonates (such as aragonite, Mg-calcite and nesquehonite, etc.) indicates how old is study area (Jubaila Formation). Transformation of Mg-calcite to calcite, supply Mg ion which is necessary of smectite generated by transformation of illite. Because of, soils strap was stuck between limestone rocks made low leaching and drainage environment which supported generation of illite/smectite mineral.
The author wishes to thank Prof. Abdulazeem Salam, Khalid Al-Enazi and Saaed Solih, Soil Science Department, King Saud Univ., Riyadh, Saudi Arabia, for their assistance and efforts during laboratory work.
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