Alunite or Alumstone, a mineral first observed in the 15th century at Tofla, near Rome-Italy, where it is mined for the alum. Alunite is a very fine hydrous sulfate mineral with hydroxyl in its structure. The formula for pure alunite is KAl3(SO4)2(OH)6. It has a rhombohedral crystal structure.
Alunite was recorded for the first time in Jordan by Khoury (1987) in the kaolinite bed of the Kurnub Sandstone Formation of the Lower Cretaceous age. Khoury (1987) indicated that alunite is possibly formed as a result of both diagenetic and supergene processes. Sulfuric acid originated from the oxidation of pyrite. The source of potassium is the result of hydrolysis reactions of feldspars and illite-muscovite. The reaction of such water with kaolinite could form alunite.
In general, different types of alunite were investigated by many authors in different areas around the world, mainly in Turkey, Italy and Australia. Frost et al. (2006a) discussed a Raman spectroscopic study of alunites. They mention that the minerals are characterized by well-defined hydroxyl stretching patterns, which give two groups of hydrogen bonds with calculated hydrogen bond distances of 2.90 and 2.84Å. Frost et al. (2006b) discussed a thermogravimetric study of the alunites of sodium, potassium and ammonium. They mention that three mechanisms of decomposition are observed (a) dehydration, (b) dehydroxylation and (c) desulphation. The thermal decomposition of the three alunites is different. Küçük and Yildiz (2006) studied the decomposition kinetics of mechanically activated alunite ore in air atmosphere by thermogravimetry. They concluded that it can be verified that alunite decomposes in two steps, which are dehydration and desulphation. Mutlu et al. (2005) studied the geochemistry and origin of the Şaphane alunite deposit, Western Anatolia, Turkey. They mention a replacement type alunite deposit occurs in the studied area.
The objectives of this study are to explain the possible mode of origin and to record the occurrences of the alunite in the lower part of the Shale Member, South Jordan. This will enhance the understanding of nature of alunite in the lower part of the Shale Member, South Jordan.
LOCATION AND GEOLOGICAL SETTING OF THE STUDIED AREA
The investigated area is situated in southern Jordan. The area is located about 100 km east of Aqaba at a distance of 20 km to the southeast of Ad-Disa Village (Fig. 1).
The oldest rocks, which crop out in the study area, are of paleozoic age. Three formations are cropping out in the study area, with a general dip towards the southeast. These are from bottom to the top: Bedded brownish weathered sandstone formation; Graptolite sandstone formation and Sabellarifex sandstone formation, respectively.
Graptolite sandstone formation covers most of the studied area in scattered
outcrops and at the surface. It is subdivided into the shale member and the
sandstone-siltstone member. The shale member, which belongs to the lower part
of the graptolite sandstone formation, consists of kaolinite of various colors,
which range from light gray, dark gray, violet, brown-red and red.
map showing the study area|
Alunite patches, mainly along the bedding planes|
It is intercalated
by few thin beds (10-20 cm) of ferruginous and hard siltstone. The kaolinite deposits are the essential constituents of the shale member.
Alunite is present as white soft separated patches mainly along the bedding
planes (Fig. 2). Graptolite sandstone formation contains well-preserved
graptolites (Didymograptus sp., Fig. 3).
Sampling and laboratory techniques: Field work was carried and focused on Shale Member in March-April 2005. Sixteen samples were collected from the lower part of the Shale Member in accordance to the lithological variations.
Scanning Electron Microscopy (SEM) analyses on the selected samples were carried
out using (SEM FEI Quanta 200-Holland) and chemical analyses of spot samples
carried out using EDAX XR Microanalyzer at the Department of Earth and Environmental
Sciences, Yarmouk University, Irbid, Jordan. The mineralogy of the samples was
determined by X-ray powder diffraction (XRD).
preserved graptolites (Didymograptus sp.)
The random samples were scanned from (2 to 65) degree 2θ using Cu Kα
radiation at 35 kV and 25 mA with a Philips-Xpert MPD diffractometer available
at the laboratories of Natural Resources Authority (NRA), Amman Jordan.
RESULTS AND DISCUSSION
The Scanning Electron Microscopy (SEM) photomicrographs (Fig. 4) indicated the presence of rhombohedral idiomorphic alunite crystals within the kaolinite beds, South Jordan.
The results of the X-ray diffraction analyses for the bulk rock samples are
shown in Fig. 5a-c). The results indicated
that the identified clay and non-clay minerals are kaolinite, quartz, alunite,
muscovite, gypsum and hematite (Fig. 5a). Kaolinite, quartz,
muscovite and alunite are present in all samples (Fig. 5a-c).
In addition, the results of the X-ray diffraction analyses of the bulk rock
samples indicated that kaolinite and quartz are the most abundant minerals.
Alunite is present as a minor component.
||Scanning electron micrograph showing aggregates of rhombohedral
||Typical X-ray diffractograms of the bulk rock samples
Gypsum is identified as a filling material in joints and fractures. Hematite is present in a few samples in low concentrations (Fig. 5b).
The chemical composition of the analyzed spot samples is given in Table 1.
The important major oxides (Al2O3, SO3 and K2O) are usually present in all of the studied spot samples as major chemical constituents of alunite.
The contents of Al2O3, SO3 and K2O range from 32.30 to 42.46%, 31.48 to 41.75% and 5.81 to 15.92%, respectively. The results indicated that the average chemical composition of the analyzed spot samples (37.11 Al2O3, 38.80 SO3 and 10.86% K2O) is equivalent with the important major chemical constituents of pure alunite (36.92% Al2O3, 38.66% SO3 and 11.37% K2O).
The results of the field, chemical and mineralogical (XRD and SEM) studies indicated that the alunite occurs as minor constituents in clay beds of the lower part of the Shale Member, South Jordan. The Shale Member which belongs to the lower part of Graptolite Sandstone Formation contains well-preserved graptolites (Didymograptus sp. Fig. 3). The occurrence of well preserved graptolite molds indicate that shale was deposited in a quite marine environment (deep marine depositional environment).
Graptolites are marine organisms usually form simple and chitinous colonies,
belong to the order Graptoloidea. They are considered as index fossils and are
used for stratigraphic correlation and for age determination. They have particularly
played an important role as stratigraphical markers for Silurian and Ordovician
periods being worldwide in distribution. Graptolites have short vertical range
in time, abundance and show rapid evolution in their species.
Didymograptus can be confused with uniserial straight Monograptus|
In most cases, Didymograptus consists of two symmetrical branches diverging
from at any angle between 0 to 180° (Fig. 3a) and are
characteristic to a pelagic (epiplanktonic) marine environment. It is important
to note that broken Didymograptus can be confused with uniserial straight
Monograptus (Fig. 6) (McKinney, 1991; Aiyengar and
Prasad, 1996; Moore et al., 1997).
The sulfur trioxide (SO3) content (Table 1) is related to the presence of gypsum and alunite (Fig. 5). Iron sulfides (source of sulfur) could be originally produced by mineral replacement of chitinous tests of graptolites (Fig. 3) in oxygen deficient marine environment. Alunite is a very fine sulfate mineral (KAl3(SO4)2(OH)6), secondary precipitation of alunite is present in voids and along the bedding planes. The formation of alunite within the clay beds indicates the presence of a sulfur source, which is a possibly sulfide.
Iron sulfides (pyrite; FeS2) could be originally produced by the mineral replacement of the chitinous tests of graptolites in oxygen-deficient marine environment and pyrite could be the source for sulfur. The oxidation of pyrite and the formation of sulfuric acid could explain the possible source for the (SO4)¯ group, which is presented in the alunite and/or gypsum.
The research was supported by the Deanship of the Academic Research, Yarmouk University, Irbid, Jordan.