The Characterization of Libyan Raw Dolomite Samples Using Chemical Techniques

Dolomite known as dolostone is a double carbonate of calcium and magnesium, CaCO3. MgCO3. After calcite, it is the second most important and abundant of the carbonate minerals. This work aims at processing, investigation and characterization of four different Libyan raw dolomite samples via simple chemical method. Based on a previous geological survey carried out by the industrial research centre (Tripoli), four dolomite samples selected from different locations of Libya namely; El-azizia, Souk El-khamees (sample P1), Nalout, Abu Rashada route, Gherian (sample P2), Sedi El-said, Abu Reshada route, El-gabl El-gharby (sample P3) and Abu Ghilan, north KafTakoot (sample P4), were processed (crushed, ground and sieved), then investigated through their chemical composition using x-ray fluorescence (XRF) technique. Their mineralogical compositions were investigated using x-ray diffraction technique (XRD) and scanning electron microscope (SEM). All the measurement techniques in this work indicated that qualitatively the four investigated samples have a dolomitic nature. It was conformed that the percent of CaO in sample P1 (31.19%) is relatively higher than the standard dolomite (30.8%) which confirms the prediction of the presence of some content of calcite CaCO3 accompanied with dolomite sample P1. In spite of the limited variance in the chemical constitutions determined by x-ray fluorescence compared with classical wet method, the results confirm that the four investigated samples are mainly dolomite and that samples P1 and P2 are relatively purer than samples P3 and P4 which contain larger contents of Fe2O3, Al2O3 and SiO2.


Introduction
The mineral dolomite is a combination of the elements calcium, magnesium, carbon and oxygen. Chemically, it is calcium, magnesium carbonate. A large number of other elements may substitute for the calcium, magnesiu, varieties containing iron, manganese, cobalt, lead and zinc. When a large amount of iron is present, the mineral ankerite forms and when excess manganese is present the mineral kutnahorite forms instead of dolomite. All these minerals have the same internal structure, but differ chemically from each other [1]. It's well known that dolomite is rarely found in some sedimentary environments, but dolostones are very common in the rock record. Also, most rocks that are rich in dolomite were originally deposited as calcium carbonate mud's that were postdepositionally altered by magnesiumrich pore water to form dolomite [2]. In addition, in literature the mode of formation, dolomites can be divided into two groups: primary dolomite and secondary dolomite [3]. Dolomites can be hard than calcite, CaCO 3 , this means it is harder than a piece of copper and softer than mild steel. The hardness of a mineral is also another characteristic used to identify it from other minerals [4]. Dolomite can be found in several different colors ranges from colorless to white with green, brown, or pink tints, it is often has a light pinkish color, but colorless and white are very common. However, it is dolomite's pink color that sets another unique characteristic for dolomite. Crystals of dolomite are well known for their typical beautiful pink color, pearly luster and unusual crystal habit and it is these clusters that make very attractive specimens [5]. In additions, Dolomite is slightly soluble in water (0.032 gram per liter of water at 17°C). Like calcite (in limestones), dolomite is also dissolve more readily in groundwater that is saturated with carbon dioxide gas, and like calcite, will form caves and sinkholes. Dolomite will also undergo de-dolomitization, or removal of magnesium, when subjected to weathering, or the action of fluids, and will convert to calcite. Dolomite dissolved by groundwater will also give rise to stalactites, stalagmites and travertine when dissolved carbon dioxide gas evaporates, but the formations that precipitate out are almost always the calcium carbonate minerals calcite [6] or aragonite, rather than dolomite. Deposits dolomite are widespread over large areas throughout Libya. The most important areas are Tripoli and Bany-Waleed. Mainly in the north zones, particularly in Elazizia, Souk El-khamees, Gharyan, Nalot, El-gabl El-gharby and surroundings, there are large reserves of the dolomite deposits. The application of dolomite is quarried for building and ornamental stone, road stone, and the production of refractory brick. It is the principal ore of magnesium metal and the source of the magnesium used by the chemical industry [7]. Dolomite is used to make magnesia, which has important medical applications. Dolomite specimens from Iran are very popular among mineral collectors and dealers. Dolomite rock is used as an ornamental and structural stone, and for extracting certain metals from their ores [8]. It is useful in the chemical industry in the preparation of magnesium salts. Dolomite occurs in a different crystal class than the calcite group. This can be noted by the fact that dolomite generally forms more elongated crystals than the calcite group. In addition, dolomite never occurs in scale no hadrons, whereas minerals of the calcite group do. This current work aims processing, investigation and characterization of different Libyan raw dolomite samples.

Materials Used
Four raw dolomite samples were selected from different locations in Libya based on a previous geological survey, carried out by Industrial researches centre (Tripoli) [8]. The first sample was taken from El-azizia, Souk El-khamees, the second from Nalout, Abu Rashada route, Gherian, the third from Sedi El-said, Abu Reshada route, El-gabl El-gharby and the fourth from Abu Ghilan, north Kaf-Katkoot. About 5 Kg of each dolomite sample was taken two meters deeply far from the surface, then processed and investigated through their mineralogical compositions. The selected dolomite batches were crushed individually, then grinded until completely passed through (125 micron) standard sieve. To obtain well representative samples, the individual batches were well hand mixed then quartered into four equal portions, two of them, from the opposite sides, were removed while the two other portions were taken, well hand mixed and quartering is repeated several times until the finally obtained 200g from each dolomite batch. Figure 1 shows the quartering technique of the dolomite batches [9]. Visual observations shown in Figure 2 indicate that the four investigated dolomite samples are translucent to transparent, have a pearly luster and are characterized with different colors commonly white, (sample P1 selected from El-azizia, Souk Elkhamees), yellowish white (sample P2 selected from Nalout, Abu Rashada route, Gherian), light, brown, (sample P3 selected from Sedi El-said, Abu Reshada route, El-gabl Elgharby) and, dark brown (sample P4 selected from Abu Ghilan, north Kaf-Katkoot). The samples are characterized with perfect cleavage. These observations give a first impression that the selected samples under investigation are dolomite rocks as reported by J. George et al [10].

X-ray Fluorescence (XRF)
About 10 g of each finely ground dolomite sample (passed completely through 75 µm standard sieve) was used to investigate their chemical constitution using x-ray fluorescence technique (XRF) through comparing tested samples with a standard dolomite sample as a reference material. An instrument XRF (BRUKER-XRF-3400, Egypt) was used for this purpose.

Infrared (IR) Spectra
About 10 mg of the substance as a fine powder was mixed well with spectroscopic pure KBr powder in agate mortar.
The mixture was then pressed about 8 ton/cm 2 in a special disc under vacuum, using a hydraulic press.

X-ray Diffraction (XRD)
About 0.5 g of each dolomite sample was investigated through its solid phase mineralogical compositions using xray diffraction technique (XRD). A Philips 1730 diffractometer with Ni filtered Cu Kα radiation at a scan speed of 120 min -1 was used.

Scanning Electron Microscope (SEM)
The microstructure and solid phase analysis of some selected samples was investigated using scanning electron microscope (A Philips XL30 scanning electron microscope attached with an EDX unit with accelerating voltage 30 kv and magnification up to 20.000 x). The polished surfaces were then cleaned under water by ultrasonic before coating under vacuum with thin layer of gold for SEM investigation.  [11]. The table also indicates that samples (P1) and (P2), have the closest chemical composition to the standard dolomite sample. In addition to the main constituents of dolomite (CaO, MgO and CO 2 ), some other impurities are present namely; SiO 2 which is very limited in samples (P1) and (P2) 0.85% and 1.22% respectively, but increases to 7.29% and 15.89% in samples (P3) Figure 3 shows the IR spectra of the four investigated samples. Comparing with the standard behavior of dolomite [12]. The results indicate that qualitatively the four investigated samples have a dolomitic nature. The characteristic dolomite bands observed at 726 cm -1 , 878 cm -1 and the broad band at 1438 cm -1 representing out of plane bending (ν 2 ), the asymmetric stretching (ν 3 ), and the in plane bending (ν 4 ), modes of the carbonate group are found to be active [13]. Additionally, bands due to silicates (1033 cm -1 ) are visible especially in samples (P3) and (P4) which indicates the presence of same impurities in such samples. Peaks due to H-bonded water (at ≈ 3400 cm -1 ) are also visible. The bread bands at 3100-3700 cm -1 can be interpreted as two H 2 O stretching modes. Free water molecules have stretching mode at 3700 cm -1 . The modes at 3428-3444 cm -1 would correspond to O...H-O-H bond [14].  Figures (4-7) show x-ray diffraction patterns of the powders of the investigated samples. Figure 4 shows the sample P1. It is composed mainly of dolomite mineral CaMg (CO 3 ) 2 in addition to a little content of calcite mineral CaCO 3 . All of the main peaks characterizing dolomite mineral could be detected including the main peak (100% intensity) at 2Ө = 31º, in addition to the other peaks with relatively lower intensities at 2Ө =24º, 33.5º, 37.5º, 41º, 45º, 50.5º and 51º (JCPDS card no. 84-1208) [15]. Also, few peaks with very low intensity characterizing calcite mineral CaCO 3 could be observed at 2 Ө= 29.5º, 39.2º, 47.2º, and 48.5º (JCPDS card no. 86-0174). (Joint Committee on Powder Diffraction and Standards) Figure 5 shows the sample P2, it is pure dolomite, no peaks characterizing any other minerals could be detected. Figure 6 shows the sample P3 it was mainly dolomite mineral with a little content of quartz mineral (SiO 2 ). In addition to the peaks characterizing dolomite some peaks with relatively lower intensity characterizing quartz could also be observed at 2Ө= 21º, 26.5º and 36.5º (JCPDS card no. 89-1813). Figure 7 shows the sample P4, it is composed mainly of dolomite in addition to quartz, but the content of quartz in sample (P4) is relatively higher than in sample (P3), this was observed from the relatively higher intensity of quartz peaks in sample (P4) than in sample (P3). The X-ray diffraction results confirm the results of chemical analysis which predict that (i) sample (P1) is mainly dolomite with very low content of calcite, (ii) sample (P2) is pure dolomite,(iii) samples (P3) and (P4) are mainly dolomite with some contents of quartz. Similar results [16] indicated that more carbonate phase was present in dolomite sample.     Figure 8 shows the photomicrographs of representative samples taken from the four investigated raw dolomite batches. The figure shows that the dolomitic nature predominates all over the matrices of the four samples, planar subhedral to euhedral crystalline characterizing dolomite [17,18] constitute the majority of the matrices in the photos of the four samples. It could also recognize the homogeneity of the matrix in sample (P2) (Figure 8b) which reflect the purity of this sample. On the other hand, some white crystals characterizing calcite could be observed deposited on the dolomite crystals (Figure 8a) while the glassy crystals with sharp edges [19] and walls characterizing quarts could be observed in sample (P3) (Figure 8c), which are larger more obvious in sample (P4) (Figure 8d), these crystals could not be recognized in samples (P1) or (P2). These results confirm those results which obtained from XRD and XRF.

Conclusions
Four different dolomite samples were selected from different locations of the north of Libya. The main findings can be summarized in the following points: 1. and the broad band at 1438 cm -1 representing out of plane bending (ν 2 ), the asymmetric stretching (ν 3 ), and the in plane bending (ν 4 ), modes of the carbonate group are found to be active. 5. The scanning using the electron microscope indicatesthat the dolomitic nature predominates all over the four samples, the euhedralrhobohedral crystals characterizing dolomite constitute the majority of the matrices in the photos of the four samples.