Iraqi Geological

Abstract


Introduction
Water flow is important for water resource planning and management, especially for assessing water availability and drought identifiers; Therefore, the presence of Sabkhas is an indication of severe drought (Yaseen et al., 2018;Khosravi et al., 2019).Sabkhas are low-lying areas that collect water in arid and semi-arid climates (Awadh et al., 2021).The word Sabkha is of Arabic origin.In Central Asia, it is called "Noor"; in North Africa, it is known as "Sabkha" and in North America "Playa" (Hamdona and Al Hadad, 2008;Briere, 2000).There are two types of Sabkha, continental and coastal.The continental Sabkha is a sedimentary basin with internal drainage in which salts accumulate, due to the proximity of groundwater to the surface (Hsü, 2013;Friedman, 1978).Soil salinization occurs as a result of the shallow groundwater level rise (Quraishi et al., 2013).The report conducted by the USAID (2004) states that the rising groundwater table and the consequent problems of soil salinity lead to land degradation and destroys 5% of the cultivated land annually.Poorly permeable soils do not allow water to penetrate during conventional irrigation and therefore are not washed away (Awadh and Al-Hamdani, 2019).Excess water evaporates due to higher temperatures in dry and semi-arid climates, and salts precipitate over time, causing a large amount of salt to accumulate on the soil surface (Awadh et al., 2022;Boschetti et al., 2020;Awadh, 2018;Al-Hamdani et al., 2016).The sources of salts and their mineral components in Iraq were determined with an estimate of the salt reserves (Jassim and Al-Badri, 2019).Many lakes formed due to structural depression defined as a sag pond (Awadh and Muslim, 2014;Awadh. 2016;Boschetti et al., 2020).One of the large and well-known Sabkhas in Iraq is the Sunaisla Sabkha, which represents a closed continental Sabkha in western Iraq in which rainwater collects, washing the surrounding soil and some of the outcrops of the Fatha and Injana formations, which recently mixed with the groundwater (Abdul Amir, 2005;Awadh and Ahmed, 2013).The Sunaisla Sabkha is mainly composed of halite in an amount of 97%.Some Sabkhas may concentrate elements after the evaporation of water during several cycles of evaporation, which makes them economically valuable or at least pay attention as an area of interest (Bhagat et al., 2021).The study area is located in Abu-Ghraib, western Baghdad, Iraq (Fig. 1).This research is going to identify and discuss the Sabkha mineralogy using XRD and a polarized transmitted microscope to further understand the mineral component of both Sabkha salt and Sabkha soil in addition to a diagnosis of the crystallization patterns.

Sampling
The fieldwork included an exploratory trip in which the locations of the Sabkhas were selected in two areas, the village of Haj Shiaa and the village of Kadim Alaa'ab (Fig. 2).A detailed trip was conducted during which thirty soil samples were collected, twenty from the village of Haj Shiaa (A) and the remaining ten from the village of Kadim Alaa'ab (B).The systematic sampling procedure was applied, in which, samples were collected from a depth of 1-30 cm with an interval of 100 m.The samples were placed in nylon bags and then transported to the laboratory to be prepared for the analysis required.All sample locations were determined using GPS (Table 1).

Thin Section Preparation
Thin sections (31 slides) were made for each sample without using water during the cutting process to avoid salt dissolving.Also, the heat was not used during the preparation and drying of the slides to keep the crystalline shape.The adhesive consists of two parts, Sikadur 52 LP with hardener, which were used to stick the soil samples onto the glass slide.They are mixed in certain proportions (2:1) to ensure good hardening.The sample was left for 24 hours to dry.The cutting machine was used for cutting slides, and then the cut face was smoothed with a smoothing device until it reached 0.3 mm thick.Minerals were diagnosed using a polarizing transmission microscope.

Clay Mineral Separation
Clay minerals and non-clay minerals were examined by Shimadzu 6000 XRD device, a Japanese manufacturer.The samples were dried in an oven at 500 °C.Thirty grams were treated with 15% H2O2 to dissolve the organic matter (Muller, 1967 ).Then it was washed with distilled water to get rid of the salt.This process was repeated five times to ensure that the salts were removed.The clay was separated from the silt using the wet sieving method.Volume less than 63 µm was collected in a container and transferred to a 1000 ml.cylinder.Ten ml. of sodium hexametaphosphate was added as a dispersant to prevent agglomeration of fine particles, according to Folk (1974Folk ( , 1980)), and Folk et al. (1970).A sample of 20 ml was withdrawn from the cylinder at a depth of 5 cm after 4 h.The clay fraction was distributed on three glass slides and left to dry at room temperature.Samples were examined based on the procedures of Stixrude and Peacor (2002); Banat (1980), andThorez (1976) as follows: • An untreated sample with 2θ angle 2-16 where most of the basic reflections of clay minerals can be identified.
• The sample treated with ethylene glycol, the sample was tested by XRD after exposure to ethylene glycol vapor for 24 h at a temperature of 60 °C, to distinguish between chlorite and montmorillonite, in which the basal space expands due to the entry of ethylene glycol into the internal structure (Brown, 1961).• The sample was treated by heating for 2 hours at a temperature of 550 °C, as the structure of kaolin is destroyed at this temperature (Thorez, 1976).

Shapes of Sabkha Salt Crystals
Continental Sabkhas consist of a thin white saline soil layer in which halite (NaCl) predominates as the main mineral that crystallizes in the cubic system.The sodium chloride ions are arranged so that each ion is surrounded by six sodium ions, and the sodium ions are surrounded by six chloride ions.The shape and size of the cubic halite crystal depend mainly on the saturation state of the solution, and the concentration of some common elements.Several shapes of salt crystals were identified in the study.Needle form is a type of crystallization consisting of the growth of salt crystals in one direction, which makes it longitudinal needle shapes (Fig. 3).This type of crystallization is found in the Sabkha area of Haj Shiaa.The Hopper shape is another crystalline shape that was recorded which occurs as inverted pyramidal crystals (Fig. 4), and is formed due to rapid crystal growth from a supersaturated solution.Crystals accumulate more at the crystal corners than at the center of the crystal (Gorniz and Schreiber, 1981).Crystallization in the area of Hajj Shia, A dendritic shape is also noticed which arises in the form of a salt crust, which varies between 4 to 12 cm thick.It is formed in a stagnant brine solution in which no water currents.If the water is moving, this type is not formed because moving causes homogeneous concentrations (Buckley, 1951) (Fig. 5).Polygonal crystals are crystallized after the surface of the Sabkha dries up, while halite crystals continue to grow from the brine solutions underneath, leading to the rise of the edges of the salt surface that intersect, forming a network of polygonal halite crystals (Fig. 6).

Growth of Sabkha Salt Crystals
In regions with arid or semi-arid climates, the Sabkha covers a salt crust consisting mostly of halite.High temperatures cause evaporation and an increase in the concentration of salts, so the process of crystal growth begins after the concentration of the solution.Crystallization takes place at different temperatures ranging from 5 to 30 °C.Crystal forms vary with different temperatures.Some crystals appear on the surface of the solution in the daytime when the temperatures are high.Others arise at the bottom of the pelvis during the night when temperatures are low (Arthurten, 1973).The crystals that arise on the surface of the solution consist of a thin film consisting of one face of the halite crystal.Then the crystal growth continues towards the top outward, forming a hollow pyramid shape and its top towards the bottom.The crystals converge and connect, forming a layer, and then sink to the bottom of the solution when their weight is higher than the surface tension force (Arthurton, 1973).As for the crystals that grow at the bottom of the basin, they are the result of the subsidence of the salt layers that formed during the day.There are five crystal forms as shown in Fig. 7, four types arise on the surface of the solution (A, B, C, D) and only one of them arises at the bottom of the basin E.
By studying the thin section of the manure samples, it was found that halite is a common salt, which was distinguished by its cubic crystals of various shapes (Fig. 8).

Conclusions
In this research, two continental Sabkhas in the Abu Ghraib region, in western Iraq, were studied.The Sabkhas were formed in low basins that suffer from drought and intense evaporation, as well as high subsurface water levels in a semi-arid-arid climate.Halite is the predominant mineral, followed by anhydrite, and gypsum is an evaporite.The Sabkha soils consist of feldspar, calcite, quartz, dolomite, and clay minerals (kaolinite, illite, and chlorite).The salts crystallized under normal thermal conditions in various patterns.Needle form was formed as the salt crystals grew in one direction, making longitudinal needle shapes.Another type is the Hopper shape a crystalline shape formed from inverted pyramidal crystals.A dendritic shape was also noticed in the Sabkhas forming a salt crust 4 to 12 cm thick.Polygonal halite crystals grow after the Sabkha's surface dries out, they are continued to grow from the brine solutions below, generating a network of polygonal halite crystals.

Fig. 1 .
Fig. 1.Location map of the study area; A) is the Shiaa village; B) is the Kadim Alaa'ab village.

Fig. 2 .
Fig. 2. Sampling sites in the studied areas; the blue circles are the location samples in the Shiaa village (A); the red circles are the location of the samples in the Kadim Alaa'ab village (B) in Abu Ghraib, western Baghdad.

Fig. 7 .
Fig. 7.The growth phases and shapes of salt crystals.A) Horizontally suspended rectangular crystal sheets growing on the surface of the solution at a temperature of 5-25°C; B) Cubic crystals with angles pointing downward, hanging from one of its corners and then dropping to the bottom when their weight increases; C) Four pyramidal faces; D) Six hollow and inverted pyramidal faces.B, C, and D crystals are formed at a temperature of 15-25°C; E is the phase of salt crystals forming at the bottom of the basin(Arthurton, 1973).

Table 1 .
Coordinates and summarized methodology done on samples that have been collected from the village of Haj Shiaa and Kadim Alaa'ab.The x is the type of analysis performed on the samples S.

Table 2 .
Mineral components (%) of non-clay and clay minerals in the Sabkha