**2. Methods**

*Mineralogy - Significance and Applications*

Sea and in the deeps of the Red Sea (Atlantis II, Chain A, Chain B, Discovery, and Thetis) [7–10] (**Figure 1**). Precipitation of these phases occurs at hypersaline environment with elevated temperatures and varying pH. Morphology of the crystals and their size is observed by high-resolution transmission electron microscopy (HRTEM). Using electron diffraction enables identifying the crystallographic structure. Goethite and lepidocrocite have orthorhombic structure, akaganéite has monoclinic structure,

*Pictures obtained by electron microscopy: Magnetite from the Thetis deep (scanning electron microscope (SEM)); lepidocrocite from the Thetis deep; lepidocrocite from the Dead Sea; mono- and multi-domain goethite from Atlantis II deep, Red Sea; mono-domain goethite from chain a deep, Red Sea; and cluster of* 

*akaganéite from the Dead Sea (high-resolution transmission electron microscope).*

Short-range ordered ferrihydrite recrystallizes into well-crystallized phase, yet its initial morphology of plates at the size of <10 nm is preserved. Singerite (100 nm) disintegrates into iron-rich clay mineral [12]. Magnetite has a cubic morphology of octahedron or cubes 2–4 μm as observed in the Thetis Deep of the

Goethite has acicular form and changes at elevated pH from mono-domain to multi-domain rods. Crystal size at the Dead Sea area and the Red Sea varied from

Lepidocrocite usually precipitates at fast oxidation rate at the presence of chloride, and its morphology changes from plates formed at lower pH to multi-domain rods that are formed at elevated pH. Lepidocrocite crystals size varied from 100 nm

Rods and multi-domain akaganéite were observed at the Dead Sea with crystal

Feroxyhyte plates are formed at very high oxidation rate and had a plate mor-

In earlier study, synthesis was performed imitating the upper convecting layer of the Atlantis II Deep with lower temperatures and salinity [13]. In this research, iron oxides were synthesized at higher pH, elevated temperatures, and hypersaline

and feroxyhyte has hexagonal structure [11].

in the Dead Sea to 300 nm in the Red Sea deeps.

phology with crystals size that reached 300 nm.

size that varied from 100 nm to 1 μm.

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brines.

Red Sea.

**Figure 1.**

few 100 nm to 3 μm.

Iron oxides were synthesized under changing conditions of salinity, temperature, pH, and oxidation rates.

NaCl salt (Loba Chemie) was used to prepare NaCl solutions (2, 4, and 5M) that were used as a matrix and were kept in water bath at 40, 60, 70, and 80°C. Prior to synthesis, N2 was bubbled through the solutions for 20 minutes to remove dissolved oxygen. pH buffering of the solutions was accomplished by adding either NaOH (Daejung) or NaHCO3 (Carlo Erba) in small amounts, hence slightly changing Na concentration. pH of the solutions were 5.5, 7, 8.2, and 10.5. Cl concentration also changed due to dissolution of FeCl2·4H2O (Sigma-Aldrich) that was chosen for the Fe2+ solutions to yield a concentration of 0.06M.

Fe oxidation was carried out by introducing air at flow rates of 25, 40, 110, and 200 ml/min which was monitored with a flow meter and was kept stable during the hours of synthesis.

In order to isolate Si effects on the crystallization of iron oxides, Na2SiO3 (Sigma) was added to some of the solutions. To avoid any side effects, all of the samples were synthesized using the same polyethylene ware and under constant stirring speed. The precipitates were slightly washed and freeze-dried immediately after their synthesis.

Analyses of the precipitates were performed using X-ray diffraction (XRD). The fitting of the peak profiles was performed by using Pseudo-Voigt function with the APD computer program developed by Philips Export B.V. A least square process using "Celsiz" software did the unit cell refinement.

Sample morphology was observed through transmission electron microscopy (TEM) and high-resolution scanning electron microscopy (HRSEM). Transmission electron microscopy was carried out on a JEOL FasTEM 2010 electron microscope equipped with the Noran energy dispersive spectrometer (EDS) for microprobe elemental analyses.

All chemical analyses were obtained by point analyses with beam width of 25 nm and are presented as atomic ratios. A NORAN Standardless Metallurgical Thin Films program based on the *Cliff-Lorimer* ratio technique with an accuracy of about 5% was used for calculations. CuKα line was used for spectrometer calibration. Crystalline phases were identified using selected area electron diffraction (SAED) in the TEM.

## **3. Results and discussion**

Initial identification of iron oxides was obtained by using X-ray diffractions (**Figure 2**). X-ray diffraction of all samples synthesized yielded the following results: At elevated temperatures, alkaline media and concentrated brine magnetite was the main phase that precipitated. At neutral to slightly acidic conditions, lepidocrocite, akaganéite, and goethite co-precipitated. Additional NaHCO3 caused precipitation of siderite at 60°C.

In this study, pictures of iron oxides obtained by high-resolution scanning electron microscope and high-resolution transmission electron microscope are presented.

### **3.1 Magnetite FeO-Fe2O3**

Unit cell parameters of magnetite vary from 0.8373 to 0.8396 nm with upper limit of 0.8396 nm, identical to that of well-crystallized magnetite [13]. Crystallite size changes from 18 to 45 nm. The smallest crystallites were obtained at pH 7.5 in 4 or 5M NaCl at all temperatures, whereas the largest crystallites were obtained at under elevated pH (9.5) and temperatures. The lowermost unit cell parameter was obtained for samples that crystallized at 60°C pH 8.5 and in 4M NaCl matrix. The largest unit

#### **Figure 2.**

*X-ray diffraction patterns of (a) synthesized magnetite at 70°C, 5M NaCl, and pH 7.5; (b) goethite and lepidocrocite in sample from the Atlantis II deep, Red Sea.*

#### **Figure 3.**

*Lattice parameter (nm) and crystalline sizes of magnetite precipitated in brines with concentrations of 4 and 5M NaCl.*

**95**

**Figure 4.**

*Iron Oxides Synthesized in Hypersaline Solutions DOI: http://dx.doi.org/10.5772/intechopen.88948*

cell parameter was obtained for samples that crystallized under elevated pH 9.5, 5M NaCl and at 80°C. Overall, crystallite size and unit cell parameters were mainly affected by pH values, rather than by salinity or temperature changes (**Figure 3**). Morphology of the crystals formed varied between unhedral to unhedral-octahedral, cubes, and hexagonal plates. The morphology of the crystals mainly results

*Pictures obtained by HRSEM presenting the morphology of magnetite crystals precipitated at salinity of 4 and* 

Magnetite Fe3O4: FeO-Fe2O3 crystallizes in cubic system with a = 0.8396 nm. Crystallization from Fe2+ solutions usually involves crystallization of hexagonal flakes of Fe(OH)2, which transforms to magnetite in moderately alkaline solutions (pH > 8). Under slightly acid to slightly alkaline conditions, green rust phases are formed, and upon further oxidation they are transformed into goethite and/or lepidocrocite [11]. Formation of magnetite in the hypersaline brines results from slow oxidation, elevated temperatures, and higher pH. The morphology of the magnetite

from the pH of the brines in **Figures 4**–**8**.

*5M NaCl, pH 6.5, and temperature of 60–80°C.*

*Iron Oxides Synthesized in Hypersaline Solutions DOI: http://dx.doi.org/10.5772/intechopen.88948*

*Mineralogy - Significance and Applications*

**94**

**Figure 3.**

**Figure 2.**

*5M NaCl.*

*Lattice parameter (nm) and crystalline sizes of magnetite precipitated in brines with concentrations of 4 and* 

*X-ray diffraction patterns of (a) synthesized magnetite at 70°C, 5M NaCl, and pH 7.5; (b) goethite and* 

*lepidocrocite in sample from the Atlantis II deep, Red Sea.*

**Figure 4.**

*Pictures obtained by HRSEM presenting the morphology of magnetite crystals precipitated at salinity of 4 and 5M NaCl, pH 6.5, and temperature of 60–80°C.*

cell parameter was obtained for samples that crystallized under elevated pH 9.5, 5M NaCl and at 80°C. Overall, crystallite size and unit cell parameters were mainly affected by pH values, rather than by salinity or temperature changes (**Figure 3**).

Morphology of the crystals formed varied between unhedral to unhedral-octahedral, cubes, and hexagonal plates. The morphology of the crystals mainly results from the pH of the brines in **Figures 4**–**8**.

Magnetite Fe3O4: FeO-Fe2O3 crystallizes in cubic system with a = 0.8396 nm.

Crystallization from Fe2+ solutions usually involves crystallization of hexagonal flakes of Fe(OH)2, which transforms to magnetite in moderately alkaline solutions (pH > 8). Under slightly acid to slightly alkaline conditions, green rust phases are formed, and upon further oxidation they are transformed into goethite and/or lepidocrocite [11]. Formation of magnetite in the hypersaline brines results from slow oxidation, elevated temperatures, and higher pH. The morphology of the magnetite

#### **Figure 5.**

*Pictures obtained by HRSEM presenting the morphology of magnetite crystals precipitated at salinity of 4 and 5M NaCl, pH 7.5, and temperature of 60–80°C.*

crystals was mainly affected by pH. In strongly alkaline brine, hexagonal flakes of Fe(OH)2 were initially formed recrystallized into magnetite, preserving the initial morphology.

The morphology of magnetite of the Thetis Deep indicates that precipitation occurred at lower pH and lower salinity, suggesting that no brine pool filled the Thetis Deep [6].

## **3.2 Goethite (α-FeOOH)**

Goethite precipitated along with other iron oxides. The morphologies of the crystals formed varied between mono-domain and multi-domain crystals. Twinning and star-shaped multi-domain crystals were formed at elevated temperatures. In higher salinity of 5M NaCl, goethite precipitated at pH 8.2 and 40°C, and at pH 7 and

**97**

**Figure 6.**

*Iron Oxides Synthesized in Hypersaline Solutions DOI: http://dx.doi.org/10.5772/intechopen.88948*

40°C, it co-precipitated with lepidocrocite and akaganéite. Adding NaHCO3 to the solution caused co-precipitation of goethite with lepidocrocite at 40°C in 2M NaCl, co-precipitation of goethite with magnetite at 60°C and at 5M NaCl solutions, at pH

*Pictures obtained by HRSEM presenting the morphology of magnetite crystals precipitated at salinity of 4 and* 

Lepidocrocite precipitated at lower temperatures than magnetite. By using NaCl solutions of 5M and temperature of 40°C, the morphology of the crystals formed was affected by the pH. Plates crystallized at pH 5.5, rods at pH 7, and multi-domain crystals at pH 8.2 (**Figure 10**). The difference between the morphologies results from faster crystallization of 010 along c direction in crystals formed at higher pH leading to multi-domain crystals at pH 8.2. Similar results were also

8.2, only magnetite precipitated magnetite [13] (**Figure 9**).

**3.3 Lepidocrocite (γ-FeOOH)**

*5M NaCl, pH 8.5, and temperature of 60–80°C.*

obtained at 25°C [13].

*Iron Oxides Synthesized in Hypersaline Solutions DOI: http://dx.doi.org/10.5772/intechopen.88948*

*Mineralogy - Significance and Applications*

crystals was mainly affected by pH. In strongly alkaline brine, hexagonal flakes of Fe(OH)2 were initially formed recrystallized into magnetite, preserving the initial

*Pictures obtained by HRSEM presenting the morphology of magnetite crystals precipitated at salinity of 4 and* 

The morphology of magnetite of the Thetis Deep indicates that precipitation occurred at lower pH and lower salinity, suggesting that no brine pool filled the

Goethite precipitated along with other iron oxides. The morphologies of the crystals formed varied between mono-domain and multi-domain crystals. Twinning and star-shaped multi-domain crystals were formed at elevated temperatures. In higher salinity of 5M NaCl, goethite precipitated at pH 8.2 and 40°C, and at pH 7 and

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morphology.

**Figure 5.**

Thetis Deep [6].

**3.2 Goethite (α-FeOOH)**

*5M NaCl, pH 7.5, and temperature of 60–80°C.*

#### **Figure 6.**

*Pictures obtained by HRSEM presenting the morphology of magnetite crystals precipitated at salinity of 4 and 5M NaCl, pH 8.5, and temperature of 60–80°C.*

40°C, it co-precipitated with lepidocrocite and akaganéite. Adding NaHCO3 to the solution caused co-precipitation of goethite with lepidocrocite at 40°C in 2M NaCl, co-precipitation of goethite with magnetite at 60°C and at 5M NaCl solutions, at pH 8.2, only magnetite precipitated magnetite [13] (**Figure 9**).
