**2.1. Recrystallization of ferrihydrite into akaganéite in a hyper-saline environment**

Akaganeite (β-FeOOH) usually precipitates directly at acidic conditions with the presence of Cl (Cornell & Schwertmann, 2003). In the Dead Sea area close to Mount Sedom salt diaper, a hyper-saline brine discharges in a small spring. As the dissolved Fe2+ ions of the spring are exposed to air, ferrihydrite precipitates as an initial iron oxide (Figure 2) and the crystallites are preserved within halite crystals. Dissolution of the halite crystals and its reprecipitation at acidic conditions and elevated Cl concentration enables a recrystallization process into akaganéite crystallites, and they are preserved again within the newly formed halite crys‐ tals. Tiny crystallites of akaganéite preserve their initial ferrihydrite precursor's morphology and exhibit a well-crystallized pattern observed through a high resolution transmission elec‐ tron microscope (Figure 3).

**2.2. Recrystallization of ferrihydrite into hematite and goethite in sand dunes and soils**

quartz grains.

hydrite crystallites.

clay minerals (Figure 5).

Quartz grains are blown inland along the eastern Mediterranean coastline and form sand dunes. A Mediterranean climate, namely a long hot dry season and a mild winter with a November–March annual precipitation of 400-600 mm (Saaroni et al., 2010) enables the piling up of additional clay minerals that are not washed away but adhere to the quartz grains and then serve as a surface for additional precipitation and recrystallization of iron oxides. Precipitation of these iron oxides causes reddening (rubification) of the

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Ferrihydrite is the initial phase formed and it serves as the precursor for hematite (α-Fe2O3) or goethite (α-FeOOH) (Figure 4). Recrystallization of ferrihydrite into other iron oxides re‐ quires aerobic conditions in a warm climate with a wet and dry season that enables forma‐ tion of more stable iron oxides. Hematite is formed during the dry season due to aggregation process that includes short range crystallization within a ferrihydrite aggregate (Cornell & Schwertmann, 2003). Dissolution and reprecipitation processes are usually in‐ volved in goethite formation. Yet the presence of clay minerals prevented complete drying of the iron oxides, and precipitation of goethite preserved the initial morphology of the ferri‐

Small grains of ilmenite (FeTiO3) are transported along with quartz grains to the eastern coast of the Mediterranean Sea. As they are exposed to the Mediterranean climate, they recrystallize into tiny grains of hematite with Ti impurities resulting from their initial precursors. The recrystallization process is feasible since both minerals ilmenite and hem‐ atite share the same crystallographic system and even the same space group. Sometimes ilmenite and hematite even form a solid solution. By using electron microscopy, it is pos‐ sible to observe the initial ilmenite grain along with the secondary hematite crystals that result from the recrystallization process. The precursor and the recrystallized tiny hema‐ tite crystals remain close to each other since they are all kept within clay minerals. SA‐ ED detects ilmenite crystal, and point analyses obtained from the large crystal and the surrounding tiny crystals show that the large ilmenite grain lost most of its iron to the hematite crystallites; however, some of the Ti precipitated within the hematite crystals as an impurity. Other elements shown in the point analyses resulted from the surrounding

Pyrite (Fe2S) crystals precipitated within Cretaceous marl layers of the Judean hills probably under anaerobic conditions. Oxidation of these crystals yielded pseudomorphic recrystalli‐ zation into tiny crystals of hematite preserving the initial pyrite large cubic crystal. Using a higher resolution image enabled the observation of small hexagonal plates of hematite crys‐

tallites (Figure 6); hence, pyrite served as a precursor for hematite crystals.

**2.3. Recrystallization into hematite from other iron bearing minerals**

**Figure 3.** A high resolution image of akaganéite crystallites that present a recrystallization product of ferrihydrite in a hyper-saline environment in the Dead Sea area.

## **2.2. Recrystallization of ferrihydrite into hematite and goethite in sand dunes and soils**

**2.1. Recrystallization of ferrihydrite into akaganéite in a hyper-saline environment**

tron microscope (Figure 3).

166 Recent Developments in the Study of Recrystallization

hyper-saline environment in the Dead Sea area.

Akaganeite (β-FeOOH) usually precipitates directly at acidic conditions with the presence of Cl (Cornell & Schwertmann, 2003). In the Dead Sea area close to Mount Sedom salt diaper, a hyper-saline brine discharges in a small spring. As the dissolved Fe2+ ions of the spring are exposed to air, ferrihydrite precipitates as an initial iron oxide (Figure 2) and the crystallites are preserved within halite crystals. Dissolution of the halite crystals and its reprecipitation at acidic conditions and elevated Cl concentration enables a recrystallization process into akaganéite crystallites, and they are preserved again within the newly formed halite crys‐ tals. Tiny crystallites of akaganéite preserve their initial ferrihydrite precursor's morphology and exhibit a well-crystallized pattern observed through a high resolution transmission elec‐

**Figure 3.** A high resolution image of akaganéite crystallites that present a recrystallization product of ferrihydrite in a

Quartz grains are blown inland along the eastern Mediterranean coastline and form sand dunes. A Mediterranean climate, namely a long hot dry season and a mild winter with a November–March annual precipitation of 400-600 mm (Saaroni et al., 2010) enables the piling up of additional clay minerals that are not washed away but adhere to the quartz grains and then serve as a surface for additional precipitation and recrystallization of iron oxides. Precipitation of these iron oxides causes reddening (rubification) of the quartz grains.

Ferrihydrite is the initial phase formed and it serves as the precursor for hematite (α-Fe2O3) or goethite (α-FeOOH) (Figure 4). Recrystallization of ferrihydrite into other iron oxides re‐ quires aerobic conditions in a warm climate with a wet and dry season that enables forma‐ tion of more stable iron oxides. Hematite is formed during the dry season due to aggregation process that includes short range crystallization within a ferrihydrite aggregate (Cornell & Schwertmann, 2003). Dissolution and reprecipitation processes are usually in‐ volved in goethite formation. Yet the presence of clay minerals prevented complete drying of the iron oxides, and precipitation of goethite preserved the initial morphology of the ferri‐ hydrite crystallites.

#### **2.3. Recrystallization into hematite from other iron bearing minerals**

Small grains of ilmenite (FeTiO3) are transported along with quartz grains to the eastern coast of the Mediterranean Sea. As they are exposed to the Mediterranean climate, they recrystallize into tiny grains of hematite with Ti impurities resulting from their initial precursors. The recrystallization process is feasible since both minerals ilmenite and hem‐ atite share the same crystallographic system and even the same space group. Sometimes ilmenite and hematite even form a solid solution. By using electron microscopy, it is pos‐ sible to observe the initial ilmenite grain along with the secondary hematite crystals that result from the recrystallization process. The precursor and the recrystallized tiny hema‐ tite crystals remain close to each other since they are all kept within clay minerals. SA‐ ED detects ilmenite crystal, and point analyses obtained from the large crystal and the surrounding tiny crystals show that the large ilmenite grain lost most of its iron to the hematite crystallites; however, some of the Ti precipitated within the hematite crystals as an impurity. Other elements shown in the point analyses resulted from the surrounding clay minerals (Figure 5).

Pyrite (Fe2S) crystals precipitated within Cretaceous marl layers of the Judean hills probably under anaerobic conditions. Oxidation of these crystals yielded pseudomorphic recrystalli‐ zation into tiny crystals of hematite preserving the initial pyrite large cubic crystal. Using a higher resolution image enabled the observation of small hexagonal plates of hematite crys‐ tallites (Figure 6); hence, pyrite served as a precursor for hematite crystals.

**2.4. Recrystallization processes in a hydrothermal hyper-saline environment in the**

surrounding hematite crystallites; d) Point analysis of hematite crystallites showing a Ti impurity.

Iron oxides and short range ordered Si-Fe phases precipitate within the hydrothermal brine of the Atlantis II Deep, in the Red Sea (Taitel-Goldman, 2009). Multi-domain goethite is usu‐

**Figure 5.** a) An ilmenite grain surrounded by tiny hematite crystals kept within clay minerals; b) SAED of ilmenite crys‐ tal; c) Point analysis of ilmenite crystal that went through some dissolution causing Fe removal from the grain into

crystals found in the Atlantis II Deep exhibit multi-domainic character. Appearance of mul‐ ti-domainic hematite crystals in the Atlantis II Deep was quite rare, yet, some were found in the sediments, probably resulting from the recrystallization process of a multi-domainic

Rounded particles of short range ordered Si-Fe phase (suggested name: singerite) (SiFe4O6(OH)4H2O) were identified for the first time in the sediments of the Atlantis II Deep (Taitel-Goldman et al., 1999 ; Taitel-Goldman & Singer, 2002). This short range ordered phase is usually metastable and transforms into a more stable phase of iron rich clay mineral

Magnetite (Fe2+OFe3+2O3) mainly crystallizes in magmatic rocks or precipitates in a mixed Fe2+/Fe3+ solution in aqueous alkaline systems. Formation of magnetite involves an initial stage of either green rust (Fe3+xFe2+y(OH)3x+2y-zAz; A=Cl, 1/2SO4) or hexagonal flakes of Fe(OH)2 that gradually oxidizes into green rust and then recrystallizes into magnetite crys‐ tals (Cornell & Schwertmann, 2003). At elevated temperatures and salinities, less oxygen is available for the oxidation of the dissolved Fe2+, leading to magnetite (mixed Fe2+ and Fe3+ phases) precipitation. Synthesis was performed in NaCl matrix solutions (4M and 5M) that were kept in a water bath at 70ºC and 80ºC. N2 was bubbled through the solutions for 20 minutes to remove dissolved oxygen. FeCl2 4H2O salt was chosen for the Fe2+ solutions to

concentration (Cornell & Giovanoli, 1986). Most of the goethite

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**Atlantis II Deep, Red Sea**

ally formed at elevated Na+

goethite precursor (Figure 7).

like nontronite (Figure 8).

**2.5. Recrystallization in vitro of green rust into magnetite**

**Figure 4.** High resolution images of a) A short range ordered ferrihydrite; b) goethite crystals preserving ferrihydrite morphology; c) Hematite tiny crystals preserving the initial morphology of ferrihydrite.

**Figure 5.** a) An ilmenite grain surrounded by tiny hematite crystals kept within clay minerals; b) SAED of ilmenite crys‐ tal; c) Point analysis of ilmenite crystal that went through some dissolution causing Fe removal from the grain into surrounding hematite crystallites; d) Point analysis of hematite crystallites showing a Ti impurity.

#### **2.4. Recrystallization processes in a hydrothermal hyper-saline environment in the Atlantis II Deep, Red Sea**

Iron oxides and short range ordered Si-Fe phases precipitate within the hydrothermal brine of the Atlantis II Deep, in the Red Sea (Taitel-Goldman, 2009). Multi-domain goethite is usu‐ ally formed at elevated Na+ concentration (Cornell & Giovanoli, 1986). Most of the goethite crystals found in the Atlantis II Deep exhibit multi-domainic character. Appearance of mul‐ ti-domainic hematite crystals in the Atlantis II Deep was quite rare, yet, some were found in the sediments, probably resulting from the recrystallization process of a multi-domainic goethite precursor (Figure 7).

Rounded particles of short range ordered Si-Fe phase (suggested name: singerite) (SiFe4O6(OH)4H2O) were identified for the first time in the sediments of the Atlantis II Deep (Taitel-Goldman et al., 1999 ; Taitel-Goldman & Singer, 2002). This short range ordered phase is usually metastable and transforms into a more stable phase of iron rich clay mineral like nontronite (Figure 8).

### **2.5. Recrystallization in vitro of green rust into magnetite**

**Figure 4.** High resolution images of a) A short range ordered ferrihydrite; b) goethite crystals preserving ferrihydrite

morphology; c) Hematite tiny crystals preserving the initial morphology of ferrihydrite.

168 Recent Developments in the Study of Recrystallization

Magnetite (Fe2+OFe3+2O3) mainly crystallizes in magmatic rocks or precipitates in a mixed Fe2+/Fe3+ solution in aqueous alkaline systems. Formation of magnetite involves an initial stage of either green rust (Fe3+xFe2+y(OH)3x+2y-zAz; A=Cl, 1/2SO4) or hexagonal flakes of Fe(OH)2 that gradually oxidizes into green rust and then recrystallizes into magnetite crys‐ tals (Cornell & Schwertmann, 2003). At elevated temperatures and salinities, less oxygen is available for the oxidation of the dissolved Fe2+, leading to magnetite (mixed Fe2+ and Fe3+ phases) precipitation. Synthesis was performed in NaCl matrix solutions (4M and 5M) that were kept in a water bath at 70ºC and 80ºC. N2 was bubbled through the solutions for 20 minutes to remove dissolved oxygen. FeCl2 4H2O salt was chosen for the Fe2+ solutions to

**Figure 6.** Scanning electron micrograph of tiny hexagonal plates of hematite crystals formed by recrystallization of large cubic pyrite crystals. a) The cubic morphology of pyrite was preserved. b) A close observation of tiny hematite crystallites.

yield a concentration of 0.06M. Fe oxidation was carried out by introducing air at flow rates of 25ml/l, which was monitored with a flow meter, and was kept stable during the 3h of syn‐ thesis. Buffering of the pH was obtained by adding a small amount of NaOH (1M). It ap‐ pears that the pH has a major effect on the kinetics of recrystallization; hence, in samples that were prepared in a highly alkaline solution, transformation into magnetite was quicker, leading to preservation of the precursor. Usually, the recrystallization process yields a cubic morphology of magnetite but due to very fast oxidation and the recrystallization process, the crystals formed preserve the hexagonal morphology of green rust that was observed with a high resolution scanning electron microscope (Figure 9).

**Figure 7.** Iron oxides observed in the hyper-saline sediments of the Atlantis II Deep in the Red Sea. a) Multi-domainic

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goethite; b) Multi-domainic hematite that was formed in a recrystallization process probably from goethite.

Recrystallization Processes Involving Iron Oxides in Natural Environments and *In Vitro* http://dx.doi.org/10.5772/53735 171

**Figure 7.** Iron oxides observed in the hyper-saline sediments of the Atlantis II Deep in the Red Sea. a) Multi-domainic goethite; b) Multi-domainic hematite that was formed in a recrystallization process probably from goethite.

yield a concentration of 0.06M. Fe oxidation was carried out by introducing air at flow rates of 25ml/l, which was monitored with a flow meter, and was kept stable during the 3h of syn‐ thesis. Buffering of the pH was obtained by adding a small amount of NaOH (1M). It ap‐ pears that the pH has a major effect on the kinetics of recrystallization; hence, in samples that were prepared in a highly alkaline solution, transformation into magnetite was quicker, leading to preservation of the precursor. Usually, the recrystallization process yields a cubic morphology of magnetite but due to very fast oxidation and the recrystallization process, the crystals formed preserve the hexagonal morphology of green rust that was observed

**Figure 6.** Scanning electron micrograph of tiny hexagonal plates of hematite crystals formed by recrystallization of large cubic pyrite crystals. a) The cubic morphology of pyrite was preserved. b) A close observation of tiny hematite crystallites.

with a high resolution scanning electron microscope (Figure 9).

170 Recent Developments in the Study of Recrystallization

**3. Conclusions**

**Author details**

**References**

118p.

Nurit Taitel-Goldman

phology or impurities from the precursor.

iron oxides or iron bearing minerals.

The Open University of Israel, Israel

nal of climatology, , 30, 1014-1025.

muth and J.K Torrance, editors), 697-705.

Iron oxides are not the only minerals formed by recrystallization processes. However, their abundance, small crystallite size, quick formation patterns that often involve preservation of their precursor enables their observation through various electron microscopes of the mor‐

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In this chapter it was shown that the initial morphology of ferrihydrite is preserved in re‐ crystallization into more stable phases like goethite, akaganéite and hematite. It was also shown that hematite can be formed through several recrystallization processes from other

In some cases, slow recrystallization process is captured within the sample leading to obser‐ vation both the decomposing precursor and the newly formed product. For example trans‐

[1] Cornell, R. M., & Giovanoli, R. (1986). Factors that govern the formation of multi-do‐

[2] Cornell, R. M., & Schwertmann, U. (2003). The iron oxides structure, properties reac‐ tions occurrences and uses. Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 664p. [3] Saaroni, H., Halfon, N., Ziv, B., Alpert, P., & Kutiel, H. (2010). Links between the rainfall regime in Israel and location and intensity of Cyprus lows. International jour‐

[4] Taitel-Goldman, N., Singer, A., & Stoffers, P. (1999). A new short range ordered, Fe-Si phase in the Atlantis II Deep, Red Sea hydrothermal sediments. in: Proceedings 11th International Clay conference, Ottawa, Canada, 1997 (H. Kodama, A.R. Mer‐

[5] Taitel-Goldman, N., & Singer, A. (2002). Metastable Si-Fe phases in hydrothermal

[6] Taitel-Goldman, N. (2009). Nano-sized iron-oxides and clays of the Red Sea hydro‐ thermal deeps: Characterization and formation processes. VDM Verlag Dr. Miller.

sediments of Atlantis II Deep, Red Sea. Clays Minerals, 37 , 235-248.

formation of singerite into nontronite clay mineral or ilmenite into hematite.

mainic goethite. Clays and Clay minerals, , 34, 557-564.

**Figure 8.** a) A cluster of rounded particles of a short range ordered Si-Fe phase (singerite); b) Disintegration and re‐ crystallization of singerite into iron-rich clay mineral (nontronite).

**Figure 9.** A high resolution scanning electron microscope image of synthesized magnetite in varying conditions. Plates appear in both images due to quick recrystallization from green rust or Fe(OH)2 plates. Samples were synthe‐ sized at a) 70°C pH 9.4 and a solution of 4M NaCl b) 60°C pH 10.2 and in a 5M NaCl solution.
