**2.7 Short range ordered phases**

HRTEM enables observation of short range ordered phases. Ferrihydrite (Fe5 3+HO8 . 4H2O) and singerite (SiFe4O6(OH)4 . H2O) were observed using HRTEM. The size of the ferrihydrite is around 5 nm in samples from the Atlantis II Deep and it has a hexagonal outline. Ferrihydrite from the Dead Sea forms clusters. In both samples, the use of high resolution enables us to see that the phase is short range ordered (**Figure 16**).

Formation of Ferrihydrite is at fast oxidation and pH > 2. Si serves as an impurity in the phase. With time, ferrihydrite can recrystallize into more stable iron oxides like hematite, akaganéite or goethite (**Figure 17**).

In the upper layer of sediments of the Atlantis II Deep in the Red Sea, a new short range ordered phase was observed using HRTEM. The new phase has disc morphology with well-crystallized margins and short range ordered inner part and it was named singerite (SiFe4O6(OH4\*H2O) (**Figure 18**) [14]. Singerite was formed by mixing of the highly saline hydrothermal brine that discharges into the Deep and Red Sea

#### **Figure 16.**

*(a) HRTEM image of short range ordered ferryhydrite from the area of the Dead Sea, and (b) HRTEM image of ferrihydrite from Atlantis II deep with electron diffraction and fast Fourier transformation obtained by digital micrograph (Gatan) software.*

**Figure 17.**

*HRTEM images of samples from the Dead Sea area formed by recrystallization of ferrihydrite: (a) wellcrystallized hematite, (b) well-crystallized akaganéite, and (c) well-crystallized goethite.*

*Minerals Observed by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy… DOI: http://dx.doi.org/10.5772/intechopen.102477*

#### **Figure 18.**

*(a) HRTEM image of a cluster of rounded plates from the Atlantis II deep, Red Sea, (b) high resolution image of singerite with well crystallized outer part and short range ordered inner part, and (c) HRTEM image of recrystallization of singerite into clay mineral, probably nontronite.*

#### **Figure 19.**

*HRTEM image of a cluster of singerite. A small tilt of the sample enables us to see that singerite is made of rounded plates.*

#### **Figure 20.**

*(a) HRTEM image of a cluster of mineraloid of Si,Fe Mn oxihydroxide, (b) HRTEM image showing short range ordered phase, and (c) Electron diffraction shows values of 0.255, 0.22 and 0.149.*

deep water. With time, singerite recrystallizes into clay minerals, usually nontronite (iron-rich smectite). Hence singerite was found only in the upper layer of sediments in the Deeps. A similar rounded phase was synthesized under saline hydrothermal conditions [9].

A small tilt of the singerite sample enables us to see that singerite is a round plate (**Figure 19**).

A new short range ordered phase from Red Sea deeps: Mn-singerite? Si(Fe,Mn)4O6(OH)4 . H2O electron diffraction yielded 0.255, 0.22 and 0.149 (**Figure 20**).

## **3. Discussion**

Observation of minerals under electron microscopy enables observation of wellcrystallized phases, study of their chemical composition and finding of impurities in the crystals by using point analyses. Under HRTEM nano-sized short range ordered phase like ferrihydrite and singerite can be observed. A new phase Mn-singerite was also observed under HRTEM. Observation of twinning that results from the conditions in which the crystals were formed, such as salinity, pH, temperature, contributes to understanding the conditions in which formation of crystals occurred. Goethite for example appears as mono-domain, multi-domain crystals or twinning creating star shape morphology. Identification of the crystallography of the minerals observed was used by electron diffraction in transmission electron microscopy. Well-crystallized minerals with euhedral morphology indicate that they were formed in situ like samples covered by halite in the Dead Sea area. It is also possible to see the initially formed dolomite minerals and later another phase, goethite, filling the open space or attached to the initially formed phases. Goethite is also crystallized on groutite since both are isostructural.

Rounded morphology is formed due to pounding as the minerals moved from their initial location where they had crystallized to the new site. Quartz grains were observed along with Ti-rich minerals with rounded morphology as well. Other rounded quartz grains were observed in red sandy soils. Using electron microscopy enables us to see Nano-size phases that form a cluster of different minerals like clusters of dust. Clays are the main phases and iron or titanium oxides are captured between the clay layers or are adsorbed on their surfaces.

### **4. Conclusion**

In this chapter Fe-oxides, Mn-oxides, Ti-oxides, quartz, dolomite, clays and K feldspar were studied using various electron microscopies (SEM, TEM and HRTEM). Using these methods helped to identify the crystallography, morphology and chemical composition of the minerals. Nano-sized short range ordered phases like ferrihydrite and singerite were also observed and identified.

### **Acknowledgements**

Thanks to the Hebrew University of Israel for funding part of the research. Thanks to Vladimir Ezersky from Ilse katz institute in Ben Gurion University in Israel for using TEM and HRTEM.

The research of the Dead Sea was supported by the Open University grant no. 100975.

*Minerals Observed by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy… DOI: http://dx.doi.org/10.5772/intechopen.102477*
