Quartz Crystal Structure

Paragraph 1

Compare and contrast the crystal structures and crystal chemistry of quartz, α-FePO4 and β-

FePO4.

Crystal structure is defined as the orderly arrangement of atoms, ions or molecules in both liquid and solid states. Quartz has a trigonal crystal system and a six-sided prism with six-sided pyramids at each end. Quartz possesses a macromolecular structure and does not contain isolated ions. The crystal structure of quartz consists of the SiO4 silicon oxygen tetrahedra, where each oxygen is being shared between two tetrahedraI, thus yielding the net chemical formula: SiO2. The basic structure unit is SiO4, where a central silicon atom is surrounded by four oxygen atoms. Quartz is hard and brittle due to the tight and inflexible lattice

SiO2 is often found in quartz and it has a series of polymorphs other than the amorphous forms. The Si-O-Si angle can vary between 140° in α- tridymite to 180° in β-tridymite.

Crystal Structure of Quartz Crystal structure of FePO4

Paragraph 2

Illustrate and describe the symmetrical differences between α-FePO4 and β- FePO4. α-Fe in α-FePO4 has this body-centered cubic B.C.C crystal structure and it is this crystalline structure which allows steel and cast iron to possess their magnetic properties. α-Fe in α-FePO4 has a strength of 280 N/mm and also a hardness of approximately 80 Brinell. In pure iron, α-Fe is relatively stable when the temperature is below 1,670 °F. A very minimal amount of carbon can be dissolved in α- Fe and the maximum solubility is about 0.02 wt% at 1,333 °F and 0.005% carbon at

32 °F. The carbon is able to dissolve in the iron interstitially, with the carbon atoms being about twice the diameter of the interstitial "holes" and each carbon atom is surrounded by a strong local strain field. Thus, the resulting enthalpy of mixing is positive, which is unfavourable, but the contribution of entropy to the free energy of solution stabilises the structure for low carbon

When the temperature rises, there is often a change in the structures of both α-FePO4 and β- FePO4. However, there is a more significant change in α-FePO4 than in β- FePO4. The bending and tilting will be observed more greatly between the Fe, O and P bonds in α-FePO4.

On the other hand, β-Fe in β-FePO4 is the paramagnetic form of α- Fe. When α-Fe is heated above the critical temperature of 1420 °F, it causes the random thermal agitation of the atoms to exceed beyond the oriented magnetic moment of the unpaired electron spins happening in the 3d shell. It forms the low-temperature boundary of the beta iron field. β-Fe is crystallographically identical to α-

Fe, with the exception of the magnetic domains and the expanded body-centered cubic lattice parameter which exists as a function of temperature, and is therefore not critical in steel heat treating. The beta phase is not regarded as a distinct phase and is only the high-temperature end of the alpha phase