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The olivine structure, typified by the minerals forsterite, Mg₂SiO₄, and triphylite, LiFePO₄, is the hcp analogue of the spinel structure. One‐eighth of the tetrahedral sites are occupied by Si or P and half of the octahedral sites by Mg or Li, Fe within an hcp oxide array. There are two crystallographically distinct octahedral sites in olivine, which are occupied in ordered fashion by Li and Fe in LiFePO₄. The crystal structure is shown in Fig. 1.45 and some olivines are listed in Table 1.23. Olivines occur mainly with oxides but also with sulphides, selenides and some fluorides. Various cation charge combinations occur, such that in oxides the three cations have a net charge 8+.

Olivines (mainly forsterite and fayalite) are believed to be the main mineralogical constituent of the Earth's upper mantle. At high pressures, many olivines transform to the spinel structure and spinels are probably the main constituent of the Earth's lower mantle. Volume changes associated with the olivine to spinel phase transformation may have had fundamental geological consequences during the evolution of the Earth, involving the formation of mountain ranges and under‐sea ridges. When spinel material from the lower mantle was pushed upwards to the Earth's surface, it transformed to olivine due to the reduction in pressure. The reverse transformation, olivine to spinel, with a volume contraction may be a contributing factor to earthquakes.

Figure 1.45 Olivine structure of LiFePO4.

Modified from J. J. Biendicho and A. R. West, Solid State Ionics 203, 33 (2011).

Table 1.23 Some compounds with the olivine structure

General formula
Octahedral site	Tetrahedral site	hcp anion	Examples
II2	IV	O4	Mg2SiO4 (forsterite) Fe2SiO4 (fayalite) CaMgSiO4 (monticellite) γ‐Ca2SiO4 A2GeO4: A = Mg, Ca, Sr, Ba, Mn
III2	II	O4	Al2BeO4 (chrysoberyl) Cr2BeO4
II, III	III′	O4	MgAlBeO4
I, II	V	O4	LiFePO4 (triphylite) LiMnPO4 (lithiophylite)
I, III	IV	O4	LiRESiO4: RE = Ho, …, Lu NaREGeO4: RE = Sm, …, Lu LiREGeO4: RE = Dy, …, Lu
II2	IV	S4	Mn2SiS4 Mg2SnS4 Ca2GeS4
I2	II	F4	γ‐Na2BeF4

Unit cell: orthorhombic; for LiFePO₄, a = 10.33, b = 6.01, c = 4.70 Å; Z = 4.

There is much current interest in LiFePO₄ as the cathode material in rechargeable lithium batteries. On charging, Li deintercalates from the structure and a corresponding amount of Fe is oxidised from Fe²⁺ to Fe³⁺, as follows:

Lithium ions occupy channels parallel to the y axis, Fig. 1.45, which allows them to leave and enter the structure readily during cell charge and discharge. This is an example of a solid state redox reaction with a cell potential of about 3.08 V. LiFePO₄, and associated LiMnPO₄, are of interest because the redox reaction and the process of lithium removal and insertion are reversible over many cycles, giving a high cell capacity, and the materials are cheap, non‐toxic and environmentally friendly.