Other Mn-containing compounds have also been studied for their colour such as brownmillerite and apatite type structures, although these colours tend to range from turquoise to dark green depending on manganese content . The apatite material has the general formula A10(MO4)6X2 where typically A = Ca, Sr, Ba or La, M =Si, Ge, P or V and X = O, OH or halides . The apatite structure can be classed as an A4(MO4)6 framework which contains A cations (A1 site) in trigonal metaprisms that are corner-linked with the tetrahedral MO4 units. The remaining A cations (A2 site) and X anions are located within the cavities of this framework (Figure ). The apatite structure shows excellent flexibility for doping on the different sites, and such doping strategies can lead to a variety of properties, such as high ionic conductivity and luminescence . The use of different dopants can also change the colour of apatites. Previously, apatite pigments have been made by doping with transition metals, with the colours observed attributed to d-d transitions or to charge transfer bands. One such example is the doping of Mn 5+ which can be doped onto the tetrahedral site and is noted to produce an intense turquoise colour .