Atomic Positional Parameters
Ca1 4f 0.3333 0.6667 0.0012
Ca2 6h 0.2415 0.0071 0.2500
P1  6h 0.3982 0.3689 0.2500
O1  6h 0.3268 0.4850 0.2500
O2  6h 0.5881 0.4668 0.2500

O3 12i 0.3415 0.2569 0.0704
F1  2a 0.0000 0.0000 0.2500

Reference
K Sudarsanan, PE Mackie and RA Young (1972) Materials Research Bulletin, 7, 1331

Mineral Chemistry
The structure of apatite can be visualised as a series of phosphate 'tubes' internally lined by calcium ions. This first calcium site is half-surrounded by five oxygens, and half-exposed to the central space into which the fluoride ion is inserted. A second calcium site - surrounded by six oxygens - provides the link to join these 'tubes' together. In chlorapatite, F^- is replaced by Cl^-. Hydroxylapatite, an important component of bone and teeth, has F^- replaced by OH^-.

Apatite is an accessory mineral in sedimentary, metamorphic and igneous rocks. It may be found in pegmatites, crystallising from supercritical fluids, and in other hydrothermal veins. Regionally metamorphosed rocks, such as crystalline limestones, commonly contain apatite. Phosphate is a component of DNA, and of the energy providing molecules ADP and ATP - hence the mobilisation of phosphate mineralised in soils is fundamental to life.

The apatite structure is shared by a large group of relatively common minerals, which may be generally defined by the formula A₅(TO₄)₃Z. A may be Ca, Sr, Pb or Ba, T may be P, As or V, and Z may be OH, F, or Cl.

The mineral pyromorphite, Pb₅(PO₄)₃Cl, which has the apatite structure, forms from oxidative weathering of lead-bearing mineral deposits. A recent study [1] has identified fine-grained calcium-rich pyromorphite as the major lead-bearing phase in mine waste soils from the South Pennine Orefield in England.

[1] JD Cotter-Howells, PE Champness, JM Charnock and RAD Pattrick (1994) European Journal of Soil Science, 45, 393-402

The CHIME figure shows calciums surrounding fluorine (green) as dark grey spheres. Calciums coordinated by six oxygens are shown as light grey spheres. Phosphorus atoms are orange, and oxygens are red. The unit cell is outlined.