<unk>  Needle-like<unk>  crystals on stainless steel. Scanning electron microscope picture from University of Tartu. scanning probe microscope A 3D visualization of<unk> lf of a<unk>  unit cell, from x-ray crystallography<unk> , also called<unk>  <unk> , is a naturally occurring mineral form of calcium apatite with the formula Ca5(PO4)3(OH), but it is usually written Ca10(PO4)6(OH)2 to denote that the crystal unit cell comprises two entities.<unk>  is the hydroxyl endmember of the complex apatite group. The OH− ion can be replaced by fluoride, chloride or carbonate, producing fluorapatite or chlorapatite. It crystallizes in the hexagonal crystal system. Pure<unk>  powder is white. Naturally occurring apatites can, however, also<unk> ve brown, yellow, or green colorations, comparable to the discolorations of dental fluorosis. Up to 50% by volume and 70% by weight of human bone is a modified form of<unk> , known as bone mineral. Carbonated calcium-deficient<unk>  is the main mineral of which dental enamel and dentin are composed.<unk>  crystals are also found in the small calcifications, within the pineal gland and other structures, known as corpora arenacea or "brain sand".
Hydroxyapatite
Neurite
Demineralized bone matrix
TCP 
Phosphate
Cellulite
Phytic acid
Calculus 