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Hydroxyapatite (also called hydroxylapatite) is the primary mineral component of bone. It has the chemical formula of 3Ca3(PO4)2*Ca(OH)2 and forms a needle-shaped crystal which is arranged with other needles in a rosette. It melts above 1100°C and cannot be dissolved in water.

A form of the mineral is used by molecular biologists in certain column chromatography lab techniques to separate single-stranded nucleic acids from double-stranded ones (the mineral tends to hang on to the double-stranded molecules while letting the single-stranded ones pass through the column unimpeded), or to separate different proteins.

"Hydroxyapatite" is also used to refer to similar minerals which have Ba (barium) or Sr (strontium) in the place of the Ca (calcium) atoms. These minerals are found in deposits of phosphorite and in biological tissue.

The information in this writeup was taken from the science dictionary at http://biotech.icmb.utexas.edu/; I oversaw the development of the dictionary (the website was mothballed in 1998) and I believe I wrote the entry this is based on.