area of the femoral head. Traces of whitlockite have also been found in tuberculous lesions, urinary calculi and even prostatic deposits. Whitlockite can also be found in the oral cavity, where it is a primary component of dental calculi and salivary stones. Lastly, whitlockite can be found in aortic media, where it may be involved in arteriosclerosis. The presence of whitlockite at these places has not attracted much attention from biomedical scientists or clinicians, chiefly because whitlockite is not visible with the stains used to routinely examine microscopic sections of healthy or diseased tissue. However, the presence of whitlockite becomes obvious when X-ray diffraction is used to examine these sections. In part, whitlockite occurs commonly in biologic systems because of the high concentrations of proteolipids and divalent cations in biologic fluids. Formation of this type of whitlockite is magnesium rich, and is preferred at temperatures typical of biologic systems because of the smaller diameter of the magnesium ion compared to calcium. Physical properties Each of the phases of whitlockite described above exhibit approximately the same physical properties. They display no cleavage, fracture subconchoidal to uneven, and tend to be brittle. They are classified with a hardness of 5 and a density of 3.12 g/cm3. Color varieties are colorless, white, gray, yellowish or pinkish, and can be transparent to translucent. Whitlockites exhibit a vitreous to resinous luster. The typical habit of whitlockite is rhombohedral crystals, but whitlockite can also rarely be tabular. The crystal habit of whitlockite also ranges from coarse granular to earthy. See also List of minerals List of minerals named after people References Calcium minerals Magnesium minerals Phosphate minerals Trigonal minerals Minerals in space group 161 Minerals described in 1941