often represented as a type of apatite. However, it differs considerably from most other phosphate minerals, including apatite, in its chemical composition and the molar proportions of these components. The first serious studies of the mineral whitlockite were launched in 1952 on terrestrial specimens from the Palermo pegmatite quarry near North Groton, New Hampshire. These specimens were initially used to describe the composition and structure of the mineral. A decade later, the Apollo landing missions returned an impressive array of lunar rocks as well as other kinds of meteoric material. This unique resource led to an unprecedented barrage of geologic studies designed to characterize and define the composition and structure of minerals in these specimens. Throughout all studies on whitlockite, it has been found that the two most common phosphate minerals occurring in lunar rocks were apatite and whitlockite, and that they usually occur together. In the biologic literature, whitlockite and apatite are use interchangeably. Whitlockite is also associated so frequently with apatite in its biologic occurrences that it is frequently presumed to be apatite. Bobdownsite is a variety of whitlockite from Yukon, Canada, that was thought to contain fluorine bonded directly to phosphorus, giving it the chemical formula Ca9(Mg)(PO4)6(PO3F) However, subsequent investigation failed to find any monofluorophosphate in samples of bobdownsite, the mineral was discredited as a distinct species, and recommendations were made to tighten the criteria for identifying minerals as containing monofluorophosphate. Geological occurrences Whitlockite has two inorganic occurrences with geologic significance. The first, known as terrestrial whitlockite, is found as a secondary mineral in granite pegmatites in such areas as Custer County, South Dakota, as dine crystals associated with quartz at the Tip Top mine, and at the Palermo mine in North Groton, New Hampshire. The second occurrence is extraterrestrial whitlockite, which is now known as merrillite. Extraterrestrial whitlockite has been identified in lunar samples as well as martian and other types of meteorites, where it is one of the most common phosphate minerals. Studies of merrillite as an accessory mineral have provided valuable insights that have helped to unlock the petrogenesis of extraterrestrial rocks. Biological occurrences Whitlockite can also be found in biological systems and has been implicated in several human diseases. Whitlockite can be found at many sites in the human body, but is particularly concentrated in calcified tissues, such as embryonic and adult bone. The highest concentrations of whitlockite appear in the weight bearing