Also known as odontolite, this beautiful rock is formed in the depths of sedimentary stacks, as bones like this fossil jaw are preserved for the aeons during the process that turns their surrounding sediments into rock. Teeth are the most common part of the skeleton that turn into odontolite, hence its name. It was long thought to be coloured (like turquoise) by copper, and when analysis failed to reveal its presence, by the iron phosphate mineral vivianite (seehttp://tinyurl.com/mnvy6vx). Recent research has shown that it consists of crystals of blue apatite (seehttp://tinyurl.com/l5rzpag), coloured by trace impurities of manganese.
In the old days it was called occidental turquoise to distinguish it from the oriental version which is a clay rock coloured by copper oxide, and occasionally the former has been erroneously (or fraudulently) sold as the latter. They can be told apart by their hardness, with the oriental version being harder. The most famous locality is in France, especially in the southwestern Languedoc region, where they accumulated in river sediments derived from the erosion of the nearby Pyrenean chain. They preserve bone structure beautifully and are much sought after by palaeontologists.
The jaw in the photo (size 5.7 x 5.3 x 3.3 cm) is recent, and belonged to a Pleistocene rodent…
We are all intimately familiar with this mineral family, as it is the main inorganic ingredient of our bones and teeth, deposited along with collagen and various cells in a process called biomineralisation. It plays a key part in the global phosphorous cycle, an element so essential to life as we know it that Isaac Asimov referred to it as ‘life’s bottleneck, since environments impoverished in this element don’t support much of an ecosystem.
A calcium phosphate (often incorporating other elements such as chlorine or fluorine, hence the practice of adding fluoride to drinking water and toothpaste), apatite forms in volcanic or metamorphic rocks, using up the phosphorous that doesn’t fit into the crystal structure of the more common rock forming minerals. Common colours are green, blue and yellow, large gemmy crystals such as the one in the photo tend to form in pegmatites, the slow cooling remnants of granites that incorporate all the distilled rare elements from the world’s depths and crystallise them into a wide variety of beautiful gems. Once the volcanic rocks are eroded, the apatite is deposited in sedimentary environments, micro crystals being the principal constituent of phosphate rich rocks.
Crystals are usually hexagonal prisms, though 12 sided ones are not uncommon. Beryl (emerald, aquamarine) and tourmaline can be confused with it, and the name comes from a Greek word for deceit, as it is often confused with more valuable gems. The Mohs hardness of 5 gives it away, as a steel blade will scratch it, leaving the others unscathed.
It has an important use in geology, since it soaks up Uranium and Thorium, allowing it to be dated, and the tracks left by particles during fission of these elements within the crystal structure are often of use to metamorphic geologists trying to date the events that heated and squished the parent rock (known as the protolith). It is also vital to feeding the world, phosphate rich rocks (essentially fossil phosphorous, just like fossil fuels) are mined globally for use in fertilisers, without which we couldn’t sustain the current world population using the currently available sources.
It is also an ore for rare earth elements, and the hand spectroscope often reveals a maze of dark lines in the spectrum due to these impurities. Occasionally faceted for collectors, it is also used by painters who make their own pigments. Major sources of gem apatite include Brazil, Burma and Mexico.
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