Ca5(PO4)3(OH,F,Cl), The fluorine rich apatite
As a source of phosphorous to be used in fertilizer, rarely as a gemstone and as a mineral specimen
apatite is actually three different minerals depending on the predominance of either fluorine, chlorine or the hydroxyl group. These ions can freely substitute in the crystal lattice and all three are usually present in every specimen although some specimens have been close to 100% in one or the other. The rather non-inventive names of these minerals are Fluorapatite, chlorapatite and hydroxylapatite. The three are usually considered together due to the difficulty in distinguishing them in hand samples using ordinary methods.
pink crystals of fluorapatite with pale blue beryl on books of muscovite crystals
An irony of the name apatite is that apatite is the mineral that makes up the teeth in all vertebrate animals as well as their bones. The name apatite comes from a Greek word meaning to decieve in allusion to its similarity to other more valuable minerals such as olivine, peridot and beryl.
apatite is widely distributed in all rock types; igneous, sedimentary and metamorphic, but is usually just small disseminated grains or cryptocrystalline fragments. Large well formed crystals though can be found in certain contact metamorphic rocks. Very gemmy crystals of apatite can be cut as gems but the softness of apatite prevents wide distribution or acceptance of apatite as a gemstone.
- Colour : typically green but also yellow, blue, reddish brown and purple
- Luster : vitreous to greasy and gumdrop
- Transparency : Crystals are transparent to translucent
- Crystal System : hexagonal; 6/m
- Crystal Habits : include the typical hexagonal prism with the hexagonal pyramid or a pinacoid or both as a termination. Also accicular, granular, reniform and massive. A cryptocrystalline variety is called collophane and can make up a rock type called phosphorite and also can replace fossil fragments
- Cleavage : indistinct in one basal direction
- Fracture : conchoidal
- Hardness : 5
- Specific Gravity : approx. 3.1 – 3.2 (average for translucent minerals)
- Streak : white
- Other : An unusual “partially dissolved” look similar to the look of previously sucked on hard candy
- Associated Minerals : hornblende, micas, nepheline and calcite
- Major Occurrences : include Durango, Mexico; Bancroft, Ontario; Germany and Russia
- Best Indicators : crystal habit, colour, hardness and look