| General Formula: Ca5(PO4)3(F,OH,Cl) |
Sample: PT-10C | System: Hexagonal |
|---|---|---|
 |
 |
 |
| Hexagonal and elongated Apatite needles Fine grained hexagonal and elongated needles of apatite included in plagioclase and clinopyroxene within an olivine diabase. The long axis of the image is 0.9 mm, plane light view |
Hexagonal and elongated Apatite needles The hexagonal grains are cross sections of apatite needles, cut perpendicular to the long axis which also corresponds to the c crystallographic axis and the optic axis, thus the needles appear black. The long axis of the image is 0.9 mm,crossed polar view |
Block diagram showing the relationship between the crystallographic axes and the indicatrix axes. |
Optical Properties
| Colour Pleochroism |
usually colourless non pleochroic in thin section |
Form | small euhedral to subhedral elongate prismatic crystals with hexagonal cross sections are most common, also found as anhedral grains and granular or columnar aggregates |
| Relief/ RI |
moderate high positive nw = 1.633-1.667 ne = 1.629-1.665 |
Cleavage | poor basal and prismatic, not readily visible in thin section |
| Birefringence Interference Colours |
0.001 - 0.007 first order grey |
Twinning | rare |
| Interference Figure Optic Sign 2V |
uniaxial negative |
Optic Orientation | elongate sections show parallel extinction and are length fast |
| Composition | widest variation in composition is associated with the hydroxyl site, e.g. F for OH for Cl | Alteration | stable in most geologic environments |
| Occurrence | present as an accessory in a wide variety of igneous and metamorphic rocks and as detrital grains in sedimentary rocks | Distinguishing Features | moderate to high relief, low birefringence and uniaxial character |
