APATITE

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


Back to: