CHLORITE

General Formula:
(Mg,Fe,Al)3(Si,Al)4O10(OH)2*(Mg,Fe,Al)3(OH)6
Sample: PT-115D System:
Monoclinic and Triclinic
Chlorite Porphyroblast
A relict chlorite porphyroblast within a Meguma Group pelite, from Nova Scotia. Note the very weak colour visible in plane light.
Field of view = 2.7 mm, plane light
Chlorite Porphyroblast
The interference colour displayed by the chlorite is masked to some degree by the colour of teh grain.
Field of view = 2.7 mm, crossed polars
Block diagram showing the relationship between the crystallographic axes and the indicatrix axes.

Optical Properties

Colour
Pleochroism
light to medium green
pleochroic, expressed in shades of green, darker varieties are Fe-rich
Form commonly found as plates similar to micas
Relief
RI
moderate to moderately high positive
nalpha = 1.55-1.67
nbeta = 1.55-1.67
ngamma = 1.55-1.69
Cleavage perfect on {001}
Birefringence
Interference Colours
0.0-0.015
first order white to yellow, may exhibit anomalous blue or purplish interference colours
Twinning none
Interference Figure
Optic Sign
2V
biaxial
positive or negative
0-60° (positive)
0-40° (negative)
Optic Orientation extinction angle up to 9°, cleavage trace in optically positive varieties is length fast and in optically negative varieties is length slow
Composition highly variable Alteration oxidation may produce iron stains, but chlorite is relatively stable.
Occurrence widespread mineral in contact and regional metamorphic rocks of low grade, as an alteration product of mafic minerals in igneous rocks Distinguishing Features colour, weak pleochroism, and weak birefringence


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