| General Formula: (Mg,Fe,Al)3(Si,Al)4O10(OH)2*(Mg,Fe,Al)3(OH)6 |
Sample: PT-115D | System: Monoclinic and Triclinic |
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| 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 |
