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| Formula | (Mg,Fe2+,Al,Fe3+)6(Si,Al)4O10(OH)8 |
| | Optic class & sign | Biaxial positive or negative |
| | Optical orientation | X near a or c, Y = b, Z near c or a |
| | Optical plane | (010) |
| | Relief | Low-positive to high |
| | Refractive indices | nx = 1.562 -1.671
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ny = 1.565 -1.684
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nz = 1.565 -1.685
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| n increases with increasing Fe, Mn, Cr, Al (Tschermak substitution) |
| | Birefringence (max.) | 0.00 - 0.02, commonly below 0.012 |
| | | Δn decreases from MgAl-rich Chl to intermediate composition with Δn = 0, then increases to Fe-rich and/or Al-poor Chl.
Characteristic anomalous interference colours of brown, violet and steel blue are associated with low-birefringent varieties that are close to optical isotropy.
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| | Optic Angle
| 2Vx
= 0 - 20° |
| | 2Vz
= 0 - 70° |
| | Sign of elongation | Length-fast, l (-): opt. negative; length-slow, l (+): opt. positive varieties |
| | Interference figure | Acute bisectrix figures in (001) sections, with broad, diffuse isogyres |
| | Colour / pleochroism | Typically pleochroic in shades of green (opt. pos.: X ≈ Y > Z; opt. neg.: Z ≈ Y > X). Oxidation results in brown colours. Mg-rich chlorite is colourless. |
| | Zoning | |
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| Form | Habit | Platy, flaky, intergrown fine-grained aggregates. worm-like stacks |
| | | Surface | Subhedral, anhedral, euhedral |
| | Cleavage | (001) perfect |
| | Twinning | Multiple twins on (001); can only be observed in sections close to (010) if extinction to cleavage traces deviates from 0°. |
| | Extinction | Inclined to straight. Max extinction angle is 9°. “Bird’s-eye-maple” mottled structure. |
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| Reaction textures | Hydrous breakdown product of garnet and other Fe-Mg silicates |
| | Alteration / decomposition | May show oxidation effects, but otherwise relatively stable |
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| Occurence | Ign | Only as secondary alteration or weathering product |
| | | Met | Metapelites and metabasites from lowest grade to greenschist/amphibolite facies transition; serpentinites |
| | | Sed | Common constituent of argillaceous sediments and sedimentary rocks |
| | | Hyd | Amygdales and fractures in volcanic rocks, hydrothermal veins, exhalites, black smokers at MOR, propylitic alteration zones of ore deposits |
| | | Other | |
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| Distinctive properties | Habit, single perfect cleavage, colour, low Δn, anomalous interference colours, mineral association. Mottled extinction (a property shared with other sheet silicates). |
| | Additional comments | |
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