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| Formula | Ca2(Mg,Fe)5Si8O22(OH)2 |
| | Optic class & sign | Biaxial negative |
| | Optical orientation | X close to a, Y = b, Z near c |
| | Optical plane | (010) |
| | Relief | Moderate to high |
| | Refractive indices | nx = 1.599 -1.688
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ny = 1.610 -1.697
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nz = 1.620 -1.705
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| n increases with increasing Fe |
| | Birefringence (max.) | 0.017 - 0.027 |
| | | Δn decreases with increasing Fe |
| | Optic Angle
| 2Vx
= 62 - 88° |
| | 2Vz
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| | Sign of elongation | Length-slow, l (+) |
| | Interference figure | High 2V, few isochromes |
| | Colour / pleochroism | Colourless (tremolite) to pale green or blue-green (actinolite)
Pleochroism: X colourless, pale yellow green, Y pale yellow green to blue green, Z pale green, blue green; absorption Z > Y > X
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| | Zoning | |
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| Form | Habit | Columnar, long-prismatic, acicular, asbestiform; 4-sided diamond-shaped or 6-sided cross-sections Ʇ c |
| | | Surface | Typically subhedral to euhedral |
| | Cleavage | 2 sets {110} perfect, at 124 and 56° (seen in sections Ʇ c). In prismatic sections, the traces of the two principal cleavage sets are parallel. |
| | Twinning | {100} simple or multiple |
| | Extinction | Inclined, cɅZ from 21° (tremolite) to 13° (Fe-rich actinolite); in sections Ʇ c, symmetrical to {110} cleavage and prism faces, and straight to {010} faces
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| Reaction textures | Retrograde: actinolite replacing hornblende in amphibolites |
| | Alteration / decomposition | Chlorite, calcite/dolomite. Actinolite is itself an alteration product of pyroxene. |
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| Occurence | Ign | |
| | | Met | Tremolite: siliceous dolomitic marble, ultramafic rocks. Actinolite: greenschist-facies metabasites and ultramafic rocks |
| | | Sed | |
| | | Hyd | |
| | | Other | |
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| Distinctive properties | Habit, including characteristic cross-sections Ʇ c, either diamond-shaped or six-sided combination of {110} + {010} faces; characteristic amphibole cleavage, Δn, n, small extinction angle cɅZ |
| | Additional comments | Nephrite is a felty aggregate of interwoven Tr/Act fibres |
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