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Actinolite thin section

About Actinolite
From Greek for ray, in allusion to its frequent occurrence in bundles of radiating needles.
Actinolite hand-specimen
Formula: ▯Ca2(Mg4.5-2.5Fe2+0.5-2.5)Si8O22(OH)2
System: Monoclinic
Color: Green, green-black, grey-green, or black
Lustre: Vitreous, Silky
Hardness: 5–6
Density: 3.03–3.24
Actinolite #1 thin section (hFOV 2mm)
Actinolite #2 thin section (hFOV 2mm)
Actinolite PPL properties
Relief: Moderate positive
Habit/Form: Commonly as columnar, bladed, or acicular crystals elongate parallel to the c axis. Sometimes fibrous or asbestiform. Basal sections are diamond shaped and show the typical amphibole cleavage. Longitudinal sections (parallel to the c axis) are roughly rectangular and show only one cleavage trace.
Color: Colorless to pale green; pale yellow, yellowish green, pale yellow-green, green, deep greenish-blue
Pleochroism: Very low to moderate, related to iron content
Cleavage: Good in two directions {110} – in two directions are at nearly obtuse angles (124° with 56°), and in one direction for longitudinal section. Sections parallel to (100) which display no cleavage since both cleavages are at too acute an angle to the section to be visible.
Actinolite XPL properties
Isotropy/Anisotropy: Anisotropic
Interference color: Order II blue to order II green
Extinction angle: Basal sections show symmetrical extinction. The extinction angle of 13 to 17° is seen in sections parallel to (010). In most cases, the extinction angle is between 14 and 25°.
Twins: Simple and lamellar twins on {100}
Uniaxial/Biaxial: Biaxial (-)
Optic axial angle (2V): 2V measured: 79 – 86°, calculated: 78 – 82°
Actinolite distinguishing features under the microscope
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  • Deer, W. A., Howie, R. A., & Zussman, J. (2013). An introduction to the rock-forming minerals (pp. 498). Mineralogical Society of Great Britain and Ireland, London.
  • mindat.org – The Mineral Database