Actinolite

Actinolite
Property	Value	Comments
Formula	Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂
Crystal System	monoclinic	beta = 105^o-107^o
Crystal Habit	Radiating habit common	Splintery surface
Cleavage	[110] Perfect	120° and 60° cleavage intersections when viewed along the c-axis
Twinning	Fairly common simple and lamellar twin
Color/Pleochroism	Colorless to pale green to dark green, darker colors and stronger pleochroism associated with high Fe contents X = colorless, pale yellow green Y = pale yellow-green, pale blue-green Z = pale green, green, blue-green
Relief	Moderate relief
Optic Sign	Biaxial (-)
2V	65-86^o	Not easy to measure due to natural cleavages causing specific orientations to be more common
Optic Orientation	X ^ a = +5° to -6° Y = b, Z ^ c = +10° to +21° optic plane = (010)	length slow
Refractive Indices alpha = beta = gamma =	1.599-1.688 1.612-1.697 1.622-1.705
Birefringence	0.017-0.027
Distinguishing Features	Can and will be confused with hornblende Actinolite differs from hornblende by: Higher 2V Lower extinction angle Paler color, less pleochroism
Occurrence	Common occurrence in contact and regional metamorphosed limestone and dolomite. Also found in metamorphosed mafic (greenschist facies) and ultramafic rocks. It is a common fine-grained alteration product of pyroxenes.
Editors	Lizz Chiarelli ('08), Rebecca Matecha ('18)

	Thin section photograph of an actinolite crystal. Note characteristic green-brown color. Click on the image to see a larger version with ppl/xpl rollover.
	Thin section photograph of an actinolite crystal that shows the intersection of two cleavages at 60° and 120°. Click on the image to see a larger version with ppl/xpl rollover. (Sample RT-2a)
	Thin section photograph of an actinolite crystal in an orientation where only one cleavage is visible. Click on the image to see a larger version with ppl/xpl rollover. (Sample RT-2a)