Smith College - Geology 222b - Petrology

Petrographic Data File

Actinolite
Property
Value
Comments
Formula Ca2(Mg,Fe++)5Si8O22(OH)2 
Crystal System monoclinic beta = 105o-107o
Crystal Habit   Splintery surface
Cleavage 120 degrees and 60 degrees [110] Perfect, [110] Perfect  
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 65-86o   
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 metamoprhosed mafic and ultramafic rocks. It is the common fine-grained alteration product of pyroxenes.

Editors Lizz Chiarelli ('08)

 
Thin section photograph of aetinolite crystal. Note characteristic green-brown color. Click on the image to see a larger version with ppl/xpl rollover.

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