Smith College - Geology 222b - Petrology

Petrographic Data File

Hornblende
Property
Value
Comments
Formula (Ca,Na)2-3(Mg,Fe+2,Fe+3,Al)5Si6(Si,Al)2O22(OH)2
Crystal System Monoclinic, inosilicate, 2/m forms a complete solid solution
Crystal Habit May be columnar or fibrous; coarse to fine grained. May have simple and lamellar twins on {100}.
Cleavage {110} perfect - intersect at 56 and 124 degrees. Also partings on {100} and {001}. .
Color/Pleochroism Pleochroic in various shades of green and brown.  In PPL a thin section of Hornblende ranges from yellow -green to dark brown. Green varieties usually have X= light yellow green, Y=green or grey-green and Z=dark green. Brownish varieties have X=greenish-yelow/brown, Y=yellowish to reddish brown and Z=grey to dark brown. Has moderate to high positive relief in thin section.
Optic Sign Biaxial (-) .
2V 52-85° .
Optic Orientation Y=b 
Z^c		 .
Refractive Indices
alpha = 
beta = 
gamma = 
delta = 
1.614-1.675 
1.618-1.691 
1.633-1.701 
0.019-0.026		 .Indicies of refraction show an increase with increasing iron content
Max Birefringence 2nd to 4th order with highest interference colors in thin section in upper first or lower second order.
Elongation Prismatic crystal that can be but is not necessarily elongated.  it is often hexigonal  
Extinction symmetrical to cleavages  
Dispersion n/a .
Distinguishing Features Cleavages at 60 and 120 degrees which form a distinctive diamond shape in cross section.  Hornblende is easly confused with biotite.  Distiguishing factors are the lack of birds eye extinction and the two distinct cleavages.  Simple twinning is relatively common. Crystal habit and cleavage distinguish hornblende from dark-colored pyroxenes.
Occurrence Is a common mineral occuring in both igneous and metammorphic rocks.  It characteristically forms near other amphiboles due to the complete solid solution.  End members include: edenite, pargasite, hastingsite and tschermakite.
Editors Kim Cheney (00), Laura Frye-Levine (03), Derek Prill (AC-08).

 
Photomicrograph in thin section of hornblende.  See U.C.L.A. Petrographic Workshop for excellent images.
Hornblende from K-84 showing two cleavages intersecting at 60 and 120 degrees.


Hornblende from K-84 in plane light. Width of the grain is approximatly 500 microns.

*Click on the image to jump to a link with a view under cross-polarized light!

Close up of the same grain showing 60 and 120 degree cleavages. Width of field of view is 400 microns.
Beautiful hornblende grain with simple twinning.
Characteristic angles apparent. 60-120 degrees in a crisp hornblende crystal.

WWW Images

 U.C.L.A. Petrographic Workshop

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