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 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.  Crystals are often hexagonal. Some crystals are double terminated, or they come to a point at both ends. Example photo below.
Extinction Symmetrical to cleavages  
Dispersion n/a  
Distinguishing Features Cleavages at 56 and 124 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 metamorphic 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), Sara Gonzalez (12), Emilty Padova ('15)

 
Photomicrograph in thin section of hornblende. 
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.

Close up of the same grain showing 56 and 124 degree cleavages. Width of field of view is 400 microns.
Beautiful hornblende grain with simple twinning visible.
Characteristic 56-124 degree angles apparent in a crisp hornblende crystal.
Green, double terminated hornblende crystals in a metamorphic rock. Pleochroism varies from light yellow to medium green as the stage is rotated. Rollover image shows the same image betwen crossed polars. The lower crystal is extinct.
Another double terminated crystal from the same rock between crossed polars. The rollover image shows the same crystal with the gypsum/quartz plate pushed in. This crystal is length fast because the white areas turn yellow, black areas turn magenta, etc...
Hornblende crystal in andesite (K-84), showing 120° cleavage and, in cross-polarized light, simple twinning. Note also the opaque minerals surrounding the crystal. These are magnetite crystals due to oxidation of the hornblende.
Photomicrograph of a double terminated hornblende crystal in plane polarized light. The pleuochroism is easily seen here with dark green along the Z plane, and light yellow/white along the X plane. The 120 degree cleavage fractures can be seen within the grain.

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U.C.L.A. Petrographic Workshop


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