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Hornblende

The term hornblende is here used as a collective term for several amphiboles not optically distinguishable from each other.

 

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Hornblende  For other amphiboles see index. (Click PPL image to show labels)

Olivine

 

Hornblende in andesite lava. Brown and pleochroic with dark rims, (possibly caused by dehydration), zoned. The phenocrysts from this source have natural holes in the centre of tht crystal. This is not a general characteristic.

 

Width of central crystal cluster: 6mm.

 

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Pleochroism

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Olivine XPL view

 

Hornblende in andesite lava. Bright interference colours somewhat masked by the strong body colour of the phenocryst. Simple twinning seen in crystal centre right.

 

Width of central crystal cluster: 6mm.

 

 

   

 

   

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Olivine rollover

 

Hornblende in a Lewisian gneiss. Green/brown/bluish pleochroism. Random shapes aligned in layers.

 

Width of field of view: 10mm

 

 

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Hornblende In gneiss. Moderate birefringence but muted pleochroism, brighter colours shown by epidote.

 

Width of field of view: 10mm

 

 

 

   

 

   

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Olivine rollover

 

Hornblende in a schist that has undergone subsequent heating resulting in a granular structure. Highly pleochroic, with characteristic cleavage angles 60°/120° (aprox), in most thin sections containing hornblende it is possible to find a few crystals with this characteristic, and when seen it is most useful in differentiating hornblende from biotite.

 

Width of field of view: 10mm

 

 

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Olivine in schist, moderate birefringence. Garnets always black in XPL.

 

Width of field of view: 10mm

 

 

 

   

 

   

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Olivine rollover

 

Hornblende in a phonolite lava. brown euhedral phenocrysts accompanied by green pyroxene (aegirine/augite)

 

Width of field of view: 12mm

 

 

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Olivine in a phonolite. Zoning. Body colour masks interference colours.

 

Width of field of view: 12mm

 

 

 

   

 

   

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