Hornblende (an amphibole)

Hornblende is black and, like all amphibole minerals, is characterized by two perfect cleavages that intersect at about 60° or 120°. Instead of breaking in broad, single cleavage planes like the micas, the cleavage surface of an amphibole mineral tends to display a large number of small, offset planes that help give the mineral a splintery appearance.

The large number of small offset cleavage planes may make it tough to spot the angles at which the two cleavages intersect. The solution is to slowly rotate the mineral in your hand and spot the flashes of reflected light. Even if offset, multiple parallel cleavage surfaces will reflect the light at one time. If you keep track of how far you rotate a given sample, you can estimate whether the two sets of flashes occur at 90° or 60°/120° from each other.

It is tough to see the 60°/120° cleavage plane intersections, but it is easy to see how dark hornblende is. The white is calcite
This close-up of a rough hornblende cleavage surface shows how the many offset but parallel cleavage surfaces reflect the light all at once. Contrast this with the almost mirror-smooth cleavage surfaces of the micas.
These hornblende pieces show how slightly weathered amphiboles display a splintery appearance.

All pieces are black; the grayer looking pieces are reflecting more light from their cleavage surfaces.

Click here to see a super-close up (or on the photo for a regular close-up).

In a rock, hornblende frequently forms rectangular crystals that show highly reflective cleavages.

In contrast, augite (a pyroxene) tends to be blocky with dull, difficult-to-spot cleavage surfaces.

Crystallization of hornblende (black) in an intrusive igneous rock can form spectacular patterns and even layers (unusual in igneous rocks).

Although the highly reflective cleavage surfaces are a good clue, it takes careful examination with a hand lens to spot the diagnostic 60°/120° cleavage intersections in an exposure such as this.

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