Augite (a pyroxene)

Augite is very dark green to black in color. The diagnostic feature of any pyroxene is two cleavages that intersect at 89° or 91°. Unlike the feldspars or hornblende, pyroxenes show mediocre cleavages that are hard to spot.

Augite is most easily confused with hornblende. Hornblende has highly reflective cleavages whereas augite cleavages are dull. Hornblende tends to form elongate rectangular crystals whereas augite crystals tend to be blocky. Finally, if you can find two intersecting cleavage planes, hornblende cleavages always intersect at ~60° or 120° whereas augite cleavages always intersect at about 90°.

This shows the typical very dark green color of augite as well as its typically mediocre cleavage. On the right one ~90° angle is formed by the intersection of the two cleavages.

Note how rough the two cleavage surfaces are compared to the micas or feldspars.

These three cleavage fragments illustrate three things: the pyroxene cleavage is not great, the cleavages intersect at about 90° (horizontal and vertical surfaces, here viewed looking down on horizontal surface), and the cleavage surfaces are not splintery or shiny like hornblende.
These are little well-formed crystals illustrate the nearly black color and blocky shape typical of augite.

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