Enargite

Enargite

I purchased a small bucket of "tumbler rocks" at the Richland Lithic and Lapidary Society annual show last summer.  I finally started to look through them the other day looking for a few to add to my tumbler for the optimal charge.  This rock looked pretty nondescript except on one tip.  I found, as you can see, that it had some material embedded in it.  Surrounding it was a milky translucent material.  The thing with stood out to me about the dark part was that it seemed to me to be some sort of crystal.  It looked like maybe a double cube.  I decided that because of the crystal formation this must be a mineral.  I read that the term for this particular type of crystal is isometric.  Cubes fall into this category.  I had trouble finding it in my guide until I picked up on the property of its metallic luster shown in this shot.  I checked its hardness and found it to be between 2 and 3.  I checked its streak and found that it was grayish black.  I tried prying it lightly with my fingernail and it split off.  

Enargite

The crystal is just slightly over 1/8" long.  As you can see, the face to the left is almost perfect.

Enargite

This view should be mentally rotated 180 end for end.  It should then be rotated along the long axis 90 degrees to position it as you see it in the first shot.  

The chemical composition is Cu3AsS4.  It is a common copper mineral probably from the West.

I am very glad that I examined the mix of rocks because this would have been ruined in the tumbler.

 

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