Argentite

[2][3] The International Mineralogical Association has decided to reject argentite as a proper mineral.

[3] The name "argentite" sometimes also refers to pseudomorphs of argentite: specimens of acanthite which still display some of the outward signs of the cubic crystal form, even though their actual crystal structure is monoclinic due to the lower temperature.

[2][1] This form of acanthite is occasionally found as uneven cubes and octahedra, but more often as dendritic or earthy masses, with a blackish lead-grey color and metallic luster.

The mineral is perfectly sectile and has a shining streak; hardness 2.5, specific gravity is 7.2–7.4.

It occurs in mineral veins, and when found in large masses, as in Mexico and in the Comstock Lode in Nevada, it forms an important ore of silver.