Chalcocite << KAL kuh syt >> is a mineral made up of copper and sulfur. It is shiny and lead-gray when freshly broken, but it quickly turns black when exposed to air. Exposure to air also softens chalcocite, so the mineral may make sooty, black streaks when rubbed. Chalcocite is an important source of copper. Its crystal structure is orthorhombic, and the crystals are flat tablets (see Crystal (Classifying crystals)). Chalcocite forms when sulfuric acid and copper sulfate solutions interact chemically with rock that contains other copper minerals. This process concentrates copper, thereby helping to form minable deposits of copper ore. Chalcocite occurs in copper deposits throughout the world.
