Talc

Talc is the softest mineral on Earth, defining position 1 on the Mohs hardness scale. You can scratch it with your fingernail. It forms through the hydrothermal alteration of magnesium-rich rocks, particularly when ultramafic rocks (like peridotite) react with silica-bearing fluids at temperatures below 800°C.

The softness comes from talc's crystal structure: stacked sheets of silica-magnesia-silica connected by only weak van der Waals forces. The sheets slide over each other with minimal effort, creating the characteristic greasy, soapy feel.

Talc is the primary ingredient in talcum powder and a major industrial mineral used in ceramics, paint, paper, plastics, and cosmetics. Soapstone is a rock composed primarily of talc. French chalk (used by tailors), and the chalk used on pool cues are talc products. The mineral is so soft that massive talc can be carved with a fingernail.

Talc is identified by its extreme softness (Mohs 1, scratched by everything), greasy or soapy feel, pearly luster, and pale green to white color. It can be scratched by a fingernail and leaves a mark on cloth.

Distinguish from gypsum (slightly harder at 2, different crystal habit), chlorite (slightly harder, different feel), and serpentine (harder, not as soft-feeling). Talc's combination of extreme softness and soapy feel is unmistakable.