Why is calcium not found naturally as a free metal on Earth?

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A labeled diagram of the periodic table entry for calcium, showing its atomic number 20, symbol Ca, atomic weight 40.078, electron configuration, and classification as an alkaline-earth metal. Icons indicate calcium is a solid with a face-centered cubic crystal structure and strongly basic oxides at room temperature. — calcium The chemical element calcium is the most abundant metallic element in the human body.

Calcium is not found naturally as a free metal on Earth because it is highly reactive. As an alkaline-earth metal, calcium readily loses electrons to form stable compounds, such as calcium carbonate and calcium oxide.123

Because of this high reactivity, the pure metal reacts readily with oxygen, water vapor, and nitrogen in the air. When exposed to these elements, it quickly forms a coating of oxide, hydroxide, and nitride. Consequently, while calcium is widely distributed across the Earth's crust in various mineral forms, it must be isolated through industrial processes—such as heating lime with aluminum or the electrolysis of calcium salts—to exist in its metallic state.1