Calcium (Ca) is an alkaline earth metal, which places it in Group 2 of the periodic table alongside elements like magnesium and barium. It is highly abundant, ranking as the fifth most common element and the third most abundant metal in the Earth’s crust. As a metal, calcium has a characteristic silvery-white appearance, and its resistance to scratching is known as hardness. It is rarely encountered in its pure form because its two outer electrons make it chemically reactive, causing it to rapidly form compounds in nature.
Understanding Mineral Hardness Scales
The Mohs scale of hardness is the standard method used by geologists and material scientists to characterize the scratch resistance of minerals. Developed in 1812 by Friedrich Mohs, this system ranks materials from 1 to 10 based on their ability to scratch one another. A substance with a higher Mohs number can successfully create a permanent groove on the surface of any substance with a lower number. The scale is not linear. It uses ten reference minerals, starting with Talc (1) and culminating with Diamond (10), the hardest known naturally occurring mineral.
The Hardness of Pure Elemental Calcium
The Mohs hardness value for pure elemental calcium metal is quite low, typically cited at approximately 1.75. This places the pure element closer to the softest end of the scale, making it comparable in softness to materials like Talc or Gypsum. In its refined, metallic state, calcium is characterized as a soft, silvery-white metal that exhibits both malleability and ductility.
This low hardness results from the metallic bonding structure, which allows layers of calcium atoms to slide past one another easily when scratched. Consequently, a piece of pure calcium metal is soft enough to be cut with an ordinary knife blade. Due to its high chemical reactivity, it is not found in nature and is primarily handled in controlled laboratory or industrial environments.
How Calcium Affects the Hardness of Common Minerals
While pure calcium is soft, its presence in mineral compounds dramatically changes the resulting material’s hardness. In nature, calcium atoms readily lose their two valence electrons to exist as a divalent cation, which then forms strong ionic bonds within a crystalline lattice structure. The strength of these ionic bonds, rather than the metallic bonds of the pure element, dictates the mineral’s final hardness.
For example, calcium sulfate dihydrate, commonly known as Gypsum, has a Mohs hardness of 2. Calcium carbonate, which forms the abundant mineral Calcite found in limestone and marble, is significantly harder, possessing a Mohs rating of 3. Apatite, a calcium phosphate mineral that is a component of tooth enamel, reaches a Mohs hardness of 5. These examples show that mineral hardness depends on the specific chemical structure and the bond strength between the calcium ion and its bonding partners.