Hardness, Cleavage, Fracture, Tenacity, Special Gravity
The atomic structure of all minerals determines their physical properties.
Hardness in a mineral means the degree of resistance offered by its smooth surface to being scratched. Minerals having degrees of hardness corresponding to values between 1 and 10 on the Mohs Scale of Hardness are shown to the upper right. The points on the scale are determined by the hardness of these minerals. Any mineral in the sequence will be scratched by a harder mineral.
Cleavage is the tendency in a mineral to break with smooth surfaces in directions which correspond to weakly bonded planes of atoms in the crystal structure. Minerals may have more than one plane of weakness, and the corresponding relationship among cleavage directions can help identify the crystal structure, thereby helping to distinguish among similar-looking minerals.
Minerals may break apart by cleavage or by fracture, which takes several forms. Fracture may be conchoidal as in obsidian, hackly as in copper, earthy as in clay, splintery as in hematite, or subsconchoidal as in siderite.
As with fracture, there are various forms of tenacity, the resistance that a mineral offers to being pulled apart or deformed. Tenacity may be tough as in jade, flexible as in gypsum, elastic as in muscovite, sectile as in gypsum, or malleable as in copper.
The specific gravity of a mineral represents its weight compared to the weight of an equal volume of pure water. Thus, a mineral with a specific gravity of 5 is five times as heavy as water. This characteristic property of minerals depends on the atomic weights of the constituent elements and their structural arrangement.