Galena is a common sulfide mineral (PbS). Mineral cleavage is the physical property describing a crystal’s tendency to split along specific, flat planes of structural weakness when subjected to stress. Galena exhibits this property strongly, and its specific cleavage pattern is one of its most recognizable features.
The Specifics of Galena’s Cubic Cleavage
Galena is known for its perfect cubic cleavage. When a sample breaks, it consistently splits along three planes that intersect at precise 90-degree angles, resulting in fragments that are geometrically perfect cubes or rectangular steps.
This cleavage is designated as “perfect” because the mineral breaks cleanly and easily along these planes, creating remarkably smooth, mirror-like surfaces. Even crushed pieces of galena will maintain this distinct angularity, appearing as a collection of tiny, silver-colored cubes.
The perfect cubic cleavage is parallel to the {001} crystallographic planes, which correspond to the faces of an ideal cube. This property is so pronounced that it often overshadows the mineral’s natural crystal habit, which can also form cubes or octahedrons.
The Atomic Basis for Cleavage Planes
Galena’s cubic cleavage is rooted in its simple, highly ordered internal atomic structure. The mineral crystallizes in the isometric crystal system, forming a structure analogous to that of common table salt (halite). This arrangement consists of alternating lead ions (Pb²⁺) and sulfide ions (S²⁻) held together by ionic bonds in a face-centered cubic lattice.
Each lead ion is surrounded by six sulfide ions, creating a repeating pattern of octahedra that stack to form the cubic crystal. The cleavage planes represent directions within this lattice where the attractive forces between the ions are the weakest. When force is applied, the crystal preferentially breaks along these planes rather than through the stronger ionic bonds.
The cubic arrangement dictates that the planes of structural weakness exist in three mutually perpendicular directions, explaining the 90-degree angles of the cleavage fragments. The bonds are equally weak along these three axes, producing the characteristic perfect cleavage that yields perfect cubes.
Identifying Galena and Its Primary Use
The cubic cleavage, combined with the mineral’s high specific gravity, sets it apart from many other metallic-looking minerals. Galena’s high density, ranging from 7.4 to 7.6, is instantly noticeable, making samples feel heavy for their size.
The mineral also exhibits a bright, metallic luster on freshly exposed cleavage surfaces, which quickly dulls to a lead-gray color as it tarnishes upon exposure to air. Its relative softness, measuring 2.5 on the Mohs scale, means it can be easily scratched by a copper coin.
Economically, galena is one of the most significant minerals, serving as the world’s main ore of lead. Lead extracted from galena is a widely used industrial material, with its most common application today being in the production of lead-acid batteries for automobiles. Certain deposits of galena also contain small but recoverable amounts of silver, making it an important source for that precious metal as well.