Fracture of Minerals

the nature of the surfaces formed when crystals or grains of a mineral are split. The appearance of the fracture depends on a number of mechanical properties of the mineral (including brittleness and malleability), its crystalline structure (the presence or absence of cleavage and the degree of its perfection), the nature of aggregate intergrowths in aggregate minerals, and the size and shape of grains in them. Crystals of minerals with perfect or good cleavage form even fractures with glistening surfaces (mica, rock salt, galenite, and others). Some minerals with perfect cleavage in certain directions can produce fractures that do not reveal the cleavage planes (for example, the conchoidal fracture in calcite). In cryptocrystalline aggregates and minerals with imperfect cleavage or none at all, a distinction is made among splintery fractures (hornfels and silica), conchoidal fractures (quartz), earthy fractures (chalk and clay), and columnar fractures (for example, an aggregate of prismatic actinolite crystals). Malleable native metals (copper, silver, gold, and others) produce the so-called hackly fracture. The fracture may be a qualitative characteristic in mineral diagnosis.