Minerals are naturally occurring solid substances, often forming intricate, geometric shapes known as crystals. However, this ideal appearance is not always achieved in nature. Many factors can prevent a mineral from developing its characteristic external shape, known as its habit. This article will explore the conditions and influences that discourage minerals from realizing their full crystalline potential.
The Ideal of Mineral Habit
Mineral habit refers to the characteristic external shape a mineral crystal typically develops when it grows unimpeded in a favorable environment. This shape is a macroscopic expression of the mineral’s internal atomic structure. For example, quartz often forms hexagonal prisms, while halite commonly grows as cubes. Understanding a mineral’s typical habit aids in identification and provides insights into formation conditions. Optimal growth conditions allow atoms to arrange in a highly ordered, repetitive pattern, leading to distinct crystal faces and a perfect form.
Physical Environmental Constraints
The physical environment surrounding a growing mineral can impede ideal habit achievement. Restricted growth space, within cracks or crowded environments, is a common limitation. This results in irregular, anhedral (lacking well-formed faces), or granular forms, rather than distinct, euhedral (well-formed) crystals. The atoms lack the room to systematically attach and extend the crystal lattice in all directions.
Rapid changes in temperature or pressure can hinder perfect crystal habit development. When molten rock cools too quickly, or mineral-rich solutions experience a sudden drop in pressure, atoms lack sufficient time to arrange into an ordered, characteristic structure. They solidify rapidly, often forming microscopic or amorphous (non-crystalline) solids. Such accelerated crystallization bypasses the methodical atomic arrangement necessary for well-defined crystal faces.
Additionally, physical agitation or turbulence within the growth medium, such as flowing water or magma, can disrupt orderly atomic attachment. This constant movement prevents new atomic layers from settling precisely, leading to distorted or imperfect crystal growth.
Chemical and Growth Rate Influences
Chemical composition and crystallization speed impact a mineral’s characteristic habit. Impurities can interfere with regular atomic addition to the crystal lattice. These impurities can adsorb onto crystal faces, blocking attachment sites, leading to distorted or stunted growth. For instance, even trace amounts of certain ions can alter the typical growth directions of a crystal, resulting in an unusual or imperfect shape.
The concentration of dissolved minerals in the growth solution, known as saturation, is also significant. Very high supersaturation often leads to rapid crystal growth, where atoms precipitate too quickly to find ideal positions within the lattice. This rapid deposition results in poorly formed or skeletal crystals. Variable chemical conditions, like pH, redox potential, or specific ion availability, can alter a crystal’s form. These changes can modify mineral solubility, alter crystal face surface energy, or even stop growth, preventing the mineral from completing its characteristic shape.
Internal and Inter-Crystal Disruptions
Internal crystal structure factors or interactions with other growing crystals can also discourage ideal habit formation. Internal imperfections, such as dislocations or point defects, can accumulate during growth. These structural irregularities can prevent the perfect propagation of the crystal lattice, leading to a less than ideal external form. These defects disrupt the overall symmetry and development of crystal faces.
Minerals frequently grow alongside other crystals. Twinning occurs when two or more crystals of the same mineral grow together in a symmetrical orientation, sharing some crystal lattice points. These twins prevent any single crystal from achieving its isolated ideal habit.
Similarly, intergrowths involve multiple crystals growing in close proximity, often impinging, preventing full development of individual faces. A mineral that has already formed its characteristic shape can undergo partial dissolution or resorption if environmental chemical conditions change. This process erodes the crystal’s surface, destroying its original faces and characteristic habit.