Final answer:
Crystal habit refers to the characteristic external shape of a mineral, which is determined by the repetitive stacking of its unit cell. Minerals like fluorite form their crystal habit through hydrothermal precipitation, where dissolved ions combine to form a solid crystalline structure. The specific arrangement of atoms in each unit cell leads to the unique crystal habit of each mineral.
Step-by-step explanation:
Crystal habit is the shape that a mineral develops by the repetition of its unit cell. The term 'unit cell' refers to the smallest repeating unit that makes up the crystal lattice of a mineral. In essence, the vast lattice structure of a mineral is an extended repetition of its unit cell, thereby shaping the crystal habit. When minerals form in environments where they are unconstrained, such as during precipitation from an aqueous solution or growth in open space, they can develop their characteristic external crystal shape.
For instance, the mineral fluorite (CaF2) is formed when dissolved calcium and fluoride ions precipitate from groundwater. This process results in the creation of a solid crystalline substance where the fluorite's unit cells are stacked in a repeating, orderly pattern to form the crystal. The idealized concept of growth from a simple unit cell is important for understanding and classifying mineral crystal habits, even though real crystal growth is more complex.
It is also important to note that the precise arrangement of atoms within each unit cell dictates the overall shape and angles of the crystal faces, which are unique to each mineral. This arrangement is what creates the distinctive external crystal habit, which can be classified into one of the seven basic crystal systems depending on the angles and lengths of their edges and crystal faces.