Rock cleavage refers to the tendency of rocks to split along closely spaced, parallel planes, distinct from bedding planes. It is a pervasive planar fabric developed in fine-grained rocks, typically formed under directed stress during deformation and metamorphism. Morphological classification categorizes cleavage based on its appearance and relationship to the rock's mineral grains.

Main Types of Cleavage:

1. Slaty Cleavage: This is the most perfect and pervasive type, characterized by extremely closely spaced, parallel planar surfaces along which the rock (slate) splits readily. It forms in very fine-grained rocks (e.g., shales) under low-grade metamorphic conditions. The cleavage planes are typically defined by the parallel alignment of platy minerals like micas and chlorite, which recrystallize perpendicular to the maximum principal stress.

2. Phyllitic Cleavage: Found in phyllites, this cleavage is similar to slaty cleavage but is slightly coarser and often has a distinctive silky or satiny sheen due to the larger size and greater abundance of aligned mica minerals. The cleavage surfaces are still very planar and closely spaced.

3. Crenulation Cleavage: This type develops in rocks that have undergone multiple phases of deformation. It forms by the micro-folding or crenulation of a pre-existing planar fabric (like slaty cleavage or bedding). The new cleavage planes are defined by the axial planes of these micro-folds, often appearing as discrete zones of dissolution or recrystallization. It can be either discrete (sharp, distinct planes) or continuous (more diffuse).

4. Schistosity: While often considered a type of foliation rather than cleavage, schistosity represents a more advanced stage of metamorphic fabric development than slaty or phyllitic cleavage. It is characterized by the parallel alignment of coarse-grained platy or elongate minerals (e.g., micas, amphiboles) in medium- to high-grade metamorphic rocks (schists). The surfaces are typically wavy or irregular due to the larger mineral grains.

5. Fracture Cleavage: This is a less perfect form of cleavage, characterized by closely spaced, parallel fractures or micro-faults that cut across the rock. Unlike slaty or phyllitic cleavage, it is not primarily defined by mineral alignment but by mechanical fracturing. It often forms in coarser-grained rocks or at lower metamorphic grades and may be associated with brittle deformation.

These classifications help geologists understand the deformational history, stress regimes, and metamorphic conditions experienced by rocks.