Cleavage
Cleavage
 “Fundamental internal grain”
 Presence of cleavage permits
a rock to be split into thin
plates and slabs
 Cleavage
Relationship to Folding
planes tend to be parallel to the axial plane of
the fold
Sometimes cleavage planes fan symmetrically about the axial
plane of a fold
 In either case, we refer to it as axial plane cleavage

Axial Plane Cleavage
Using Cleavage
 Bedding-cleavage
relationships tell us where we are on a
fold and which way the rock units face
 At
Using Cleavage
Using Cleavage
Using Cleavage
Using Cleavage
Bedding-Cleavage Relationships
any one point, the bedding cleavage relationship does
not accurately reflect the axial plane of the fold
Cleavage tends to fan, trying to remain nearly perpendicular to
bedding
 Cleavage refracts, staying more nearly perpendicular to bedding
in sandy layers and more nearly parallel to the axial plane in
shaly layers

Cleavage Refraction
Axial Plane Cleavage
 Microscopic

Cleavage Domains
variations in mineralogy and structure
Cleavage domains
Anastomosing to subparallel, mica-rich laminae
Fabric of the host rock has been rearranged
 Strong preferred dimensional and crystallographic orientation



Microlithon domains or microlithons
Narrow zones in which the original mineralogy and texture of the rock
are preserved
 Little preferred orientation of grains

Microscopic Properties of Cleavage
 Slaty
cleavage
Overgrowths
 Beards
 QF-domains
 M-domains

Domain Structure
Photomicrograph of Domains
Types of Cleavage

Continuous—Present throughout the rock




Disjunctive



Slaty cleavage
Phyllitic structure
Schistosity
Crenulation cleavage
Spaced cleavage
Slaty cleavage



Typically associated with slates (fine-grained, pelitic rocks, subjected to
low-grade metamorphism
This is what makes roofing slate and pool table tops
Schistosity
Slaty Cleavage
Types of Cleavage
Medium grained rocks containing visible flakes of mica
 Schists seldom split cleanly
 Developed in metasedimentary rocks and certain volcanic rocks

metamorphosed to medium-high grade

Phyllitic Structure
Intermediate between slaty cleavage and schistosity
 Rocks have a satiny luster

 Disjunctive

Domainal Fabric in Schist
Types of Cleavage
Crenulation cleavage
A pre-existing continuous cleavage is crenulated into microfolds
 Discrete

– Very narrow cleavage domains sharply truncate the continuous cleavage of the
microlithons
– Typically forms in slates

Zonal
– Wider cleavage domains that correspond to the tight, appressed limbs of
microfolds in the preexisting continuous cleavage preserved within the microlithons
– Typically forms in phyllites and schists
Discrete Crenulation Cleavage
Discrete Crenulation Cleavage
Zonal Crenulation Cleavage
Zonal Crenulation Photomicrograph

Spaced cleavage
Types of Cleavage
Parallel to anastomosing, stylolitic to smooth, fracture-like partings that
are often occupied by clayey and carbonaceous matter
 Billings used to say, “closely spaced joints.”
 Offsets of bedding due to pressure-solution

Overgrowth Beards
Spaced Cleavage Photomicrograph
SEM Photo of Spaced Cleavage