Foliation

Department of Geology and Mineral Science
University of Ilorin, Ilorin, Nigeria
FOLIATION
Prepared by : Atolagbe Joshua Mayowa

Focus
 Introduction
 Characteristic features of foliation
 Foliation genesis
 Tectonites
 Foliation types
 Cleavage and foliation
 Geological significance of foliation

Introduction
FOLIATION is a planar structure in a rock that is homogeneously
distributed throughout the volume.
Examples of foliations include the planar structure defined by the
parallel alignment of platy minerals in rocks such as schist in slate,
the parallel alignment of flattened mineral grains in gneisses
NOTE: FOLIATION is not LINEATION. They are distinct.
LINEATION is a homogeneously distributed linear structure. Lineations
are surficial if they are present along discrete surfaces, and they are
penetrative if they occur throughout the volume of a rock. Examples
of surficial lineation is parallel alignment of mineral fibers that
develop along some fault surfaces.

 Foliation in geology refers to
repetitive layering in metamorphic
rocks. Each layer may be as thin as
a sheet of paper, or over a meter in
thickness.
 It is caused by shearing forces
(pressures pushing different
sections of the rock in different
directions), or differential pressure
(higher pressure from one direction
than in others)
 Foliation is any penetrative planar
fabric present in metamorphic
rocks. Penetrative structures are
such which do not occur as
individual features (like fractures
or bedding planes) but rather
affect the whole of the intact rock,
usually as a preferred shape and/or
crystallographic orientation of
mineral grains and/or aggregates
of mineral grains. OR if the spacing
or the scale of the structure in a
rock is very small compared to the
size of the rock volume, it’s
considered penetrative.
 They are formed by the prograde
metamorphism (increase in
temperature and pressure) of
mudrocks; slate, phyllite, schist
and gneiss.

1. Grain shape orientation: grains of
minerals which are normally
equidimensional, such as quartz,
feldspar, calcite, etc., show flattened
ellipsoidal shapes with the short axes
perpendicular to the foliation plane,
independent of crystallographic
orientation (Figure 2-1, upper left).
2. Paleosome/neosome: in migmatites,
alternating layers of mafic and felsic
material occur, where the mafic
paleosome represents the remains of
the protolith, and newly crystallised
neosome layers the felsic components
which formed by partial melting (Figure
2-1, upper right).
3. Crystallographic preferred
orientation: tabular and prismatic
minerals, such as mica and amphibole,
are arranged such that crystallographic
axes and planes, and
crystallographically determined grain
boundaries, tend to be parallel (Figure
2-1, lower left).
4. Aggregate shape orientation:
aggregates of grains of the same
mineral, often derived from original
equidimensional grains or aggregates in
the protolith which have been
transformed in the course of
deformation to mono-mineralic lenses
and bands
Characteristic Features of Foliation

Figure 2-1. Schematic representation of the fabric
elements which, alone or in combination, form the
most common types of foliation in metamorphic
tectonites (from Passchier & Trouw 1996, Figure
4.1). Upper left: grain shape orientation; upper
right: paleosome/neosome; lower left:
crystallographic preferred orientation; lower right:
aggregate shape orientation. These fabric elements,
together with the effects of isoclinal folding,
combine to form the “composite foliation” which is
a characteristic feature of the Olkiluoto bedrock.

1. Primary – Foliations are
primary if they originate by
primary sedimentary or
magmatic processes. Primary
sedimentary processes such as
sediment transport and
deposition produce, for
example, linear tool marks, a
preferred orientation of
sedimentary clasts, and
bedding. Primary igneous
processes, such as flow and
crystallization, result in the
preferred orientation of
bubbles and pumice fragments
or compositional streaks and
bands.
2. Secondary — Foliations are
secondary if they originate by
secondary processes such as
tectonic deformation or
metamorphism. Forms after
lithification and/or
crystallization of rocks.
Secondary foliation is the
product of stress and strain
(tectonic foliation). Examples
are cleavage, schistosity.
NB: We use terms sedimentary
foliation, igneous lineation, or
tectonite foliation to specify the
inferred origin of a foliation
Foliation Genesis

Tectonites
 They are rocks whose structure is a product of deformation
and which are commonly, but not necessarily,
metamorphosed.
 They rocks that are pervaded by foliation and or lineation-
flowed in solid state.
 Most tectonite foliations and lineations are secondary in
origin and develop as a result of the deformation, although
some may be inherited primary features.
 Although most tectonites have both foliations and
lineations, if the tectonite structure is dominated by a
foliation, we call it an S-tectonite, and if is dominated by
a lineation, we call it an L-tectonite.

Tectonites are defined by the
fabric they form in rocks.
Fabric is a configuration of
objects penetrating the rock.
Linear (stack of straws)
objects form L-fabrics while
Planar (platy or leafy) objects
constitute S-fabrics.
The rocks are known as L and
S-tectonites respectively.

1. L –tectonites -Rocks that
show a marked linear fabric
(indicate constrictional
strain).
2. S tectonites – Rocks showing
pronounced planar fabric
(flattening strain).
3. LS tectonites – Deformed
rocks that contain both
linear and planar fabric
(plane strain)

Types of Foliation
1.Slaty cleavage
2.Phyllitic cleavage
3.Schistosity
4.Gneissic foliation

1. Gneissic Foliation
 Gneissic foliation refers to foliations that
develop in gneisses, which are coarse-grained,
high-grade metamorphic rocks in which platy
minerals are sparse or absent.
 The foliation generally provides at best a weak
cleavage.
 Gneissic textures occur when the silicate
minerals in the rock separate and
recrystallized into alternating bands of light
(quartz and feldspar) and dark (biotite,
amphibole, or hornblende) grains of silicate
minerals.

2. Slaty cleavage
 Slaty cleavage refers to fine continuous foliations
characteristic of slates. Slates are very fine-grained, low-
grade metamorphic rocks that contain abundant sheet
silicates (generally clays, chlorites, and micas).
 develops due to tectonic shortening, reorientation of clay
grains, and solution of quartz. QF- and M-domains.
 Slaty cleavage is used to describe rocks that split into thin,
planar slabs when hit with a hammer.
 They may also contain subordinate amounts of silty and
carbonaceous material. The foliation provides a very strong
cleavage to the rock, along which the rock breaks easily and
tends to weather preferentially.
 Rocks with slaty cleavage traditionally have been a valuable
source of materials for such uses as roofing slates and
blackboards.

3. Phyllitic cleavage (thin wave foliation)
 Phyllitic cleavage resembles slaty cleavage except
that the QF- and M-domains become more pronounced
because grain size of the rock is slightly coarser as
temperature increase with depth.
 It characterizes phyllites, which are low-grade
(greenschist), fine-grained, metamorphic rocks
containing abundant micas, chlorite, or both. In hand
samples, the surface of the foliation has a sheen to it,
and individual sheet silicate flakes may just be
resolvable with a good hand lens.
 The foliation strongly affects the rock's weathering
pattern

4. Schistocity
 Schistosity refers to the irregular planar foliation
found in coarse-grained, mica-rich, high-grade
metamorphic rocks.
 Chlorite, biotite or muscovite defines the foliation,
and the mineral grains are coarse enough to be visible
with the unaided eye (without the use of microscope.
 It provides a strong cleavage to the rock.
 Rocks with schistosity are generally referred to as
schist.

Rock cleavage, or simply cleavage, is the tendency of a rock to
break or cleave along surfaces of a specific orientation.
All cleavages are foliations, but not all foliations are cleavage
because rocks includes planar geometric features that do not
necessarily result in a cleavage.
 The planar alignment of slightly flattened grains, like quartz in a
quartzite or olivine in a peridotite, or the compositional banding
in a gneiss, would define a foliation but would not result in a
cleavage.
Cleavage and Foliation

 In geotechnical engineering, a foliation plane may form a discontinuity
that may have a large influence on the mechanical behavior (strength,
deformation, etc.) of rock masses in, for example, tunnel, foundation, or
slope construction.
 The effect of foliation on rock stiffness
 Effect of foliation on the rock strength
 Influence on The Excavation Disturbed Zone (Edz) and Permeability
 Influence on Thermal Properties
 Influence on In-Situ Stress Estimation
 Influence on Repository Layout and Construction Activities (Blasting,
Grouting)
Geological Significance of Foliation

Read more on:
https://www.britannica.com/science/foliation-geology

Foliation

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