Sedimentary Petrology
GEO 333
Lab (5)
Grain Morphology (Grain Shape)
2009
Mansour Al-Hashim
Preview of Lab 4

Classification of sediments

Reading a ternary diagram

Finding Clay : Silt Ratio

Finding Mud : Sand Ratio
Objectives of Lab 5

Principle axes of sedimentary particles

Grain shape

Roundness

Sphericity

Form
Principle Axes of Sedimentary Particles
•
Each sedimentary particle has three
main axes:
1) L (a): Long axis
2) I (b): Intermediate axis
3) S (c): Short axis
Principle Axes of Sedimentary Particles
From Cheel (2005)
Principle Axes of Sedimentary Particles
From Cheel (2005)
Grain Shape (1)

Shape is a fundamental property of particles.

May provide important information about the
history of sediments.

The three shape properties are: Roundness,
Sphericity, and Form.
Grain Shape (2)
Grain shape depends on:
1) The original shape of the particle.
Mica particles are usually flat, whereas quartz
grains are rounded.
2) The internal structure and hardness.
3) The length of transportation (distance of
source area).
The longer a grain is transported, the more round
it becomes.
Roundness (angularity)
1.
A description of the degree of sharpness of the
corners and edges of grains.
2.
Gives information about the distance of transport,
the energy of transporting mechanisms, and the cycles
of erosion and redeposition.
3.
The simplest way to determine the roundness is by
visual comparison with standard forms.
4.
The most widely used chart for roundness is Powers’
chart.
Powers’ visual comparison chart
Roundness classes
After Pettijohn et al. (1973)
Wadell’s Roundness (Rw)
1.
The most accurate method, but it involves the
greatest effort and time.
2.
Rw is the ratio of the average radii of
curvature of the corners of a grain to the
radius of the largest inscribed circle within the
particle.
3.
The maximum possible value of Rw is 1.
Wadell’s Roundness (Rw)
N : number of corners
R : radius of the largest inscribed
circle within the particle
From Cheel (2005)
Sphericity
1.
The degree to which a particle resembles a
sphere.
2.
May be useful for understanding other properties
of the particle (e.g. settling velocity)
3.
Stoke’s Law of Settling applies accurately only to
spherical particles, and its error increases as the
sphericity decreases.
4.
Normally given the symbol “ψ”
Wadell’s Sphericity (ψ)
Wadell Operational Sphericity (WOS). Wadell (1932)
Sneed and Folk Sphericity (ψP)
• Also known as maximum projection sphericity (MPS).
• The most widely-used expression of sphericity.
Maximum Projection Sphericity (MPS). Sneed and Folk (1958)
Note

Calculating ψ and ψP requires the measurement
of dS, which is impractical for sand-size
sediments.
Riley Sphericity (ψR)

Riley sphericity relies on measurements that can
be taken from the two-dimensional view of a
sand grain as seen through a microscope.
Riley Sphericity (ψR)
From Cheel (2005)
Corey Shape Factor (S.F.)
• Very similar to MPS.
• Widely used by engineers to describe the overall shape
of grains
Corey Shape Factor (CSF). Corey (1949)
Form

The geometric form of detrital grains, and is expressed
using specific terms.

The two commonly used methods of describing the
form of particles are based on the ratios of dL, dI, and
dS.
1- Zingg diagram (1935)
2- Sneed and Folk classification (1958).
Zingg Diagram
From Cheel (2005)
Figure by MIT OCW
Sneed and Folk classification of grain form (1958)
From Cheel (2005)
Illenberger Form Diagram 1991
From Illenberger (1991)
Summary Chart
Quiz
• Find the form, MPS, and CSF for the
following grains
• Grain 1: L= 56, I= 49.5, S=39 (mm)
• Grain 2: L= 46, I= 36, S= 13 (mm)
Assignment 5
From Folk (1974)
(1) Determine the roundness and Riley sphericity of the given grain.
Assignment 5
(2) Using a caliper, measure the L, I, and S axes of the given grains then
complete the above table.
References

Cheel, R.J., 2005. Introduction to clastic sedimentology, Department of Earth
Sciences, Brock University, Ontario, Canada.

Folk, 1974.Petrology of sedimentary rocks.

Pettijohn F. J., Potter, P.E. and Siever, R., 1973. Sand and sandstone, pp. 617.
Springer-Verlag, Berlin.

Illenberger, W.K., 1991. Pebble shape (and size!). Journal of Sedimentary
Petrology, v. 61, p. 756–767.
The End