Grain Morphology
A) Controlled by:
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provenance [e.g. original crystal and grain shape (eu-, sub-, or anhedral;
prismatic, rounded)]
o Quartz grains are typically slightly elongated (L:s = 1 to 2.5, avg = 1.5)
o Detrital quartz elongation is typically along the c-axis (Wayland, 1939)
o Ingerson and Ramisch (1942) determined that quartz grains in Igneous and
metamorphic source rocks are typically elongated along their c-axis
o Quartz also has both weak prismatic (parallel to c-axis) and rhombehedral
cleavages (at a fixed angle to the c-axis), causing quartz fragments to be
preferentially elongated along the c-axis (Bloss, 1957; Moss, 1966)
weathering (spheroidal, etc.)
abrasion (e.g. waves vs. fluvial transport)
differential sorting in depositional systems
o Settling velocities vary as much by shape as they do by density (Briggs,
McCulloch, and Moser, 1962; Krumbein, 1942; and Sneed and Folk,
1958)
diagenesis (e.g. overgrowths)
Folk (1974) suggested morphology is the result of structure (original material), process,
and stage (amount of time for alteration).
B) Four characteristics of grain morphology
1.
2.
3.
4.
Form
Roundness
Sphericity
Surficial features
© WB Leatham, 2005
1. Form
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3-D relationships of principle grain axes
o All grain axes perpendicular to one another
o Long (L), intermediate (i), and short (s)
Four classes, first used by Zingg (1935) for pebble classification
2/3
1
di/dL
0
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Oblate
Equant
(disc or plate)
(L=i=s)
(cube or sphere)
(L=i=s)
Bladed
Prolate
(L≠i≠s)
(cylinder or rod)
(L≠i≠s)
ds/di
2/3
1
2. Roundness
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recognized as the curvature of corners and
protuberances
1st established by Wentworth (1919)
o ri/R (where ri is the radius of
curvature of the sharpest corner,
and R is the ½ the largest
diameter.
Wadell(1932) refined roundness to as
the ratio of the average radius of
curvature of the corners to the radius of
curvature of the maximum inscribed sphere
(sort of a two-dimensional section of a grain)
© WB Leatham, 2005
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Powers (1953) and Pettijohn (1977) defined six roundness classes where the
class limits are basic a 2X geometric progression. Folk (1955) assigned a rho
values to these class limits similar to Krumbeins (1938) phi scale for size.
o ρ (rho) = ∑(ri/R)/N
 Where ri = indiv radii of corners, R= Radius of max inscribed
circle, and N= number of corners)
o Very angular, 0.15 sd, ρ =0-1.0
o Angular, 0.2 sd, ρ =1-2
o Subangular, 0.3 sd, ρ =2-3
o Subround, 0.4 sd, ρ =3-4
o Round, 0.5 sd, ρ =4-5
o Well round, 0.65 sd, ρ =5-6
Use of graphical comparator
Perfect balls =6
Average grain 2.5 ρ (subangular)
47% reduction in volume from cube to sphere through corner abrasion (try
salt)
roundness and length of travel closely related in large clasts (pebbles), rapid at
first, slower later.
o Limiting roundness (compositionally controlled)
Pebbles round quickly—
o angular to well rounded requires between 11 miles (limestone
(Plumley, 1948 in black hills) 45 miles (quartzite (Schlee, 1957) in
Maryland; granite pebbles in Poland (78 miles)(Unrug, 1957); and
gabbro (Kuenen, 1956) in 87 miles.
o Granodiorite in San Gabriel Canyon (Krumbein, 1940) rounded from
.28 to .44 in 5.5 miles from source.
Sand is slow—
o Quartz = 1% loss of weight in 10,000 km of transport experimentally
(Kuenen 1958). Thiel (1940) found a 22% loss in 100 hours of
abrasion mill, equating to around 5000 miles of transport!
o Most stream transport is less than 1000 km, therefore….
o Eolian action is 100 to 1000 times greater than aqueous transport for
same distance (cubes to spheres, Kuenen 1960).
© WB Leatham, 2005
3. Surface Features
o Not quantifiable
o Frosted, pitted, etched polished dull, percussion fractures, chatter
marks, striations
o Generally show last sedimentary process, easily removed
 0.35 mile transport of limestone pebbles in fluvial system
removes glacial striae w/ no change in shape (Krumbein 1935).
 Aeolian frosted grains of the Kalahari lose frost in less than 40
miles of transport in Zambezi River (Bond 1964).
o Overprinting common—
o Problems with inheritance of features from mixed provenance
4. Sphericity
o Not the same as roundness
o Important for settling velocities
o Wadell (1935)
 Cube root of the ratio of the volume of a grain determined by
immersing in water to to volume of a sphere circumscribed
around the longest dimension of the clast.
 Impractical. Why?
o Krumbein (1940’s)
 Cube root of the ratio of the multiplicand of the 3 principle
grain axes to the Longest dimension cubed
o Riley Sphericity (2-D)
 Easy to calculate, thin section or drawing
 Square root of the ratio of the diameter of the largest inscribed
circle to the diameter of the largest circumscribed circle around
a grain)
© WB Leatham, 2005