Grain Morphology

A) Controlled by:

  • provenance[e.g. original crystal and grain shape (eu-, sub-, or anhedral; prismatic, rounded)]
  • Quartz grains are typically slightly elongated (L:s = 1 to 2.5, avg = 1.5)
  • Detrital quartz elongation is typically along the c-axis (Wayland, 1939)
  • Ingerson and Ramisch (1942) determined that quartz grains in Igneous and metamorphic source rocks are typically elongated along their c-axis
  • 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
  • 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. Form
  2. Roundness
  3. Sphericity
  4. Surficial features

1. Form

  • 3-D relationships of principle grain axes
  • All grain axes perpendicular to one another
  • Long (L), intermediate (i), and short (s)
  • Four classes, first used by Zingg (1935) for pebble classification

2. Roundness

  • recognized as the curvature of corners and protuberances
  • 1st established by Wentworth (1919)
  • 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)
  • 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.
  • ρ (rho) = ∑(ri/R)/N
  • Where ri = indiv radii of corners, R= Radius of max inscribed circle, and N= number of corners)
  • Very angular, 0.15 sd, ρ =0-1.0
  • Angular, 0.2 sd, ρ =1-2
  • Subangular, 0.3 sd, ρ =2-3
  • Subround, 0.4 sd, ρ =3-4
  • Round, 0.5 sd, ρ =4-5
  • 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.
  • Limiting roundness (compositionally controlled)
  • Pebbles round quickly—
  • 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.
  • Granodiorite in San GabrielCanyon (Krumbein, 1940) rounded from .28 to .44 in 5.5 miles from source.
  • Sand is slow—
  • 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!
  • Most stream transport is less than 1000 km, therefore….
  • Eolian action is 100 to 1000 times greater than aqueous transport for same distance (cubes to spheres, Kuenen 1960).

3. Surface Features

  • Not quantifiable
  • Frosted, pitted, etched polished dull, percussion fractures, chatter marks, striations
  • 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 ZambeziRiver (Bond 1964).
  • Overprinting common—
  • Problems with inheritance of features from mixed provenance

4. Sphericity

  • Not the same as roundness
  • Important for settling velocities
  • 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?
  • Krumbein (1940’s)
  • Cube root of the ratio of the multiplicand of the 3 principle grain axes to the Longest dimension cubed
  • 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