Classification of Sedimentary Rocks

TEXTURES:  For the purpose of classification, the primary attribute of texture is the grain size.  The table below describes the important grain size ranges.

      
RANGE of 
GRAIN SIZE
(in mm)
SIZE NAME
DESCRIPTION
  
APHANITIC
< 1/256
MUD
 About the grain size of flour or baby powder.  Luster can be vitreous, waxy, or earthy.  Surface of hard material generally feels smooth like glass.  Soft materials feel like powder.
  
PHANERITIC
1/256 to 1/16
SILT
 Even at the fine end of the range, surface of material feels ever so slightly abrasive or gritty, like the very finest sand paper.  At the coarse end of the range, materials feel like fine sand paper.
1/16 to 2
SAND
 Surface of material typically feels like medium to coarse sand paper.  Think in terms of the grain size of beach sand.  At the coarse end of the range, you can measure grains up to about the size of a match head.
> 2
GRAVEL
 Coarser than the size of a match head.
  


MINERALOGY and OTHER CONSTITUENTS:  Sedimentary rocks consist of the (1) products of mechanical weathering, (2) products of chemical weathering, (3) chemical precipitates, (4) fragments of the hard parts of animals, plant material, etc.  The table below describes the important minerals and other materials.
 MATERIAL
  
PROPERTIES
Why it's in 
Sedimentary Rock
   
 MINERALS Quartz Colorless, Gray, other colors too.  No cleavage. H=7 (Scratches glass). Resistant to chemical and mechanical weathering.
Chert Looks quartz, but luster is waxy, and generally has a "milky" look.  No cleavage. H=7 (Scratches glass). Chemical precipitate, usually in limestone.
Clay minerals Any earthy color.  Fresh surface smell musty, especially when you breath on the surface Product of chemical weathering of feldspar.
Potassium Feldspar Usually white or pink in sedimentary rocks.  Cleavage. H=6 (Scratches glass). There's so much feldspar in the Earth's crust that a lot of it gets buried in sediment before it breaks down to clay minerals.
Calcite Most often white or gray, but can be any color.  H=3 (Softer than glass).  Reacts vigorously (effervesces) with dilute HCL.  When coarse, 3-way cleavage is observable.  Chemical precipitate from sea water. Also lots of fossil material.
Dolomite Most often white or gray, but can be any color.  H=3 (Softer than glass).  Reacts with dilute HCl, but reluctantly (may react only in powder form).  When coarse, 3-way cleavage is observable. Chemical precipitate from sea water.
Gypsum Typically earthy, white, but may show red staining by hematite. H=2 (Softer than glass).  Does not react with HCl. Chemical precipitate from sea water.
Halite Colorless, white, pink. H=2.5 (Softer than glass).  Tastes SALTY.  May show 3-way cleavage. Chemical precipitate from sea water.
  
OTHER MATERIALS Rock Fragments Igneous, Metamorphic, Sedimentary rock fragments are all possible. Incomplete mechanical breakdown.
Coal JET BLACK.  May be earthy (bituminous) or glassy (anthracitic).  H=2-2.5. BURNS. Forms from plant material when buried in sediment.
Fossils Shell fragments, bone fragments, etc. Incomplete mechanical or chemical breakdown.
Ooids Curious little (~1mm) spheres of calcite in limestone.  Show concentric layered structure. Chemical precipitate.
   


CLASSIFICATION
Determine whether the rock is APHANITIC or PHANERITIC.  Then proceed to the appropriate chart:

[APHANITIC]---[PHANERITIC]

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