Although coal lithotypes were not categorized using the same methodology as for other kinds of rock, the Ferm classification includes codes for the spectrum of descriptions commonly found in company drillhole records. The focus of those descriptions is on distinguishing gross coal quality on the basis of observable properties. The main properties are brightness, banding, presence of sulfur-bearing minerals, and relative density.

Most coal types contain layers or bands of bright, shiny coal separated by duller material. The dull material can range from siliclastic rock to low-density coal. Distinguishing density can be very subjective, so a scratch test should be performed on the material. Lighter-colored powder usually indicates more siliclastic content, whereas coaly material will generate a darker brown powder.

Three densities of material form the basis of classification:

  • Shale: High density
  • Bone: Medium density
  • Coal: Low density

Mixtures of these constituents in different proportions result in the principal classes of coal.

021: Common banded coal  
022: Coal with bone layers 032: Bone with coal layers
023: Coal with bone streaks 033: Bone with coal streaks
024: Dull or cannel coal 034: Bone or impure coal
027: Coal with shale layers 037: Bone with shale layers
028: Coal with shale streaks 038: Bone with shale streaks
029: Coal with pyrite streaks 039: Bone with pyrite streaks

In low-density banded coals (021), the matrix material ranges from very dull to very bright, depending on its particulate composition.

Hand sample of a bright banded coal where the matrix material is almost as bright as the bands.

Hand sample of a bright banded coal where the matrix material is duller than the bands.

There are two varieties of low-density dull coal (024): cannel and splint. Cannel is usually dark colored, homogeneous, and has a conchoidal fracture. Splint has a light steely gray color and is commonly associated with very thick bands of bright coal.

Hand sample of a dull cannel coal. Note the uniform, smooth texture and conchoidal fracture.
Hand sample of a dull splint coal. Note the contrast between the bands and matrix and the unusually thick bands.

To learn more about coal lithotypes (not using the Ferm classification) go to the Coal Rank, Type, and Grade pages.

 

Last Modified on 2017-06-30
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