Describing Coal 02X 03X
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.
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.