Limestones 8XX 9XX 51X 7X7
Although limestones constitute a relatively small volume of coal-bearing strata, recognizing them is important because they are useful for establishing correlation and have distinctive properties that have an impact on mines’ roofs and floors. Most limestones are light colored (white, buff, or light gray) and very hard, which makes them easy to recognize in typical coal-bearing rock core. Some varieties of limestone are darker and fine-grained, however, and can easily be confused with shale. The definitive test is to apply dilute hydrochloric acid, which should result in distinctive effervescence.
Some shales and sandy shales contain irregular masses of carbonate material that will effervesce, but the darker matrix material does not, so testing the different components of rocks you suspect to contain limestone is important. Go to Rocks with Limestone Nodules to classify these specimens.
Some sandstones have carbonate cement between the sand grains and will effervesce, but the rock is not considered a limestone. If the rock is primarily made of quartz grains, go to the sandstone page to classify the specimen.
Some limestones are light colored and very hard, whereas others are darker, softer, having the appearance of shale or mudstone. A third variety has distinct rounded, sand-sized grains of carbonate material. These properties lead to the three main subdivisions of limestones:
- Hard limestone 9XX
- Shaly limestone 8XX
- Carbonate sandstone 51X
To complete the classification of the limestone specimen, note the presence or absence of fossil shell material (2nd digit) and determine the structure of the sample (3rd digit) according to the following categories:
(89X, 99X, 519) Has fossil shells
(80X, 90X, 51X) No fossil shells
Inclined lines (laminae) in a carbonate sandstone
Layered (512, 8X2, 9X2)
Alternating layers of dark and light limestone material
Massive (514, 8X4 9X4)
Homogeneous material with no obvious lines or masses
Mosaic (8X5 9X5)
Homogeneous material with angular fragments separated by lighter colored material; fragments maintain original orientation relative to adjacent pieces
Nodular (8X6 9X6)
Rounded or lumpy masses in a darker limestone matrix
Brecciated (787, 797)
Angular limestone fragments in a limestone matrix, in which fragments are rotated from original position