Limestones 8XX 9XX 51X 7X7

Limestones

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.

Testing core with dilute hydrochloric acid. If all parts of the rock fizz a lot, the rock is limestone.
Testing core with dilute hydrochloric acid. If all parts of the rock fizz a lot, the rock is limestone.

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.

Limestone nodules (react with acid) in dark claystone (dark part does not react) in core (234). These types of nodules are common in ancient soils (paleosols).
Limestone nodules (react with acid) in dark claystone (dark part does not react) in core (234). These types of nodules are common in ancient soils (paleosols).

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
Hard, fine-grained limestone in core (904) (it would fizz with acid).
Hard, fine-grained limestone in core (904) (it would fizz with acid).
Shaly limestone in core (804). Dark-colored limestones are easily mistaken for siltstones and mudstones if not tested with acid. This sample would fizz (react) with acid, indicating it is limestone.
Shaly limestone in core (804). Dark-colored limestones are easily mistaken for siltstones and mudstones if not tested with acid. This sample would fizz (react) with acid, indicating it is limestone.
Carbonate sandstone in core (514). Carbonate sandstones are easily mistaken for normal (noncalcareous) sandstones if not tested with acid. The grains in these specimens are made of carbonate material, often rounded (oolitic), but sometimes angular.
Carbonate sandstone in core (514). Carbonate sandstones are easily mistaken for normal (noncalcareous) sandstones if not tested with acid. The grains in these specimens are made of carbonate material, often rounded (oolitic), but sometimes angular.

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:

2nd Digit

(89X, 99X, 519) Has fossil shells

(80X, 90X, 51X) No fossil shells

3rd Digit

Crossbedded (511)

Inclined lines (laminae) in a carbonate sandstone

Layered (512, 8X2, 9X2)

Alternating layers of dark and light limestone material

Layered limestone in core (902). This sample could easily be mistaken for interlayered siltstone and shale, if not tested with acid.
Layered limestone in core (902). This sample could easily be mistaken for interlayered siltstone and shale, if not tested with acid.

Massive (514, 8X4 9X4)

Homogeneous material with no obvious lines or masses

Massive (homogeneous) shaly limestone in core (804). This sample could easily be mistaken for a massive siltstone, if not tested with acid. The faint lines on this core are from core drilling, and do not represent true bedding.
Massive (homogeneous) shaly limestone in core (804). This sample could easily be mistaken for a massive siltstone, if not tested with acid. The faint lines on this core are from core drilling, and do not represent true bedding.

Mosaic (8X5 9X5)

Homogeneous material with angular fragments separated by lighter colored material; fragments maintain original orientation relative to adjacent pieces

Mosaic limestone in core (805). This type of feature in a carbonate generally represents ancient soil processes. It reacts with acid.
Mosaic limestone in core (805). This type of feature in a carbonate generally represents ancient soil processes. It reacts with acid.

Nodular (8X6 9X6)

Rounded or lumpy masses in a darker limestone matrix

Nodular limestone in core (906).
Nodular limestone in core (906). This type of feature could be mistaken for pebbles in a sandstone or siltstone, but both the nodules and matrix react with acid. The right-hand specimen also shows siderite-fillet root structures. This type of feature is common in some types of ancient soils.
Nodular limestone in core (906). This type of feature could be mistaken for pebbles in a sandstone or siltstone, but both the nodules and matrix react with acid. The right-hand specimen also shows siderite-fillet root structures. This type of feature is common in some types of ancient soils.

Brecciated (787, 797)

Angular limestone fragments in a limestone matrix, in which fragments are rotated from original position

Brecciated limestone in core (797). This variety is different from a nodular limestone in that the large particles are angular rather than smooth and rounded.
Brecciated limestone in core (797). This variety is different from a nodular limestone in that the large particles are angular rather than smooth and rounded.

 

Go Back to Step 1: Determing the major rock groups (first digit)

 

Last Modified on 2023-01-05
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