These two rock groups are easily identified by their color and texture. Unweathered specimens of both have distinctive red, orange, yellow, or brown colors.

Ironstone is very hard, and if you scrape a powder with a knife blade, it will effervesce weakly with dilute hydrochloric acid. Ironstones are usually composed of iron-carbonate minerals such as siderite and iron oxides such as goethite, hematite, and limonite, but the exact mineral composition is not required for classification. Most shales contain some ironstone as nodules or void fillings, and this class is restricted to rocks that have more than 50 percent ironstone.

Flint clays are highly variable in color, have a distinctive milky luster, and a conchoidal fracture. Flint clay is a variety of clay-rich rock in which the clay mineral is predominantly kaolinite.

Example of a massive ironstone bed in core (074).
Photo of the butt end of a flint clay (095) core specimen showing the distinctive milky luster and conchoidal fracture.

Ironstones and flint clays have additional fabrics according to one of the following categories:

  • 0X4 Massive: Homogeneous material with no obvious lines or marks
  • 0X5 Mosaic: Divided into fragments; each piece maintains it original position relative to adjacent pieces
  • 0X1 Brecciated: Divided into fragments; pieces have rotated relative to adjacent pieces
  • 0X6 Nodular: Rounded or irregular masses floating in a matrix of different material
  • 0X7 Oolitic (Pisolitic): Numerous, small rounded grains set in a matrix of different material
Mosaic flint clay in core (095). Note the fragments maintain their original positions.
Brecciated flint clay in core (091). Note the fragments have rotated from their original position.
Nodular ironstone in core (076).
Oolitic ironstone in core (077).


To learn more about weaknesses caused by churned and root-penetrated clays in mine roofs go to Weakly-bedded shale roofs for cores that exhibit shale and clay (but not coal) above the mined coal, and Coal rider roofs for core with shale, clay and coal rider in the expected roof rock above the mined coal.


Last Modified on 2018-06-22
Back to Top