Mudstone is an extremely fine-grained sedimentary rock consisting of a mixture of clay and silt-sized particles. Terms such as claystone and siltstone are often used in place of mudstone, although these refer to rocks whose grain size falls within much narrower ranges and under close examination these are often technically mudstones. Shale is often used to describe mudstones which are hard and fissile (break along bedding planes). Marl is often used to describe carbonate-rich soft mudstones.


mudstone

Other specimens - Click the thumbnails to enlarge

Texture - clastic (only noticeable with a microscope).
Grain size - very fine-grained (< 0.06mm); clasts not visible to the naked eye.
Hardness - generally quite soft, but can be hard and brittle.
Colour - variable - black, white, grey, brown, red, green, blue etc.
Clasts - generally a mixture of clay minerals with any or all of quartz, feldspar (orthoclase, plagioclase), mica (biotite, chlorite, muscovite); can contain iron oxides (cause red or yellowish colouring); black colouring due to carbonaceous content and / or pyrite.
Other features - smooth to touch.
Uses - generally too soft to be of use.
New Zealand occurrences - extensive occurrence throughout the country (especially North Island), well exposed on many coastlines, e.g. Auckland, Taranaki, Wanganui, Wairarapa, Southland; generally found interbedded with sandstone.