(possibly obsolete) A soft earthymineral, of a brownish colour, which has a greasy or soapy feel and is used as a filler in crayons.
1805, Robert Jameson, A Treatise on the External Characters of Minerals, page 76:
It soils either strongly, as chalk and mountain soap; or slightly, as molybdana, lead glauce, and graphite. Besides this, there are three other distinctions connected with this character to be attended to.
1848, American Journal of Science and Arts, page 72:
[…] seven different minerals possessing the same formula, namely, fahlunite, esmarkite, pyrargillite, bole, iron lithomarge, halloylite from La Vouth and from Thiviers, and mountain soap from Thuringia.
1859, Gustav Bischof, Elements of Chemical and Physical Geology ..., page 311:
Sometimes, after removing the coating of calcspar from the crystals, they appear to be covered with a mass resembling steinmark or mountain soap. The compact and crystalline hornblende occurs almost everywhere as a constituent of the[…]
