mountain soapn. [after German Bergseife (A. G. Werner Cronstedt's Versuch einer Mineralogie (1780) 189)] Mineralogy (now historical) a kind of dark earthy clay, probably a halloysite.
ΚΠ
1794 R. KirwanElements Mineral. (ed. 2) I. 189 The earth called mountain soap.
1993 A. M. ClarkHey's Mineral Index (ed. 3) 427/2Mountain soap, syn. of Bergseife (= halloysite).