pentahydrocalcite
/ˌpɛntəhʌɪdrə(ʊ)ˈkalsʌɪt/Mineralogy
noun
A mineral supposedly consisting of a pentahydrated form of calcium carbonate, CaCO3·5H2O, the natural occurrence of which is uncertain.
Origin
Early 20th century; earliest use found in Mineralogy Magazine. From penta- + hydro- + calcite, after Russian pentagidrokal′cit.