Halloysite Halloysite is an aluminosilicate clay mineral with the empirical formula Al2Si2O5(OH)4. Its main constituents are aluminium (20.90%), silicon (21.76%) and hydrogen (1.56%). Wikipedia https://en.wikipedia.org/wiki/Halloysite Added to support Critical Zone Observatory (CZO) data use cases. ODM2 Variable Name Controlled Vocabulary ODM2 Working Group A vocabulary for describing the name of Variables.