coloured by mineral or organic admixtures
1clay mineral — any of a group of hydrous aluminum silicate minerals, as kaolinite, illite, and montmorillonite, that constitute the major portion of most clays. [1945 50] * * * Any of a group of important hydrous aluminum silicates with a layered structure and… …
2silica mineral — Any of the forms of silicon dioxide (SiO2), including quartz, tridymite, cristobalite, coesite, stishovite, melanophlogite, lechatelierite, and chalcedony. Various kinds of silica minerals have been produced synthetically. * * * Introduction… …
3duricrust — /door i krust , dyoor /, n. a hard crust that forms on or in soil in semiarid climates owing to cementation of soil particles. Cf. caliche, hardpan (defs. 1, 2). [ < L dur(us) hard + I + CRUST] * * * ▪ geology Introduction surface or near… …
