An iron-rich dioctahedral mica that shows K deficiency and limited substitutions of Al in the tetrahedral sites. Glauconite is a series name (Rieder et al., 1998) with a generalized composition of K_0.8R³⁺_1.33R²⁺_0.67(Al_0.65Si_3.87)O₁₀(OH)₂, where ^viR²⁺/(^viR²⁺ + ^viR³⁺) >= 0.15, and ^viAl/(^viAl + ^viFe³⁺) <= 0.5 and shows no compositional overlap with celadonite. Often interstratified with smectite as the mixed-layered mineral glauconite/smectite. When mixed with other minerals or when referring to morphological features, the term “glauconitic” is appropriate. The mode of origin is not a criterion for identification.