Glauconite

Not an IMA-approved name.

This mineral has long been described in the literature and recognised by most geologists but is not easily defined. The green "glaucony" mineral commonly found in marine sediments has been defined as a mixed-layer glauconite-smectite (Odin & Létolle, 1980), but in part it can actually be a mixture of minerals from the mica, serpentine and chlorite groups, rather than a pure glauconite mineral.

Rieder et al. (1998) defined glauconites somewhat vaguely as a counterpart of illite, as dioctahedral interlayer-deficient (i.e., (K+Na)<0.85) micas with (Mg+Fe)>Al and Fe³⁺ > Al in the octahedral sites (basically being interlayer-deficient celadonite subgroup minerals). The range of interlayer cation site occupancy is defined as 0.6-0.85; in the octahedral layer (Mg+Fe)>Al. It could be a subgroup with members defined similarly to the celadonite subgroup or just represent varieties of the celadonite subgroup.

Typically found as rounded aggregates or "pellets" of very fine-grained scaly clay particles, having a blue-green to yellow-green or grey-green colour. True glauconites may have been found in other environments (e.g., igneous rocks), but, before 1998, may have been classed as celadonite.