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Abstract A system based on variation of the octahedrally coordinated cations is proposed for graphical presentation and subdivision of tri- and dioctahedral K micas, which makes use of elemental differences (in a.p.f.u.): (Mg – Li) [= mgli ] and (Fe tot + Mn + Ti – VI Al) [= feal ]. All common true tri- and dioctahedral K micas are shown in a single polygon outlined by seven main compositional points forming its vertices. Sequentially clockwise, starting from Mg 3 (phlogopite), these points are: Mg 2.5 Al 0.5 , Al 2.167 □ 0.833 , Al 1.75 Li 1.25 , Li 2 Al (polylithionite), Fe 2 2+ Li, and Fe 3 2+ (annite). Trilithionite (Li 1.5 Al 1.5 ), Li 1.5 Fe 2+ Al 0.5 , Fe 2 2+ Mg, and Mg 2 Fe 2+ are also located on the perimeter of the polygon. IMA-siderophyllite (Fe 2+ 2 Al) and muscovite (Al 2 □) plot inside. The classification conforms with the IMA-approved mica nomenclature and differentiates among the following mica species according to their position in a diagram consisting of mgli and feal axes plotted orthogonally; trioctahedral: phlogopite, biotite, siderophyllite, annite, zinnwaldite, lepidolite and tainiolite; dioctahedral: muscovite, phengite and celadonite. Potassium micas with [Si] <2.5 a.p.f.u. including IMA-siderophyllite, KFe 2 2+ AlAl 2 Si 2 O 10 (OH) 2 , and IMA-eastonite, KMg 2 AlAl 2 Si 2 O 10 (OH) 2 seem not to form in nature. The proposed subdivision has several advantages. All common true, trioctahedral and dioctahedral K micas, whether Li-bearing or Li-free, are shown within one diagram, which is easy to use and gives every mica composition an unambiguously defined name. Mica analyses with Fe 2+ , Fe 3+ , Fe 2+ + Fe 3+ , or Fe tot can be considered, which is particularly valuable for microprobe analyses. It facilitates easy reconstruction of evolutionary pathways of mica compositions during crystallization, a feature having key importance in petrologically oriented research. Equally important, the subdivision has great potential for understanding many of the crystal-chemistry features of the K micas. In turn this may allow one to recognize and discriminate the extent to which crystal chemistry or bulk composition controls the occurrence of some seemingly possible or hypothetical K mica.