- 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 - Al-VI) [= 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: Mg2.5Al0.5, Al(2.167)square(0.833), Al1.75Li1.25, Li2Al (polylithionite), Fe22+Li, and Fe-3(2+) (annite). Trilithionite (Li1.5Al1.5), Li1.5Fe2+Al0.5, Fe22+Mg, and Mg2Fe2+ are also located on the perimeter of the polygon. IMA-siderophyllite (Fe22+Al) and muscovite (Al(2)square) 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 nigh and feal axes plotted orthogonally; trioctahedral: phlogopite, biotite, siderophyllite,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 - Al-VI) [= 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: Mg2.5Al0.5, Al(2.167)square(0.833), Al1.75Li1.25, Li2Al (polylithionite), Fe22+Li, and Fe-3(2+) (annite). Trilithionite (Li1.5Al1.5), Li1.5Fe2+Al0.5, Fe22+Mg, and Mg2Fe2+ are also located on the perimeter of the polygon. IMA-siderophyllite (Fe22+Al) and muscovite (Al(2)square) 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 nigh 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, KFe22+AlAl2Si2O10(OH)(2), and IMA-eastonite, KMg2AlAl2Si2O10(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 Fe2+, Fe3+, Fe2+ + Fe3+, 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…
