A NEW GRAPHICAL PRESENTATION AND SUBDIVISION OF POTASSIUM MICAS

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 (Fetot + Mn + Ti - VIAl) [= 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 Mg3 (phlogopite), these points are: Mg2.5Al0.5, Al2.167⃞0.833, Al1.75Li1,25, Li2Al (polylithionite), Fe2+2Li, and Fe2+3 (annite). Trilithionite (Li1.5Al1.5), Li1.5Fe2+Al0.5, Fe2+2Mg, and Mg2Fe2+ are also located on the perimeter of the polygon. IMA-siderophyllite (Fe2+2Al) and muscovite (Al2⃞) 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, KFe2+2AlAl2Si2O10O(OH)2, and IMA-eastonite, KMg2AlAl2Si2O10O(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 Fetot 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.