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A new graphical presentation and subdivision of potassium micas

¹ Bautzner Straße 16, D-02763 Zittau, Germany
² Department of Geochemistry, Mineralogy, and Mineral Resources, Charles University, Albertov 6, CZ-12843 Praha 2, Czech Republic
³ Institute of Earth Sciences, University of Potsdam, P.O. Box 601553, D-14415 Potsdam, Germany
⁴ GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany
⁵ Department of Geological Sciences, University of Maine, Orono, ME 04469-5790, USA

^* E-mail: for{at}geo.uni-potsdam.de

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 –^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: Mg_2.5Al_0.5, Al_{2.167 0.833}, Al_1.75Li_1.25, Li₂Al (polylithionite), Fe₂²⁺ Li, and Fe₃²⁺ (annite). Trilithionite (Li_1.5Al_1.5), Li_1.5Fe²⁺ Al_0.5, Fe₂²⁺Mg, and Mg₂Fe²⁺ are also located on the perimeter of the polygon. IMA-siderophyllite (Fe²⁺₂Al) and muscovite (Al₂) 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, 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.

KEYWORDS: K micas, trioctahedral, dioctahedral, celadonite, classification

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