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True and brittle micas: composition and solid-solution series

¹ Bautzner Strasse 16, D-02763 Zittau, Germany
² Institute of Earth Sciences, University of Potsdam, P.O. Box 601553, D-14415 Potsdam, Germany
³ GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany
⁴ Institute of Materials Chemistry, TU Ostrava, 17. listopadu 15/2172, CZ-708 33 Ostrava-Poruba, Czech Republic

^* E-mail: forhj{at}gfz-potsdam.de

Micas incorporate a wide variety of elements in their crystal structures. Elements occurring in significant concentrations in micas include: Si, ^IVAl, ^IVFe³⁺, B and Be in the tetrahedral sheet; Ti, ^VIAl, ^VIFe³⁺, Mn³⁺, Cr, V, Fe²⁺, Mn²⁺, Mg and Li in the octahedral sheet; K, Na, Rb, Cs, NH₄, Ca and Ba in the interlayer; and O, OH, F, Cl and S as anions. Extensive substitutions within these groups of elements form compositionally varied micas as members of different solid-solution series. The most common true K micas (94% of almost 6750 mica analyses) belong to three dominant solid-solution series (phlogopite–annite, siderophyllite–polylithionite and muscovite–celadonite). Their classification parameters include: Mg/(Mg+Fe_tot) [=Mg#] for micas with ^VIR >2.5 a.p.f.u. and ^VIAl <0.5 a.p.f.u.; Fe_tot/(Fe_tot+Li) [=Fe#] for micas with ^VIR >2.5 a.p.f.u. and ^VIAl >0.5 a.p.f.u.; and ^VIAl/(^VI Al+Fe_tot+Mg) [=Al#] for micas with ^VIR <2.5 a.p.f.u. The common true K micas plot predominantly within and between these series and have MgxLi <0.3 a.p.f.u. Tainiolite is a mica with MgxLi >0.7 a.p.f.u., or, for transitional stages, 0.3–0.7 a.p.f.u. Some true K mica end-members, especially phlogopite, annite and muscovite, form binary solid solutions with non-K true micas and with brittle micas (6% of the micas studied). Graphical presentation of true K micas using the coordinates Mg minus Li (= mgli) and ^VIFe_tot+Mn+Ti minus ^VIAl (= feal) depends on their classification according to ^VIR and ^VIAl, complemented with the 50/50 rule.

KEYWORDS: true micas, brittle micas, classification, solid-solution series, composition