True and brittle micas: composition and solid-solution series

Abstract Micas incorporate a wide variety of elements in their crystal structures. Elements occurring in significant concentrations in micas include: Si, IV Al, IV Fe 3+ , B and Be in the tetrahedral sheet; Ti, VI Al, VI Fe 3+ , Mn 3+ , Cr, V, Fe 2+ , Mn 2+ , Mg and Li in the octahedral sheet; K, Na, Rb, Cs, NH 4 , 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). Theirclassification parameters include: Mg/(Mg+Fe tot ) [=Mg#] formicas with VI R >2.5 a.p.f.u. and VI Al <0.5 a.p.f.u.; Fe tot /(Fe tot +Li) [=Fe#] formicas with VI R >2.5 a.p.f.u. and VI Al >0.5 a.p.f.u.; and VI Al/( VI Al+Fe tot +Mg) [=Al#] formicas with VI R <2.5 a.p.f.u. The common true K micas plot predominantly within and between these series and have Mg6Li <0.3 a.p.f.u. Tainiolite is a mica with Mg6Li >0.7 a.p.f.u., or, fortr ansitional 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 VI Al (= feal) depends on theirclassification according to VI R and VI Al, complemented with the 50/50 rule.