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, 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). Theirclassification parameters include: Mg/(Mg+Fetot) [=Mg#] formicas with VIR >2.5 a.p.f.u. and VIAl <0.5 a.p.f.u.; Fetot/(Fetot+Li) [=Fe#] formicas with VIR >2.5 a.p.f.u. and VIAl >0.5 a.p.f.u.; and VIAl/(VIAl+Fetot+Mg) [=Al#] formicas with VIR <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 VIFetot+Mn+Ti minus VIAl (= feal) depends on theirclassification according to VIR and VIAl, complemented with the 50/50 rule.

The petrogenesis of sodic island arc magmas at Savo volcano, Solomon Islands

“…Biotites are relatively magnesium rich (Mg# typically 0.6-0.7). The majority of samples analysed are part of the annite-phlogopite series (sensu Tischendorf et al 2007), with a significant proportion sufficiently Mg-rich to be classified as phlogopite.…”

High-strain deformation and fluid infiltration diachronism of the middle crust: New Devonian–Permian Alice Springs ages (365–290 Ma) of shear zones in the Strangways Metamorphic Complex, Central Australia

“…Biotite is classified as annite (Tischendorf et al, 2007), although samples ASP-38 and ASP-70 are richer in Mg and are plotted close to the phlogopite field (see Fig. 4a).…”

“…Mineral structural formulae were calculated using the program MICA+ (Yavuz, 2003) for micas. The micas were classified on composition and octahedral site occupancy using the scheme of Tischendorf et al (2007).…”

The granite system near Betliar village (Gemeric Superunit, Western Carpathians): evolution of a composite silicic reservoir

“…Dioctahedral white mica corresponds to muscovite of secondary origin or magnesian and ferroan "phengite" in all granite types (Tischendorf et al 2007). Some of the white micas show low Fe concentrations (1.7 wt.…”