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True and Brittle Micas: Composition and Solid-Solution Series

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True and Brittle Micas: Composition and Solid-Solution Series Mineralogical Magazine, June 2007, Vol. 71(3), pp. 285–320 True and brittle micas: composition and solid-solution series 1 2, 3 4 G. TISCHENDORF , H.-J. FO¨ RSTER *, B. GOTTESMANN AND M. RIEDER 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 [Received 8 May 2007; Accepted 11 September 2007] 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, VI 3+ 3+ 2+ 2+ Fe ,Mn ,Cr,V,Fe ,Mn , 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, siderophylliteÀpolylithionite and muscoviteÀceladonite). Theirclassification VI VI parameters include: Mg/(Mg+Fetot) [=Mg#] formicas with R >2.5 a.p.f.u. and Al <0.5 a.p.f.u.; VI VI VI VI Fetot/(Fetot+Li) [=Fe#] formicas with R >2.5 a.p.f.u. and Al >0.5 a.p.f.u.; and Al/( Al+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, fortransitionalstages, 0.3 À0.7 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 VI VI VI VI Fetot+Mn+Ti minus Al (= feal) depends on theirclassification accordingto R and Al, complemented with the 50/50 rule. KEYWORDS: true micas, brittle micas, classification, solid-solution series, composition. Introduction Following an idea and proposal of Charles { MICAS are widespread in igneous, metamorphic Guidotti , ourcolleague, friendand co-authorof a and sedimentary rocks. Their crystal structure recent paper on micas (Tischendorf et al., 2004), accommodates a plethora of elements, leading to we present in this paper a survey and analysis of a large and diverse mineral group. The composi- composition and solid solution in the mica group, tional diversity of micas has led to numerous comprising trioctahedral and dioctahedral, attempts at classification and graphical presenta- common and uncommon true K micas, other tion (Foster, 1960a,b;Tro¨ger, 1962; Rieder et al., alkali-element-bearing micas, and brittle micas. 1970, 1998; Koval et al., 1972; Gottesmann and The principles behind the subdivision, and the Tischendorf, 1978; Cˇ erny´ and Burt, 1984; Monier graphical presentation adopted, follow the recom- and Robert, 1986; Jolliff et al., 1987; Burt, 1991; mendations of the Mica Sub-committee of the Tischendorf et al., 1997, 2004; Sun Shihua and International Mineralogical Association’s Yu Jie, 1999, 2000). Commission on New Minerals, Nomenclature * E-mail: [email protected] DOI: 10.1180/minmag.2007.071.3.285 { Died 19 May 2005 # 2007 The Mineralogical Society TISCHENDORF ET AL. and Classification (IMA-CNMNC) (Rieder et al., (2) uncommon brittle micas (0.4%): contain V, 1998) and the IMA principles of mineral Be, Fe3+, Ti orS and O as majorelements, in classification. This papertreatsthe micas only in addition to Ca orBa [in anandite, bityite, terms of their compositions, an approach that chernykhite, oxykinoshitalite]. permits a quick and easy classification of any mica. Common true K micas Principles of classif|cation Methods Ourclassification scheme uses fourmajor, This study is based on mica analyses obtained by octahedrally-coordinated cations (Mg, Fetot, different analytical methods (wet chemical, X-ray VIAl, Li) togetherwith the existence of solid fluorescence, electron- and ion-microprobe solutions. It considers only IMA-approved end- analysis). Data sources not listed in the member names and strictly applies the 50/50 rule References are given in previous publications (e.g. Nickel, 1992). (e.g. Tischendorf et al., 1997, 1999, 2001a,b, The main parameters in this classification are 2004) orarenoted in Deer et al. (2003). VIR, VIAl and the product Mg6Li (all in a.p.f.u.). The crystallo-chemical formulae were calcu- The value of VIR = 2.5 differentiates trioctahedral lated on the basis of 22 cation charges, except for from dioctahedral micas. The limiting value oxy-micas with 24 cation charges. The concentra- between micas of the phlogopite–annite and VI tion of Li2O, if essential but not known, was siderophyllite–polylithionite series ( Al = 0.5) estimated using the empirical equations published results from the application of the 50/50 rule. The by Tischendorf et al. (2004, theirAppendix). same is valid forthe parameterMg 6Li, which separates tainiolite micas (Mg6Li >0.3) from all other trioctahedral micas (Mg6Li <0.3). Results Compositionally, micas are subdivided into true (1) Phlogopite–annite series micas, with monovalent cations in the interlayer, trioctahedral (VIR >2.5); VIAl <0.5; Mg6Li <0.3 and brittle micas containing divalent cations in the end-members: phlogopite KMg3[AlSi3O10](OH)2, 2+ interlayer. Our evaluation of mica analyses yielded annite KFe3 [AlSi3O10](OH)2 the following quantitative subdivisions (in percen- classification according to the ratio Mg/ tages of the total population of ~6750 analyses). (Mg+Fetot) [= Mg#] True micas (96.8% of all analyses) comprise: phlogopite: Mg# >0.5 (1) common true K micas (93.1%): [annite, annite: Mg# <0.5 celadonite, muscovite, phlogopite, polylithionite, (2) SiderophylliteÀpolylithionite series siderophyllite, tainiolite]; trioctahedral (VIR >2.5); VIAl >0.5; Mg6Li <0.3 2+ (2) uncommon true K micas (1.6%): contain a end-members: siderophyllite KFe2 Al 2+ 3+ minorelement (Mn ,Fe orF) in an above- [Al2 Si2 O 10](OH)2 , polylithionite average concentration [fluorannite, masutomilite, KLi2Al[Si4O10]F2; montdorite, shirozulite, tetra-ferriannite, tetra- classification according to the ratio Fetot/ ferriphlogopite], an uncommon element (Zn, V, (Fetot+Li) [= Fe#] Cr, Mn3+ orB) as majorelement [in boromusco- siderophyllite: Fe# >0.5 vite, chromphyllite, hendricksite, roscoelite, polylithionite: Fe# <0.5 norrishite] or a common element in an uncommon (3) Tainiolite group coordination (e.g. Na+ in shirokshinite); trioctahedral (VIR >2.5); Mg6Li fortainiolite (3) uncommon true non-K micas (2.1%): sensustricto >0.7, fortainiolitic micas 0.3 À0.7 contain the monovalent cations Na, Rb, Cs or end-member: tainiolite KLiMg2[Si4O10]F2 NH4 as majorelement substituting forK [in (4) MuscoviteÀceladonite series aspidolite, ephesite, nanpingite, paragonite, preis- dioctahedral (VIR <2.5); Mg6Li <0.3 werkite, sokolovaite, tobelite]. end-members: muscovite KAl2&[AlSi3O10] 3+ Brittle micas (3.2% of all analyses) comprise: (OH)2, celadonite: KMgFe &[Si4O10](OH)2; VI VI (1) common brittle micas (2.8%): contain Ca or classification according to Al/( Al+Fetot+Mg) Ba as majorcations proxyingforK [in clintonite, [= Al#] ferrokinoshitalite, ganterite, kinoshitalite, muscovite: Al# >0.5 margarite]; celadonite: Al# <0.5 286 CLASSIFICATION OF MICAS Celadonites are further subdivided according to Li maximum in the frequency distribution of natural et al. (1997) as confirmed by Rieder et al. (1998). muscovite compositions is close to the end- membercomposition. Two frequency peaks Distribution of natural compositions in the mgliÀfeal occurin the phlogopite–annite series, and one plot occurs in the siderophyllite–polylithionite join. Mica compositions may be described in two- Very few compositions plot in the relatively large dimensional triangular or three-dimensional plots areas in the Mg-Al sector (lower right) and in (cf. Tischendorf et al., 2004, fora compilation). smallerareas in the Fe-Li sector(upperleft) of the We have proposed a simple two-dimensional plot. Figure 2 shows the numbers of cations per presentation according to the occupancy of the formula unit for compositions in the phlogopite– octahedral sheet, using the parameters Mg minus annite, siderophyllite–polylithionite and VI VI Li (= mgli) and Fetot+Mn+Ti minus Al (= muscovite–celadonite series. Figure 3 shows feal) a.p.f.u. (Tischendorf et al., 1997, 2004). species resulting from the application of the Figure 1 shows common true K micas, 50/50 rule. Joins combining related end-members excluding only tainiolite and celadonite. The are displayed and so are the half-way divides. FIG.1.mgli/feal plot of ~6100 common true K-mica compositions (excluding tainiolite and celadonites). Mica end- members, ideal members, and one theoretical component are indicated. Isolines show relative densities of composition points (1, 5, 10, 20, 30%) normalized to the density maximum at mgli = 0.05 and feal = À1.70 (the most frequent muscovite composition), which is taken as 100%. Abbreviations: ann À annite, eas À eastonite, hyp-mus À hyper-muscovite, mus À muscovite, phl À phlogopite, pol À polylithionite, sid À siderophyllite, trans-mus À transitional muscovite, tri À trilithionite. 287 TISCHENDORF ET AL. FIG. 2. PhlogopiteÀannite and siderophylliteÀpolylithionite series, and the muscovite portion of the muscov- iteÀceladonite series plotted in the mgli/feal diagram. Mica end-members, ideal members, and one theoretical component are indicated. The boundary between the first two series (VIAl = 0.5) and theirboundarywith muscovite (VIR = 2.5) is marked by dashed lines. Note that two boundaries are shown in the transitional area between annite and siderophyllite (both for VIAl = 0.5), one at VIR = 3.0, and anotherat VIR = 2.75. See Fig. 1 forabbreviations.
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