On Li-Bearing Micas: Estimating Li from Electron Microprobe Analyses and an Improved Diagram for Graphical Representation
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On Li-bearing micas: estimating Li from electron microprobe analyses and an improved diagram for graphical representation GERHARD TISCHENDORF Neumannstr. 106, 13189 Berlin, Germany BXRBEL GOTTESMANN,HANS-JORGEN FORSTER, AND ROBERT B. TRUM]]ULL GeoForsehungsZentrum Potsdam, Dept. 4.2, Telegrafenberg A50, 14473 Potsdam, Germany Abs~a~ Lithium may constitute an essential element in micas, yet it cannot be detected by the electron microprobe. Since Li is critical for correctly classifying micas and properly calculating their formulae, several methods have been proposed to overcome this analytical deficiency. We offer empirical relationships between Li20 and SiO2, MgO, F, and Rb in trioetahedral micas, and between Li20 and F as well as Rb in dioctahedral micas. The resultant regression equations enable lithium contents to be sufficiently well estimated from EPM analyses within the range of validity discussed. Secondly, we introduce an easy to handle, new diagram with the axis variables [Mg-Li] and [Fetot + Mn + Ti-AIva] for graphical representation and discuss its scientific rationale. Being based on absolute abundances of cations in the octahedral layer, the diagram provides a simple means to classify micas in terms of composition and oetahedral site occupancy, and it also allows compositional relationships between Li-bearing and Li-free mica varieties as well as between trioctahedral and dioetahedral micas to be displayed on a single, two-dimensional diagram. KEVWORDS: mica, lithium, correlation, classification, electron microprobe. Introduction because of their importance in evolved granites and related pegmatites and aplites. The basis of the work THE importance of micas for petrologic or metallo- is a compilation of several hundred Li-mica analyses, genetic studies of metamorphic and igneous rocks is many from sources in the literature and many others well established and has been the subject of a great from published and unpublished data sets of our own. deal of research (see Bailey, 1984, for an overview). Most of the latter represent micas from the The reason for their popularity is that micas form one Erzgebirge province, hosted in Varisean granites, or of the most common mineral groups, stable over a in a few cases from associated pegrnatites as well as wide range of pressure and temperature in rocks of from greisens in tin deposits. Analyses of mica from many kinds. They have a highly variable chemical metamorphic rocks of the Erzgebirge and of nearby composition and the ability to exchange components granodioritic and granitic complexes of Cadomian readily with fluids or solid phases as external and Varisean ages are also represented in the data conditions change. Being hydrous phases, micas are compilation (Fig. 1). sensitive to the fugacities of water, oxygen, fluorine Our objective is twofold. Firstly, we expand on the and other volatile species. The micas, therefore, are works of Stone et al. (1988) and Tindle and Webb useful as monitors of the pbysico-chemieal environ- (1990), and explore ways to indirectly estimate Li ment in which they grew. This aspect is particularly concentrations in both trioctahedral and dioetahedral important in the ease of granites, in many of which micas from electron microprobe analyses by micas are the only marie and hydrous phases present. empirical approaches based on element correlations. This paper is concerned with Li-bearing micas Secondly, we introduce a new diagram with the axis Mineralogical Magazine, December 1997, 1Iol. 61, pp. 809-834 Copyright the Mineralogical Society 810 G. TISCHENDORF ETAL. + + plutonic rocks ~-,,.Harzbur( ~....~B rogcken \ . + o p,ex Atb Altenberg~ ~/4~~)" Eib Eibenstock Nbz Niederbobritzsch Pob Pobershau SIh Schellerhau Znw Zinnwald FIG. 1. Simplified geological sketch map showing the sample localities of trioctahedral micas listed in Table 1. variables [Mg-Li] and [Fetot 4- Mn 4- Ti-AlVI], of ferric and ferrous iron by stoichiometry because which provides a simple means to classify micas in site vacancies are common, and because manganese terms of composition and octahedral site occupancy can be present in the divalent and trivalent state. (trioctahedral and dioctahedral series), and which Therefore Fe2+ and Fe 3+ must be treated together as also allows the compositional relationships between Fetot. The inability of the microprobe to analyse Li-bearing and Li-free mica varieties to be displayed lithium is a serious problem in evolved granites, on a single, two-dimensional diagram. aplites and pegrnatites, where Li is an essential Today, the vast majority of chemical analyses of component in both dioctahedral and trioctahedral mica composition for petrologic studies are micas (Tischendorf et al., 1969; Lapides et al., 1977; performed in situ by electron microprobe. Bulk Stone et al., 1988). Failure to analyse Li in micas analysis of mineral separates can be misleading or from this environment means a loss of important non-representative because more than one generation petrogenetic information as well as systematic errors of micas may be present in a given sample, and in calculating structural formulae (Tindle and Webb, because individual mica grains are commonly zoned, 1990). There are established methods of micro- especially in fractionated granites and related aplites analysis capable of analysing lithium, SIMS (FonteiUes, 1987; Monier et al., 1987; Hecht, 1993; (Henderson et al., 1989; Charoy et al., 1995) and Gottesmann et al., 1994a; Charoy et al., 1995). laser-ablation ICP-MS (Bea et aL, 1994), but they are Intergrowths, overgrowths and inclusions of other expensive and out of reach for most researchers; thus minerals in mica are other factors which pose no they do not represent practical alternatives to the great problem for the microprobe method but can electron microprobe at present. seriously affect the results of bulk chemical analysis. Fortunately, it turns out that lithium correlates One disadvantage of electron microprobe analyses strongly with several other elements in micas which of micas is that ferric and ferrous iron cannot be can be measured routinely by electron microprobe, distinguished. Micas are poorly suited for calculation and this offers a way to estimate Li contents from a Li-BEARING MICAS 811 microprobe analysis. Tindle and Webb (1990) Li utilized this in a study of element correlations in Li micas from a literature compilation, and they suggested the following equation to estimate Li from the Si content: Li20 = (0.287 x SiO2) - 9.552. nite~ These authors limit application of their equation to trioctahedral micas with less than 8 wt.% MgO. In this paper we take the same approach and examine element correlations in both trioctahedral and dioctahedral lithian micas separately, making a particular effort to find workable solutions for the ..~ \ ............... \ cases which Tindle and Webb (1990) found problematical, namely, the Mg-rich trioctahedral and the dioctahedral micas. OctahedralR 3+ (AI, 103+)+14+ R2+ (Ie 2+, In 2+, Mg) Mica compositions and covariances with lithium Fro. 2. Relationship between Li and octahedral cations R 2+ (Fe 2+, MI12+, Mg) and R 3+ (A1TM, Fe3+) + Ti 4+ Trioctahedral micas. The most frequently applied (according to Foster, 1960b). nomenclature for trioetahedral micas was introduced by Foster (1960a, b), and is based on the octahedral site occupancy. Li-bearing micas are represented in a triangular diagram with the comer variables Li; R 2+ large and irregular, with a weak negative correlation (Fe2+, Mn 2+, Mg); and octahedral R 3+ (A1, Fe3+) + and considerable overlap among the three composi- Ti4§ (Fig. 2). tional series which is partly caused by the presence of We examine below the correlations of Li20 with A1TM and A1xa. The Li-AI series shows the greatest other elements in the octahedral layer (TiO2, A1203, range in composition, with A1203 contents from less FeOtot, MnO, MgO) as well as with SIO2, F, and than 15 wt.% (Bargar et al., 1973; Skosyreva and Rb20. As recognized by Tindle and Webb (1990), Vlasova, 1983; Uhlig, 1992) to over 25 wt.% SiO2 alone does not permit estimation of lithium over (Heinrich, 1967). Total iron-silica distributions the entire range of mica compositions. Thus we (Fig. 3b) are more regular than alumina-silica, but focussed on the search for elements alternative or there is complete overlap of the Mg-biotite-Fe- supplementary to silica. The data used were taken biotite and siderophyllite varieties. The diagram of from our own analyses and from the literature which MgO and SiO2 (Fig. 3c) shows the most regular met the following criteria: (a) Li was determined, (b) variation and good discrimination among the three the analyses are commonly not older than 25 years mica series, which has been recognized before by (to ensure comparable data quality), and (c) other workers (e.g. Tindle and Webb, 1990). Note compositions of concentrates appeared to represent that we use a logarithmic scale for MgO to better pure trioctahedral compositions not contaminated by illustrate the correlation with SiO2 over the extreme dioctahedral micas. compositional range of MgO. The important feature Figures 3-6 plot analyses of trioctahedral micas of this diagram is that the correlation between MgO (screened by the above criteria a-c) which were and SiO2 is negative in the Li-Fe and Li-A1 series taken from the literature, our own published data (< 6 wt.% MgO), and positive in the Mg-Fe series. (Tischendorf et al., 1969, n = 66; Gottesmann et al., This transition in behaviour has an effect on the Li 1994b, n = 4) and about 100 unpublished analyses distribution in the low-Mg and high-Mg micas, and (see Appendix). Several representative analyses from must be born in mind when one considers the the unpublished data are given in Table 1 (see Fig. 1 correlations of Li with Mg and Si (see below). for sample localities). Details on the petrology and The variation of Li20 with other chemical geochemistry of the rocks from which the micas were components of trioctahedral micas is examined in separated are given by Tischendorf et al.