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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 Li2O and SiO2, MgO, F, and Rb in trioctahedral micas, and between Li2O 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-AlVI] 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 octahedral site occupancy, and it also allows compositional relationships between Li-bearing and Li-free mica varieties as well as between trioctahedral and dioctahedral micas to be displayed on a single, two-dimensional diagram.

Xenotime, from a geochemically heterogeneous series of mildly to strongly peraluminous granites of the Erzgebirge, Germany, displays extended compositional variability with respect to abundances of HREE, Y, U, and Th. With few exceptions, the maximum and minimum concentrations for the lanthanides and actinides exceed those noted for this mineral from other geologic environments. Xenotime chemistry is dominated by the theoretical end-members HREE-PO 4 and YPO 4 (Ͼ90 mol% in total). Typical xenotime grains from the Erzgebirge granites and from other granites worldwide contain 70-80 mol% YPO 4 , and 16-25 mol% HREE-PO 4. This study documents the occurrence of xenotime, abnormally rich in HREE in place of Y, with up to 45 mol% HREE-PO 4. Substitutions of thorite-coffinite, (Th,U,Pb)SiO 4 , brabantite, (Ca,Th,U)(PO 4) 2 , and monazite, (La-Sm)PO 4 , are of minor importance, reaching maximum levels of 5-6 mol% each with a total contribution typically K10 mol%. Typically, the incorporation of actinides in xenotime from these and other granites, as well as from other rocks, is dominated by thorite-coffinite substitutions. However, in some xenotime grains, the U and Th concentrations are largely accounted for by the substitution mechanism 2 (REE,Y) 3ϩ ↔ (Th,U) 4ϩ ϩ Ca 2ϩ. The total lanthanide and actinide contents in xenotime and host granite are not strongly correlated. However, formation of xenotime unusually rich in HREE occurs only in the A-type Limica granites that are strongly enriched in these elements. The shapes of HREE patterns of xenotime and host rock are similar which, combined with mass-balance calculations, indicate the importance of this mineral in affecting the HREE as well as Y evolution of late-stage granitic melts. Late xenotime may show strong Y/Ho fractionation, thus recording the decoupling of Y and the HREE, which can occur in the latest stages of crystallization of evolved granites and which may be associated with deuteric alteration. Note: A/CNK ϭ molar Al 2 O 3 /(Na 2 O ϩ K 2 O ϩ CaO). * Lower values are due to post-magmatic U mobility.

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