CRYSTAL CHEMISTRY OF SOME TRIOCTAHEDRAL AND DIOCTAHEDRAL MICAS COEXISTING IN METAMORPHIC SEQUENCES FROM N. W. MAINE

Samples under study are coexisting trioctahedral and dioctahedral micas from North Western Maine, namely from Oquossoc and Bryant Pond. This research, promoted by C. V. Guidotti, was finalized after his sorrow death. Mica samples are either from a two-mica peraluminous granite or from amphibolite facies metapelites of increasing metamorphic grade, from the Lower Sillimanite Zone (LSZ), to the Upper Sillimanite Zone (USZ), and up to the K-feldspar + Sillimanite Zone (K+S Z).

Trioctahedral micas range from ferroan phlogopite to magnesian annite and show different octahedral substitutions (Al = 0.48-0.72 apfu; Ti = 0.10-0.15 apfu and Mn = 0.01-0.07 apfu) and different polytype (1M and 2M ₁). Tetrahedral mean bond distances < T-O > varies from 1.659 to 1.672 Å and octahedral M1 site (< M1-O >: 2.076-2.114 Å) is smaller than octahedral M2 site (< M2-O >: 2.061-2.083 Å). Mean electron count points out to a compositional disorder among octahedral sites. The charge balancing, connected to octahedral Ti substitution can involve the octahedral cations and the deprotonation of anionic octahedral site. Dioctahedral micas plot relatively close to muscovite end member in diagrams from Tischendorf et al. (2004). The compositional ranges are: Si = 5.881-6.243; ^[vi]Al = 3.453-3.791; Fe = 0.087-0.451; Mg = 0.089–0.249 and Ti = 0.050–0.096 apfu. Polytype is 2M₁. These micas show mean electron count greater than 13 in M2 site and ranging from 0.37 to 0.69 in M1, thus confirming the substitution of heavier cations for ^[vi]Al and a partial occupancy of M1. In coexisting dioctahedral and trioctahedral micas, the ^[vi]Fe²⁺ partition coefficient is directly related to ^[vi]Al in dioctahedral micas. Chemistry of both micas changes according to previously established trends with metamorphic grade, whereas the control of this latter on crystallographic parameters was mainly recognized for trioctahedral micas.