Mineralogy of dark mica from the Wiborg rapakivi batholith, southeastern Finland
Rieder, Milan; Haapala, Ilmari; Povondra, Pavel
European Journal of Mineralogy Volume 8 Number 3 (1996), p. 593 - 606
published: Jun 17, 1996
manuscript accepted: Jan 4, 1996
manuscript received: Mar 28, 1995
ArtNo. ESP147050803011, Price: 29.00 €
Abstract A mineralogical study was performed on biotite separated from nine chemically analyzed rock specimens representing the main granite types of the classic Wiborg batholith in southeastern Finland: dark wiborgite, pyterlite, various biotite granites and topaz-bearing granites. Micas were chemically analyzed and their unit-cell dimensions, polytypes, and some other physical properties determined. All compositions plot near the annite-siderophyllite corner of the Fe+2-Mg-Li triangle used for trioctahedral micas. Their Fe2+/(Fe+2 + Mg) ratio varies from 0.8 to 1.0, being lowest (0.80-0.88) in biotites from biotite-hornblende-fayalite granites (dark wiborgite, wiborgite, tirilite) and highest (0.92-1.00) in lithian siderophyllite (containing 1.2-1.6% Li2O) from a topaz-bearing granite. The TiO2 content of the micas varies inversely with Fetot/(Fetot + Mg), it is highest in dark wiborgite (3.98%) and lowest in topaz-bearing granites (0.18%). The content of fluorine is high only in lithian siderophyllite (4.2-4.9%). In the annite-phlogopite-K Fe3+3 AlSi3 O12 (H-1) diagram, all biotites plot below the magnetite-hematite oxygen buffer line. Lowest oxygen fugacity (below the fayalite-quartz-magnetite buffer line) is indicated by annite from dark wiborgite, higher oxygen fugacities, by siderophyllites from the topaz-bearing granites (between Ni-NiO and magnetite-hematite buffer lines). It is likely that the composition of micas from topaz-bearing granites changed by subsolidus reactions that took place during late magmatic or postmagmatic stages. Four crystals from each mica sample were examined for polytypism. Although \M was expected and found to be most frequent, an unusual wealth of polytypic stackings was observed: siderophyllite from pyterlite contains polytypes 5M, 37c and 1M, whereas lithian siderophyllite ("protolithionite") from the topaz-bearing granite at Kymi yielded polytypes 2MU 3T and unresolved complex structures. Wide variety of polytypic structures is usually reported from pegmatites or hydrothermal veins, where physicochemical conditions changed quickly causing spontaneous nucleation, whereas a relative monotony is reported from abyssal or metamorphic rocks. The different pattern observed here may be associated with the rather unique chemistry of this mica and its structural consequences.