2 M1 e 3 T polimorfi delle miche diottaedriche coesistenti nei graniti del Massiccio dell'Argentera (Alpi Marittime)

Blasi, Achille ; Blasi De Pol, Carla

Atti della Accademia nazionale dei Lincei. Rendiconti della Classe di scienze fisiche, matematiche e naturali, Série 8, Tome 55 (1973) no. 5, pp. 528-545

Résumé

Dioctahedral micas from granites of the central nucleus of the Argentera Crystalline Massif (Maritime Alps) have been investigated optically with the universal stage and by X-rays using the precession camera. These minerals crystallized during the main granitization process affecting the massif; they are always present in the granites, even if in limited amounts. Micas were separated from 12 samples of the 3 commonest varieties of granite (Table I and fig. 1), i.e. "feldspar macrocrysts-bearing granite", "fundamental granite" and "aplitic microgranite". Optical investigations showed that biaxial and uniaxial micas always coexist in these samples, the former strongly prevailing over the latter ones (Table II). The average 2 Vx values of the biaxial micas range from 39° to 43°; thus they cannot be assumed to be pure muscovites, rather they tend toward phengites. Such a trend appears to be more pronounced for micas occurring in the feldspar macrocrysts-bearing and fundamental granites than for those occurring in the aplitic microgranites. X-ray investigations (Table III) showed for all the biaxial crystals a diffraction symbol 2/mC-/c and a "vector stacking symbol" 2M1 [$2\bar{2}$]; in these variants the hkl reflections where the conditions $k \neq 0$, $3h \pm k \equiv 0$ (mod 6), $k \pm 3l \equiv 3$ (mod 6), [$h \pm l \equiv 1$ (mod 2)] are simultaneously satisfied appear to be systematically weak. Examination of the presence criteria of all observed reflections (fig. 2) and geometrical features of precession photographs (Table I and II) suggest that no epitactic overgrowths or twins occur in these micas. The uniaxial crystals show a diffraction symbol $\bar{3}2/mP3_{1}$ -- and a "vector stacking symbol" 3T [222]; in these variants the hkl reflections where the conditions $h \neq 0$ if $k = 0$, $k \neq 0$ if $h = 0$, $h-k \equiv 0$ (mod 3), $k+l \not\equiv 0$ (mod 3), [$h+l \not\equiv 0$ (mod 3)] are simultaneously satisfied are weak, but clearly observable. Examination of the presence criteria of all observed reflections (fig. 3) and geometrical features of precession photographs (Table II) again suggest that no twins or epitactic overgrowths are present. According to their cell parameters (Table IV) most crystals of the 2M1 polymorph differ more or less sensibly from pure muscovite, tending toward phengite. Such a trend is fairly uniform in micas from the feldspar macrocryst-bearing and fundamental granites, whilst it is irregular in those from the aplitic microgranites. On the basis of its cell parameters the 3T polymorph too is fairly phengitic.