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.

Bibliographie

Abbona F. e Aquilano D. (1970) - Politipismo nelle miche. Identificazione di tre politipi nell'alurgite, «Periodico Mineral.», 39.

Bernardini G. P., Cipriani C. e Manetti P. (1967) - Metodi di determinazioni microscopiche su lamine sottili di miche, «Rend. Soc. Min. It.», 23.

Beugnies A., Godfriaux I. e Robaszynski F. (1969) - Contribution à l'étude des phengites, «Bull. Soc. Belge Géol. Pal. Hydr.», 77 (1968).

Blasi A. (1972) - "Iso-microclino" ed altre varianti strutturali del K-feldspato coesistenti in uno stesso cristallo nei graniti del Massiccio dell'Argentera (Alpi Marittime), «Rend. Soc. It. Min. Petr.», 28.

Buerger M. J. (1964) - The precession method in x-ray crystallography. John Wiley e Sons, Inc., New York.

Chatterjee N. D. (1971) - Phase equilibria in the Alpine metamorphic rocks of the environs of the Dora-Maira-Massif, western Italian Alps. Part I and Part II, «N. Jb. Miner. Abh.», 114.

Cipriani C., Sassi F. P. e Viterbo Bassani C. (1968) - La composizione delle miche chiare in rapporto con le costanti reticolari e col grado metamorfico, «Rend. Soc. It. Min. Petr.», 24.

De Pol C. (1970) - Stato strutturale e composizione del feldspato potassico dei graniti dell'Argentera (zona centrale del Massiccio), «Rend. Acc. Naz. Lincei», ser. VIII, 48.

De Pol C., Schiavinato G. e Minutti L. (1968) - Distribuzione di Na, Rb, Ca, Sr, Ba nel feldspato potassico dei graniti dell'Argentera (zona centrale del Massiccio), «Boll. Soc. Geol. It.», 87.

Ernst W. G. (1963) - Significance of phengitic micas from low-grade schists, «Am. Min.», 48.

Fairbairn H. W. e Podolski T. (1951) - Notes on precision and accuracy of optic angle determination with the universal stage, «Am. Min.», 36.

Ferrara G. e Malaroda R. (1969) - Radiometric age of granitic rocks from the Argentera Massif (Maritime Alps), «Boll. Soc. Geol. It.», 88.

Franzini M. e Schiaffino L. (1963) - Polimorfismo e leggi di geminazione delle biotiti, «Atti Soc. Toscana Sci. Nat., Mem.», ser. A, 70.

Güven N. (1968) - The crystal structures of 2M1 phengite and 2M1 muscovite, «Carnegie Inst. Wash.», Year Book, 66.

Güven N. (1971) - The crystal structures of 2M1 phengite and 2Mi muscovite, «Z. Krist.», 134.

Güven N. e Burnham C. W. (1967) - The crystal structure of 3T muscovite, «Z. Krist.», 125.

Hendricks S. B. e Jefferson M. E. (1939) - Polymorphism of the micas with optical measurements, «Am. Min.», 24.

International tables for x-ray crystallography: I (1952); II (1956); III (1962). The Kynoch Press, Birmingham, England.

Jackson W. W. e West J. (1931) - The crystal structure of muscovite KAl2(AlSi3)O10(OH)2, «Z. Krist.», 76, (1930).

Levinson A. A. (1953) - Studies in the mica group: relationship between polymorphism and composition in the muscovite-lepidolite series, «Am. Min.», 38.

Malaroda R. (1968) - Atlante delle facies petrografiche del Massiccio Cristallino dell'Argentera, «Mem. Soc. Geol. It.», 7.

Malaroda R. (1970) - Carta geologica del Massiccio dell'Argentera alla scala 1 : 50.000 e Note illustrative (a cura di Carraro F., dal Piaz G. V., Franceschetti B., Malaroda R., Sturani C. e Zanella E.), «Mem. Soc. Geol. It.», 9.

Malaroda R. e Schiavinato G. (1957) - Osservazioni preliminari sui fenomeni di anatessi nel settore italiano del Massiccio dell'Argentera ((Alpi Marittime), «Boll. Soc. Geol. It.», 76.

Malaroda E. e Schiavinato G. (1958) - Le anatessiti dell'Argentera, «Rend. Soc. Min. It.», 14.

Radoslovich E. W. (1960) - The structure of muscovite, KAl2(Si3Al)O10(OH)2, «Acta Cryst.», 13.

Radoslovich E. W. e Norrish K. (1962) - The cell dimensions and symmetry of layer-lattice silicates, «Am. Min.», 47.

Rieder M. (1968) - Zinnwaldite: octahedral ordering in lithium-iron micas, «Science», 160.

Rieder M. (1970) - Lithium-iron micas from the Krušné hory Mountains (Erzgebirge): twins, epitactic overgrowths and polytypes, «Z. Krist.», 132.

Ross M., Takeda H. e Wones D. R. (1966) - Mica polyiypes: Systematic description and identification, «Science», 151.

Sadanaga R. e Takeuchi Y. (1961) - Polysynthetic twinning of micas, «Z. Krist.», 116.

Smith J. V. e Yoder H. S. (1958) - Experimental and theoretical studies of mica polymorphs, «Min. Mag.», 31 (1956).

Troger W. E. (1971) - Optische Bestimmung der gesteinsbildenden Minerale. Teil 1. Bestimmungstabellen, 4a ed., E. Sehweizerbart’sche Verlagsbuchhandlung, Stuttgart.

ZEN E-AN, Ross M. e Bearth P. (1964) - Paragonite from Täsch valley near Zermatt, Switzerland, «Am. Min.», 49.

Zvyagin B. B. (1962) - A theory of polymorphism of micas, «Soviet Physics, Crystallography», 6.