Mineral Chemistry and Paragenesis of Astrophyllite from Egypt

Abdel-Fattah M. Abdel-Rahman*
Department of Geological Sciences, McGill University, 3450 University Street, Montreal, Quebec, Canada, H3A 2A7
*Present address: Department of Geology, Concordia University, Loyola Campus, 7141 Sherbrooke Street West, Montreal, Quebec, Canada, H4B 1R6.

Abstract: Astrophyllite is a rock-forming mineral (3.4 vol.%) in the metasomatized wallrocks within 2 to 3 km of the contact with the Mount Gharib A-type complex in northeastern Egypt. In the peralkaline A-type granites of the main complex, astrophyllite occurs in accessory amounts (0.1 to 0.5 vol.%), mostly in association with arfvedsonite. Eighteen chemical analyses of astrophyllite from both rock units have been used to discuss mineral chemistry, compositional variations and isomorphism.

Substitutions in the astrophyllite unit cell [(Na,K)3Fe7Ti2Si8(O,OH)31] involve F, Ca, Mn, Zn, Mg, Nb, Zr, Sn and Al. The contents of the unit cell depart from ideal stiochiometry. Unusually high Nb contents (up to 5 wt.% Nb2O5) characterize the Egyptian astrophyllites. The replacement of Ti by Nb is balanced by cation vacancies (Nb5+ → Ti4+ + □). The niobophyllite-astrophyllite relationship and the structure of astrophyllite in relation to mica and biopyriboles are discussed.

Astrophyllite may have been formed at the expense of alkali amphiboles by metasomatic reactions involving Ti-Nb-rich alkaline fluids and arfvedsonite;

3 arfvedsonite + 3 water + liquid →

2 astrophyllite + 8 quartz + free ions + gas

3[(K0.33Na0.66)Na2Fe5Si8O22(OH,F)] + 3H2O + K3Ti3NbO10

2[(K2Na)Fe7(Ti1.5Nb0.5)Si8O26(OH4F)] + 8SiO2+

↑ ↑


Most astrophyllite reported in the literature occurs in association with alkali amphiboles, particularly arfvedsonite, thus suggesting that similar metasomatic reactions were responsible for the formation of astrophyllite in many other localities in the world.

Keywords: astrophyllite • A-type granite • arfvedsonite • metasomatic reactions • Egypt

Mineralogical Magazine; March 1992 v. 56; no. 382; p. 17-26; DOI: 10.1180/minmag.1992.056.382.02
© 1992, The Mineralogical Society
Mineralogical Society (www.minersoc.org)