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[(K_0.33Na_0.66)Na₂Fe₅Si₈O₂₂(OH,F)] + 3H₂O + K₃Ti₃NbO₁₀→

2[(K₂Na)Fe₇(Ti_1.5Nb_0.5)Si₈O₂₆(OH₄F)] + 8SiO₂+

↑ ↑

6Na⁺Fe²⁺HF+3O₂

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.