Microstructural variations in a natural cordierite from the Eifel volcanic field, Germany

A cordierite crystal (approx 3 mm) from a small metapelitic xenolith from the Bellerberg volcano, Eifel, was studied by TEM, which showed that the grain was subdivided into regions with different types of microstructure. These regions are a few mu m to 0.1 mm in size and range from metrically hexagonal to orthorhombic. They include: 1) modulated structures approx 100 (sub hex) and perpendicular it, i.e. approx the corresponding plane of type 120 (sub hex) ; 2)distinct lamellae with modulated structures sharing splitting of high order reflections <100> (super *) (sub orth) ; 3) lamellar and blocky twin domains of the mirror twin laws 110 (sub orth) and 110 (sub orth) , and 4) orthorhombic regions without modulated structures and twin domains. This variety of different but adjacent microstructures correlates with varying degrees of K + Al for Si substitution: regions (1) have the highest K contents (0.09-0.23 per formula unit with 18 O) whereas regions (2)-(4) have K <= 0.09 pfu. Thus, KAlSi (sub -1) is shown to stabilize the modulated structure, described here for the first time from a natural cordierite, and to hinder its transformation to orthorhombic cordierite.