Original paper

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

Müller, Wolfgang Friedrich

European Journal of Mineralogy Volume 3 Number 6 (1991), p. 915 - 932

35 references

published: Dec 19, 1991
manuscript accepted: Apr 16, 1991
manuscript received: Nov 14, 1990

DOI: 10.1127/ejm/3/6/0915

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Abstract

Abstract A cordierite crystal about 3 mm in size, which occurred in a small metapelitic xenolith from the Bellerberg volcano, Eifel, was studied by methods of transmission electron microscopy (TEM). TEM showed that the grain was subdivided into regions of different types of microstructures. These regions are from a few μm up to 0.1 mm in size. They range from metrically hexagonal to orthorhombic. The following microstructures and their associated electron diffraction patterns of the different regions were distinguished: (a) modulated structures about parallel to {100}hex and perpendicular to it, i.e. about parallel to the corresponding plane of the type {120}hex ; they have hexagonal metrics of the diffraction pattern, but very faint streaks of the reflections parallel [100]*hex and [120]*hex ; (b) distinct lamellae with modulated structures showing splitting of high order reflections parallel to *ortho ; (c) lamellar and blocky twin domains, a few μm in size, of the mirror twin laws {3̅10}ortho and {110}ortho ; (d) orthorhombic regions without modulated structures and twin domains. This variety of different, but directly adjacent types of microstructures correlates with varying degrees of K + Al for Si substitution as follows: Regions (a) have the highest K-contents (0.09 - 0.23 per formula unit with 18 oxygens), whereas the regions (b) to (d) have K ≤ 0.09 p.f.u. Thus, KAlSi-1 is shown to stabilize the modulated structure, which is described here for the first time from a natural cordierite, and to hinder its transformation to orthorhombic cordierite.

Keywords

cordieritetransmission electron microscopytwinsmodulated structuresphase transition