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The illitization of dickite: chemical and structural evolution of illite from diagenetic to metamorphic conditions

¹ Departamento de Química Inorgánica, Cristalografía y Mineralogía, Facultad de Ciencias, Campus de Teatinos, 29071 Málaga, Spain
² Institute of Geology, Academy of Sciences CR, Rozvojová 269, 165 00 Prague 6, Czech Republic

^* Corresponding author, e-mail: mdruiz{at}uma.es

The illitization of dickite is commonly observed at diagenetic to low-grade metamorphic conditions in Triassic rocks from the Betic-Rif Cordillera. It has been studied by X-ray diffraction and by scanning and transmission/analytical electron microscopy. Dickite is a well-ordered diagenetic phase that occurs filling the secondary porosity in sandstones and conglomerates. In diagenetic samples, the alteration of dickite to illite produces thin 1M mica crystals with incomplete interlayer occupancy (illite), wide chemical variability, and low Na contents. Under low anchizonal conditions, the illitization process is more advanced and 2M₁ K-illite coexists with minor dickite, sudoite, and 1M intermediate Na–K-illite. At higher anchizonal conditions only relics of dickite persist, whereas pyrophyllite coexists with 2M₁ illite/muscovite with K-rich composition, indicating an approach to equilibrium.

Alteration of dickite to illite and sudoite mainly progresses along the dickite layers and produces progressively larger packets of 1M and 2M₁ white mica. Alteration of dickite to pyrophyllite advances parallel and transverse to the dickite layers. The entry of appreciable amounts of Na into the illite structure seems to depend not on the rock composition but on the metamorphic conditions, i.e. temperature. The dickite to Na–K white mica transformation is an alternative, and probably common way, not previously described, for forming Na-bearing mica in coarse-grained smectite-poor rocks.

Key-words: Betic-Rif cordillera, anchizone, dickite, illite, intermediate Na—K white mica, pyrophyllite, TEM/AEM.