Mineralogy, Geochemistry and Cathodoluminescence of Authigenic Quartz from Different Sedimentary Rocks

Abstract

Authigenic quartz is present in different sedimentary rocks of North-Eastern Germany. Single crystals of euhedral quartz were detected in the Permian (Zechstein) salt deposit of Roßleben, in quartz nodules within Triassic sandstone layers (Chirotherien sandstone, Bunter) from Jena, and Tertiary lignite deposits in the Leipzig region (Zwenkau, Cospuden). Mineralogical and geochemical investigations revealed that the authigenic quartz crystals from the different geological units differ in morphology (habit), characteristic inclusions, trace-element geochemistry and cathodoluminescence properties. Accordingly, the results allow not only to clearly distinguish between authigenic and detrital quartz, but also between authigenic quartz from different sedimentary environments. Authigenic quartz from Zechstein salt deposits shows characteristic euhedral forms dominated by rhombohedral faces or a combination of rhombohedral and prism faces, and mineral inclusions (halite or anhydrite) in dependence on the saliniferous facies. The crystals exhibit a blue luminescence, which can be related to a broad emission band at 450 nm. The authigenic quartz crystals from the Bunter sandstone are often intergrown, forming aggregates of several mm up to cm in size. At least three growth zones can be distinguished: spherulithic growth starting from calcite inclusions, quartz with complex internal CL structure, and a homogeneous outer zone with no visible luminescence. The second zone exhibits a cathodoluminescence pattern similar to that of agate with three emission bands at 650, 580 and 450 nm. Authigenic quartz from Tertiary lignites is characterized by doubly terminated crystals with prism and rhombohedral faces. Intergrowth of two or more crystals was observed. The CL is dominated by a transient emission band at 650 nm, which increases in intensity during electron irradiation. The crustal signature of all quartz REE distribution patterns and high contents of Al and Fe indicate the origin of the silica-bearing fluids from weathering solutions and do not show any influence of hydrothermal fluids. On the other hand, elevated concentrations of Na, K, Mg, Ca, and B can probably be related to the influence of saliniferous fluids during quartz precipitation. Although the specific physico-chemical conditions may have been different for the various occurrences, the data suggest a formation of the authigenic quartz crystals during early diagenesis.