01 – Quartz habits and secondary pseudomorphs from secondary filled lenses in triassic grey Wesersandstein
Ano 04 (2017) - Número 03 Artigos
10.31419/ISSN.2594-942X.v42017i3a1HP
Pöllmann, H., Neumann, A., Menezes, F., Svensson, K. & Lempp, Ch., Martin Luther University of Halle/Saale
Göske, J. & Winter, S., ZWL, Lauf
The grey Wesersandstein from Germany is a typical triassic feldspatic sandstone containing some secondary modified lenses with lower density and interesting mineralogical features. These lenses contain some secondary pseudomorphys of quartz after halite.
These lenses, from 1mm to 1cm in the sandstone are interpreted by a diagenetic process during which some original NaCl formation is replaced by other sodium containing minerals like Albite. Also Halite is replaced by SiO2 (release from the alteration of feldspars). Figure 1 shows a typical slab with the various secondary filled lenses. The grey Wesersandstein is actually used as building and construction material for many applications.
Fig. 1: Slab of grey Wesersandstein with various secondary filled lenses with lower density
Due to this fact different grains and crystals of quartz occur in the feldspar.
- Original quartz grains of the sandstone
- Secondary authigenic quartz crystals
- Pseudomorphs of quartz after halite-crystal (hexaedric habit)
- Typical etching cavities due to weathering effects
The following images show the different quartz habits and formations in the grey Wesersandstein formation.
In the performed investigations for investigation of probable usage of Wesersandtein as a potential storage material, these different quartz crystals can have different geochemical and geomechanical properties. The formation and habit also differs from their varying origin. Almost rounded quartz grains in the sandstone, newly crystallized idiomorphic quartz and also pseudocubic and pseudomorphic quartz. Some newly formed quartz crystals show already etching pits as their faces are already under the influence of starting dissolution.
Fig.2: Smooth surfaces of quartz grains in the grey Wesersandstone
Fig.3: Partially regrown quartz with trigonal etching cavities
Fig.4: Typical etching cavities on quartz crystals
Fig.5: Authigenic quartz crystal covered with platey clay minerals
Fig.6: Aggregate of pseudocubic quartz in secondary mineral lenses
Fig.7: Pseudocubic and lathy quartz crystals
Quartz in the SEM images
References
Land, L. S., Milliken, K.: Regional loss of SiO2 and CaCO3, and gain of K2O during burial diagenesis of Gulf Coast mudrocks, USA. In: Worden R, Morad S, editors. Quartz cementation in sandstones, Int. Assoc Sed, Sp Publ, vol. 29, 2000, p. 183– 97.
Pöllmann, H., Neumann, A., Menezes, F., Svensson, K. & Lempp, Ch. (2017): Mineralogy and microstructure of small secondary modified lenses in the grey Weser sandstone, Geo-Bremen, Abstract.
Weber, J. (2000): Kieselsäurediagenese und gekoppelte Sedimentarchitektur – eine Becken-analyse des Reinhardswald-Troges (Norddeutsches Becken, Solling-Folge, Mittlerer Bunt-sandstein), PhD thesis, Kölner Forum für Geol. u. Pal. 7; 2000.