TL;DR: Helvetic Siliceous Limestone (Lower Cretaceous) is characterized by up to 40% of very fine (1-10 μm), evenly distributed authigenic quartz crystals as discussed by the authors.
Abstract: Helvetic Siliceous Limestone (Lower Cretaceous) is characterized by up to 40% of very fine (1-10 μm), evenly distributed authigenic quartz crystals. Early diagenetic dissolution of opaline sponge spicules led to silica enrichment of interstitial waters, which reprecipitated silica in the overlying horizons, forming tiny quartz crystals in pore spaces.