Silicate anions are formally the conjugate bases of silicic acids. Late Holocene lava compositions include basalt and andesite, but silicic lavas dominate. Topaz is commonly associated with silicic igneous rocks of the granite and rhyolite type. Rocks high in silica produce silicic acid as a weathering product. Gold is associated with silicic, kaolinitic, and pyritic alteration of greenschist-grade felsic metavolcanics. Diatoms and radiolarians have evolved to uptake silica in the form of silicic acid, Si(OH). Lenses of silicic tuff exist as evidence of limited volcanism. When the nanoparticle is coated, for example, in silica the surface exists as silicic acid. Linus Pauling predicted that silicic acid would be a very weak acid. When dissolved in water it is usually known as silicic acid. The lava flow is unusually long for a silicic lava flow. Choanoflagellate biosilicification requires the concentration of silicic acid within the cell. In addition, contact with eyes may form silicic acid with resultant irritation. Silicic acids are generally very weak and cannot be isolated in pure form.