О возможной природе сталактитоподобных образований в агатах Ольского плато (Северо-Восток России)

The features of the stalactite-like formations relationship with onyxes in the geodes of the Ola basalt plateau agates are considered. It is shown that such formations are formed from the gel in the arc parts of cavities and flow down in the shape of icicles under the influence of gravity. "Stalactites" intersect previously formed layers of opal, chalcedony, quartz. The onyx layers. consisting of the silica group (SiO2,) minerals, can be dissolved only with the participation of F or HF. This is especially important in the case of poured basalts, since they contain no silica (SiO2) minerals, and silicon crystallizes in a different coordination (SiO4, Si3O8) in feldspars, olivine, etc. Geode filling with silica can be explained by the passage of gaseous HF and F from volcanic emanations through the thickness of silicon-containing rocks, with partial Si dissolution and formation of gaseous silicon tetrafluoride (SiF4), which fills cavities (bubbles) in basalt lavas. Subsequently, when meteoric water seeps into the cavity, silicon tetrafluoride is hydrolyzed to form orthosilicic acid, which is transformed into silica gel; the latter polymerizes to form chalcedony. Simultaneously, the remains of F either evaporate through cracks or accumulate in the cavity arch, again dissolving basalt in the vault of the geode (cycle repetition), with the formation of a viscous gel-like substance containing fluorine, which flows down in jets and polymerizes, thus creating stalactite-like structures. The author uses the photographs of agates found by B. M. Sedov in his numerous hikes to the Ola basalt plateau. The photographs were published in the wonderful book, "Agates from Upstream of the Ola River (Russia)" by B. M. Sedov.