New Concept of Silica Source in Agates
N E Savva*
North-East Interdisciplinary Scientific Research Institute n. a. N. A. Shilo, Russian Academy of Sciences, Russia
Submission: April 01, 2021; Published: April 20, 2021
*Corresponding author: N E Savva, Doctor of Geological and Mineralogical Sciences, North-East Interdisciplinary Scientific Research Institute n. a. N. A. Shilo, Far East Branch, Russian Academy of Sciences, Russia
How to cite this article: N E Savva. New Concept of Silica Source in Agates (Fluoride Model). Int J Environ Sci Nat Res. 2021; 28(1): 556226.
The concept is based on the idea of partial dissolution of silicate minerals in the rocks of volcanic flows when interacting with hydrogen fluorine. HF is the only solvent of silicates. As a result of this interaction, SiF4 gas is formed, filling the bubbles in the boiling lava. The entry of meteor water into the gas cavities leads to the hydrolysis of silicon tetrafluoride to form orthosilicic acid (H4Si04), which is subject to gelation and polymerization. This leads to the formation of silicon dioxide gels, which are necessary for the structure of chalcedony. Thus, chalcedony is formed inside cavities filled with SiF4 gas, and only vadose water enters the cavities through the channels; otherwise, the channels can quickly become clogged with a viscous gel. It is shown that the thickness of the bands in agates depends on the rate of water entry into the gas voids. The slower the H2O droplets arrive, the thinner the layers (bands) in the agate. After the fluorine is depleted, the true solutions fall into the cavities, forming crystalline quartz.
Chalcedony fills cavities in both acidic and basic igneous rock, seams and voids in intrusive and metamorphic rocks, and replaces previous minerals in sedimentary rocks. This variety of growth conditions almost does not limit its paragenesis. Therefore, there are different models of natural formation and experiments on the synthesis of chalcedony have been successful in many different ways [1-6]. The idea of developing a fluoride model is largely related to the works of I A Bekbulativa , which dealt with the methods of industrial synthesis of chalcedony.
The formation of natural agates in most cases is associated with volcanic activity. Gas-rich boiling lava contains many gas bubbles, like boiling water. Hot gases come from great depths under high pressure and move upward, as evidenced by their many kilometers of emissions from the volcanic vent. The formation of chambers with an increase in their volume occurs against the background of adiabatic expansion of gases in boiling lava bubbles in a viscous, not cooled environment of magmatic flows. The expansion of the gases causes pressure on the walls of the chambers with an increase in volume, and often the merging of bubbles, that is, the chambers are being prepared for future agates.
As we ascend to the surface, the size of the chambers increases, which is well illustrated by the documentation of the outcrop in the steep wall of the Olsky basalt plateau (Figure 1).
It is this pressure, which spreads evenly throughout the entire volume of the chambers, that regulates (holds at an early stage) the deposition of chalcedony, and then the growth of quartz on the walls of the chambers from the periphery to the center. In the future, when the pressure on the walls ceases, the incoming solutions, already under the influence of gravity, will successively fill the cavity with horizontal layers, forming onyx with a typical striped texture (Figure 2).
On agate sections, channels of subsequent gas and water phase flows into the chambers (geodes) are often found on the successful cut (Figure 3).
Analysis of the available literature shows that fluorine is actively released during volcanic eruptions. This is confirmed by the findings of hyeratite-K2SiF6, which was first found in the products of basalt volcanic eruptions. Potassium fluorosilicate (hieratite–) was first found among the products of the solfataric activity of the volcano La Fossa on the Vulcano Island off the coast of Italy. The mineral got its name in honor of the ancient name of
this island-Hiera (Hiera). Hyeratite is also found in posteruptive
products of Kamchatka volcanoes. It was first mentioned by B I
Piip  when describing the eruptions of 1944-1945. It was found
in the products of the Great Fissure Tolbachinsky eruption BTTI
fumarole fields [9,10].
Sublimate fluorides often form fluorine-metasomatites, which
are depleted of aluminum, sodium and magnesium and enriched
with calcium. Sometimes ralstonite - Na0.5(Al,Mg)2(F,OH)6•H2O
is found in volcanic rocks, which also confirms the presence
of fluorine in volcanic exhalations. Observations in areas of
volcanism show that the gas condensates released from magma,
lavas, and fumaroles not only dissolve in the reservoir waters,
giving them an acidic reaction near the magmatic bodies, but
also seep through the fracture zones in the rocks and through the
cinder cones , Figure 4.
Geologists often ask the question: why in basaltic lavas that
do not contain minerals of the SiO2 group, large, often numerous
tonsils are formed, filled with agate, the bulk of which is SiO2? It
seems that the presence of F in volcanic emanations is of great
importance for the formation of silicon oxides in agates. According
to L. A. Basharina (1961), 100g of fresh ash from Bezymyanny
volcano (Kamchatka) contains easily soluble forms (in mg):
chlorine 76-530, fluorine 1.5—6.7, sulfur tetrachloride 237-938,
carbonic acid 12-104, boric acid 1.5—4.2.
F – gas in a volcanic explosion reacts with hydrogen over a
wide temperature range to form HF:
H 2 + F 2 → 4HF
HF – is the only solvent of silicates, which are mainly composed
of volcanic rocks. The heated 4HF, moving up through the pore
space and channels, dissolves Si-containing minerals (silicates)
on its way, and combines with silicon to form a gaseous silicon
40 - 100°C → Si + 4HF → SiF4 + 2H
The gaseous tetrafluoride that fills the chambers (voids in
volcanogenic rocks) is hydrolyzed by water to form orthosilicic
acid (H4SiO4), and the products formed in this process depend on
High-temperature volcanic gases that are released during the
introduction of a magmatic body are magmatic emanations. and
analogues of hydrothermal fluids (hydrotherms).
As can be seen from the formulas shown, the seepage of H2O
(VAD, surface water) leads to the dilution of HF, as well as the
possible precipitation of silica gels into the sediment with the
formation of orthosilicic acid H4SiO4. Solutions of ortosilicic acid
are unstable in time and easily pass into gels, their great tendency
to polymerization with, accompanied by polycondensation is
manifested: first, linear, then branched, layered and mixed, and
finally - three-dimensional structures are obtained . The
formation of gels and their polymerization occurs inside the
chambers (bubbles), otherwise the gels could block the channels
of H2O intake.
Depending on the size of the chambers and the volume of H2O
intake, some geodes are completely filled with chalcedony, but
in the case of slow leakage of H2O drops, the spaces adjacent to
the walls are gradually filled with layers, since they are held by
adiabatic expansion (uniform pressure on the walls of the voids).
The smaller the portion of incoming H2O, the thinner the layers in
the growing agate up to the thin – layered - “moire”.
After the depletion of colloidal solutions, as the fluorine
is consumed, true silicon-containing hydrothermal solutions
arrive and the formation and growth of quartz crystals begins
on the walls of geodes made of chalcedony. The depletion of true
solutions often leads to the fact that an empty space (drusovye)
voids remain in the geodes filled with quartz (Figure 5a).
In agates associated with basaltic and andesibasaltic
outpourings, in drusic voids on quartz, crystal accretions
of minerals of the carbonate group – calcite, aragonite,
manganocalcite, and rhodochrosite-are later deposited (Figure
Silicon fluoride reacts with water (hydrolysis) seeping into
the gas chamber, and in turn forms a hydrate of silicon oxide –
silicic acid. As the acidity of the solution decreases, it turns from
sol into a gel-like gel. I A Bekbulatova  indicates that with
strong dilution, the formation of orthosilicic acid H4SiO4, which
exists only in very dilute solutions, is possible. In the processes
of fluoride pneumatolysis, water plays an important role, which
reacts with volatile compounds. In this case, even quartz can be
SiO2 + SiF4 + 2H2O = 2SiO2 + 4HF.
Orthosilicic acid vapors are unstable in time, due to the
fact that they are more prone to polymerization, accompanied
by polycondensation: first, linear, then branched, layered and
mixed, and finally - three-dimensional structures of chalcedony,
which are well traced in transmitted light (Figure 6). And here it
is important to note that the slower the water seeps, the thinner
the layers of chalcedony in the agate. This thinly layered filling
gives rise to agates with a “moire” texture (Figure 7a). In turn,
the rapid flow of water into the chamber can lead to its complete
filling with chalcedony and the absence of stratification in the
geode (Figure 7b). The transition of orthosilicic acid to polyacids
is accompanied, by the transformation of the molecular solution
H4SiO4 into colloidal solutions - sols. Sols, in turn, either solidify,
turning into a gel.
Horizontal banded formations (onyxes) inside the geodes
indicate the adjustment of the pressure in the chambers and its
approach to atmospheric pressure. They fix the deposition of
chalcedony under the influence of gravity (gravity). There are
options when the pressure adjustment occurs at the stage of
colloid intake, and by this time the chambers are already filled.
Then crystal quartz is not formed in geodes (Figure 2) [11-14].
The presented fluorinated model perfectly corresponds to
natural processes and indicates a very likely source of silica for the
formation of agates, even in basaltic lavas. First, the cavities are
filled with SiF4 gas, and when the surface waters enter, they form
orthosilicic acid, which is prone to conversion into sols and gels,
followed by their polymerization. The model can be improved and
supplemented with experimental developments and observations
of the eruption of modern volcanoes.
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