How opal is formed by a volcanologist-
Author Marco Campos Venuti has just written a new book titled
Genesis and classification of agates and jaspers
Author is a PhD volcanologist and author of the Gemmologica Italiana
Thanks to Marco Campos for permission to publish
Opal is a hardened silica gel with the formula Si02.nH20, deposited from a concentrated solution. In the living organisms that product organic opal, such as diatoms, radiolarians, sponges and many vascular plants, silicic acid accumulates in special cellular compartments where specific proteins induce the precipitation of opal-A.
The inorganic opal instead, originates from a combination of factors. It is necessary a silica rich soil, which can be found in a volcanic area, or due to the presence of sands rich in quartz. It is generally a very basic pH (>9) that promotes the formation of brines at high concentration of silicic acid. Then we need a groundwater with strong seasonal swings of the water table which favours the concentration of silica. Then it is necessary a catalysing factor to precipitate silica. Catalysis can be started by an increase in temperature, by a shifting to an acid pH or by the presence of some molecules as iron oxide, aluminium oxide, magnesium oxide, sodium sulphate or simply sea salt.
When a precursor of the opal has been deposited in the form of a viscous solid, it can have porosity higher than 35%, partly occupied by water. If the sample dries, it would crumble and irrecoverably convert into a white powder. This is the major problem encountered in the production of synthetic opal. In the laboratory, it is necessary to fill this porosity with some resin that does not alter the optical properties of opal. Nature uses other silica gel, that in most cases lower the quality of the opal. The transformation of mineral opal into jasper can also be drived by diagenesis.
What is a?
Diatoms are algae with distinctive, transparent cell walls made of silicon dioxide hydrated with a small amount of water (Si02 + H20).
They are amoeboid protozoa that produce intricate mineral skeletons
silicic acid ?
(H4SiO4), is an important nutrient in the ocean
Brine is a solution of salt in water. In different contexts, brine may refer to salt solutions
inorganic opal ?
Inorganic compounds are traditionally viewed as being synthesized by the agency of geological systems. In contrast, organic compounds are found in biological systems.
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