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The Earth, Moon, the nearest planets and their satellites, asteroids, comets and meteorites - all of them are stone bodies consisting of crystalline substances - minerals. And since the main property of crystals is symmetry, then depending on its nature these minerals form particular systems, or groups which are called syngonies. These are cubic, hexagonal, tetragonal, trigonal, rhombic, monoclinic and triclinic (in the order of declining symmetry). How do these basic properties of "dead matter" change in the course of the evolution of the world around us? An answer to this question was provided more than 15 years ago by Prof. Nikolai Yushkin (now Academician) who said that in the process of aggregation of meteorite systems into planets like the Earth, in the process of the evolution of the Earth and other planets, the high-symmetry cubic face of the mineral world is being gradually replaced with a low-symmetry, or monoclinic one.
The former - as was established by a team of scientists from the Institute of Mineralogy of the Chelyabinsk Science Center of the RAS Ural Branch led by Dr. B. Chesnokov (Geol.&Mineral.) - is common for minerals which predominantly consist of anhydrites (they contain no H + , OH - or H 2 O). The latter incorporates minerals in which an important role belongs to hydrites (H + , OH - and H 2 O are incorporated as their structural elements).
Hydrites and anhydrites have distinctly different symmetry characteristics. Whereas the distribution by syngonies in the former group is sharply monoclinic (terrestrial type of distribution), in the latter it is of what we call a non-contrast rhombic - or monoclinal-cubic type (lunar type). Thus what one can call the crystallochemical "essence" of the evolution of the "mineral world" as defined by Acad. Nikolai Yushkin consists in a replacement, or substitution, of the anhydrite ("waterless" and "dead") mineral associations with hydride ones ("aqueous" and "live"). Acad. Chesnokov and his colleagues called this process "hydrite aggression". As for the occurrence of such compounds - they are few and far between on the Moon, amount to a significant proportion in meteorites, and, as for the Earth, the situation here can be described as a "hydrite paradise". It has been brought about by an abundance of water, oxygen and living organisms.
The hydrite aggression starts with the development of the least symmetrical (monoclinic) hydrites. In the severe space environment they, as a rule, are represented by laminary "aqueous" silicates - serpentines, chlorites, etc., and replace cubic and other anhydrites with crystal structures which contain layers of the densest packing of oxygen ions. In the crystals of the "aqueous" silicates there are also layers with the same kind of packing of oxygen ions and groups OH - , and because of that less energy is required for the replacement of anhydrites with hydrates. And that means that these laminated silicates, which are monoclinic hydrates, can be called the "pioneers" of the hydrite aggression.
In anhydrite ("waterless", "dead") objects there prevail cubic ones by
the number of mineral types, but by the volume pride of place belongs to rhombic minerals - olivines and pyroxenes. And these objects include the lower lithosphere of the Earth, its upper and middle mantle, meteorites and the lithosphere of the Moon. The matter therein is "animated" with the appearance of monoclinic hydrites therein. They usually impact olivines in the first place, which is followed by the formation of serpentinites and other laminated silicates. And rhombic pyroxenes are also subject to serpentinization - the replacement of olivines. Then laminated silicates begin to "consume" rhombic structures. Such cases can be described as "anti-rhombic aggression" of hydrites. It is very common on the Earth and is expressed in an intensive serpentinization of olivine-containing ultrabasic rocks. One case in point are the Ural Mountains where most such massifs have almost completely turned into serpentinites. They continued to change with time often producing deposits of asbestos, talcum, etc.
Other rocks are also hydrated in the conditions of the Earth's lithosphere when various geological processes push them up from the "bowels" of the planet closer to its surface or on the surface itself. The "hydration" of plutonic (anhydrite) material ends up above - in the weathering zone, where vast masses of clayey matter are concentrated, piled up by monoclinic "highly aquatic" laminated silicates.
It was due to this process that a special section of our planet was formed - the "live" lithosphere and a truly living zone of hypergenesis upon it.
Nauka Urala (Science of Urals), 2000
Prepared by Emma SOLOMATINA
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