Mechanisms of radiation damage of zircons deduced from computer simulation     

V. S. Urusov  A. E. Grechanovsky  N. N. Eremin 《Geology of Ore Deposits》2012,54(5):398-409

The radiation resistance of zircon (ZrSiO₄) was comparatively tested with computer simulations of four different sets of parameters of interatomic potentials. The energies of Frenkel pairs (FP) for Zr, Si, and O atoms were calculated using the Mott-Littleton method in approximation of isolated defects. The difference between the FP energies calculated for four potentials and the FP energies obtained ab initio is the least in the one of the four calculated potentials that takes into account the ab initio data for Zircon 3. The formation of an atomic displacement cascade (ADC) after the passage of the initially knocked-out Th atom with energy of 20 keV was investigated using the molecular dynamics method. The number of FP in ADC reaches 5300 to 61 900, depending on the chosen potential. The number of FP at the end of the simulation varies from 480 to 4970. The distribution of internodal oxygen atoms in zircon has been studied. It is shown that within a time (t) interval of 0?C0.1 ps (ballistic stage), the internodal oxygen atoms knocked-out from the initial equilibrium site are predominant (O1 defects). Thereby the Si-O bond is ruptured. The mean displacement of those defects is 2?C3 ?. The probability of their survival is 34?C73% depending on the chosen potential. After t = 0.1 ps (onset of the thermal stage), many SiO₄ tetrahedra in zircon are displaced with formation of a great amount of internodal oxygen atoms, because the Si-O bond is not ruptured. The mean displacement of those O2 defects is 1 ?; the probability of their survival is insignificant: 1.5?C3.0% depending on the chosen potential. The total amount of internodal oxygen atoms consists of O2 defects (19?C25%) and largely of O1 defects. A parameter characterizing that part of the energy of the initially knocked-out atom, which is consumed for FP, has been introduced. The physically acceptable values of this parameter (<1) are obtained only for Zircon 3 potential. The results show that a substantial discrepancy in relative amounts of Si and O atoms with displacements for 0.3?C0.5 ? from the equilibrium site exists for different potentials. In particular, the number of FP in zircon diminishes with decrease in mobility of atoms. It has been established that the lower the mobility of atoms, the lower the number of FP, and the most reasonable consistency with the preset conditions of the computer simulation is provided by Zircon 3 potential.  相似文献   

Natural selection of mineral species     

V. S. Urusov 《Geology of Ore Deposits》2010,52(8):852-871

The old issue of the limited number of mineral species is being brought up again and requires explanation. The review of successive calculations of the number of mineral species and their main varieties is given in this paper with allowance for the contemporary view on this problem. The natural selection of mineral species develops under the effect of geochemical differentiation of mineral matter, a multitude of chemical elements and their substitutions, and acid-alkali equilibria. It is noteworthy that water, oxygen, carbon dioxide, and other volatiles make a crucial contribution to the increase in the number of mineral species in the Earth’s crust.  相似文献   

Characteristics of selenium migration in soil–plant system of East Meshchera and Transbaikalia     

Vadim Ermakov  Larisa Jovanovic 《Journal of Geochemical Exploration》2010

Herein some results of comparative researches to determine the parameters of selenium migration in meadow biogeocenoses of East Meshchera and East Transbaikalia are presented. The parameters of selenium mobility in soils (i.e., the element total content, plant-accumulation coefficient (Kb) and/or mobile form ratio) in Urov biogeochemical provinces of the East Transbaikalia are quite comparable to the element migration parameters found in selenium-deficient landscapes of the same area. Selenium mobility indices and ecological status are significantly higher for meadow biogeocenoses of the East Meshchera (Moscow Region) and also for limestone soils.  相似文献