engleski

Metamict minerals: insight into relic crystal structures

Since they were firstly described in 19th century, metamict minerals have been always attracting the attention of mineralogist. The term “ metamict” refers to the minerals that are found partly or completely amorphous in nature but are supposed to be originally crystalline. The process of amorphization, here called metamictization, is facilitated by  -decay of radionuclides usually positioned in the structure of metamict minerals. In an  -decay series,  -particles and  -recoil nuclei are produced, which displace atoms from their structural positions by different mechanisms. In a simplistic view, through accumulation of these displacive defects, a mineral could become amorphous, i.e. metamict. There is a large importance of metamict minerals study. An intrinsic mineralogical interest in crystal-chemical properties of metamict minerals is important for mineral nomenclature and classification. The application of zircon and monazite in age dating of geological formations also imparts a need for understanding the processes of metamictization and related mineral alteration, since it can significantly influence the reliability of absolute geological ages obtained. Moreover, modern approaches in nuclear waste management try to find a suitable matrix for immobilization and storage of highly radioactive nuclear waste. The studies on radiation influence on different crystal structures are thus of a great importance, whereby metamict minerals represent natural models for radiation damage. Some groups of oxide minerals are frequently susceptible to metamictization. Although having different stoichiometry and crystal structure, most of metamict oxides have two cationic sites, one for larger cations like Ca, REE, U and Th, and another one for cations like Nb, Ta, and Ti. Especially are known mineral groups of aeschynite, euxenite, fergusonite, samarskite and pyrochlore. These oxide minerals are often completely metamict, and a classical approach in a crystal structure study is to recrystallize them by heating. As a rule, recrystallization rarely results in a single phase system, thus X-ray data are not unanimous regarding the identification of pre-metamict structure. Therefore, a part of studies is mostly devoted to evaluating the chemical data using sophisticated statistical methods for distinguishing different mineral groups of metamict minerals. On the other hand, some modern analytical methods, like micro-Raman spectroscopy and various transmission electron microscopy techniques (SAED, HRTEM), enable the identification of the original structure in generally amorphous matrix of metamict minerals. In the lecture mostly the aspects of the crystal structure determination of oxide metamict minerals will be discussed.

metamict minerals; crystal structure relics; XRD; Raman spectroscopy; TEM

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