Growth mechanisms, polytypism, and real structure of kaolinite microcrystals
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TAL GROWTH
Growth Mechanisms, Polytypism, and Real Structure of Kaolinite Microcrystals N. D. Samotoin Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences, Staromonetnyœ per. 35, Moscow, 119017 Russia e-mail: [email protected] Received June 28, 2007; in final form, May 5, 2008
Abstract—The mechanisms of growth of kaolinite microcrystals (0.1–5.0 µm in size) at deposits related to the eluvial weathering crust, as well as to the low-temperature and medium-temperature hydrothermal processes of transformations of minerals in different rocks in Russia, Kazakhstan, Ukraine, Czechia, Vietnam, India, Cuba, and Madagascar, are investigated using transmission electron microscopy and vacuum decoration with gold. It is established that kaolinite microcrystals grow according to two mechanisms: the mechanism of periodic formation of two-dimensional nuclei and the mechanism of spiral growth. The spiral growth of kaolinite microcrystals is dominant and occurs on steps of screw dislocations that differ in sign and magnitude of the Burgers vector along the c axis. The layered growth of kaolinite originates from a widespread source in the form of a step between polar (+ and –) dislocations, i.e., a growth analogue of the Frank–Read dislocation source. The density of growth screw dislocations varies over a wide range and can be as high as ~109 cm–2. Layered stepped kaolinite growth pyramids for all mechanisms of growth on the (001) face of kaolinite exhibit the main features of the triclinic 1Tc and real structures of this mineral. PACS numbers: 68.37.Lp, 81.10.Aj DOI: 10.1134/S1063774508050271
INTRODUCTION Although there is a great deal of evidence about mechanisms of crystal growth [1–3], mechanisms of mineral growth are poorly understood [4–6]. This is especially true in regard to minerals formed in a finegrained form with crystals from several fractions of a micrometer to several micrometers in size. Only fragmentary data on their growth are available in the literature [7–9]. However, fine-grained minerals occur widely in nature and have been frequently formed under laboratory conditions. These minerals are of great scientific and practical importance. Among all the fine-grained minerals encountered in the Earth’s crust, the most abundant are layered silicates and, especially, polytypic varieties of kaolinite group minerals (kaolinite, dickite, nacrite, halloysite), smectites, and other clay minerals [10]. The field of application of these minerals is very wide and is constantly expanding. In this respect, much attention has been focused on the study of clay minerals. Investigations in this field of science have been performed using both the traditional methods [11] and the most modern techniques [12, 13]. The purpose of this study was to establish the mechanisms of growth of microcrystalline kaolinite individuals, the occurrence of the revealed growth mechanisms, and the role they play in the variation of the per-
fection of the crystal structure of this mineral and its poly
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