Enantiomorphism of kaolinite: Manifestation at the levels of elementary layer and microcrystals
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TAL GROWTH
Enantiomorphism of Kaolinite: Manifestation at the Levels of Elementary Layer and Microcrystals N. D. Samotoin Institution of the Russian Academy of Sciences, Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Staromonetnyi per. 35, Moscow, 119017 Russia email: [email protected] Received June 10, 2010
Abstract—The right and left forms of the argillaceous mineral kaolinite (Al2 Si2O5(OH)4), which is wide spread in nature, have been revealed for the first time by transmission electron microscopy and gold decora tion in vacuum. The enantiomorphic forms of this mineral are established at the level of the elementary 7 Å layer, which determines the kaolinite structure, and at the level of nano and microcrystals typical of this min eral. Both kaolinite forms are widespread in ancient and young weathering crusts. Enantiomorphic kaolinite microcrystals are formed in two ways: through the periodic formation of 2D nuclei and via helical growth, which is dominant for both kaolinite forms. The right and lefthanded kaolinite forms are observed in the samples under study with equal probability. DOI: 10.1134/S1063774511010202
INTRODUCTION The importance of studying enantiomorphic crys tal forms was noted a long time ago by V.A. Vernadsky, who wrote: “Strange as it is, but the inevitable simul taneous manifestation of two, right and lefthanded, states of crystal space has not interested the research ers much.” [1]. However, these states have still not been very well studied to this day. The righthanded (d) and lefthanded (l) (dexter and laevus in Latin) crystal forms belong to the most peculiar and interesting manifestations of symmetry in nature. They are two mirroridentical crystalline mod ifications of the same material [2]. Right and left handed crystals can be formed in 11 out of 32 existing symmetry classes, specifically, in primitive and axial classes of all systems [1, 2]. Of all the many minerals, right and lefthanded crystals are only known for a few: quartz, pyrite, cuprite, cinnabar, and some oth ers. They have been thoroughly studied only for quartz [3, 4]. The purpose of this study was to prove the existence of enantiomorphic kaolinite forms in nature and esti mate their relative abundance. Kaolinite (Al4Si4O10(OH)8) The kaolinite structure is based on a 7Å twolevel dioctahedral 1 : 1 layer composed of octahedral (–Al– O(OH)–) and tetrahedral (–Si–O–) sheets. The lay ers in the structure are equally oriented and relatively displaced along the а axis by а/3 and in the negative direction of the b axis by ~b/30. The structure has a singlelayer period along the с axis and belongs to the
triclinic polytype 1Tc and sp. gr. P1 (C1) [5]. The unit cell parameters, according to [6], are a = 5.15 Å, b = 8.94 Å, c = 7.39 Å, α= 91.9°, β = 105°, and γ = 89.79°. Kaolinite is most widespread among argillaceous minerals. It is formed as a result of weathering and at lowtemperature hydrothermal transformations of various rocks. The largest kaolinite deposits are relate
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