Kaszab Zoltán (szerk.): A Magyar Természettudományi Múzeum évkönyve 77. (Budapest 1985)

Szendrei, G. ; N. Tóth, M.: Contribution to the study on the crystallinity degree of kaolinites

ANNALES HISTORICO-NATURALES MUSEI NATION ALI S HUNGARICI Tomus 77. Budapest, 1985 p. 5-16. Contribution to the study on the crystallinity degree of kaolinites by G. SZENDREI & M. N. TÓTH, Budapest Abstract

—The crystallinity parameters of 17 kaolinites mainly from well-known European localities were determined by X-ray diffractometry and thermal analysis. A sequence of the kaolinite specimens with decreasing crystallinity was established. An attempt was made to find relationships between the thermal characteristics and parameters calculated from X-ray diffractometry, as well as total iron content. The correlation between average domain size in the (001) direction and the degree of order was found to be close. With 5 figures. Introduction

—The exact determination of clay minerals is only possible by instrumental analysis (e.g. X-ray diffractometry, thermal analysis, etc.). Many of the clay mineral specimens belonging to the collection of the Mineralogical and Petrological Department, Hungarian Natural History Museum were collected when these apparatuses had not yet been developed and were later not available for the investigation of these samples. Some kaolinites of these specimens, wherever possible, have now been investigated. Some of these specimens proved to be pure kaolinite, others contained variable quantities of minerals like illite, smectite, quartz, etc., while a further group belonged to other minerals which were revised. For comparison kaolinites from the collection of the Laboratory for Geochemical Research of the Hungarian Academy of Sciences were used as reference minerals. Besides the revision of these specimens the crystallinity parameters were determined and an effort was made to find relationships between the parameters calculated from the X-ray diffractograms and the thermal analysis data and total iron content, respectively. Some methods for the determination of kaolinite crystallinity The crystallinity indices known from the literature are calculated from the intensity relations of certain reflection groups in the X-ray difftactograms or by using the analysis of the base reflections, respectively. Well-crystallized kaolinite is characterized by favourable peak-to-background ratio (low background), by many sharp peaks as well as by three well defined triplets. The base reflections are sharp and the angular width at the half-maximum is small. With increasing disorder the sharpness of the reflections decreases gradually, adjacent lines will be confused and triplets mistaken for doublets. The disappearance of the (110) and (111) reflections is especially characteristic for the structural disorders evoked by the structural displacements in the direction of the b-axis. Structural changes also deform the base reflections as indicated by a decrease in intensity, by d-value displacement evoked by lattice expansion, and by an increase in the angular width at the half-maximum. The various indices characterizing kaolinite crystallinity are as follows. The height ratios of reflections (110) and (020) were first used by MURRAY & LYONS (1960) as crystallinity indices. In case of well-crystallized kaolinite the index value is greater than 1, for medium crystallized samples it lies between 0.7 and 1.0 while it proves to be less than 0.17 in case of poorly crystallized material. The index proposed by HINCKLEY (1964) measures the separation of the lines of (110) and (111) The more perfect the crystal structure of the kaolinite, the better separated the two lines. The value