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

of this index changes as follows: well-crystallized kaolinite: 1.0-1.2; medium-crystallized kaolinite: 0.8-0.9; poorly crystallized samples: 0.6-0.7. According to BRINDLEY & KURTOSSY (1961) the angular width at the half-maximum of the base reflections can be used for comparison of kaolinite samples from various locations. For the best crystallized kaolinites this value lies between 0.2 and 0.3 20. Incase of pure kaolinite or samples with relatively high kaolinite content this index can be applied with high accuracy. BRINDLEY (1961) introduced the ratio of d =2.55 and 2.49 Â reflections as a crystallinity index. This ratio also shows the separation of the lines. The decrease in the order of the lattice results in the gradual disappearence of the triplet's middle peak and in a change in the intensity relations of the two lines. In case of well crystallized kaolinites the intensity of the peak at d =2.49 Â is higher. As shown by the results, this index is not or hardly disturbed by coincidences. When measuring the X-ray parameters it was found that the angular width at the half-maximum of the (060) reflection, having been considered as most suitable for quantitative analysis by KRANTZ & WIEGEMANN (1957), and could also be applied as an index for comparison of samples with various degrees of crystallinity. Independently from the crystallinity degree the intensity of (060) reflections is constant but this is a result of a reversed change of its height and of the angular width ac the halfmaximum. The change of the angular width at the half-maximum is correlated with the degree of crystallinityTo study the first basal reflection of kaolinite, variance analysis (WILSON 1963) was applied as one of the mathematically precise analytical methods of line profile determination (TÓTH 1980), by this method the deformation and domain size could be separated. Deformation is defined as the average deformation in the (001) direction, and domain size is the coherently scattering domain size in the same direction. The relation between the thermal parameters and the (dis-) order of kaolinites is obvious and has been detailed by a fairly large number of papers. Some of these references are mentioned below. MACKENZIE (1970) established that kaolinite

—M

D gave a small low temperature endothermic peak due to the loss of adsorbed moisture. TODOR (1976) also mentioned that poorly crystallized kaolinites showed an endothermic peak (100-200°C) due to (dis-)order and also dependent upon concentration and particle size of the kaolinite. According to MACKENZIE (1970) kaolinite

—T

c shows an endothermic peak temperature of about 580°C, while kaolinite

—M

D gives a lower one. Upon investigation of 19 kaolinite and halloysite samples from Tanganyika, ROBERTSON, BRINDLEY & MACKENZIE (1954) pointed out that with increasing disorder of kaolinite the temperature and symmetry of the endothermic peak are decreasing. The authors mentioned that these thermal parameters are also affected by other factors, i.e. first of all by the amount of mineral in the sample, grain size, etc. While carrying out quantitative determination of kaolinite by DTA, CARTHEW (1955) found that besides concentration and particle size the endothermic peak height is also influenced by the degree of (dis-)order (diminishes with increasing disorder). SMYKATZ-KLOSS (1974a, b), following a strictly standardized procedure and using the same grain size fraction (0.6-2^m), recognized relationships between temperature, peak heights of dehydroxylation and the degree of (dis-)order (which increase with increasing degree of order). On the basis of the dehydroxylation temperature a rough grouping as given below was given : Later on SMYKATZ-KLOSS (1975) and WERNER & SMYKATZ-KLOSS (1977) introduced an index for the degree of (dis-)order calculated by multiplying the factor dehydroxylation peak temperature minus 530°C by the peak height (A T value). To avoid the interference of impurities and changing amount of kaolinite in the sample, correction factors were applied partly based on the mineralogical composition of the sample determined by X-ray diffractometry. The relation of indices is expressed in percentage assuming the index of the most ordered reference material as 100%. Having studied the degree of order in almost 40 samples of kaolinites, mainly from desposits in Germany, the kaolinite from Murfresboro (Arkansas, U.S.A.) was taken as reference material. STOCH & WACLAWSKA(1980), investigating 8 kaolinite samples, obtained relationships between dehydroxylation peak temperature, shape index, Arrhenius activation energy, order of reaction and the degree of (dis-)order in the kaolinites. They proposed a relationship between the degree of crystallinity and loss of weight due dehydroxylation which is less influenced by grain size distribution. In order to characterize the dehydroxylation of kaolinite, HORVÁTH & KRANZ (1980) presented the starting and end temperatures, the temperature difference, the decomposition temperature at extremely disordered: strongly disordered: little disordered : well-ordered : 530°C > 530-555°C 555-575°C 575°C <