0024-4902/01/3604- $25.00 © 2001 åÄIä “Nauka /Interperiodica” 0337 Lithology and Mineral Resources, Vol. 36, No. 4, 2001, pp. 337–352. Translated from Litologiya i Poleznye Iskopaemye, No. 4, 2001, pp. 390–407. Original Russian Text Copyright © 2001 by Drits, Ivanovskaya, Sakharov, Gor’kova, Karpova, Pokrovskaya. INTRODUCTION Chlorites form in various geological settings includ- ing clastic and volcaniclastic rocks, which experienced postsedimentary transformations, sedimentary and magmatic rocks, greenschist-facies rocks, altered mafic and ultramafic rocks, hydrothermally altered rocks, marine evaporite and carbonate deposits, and others (Drits and Kossovskaya, 1991). The ideal chlorite structure is characterized by regularly alternating octa- hedral brucite and 2 : 1 layers (Bailey, 1988). Based on the relative position of these layers in the structure, chlorite occurs as 12 three-dimensional regular poly- types and 6 semirandom polytypes containing packing defects related to disordered shift along the b axis by b/3 (Bailey, 1988; Bailey and Brown, 1962; Zvyagin, 1964). Because of the variety of isomorphous substitu- tions of the octahedral and tetrahedral cations, the gen- eral formula of chlorites can be written as follows: where R 2+ is Fe 2+ and Mg, R 3+ is Fe 3+ and Al cations, and  represents the vacant octahedra. The wide isomorphism of chlorites simulated the search of relations between their formation conditions, structure, and composition (Bailey, 1988; de Caritat et al., 1993; Drits and Kossovskaya, 1991; Hayes, 1970; Walker, 1993). Particular attention was given to application of chlorite as geothermometer. Numerous observations showed that temperature increase, which is related to increase of burial depth, metamorphic grade, or heating of solutions in the hydrothermal sys- tems, causes a systematic decrease in tetrahedral Si and octahedral vacancies (Battaglia, 1999, Cathelineau, 1988; de Caritat et al., 1993; and others). Based on R z 2+ R y 3+  3- y - z ( 29 S 4- x Al x ( 29 O 10 OH ( 29 8 , chemical analyses and temperatures estimated by dif- ferent methods, several empirical equations have been proposed to relate the composition and temperature of chlorite formation. The review of the works devoted to chlorite geothermometry is given by de Caritat et al. (1993). Chlorites often occur in a mixture with other miner- als, which hampers the determination of their chemical composition. Therefore, the relations between compo- sition and unit cell parameters of chlorites have been studied (Shirozu, 1958; Brindley, 1961; Kepezhinskas, 1965; Bailey, 1975, 1988). Drits and Smoliar-Zvyagina (Drits and Smoliar-Zviagina, 1992) showed that coor- dinates of atoms in the unit cell can be deduced from the chemical composition and b parameter. Because of wide abundance of chlorites in different conditions, special attention has been given to the study of structural mechanism of their formation (Bailey, 1988; Buseck and Veblen, 1988; Drits and Koss- ovskaya, 1991). High-resolution transmission electron microscopy (HRTEM) made it possible to solve this problem at a new level (Banfield and Bailey, 1996; Banfield et al., 1994; Buseck and Veblen, 1988; Eggle- ton and Banfield, 1985). It was shown that the chlorite structure formed by solid-state reaction inherits struc- tural elements of the parental rock-forming silicates affected by weathering, methamorphism, or hydrother- mal alterations. Of interest in this respect are the works on transformations of biotite into chlorites (Buseck and Veblen, 1988; Olives and Amouric, 1984; Veblen and Ferry, 1983), serpentine into chlorite, and chlorite into serpentine (Banfield and Bailey, 1996; Banfield et al., 1994). In particular, it was shown that the transforma- tion of serpentine into chlorite is mainly accompanied Pseudomorphous Replacement of Globular Glauconite by Mixed-Layer Chlorite–Berthierine in the Outer Contact of Dike: Evidence from the Lower Riphean Ust’-Il’ya Formation, Anabar Uplift V. A. Drits, T. A. Ivanovskaya, B. A. Sakharov, N. V. Gor’kova, G. V. Karpova, and E. V. Pokrovskaya Geological Institute (GIN), Russian Academy of Sciences, Pyzhevskii per. 7, Moscow, 109017 Russia Received February 12, 2001 Abstract—Globular, platy, and fine-dispersed phyllosilicates of chloritic composition were studied in the outer contact of dike with glauconite-bearing rocks. It is shown that the globular Al-glauconite is replaced by pseudo- morphs of mixed-layer Mg- and Fe-bearing chlorite–berthierine containing 5% berthierine layers in this zone. The crystallochemical characteristics and microstructure are reported for the globular, platy, and fine-dispersed chlorites. The possible models of chlorite–berthierine formation are discussed.