Copyright  2001, The Clay Minerals Society 81 Clays and Clay Minerals, Vol. 49, No. 1, 81–91, 2001. VERMICULITIZATION OF PHLOGOPITE IN METAGABBRO, CENTRAL TURKEY FATMA TOKSOY-KO ¨ KSAL,ASUMAN G. TU ¨ RKMENOG ˇ LU, AND M. CEMAL GO ¨ NCU ¨ OG ˇ LU Middle East Technical University, Department of Geological Engineering, 06531, Ankara, Turkey Abstract—Dioctahedral vermiculite occurs in an isolated metagabbro klippe (Kuranc ¸alı Metagabbro) that belongs to the Central Anatolian Ophiolites from central Turkey. Both the metagabbro and the structurally underlying high-grade metamorphic rocks are intruded by granitic rocks. The Kuranc ¸alı Metagabbro is characterized by its well-developed compositional layering, and the presence of vermiculitized phlogopite- rich layers. Petrographic and mineralogic studies show that the primary mineral phases in the host rock are diopside, tschermakitic hornblende, Fe-rich phlogopite, and plagioclase. Secondary minerals are horn- blende, actinolitic hornblende, Fe-rich phlogopite, and vermiculite. A two-phase history of alteration involving acidic weathering and alkaline metasomatism is suggested for the dioctahedral vermiculite and secondary Fe-rich phlogopite, respectively. The alteration of phlogopite to dioctahedral vermiculite pro- ceeded both along cleavage planes and at crystal edges. The vermiculite is colorless to pale yellow with weak pleochroism and shows optical continuity with the parent mineral. Vermiculite flakes, analyzed semi-quantitatively by scanning electron microscope-energy dispersive analysis (SEM-EDS) and electron microprobe (EMP), are characterized by partially expanded interlayers, K depletion, and Mg and/or Al enrichment. X-ray diffraction (XRD) and differential thermal analysis-thermal gravimetric (DTA-TG) analyses indicate that phlogopite is not a pure phase, although it is the dominant one. The XRD patterns show the presence of both dioctahedral vermiculite having dehydrated interlayers and hydroxy-Al inter- layers, and interstratified phlogopite-vermiculite. The transformation of phlogopite to vermiculite is thought to represent an initial stage of weathering in an acidic environment. Key Words—Central Anatolian Ophiolites, Metagabbro, Mineral Weathering, Phlogopite, Vermiculite. INTRODUCTION Vermiculite is a mineral, resembling mica, which exfoliates by rapid heating. The name is for an ex- pandable 2:1 phyllosilicate structure with a layer charge of 0.6  x  0.9 per O 10 (OH) 2 and with Mg, Fe 3+ or hydroxy-Al complexes in the interlayer (Velde, 1985; de la Calle and Suquet, 1988; Moore and Reyn- olds, 1989). Numerous studies were performed on large vermiculite grains that were well characterized as trioctahedral vermiculite, according to the definition of Gruner (1934). Later studies reported vermiculite from soils that is either dioctahedral or trioctahedral (Hathaway, 1955; Barshad and Kishk, 1969; Douglas, 1989). Most vermiculite forms by alteration of micas. The major elemental composition of vermiculites is closely related to that of the parent mica, although there is partial disruption of the mica structure (Jelitto et al., 1993). Vermiculites resulting from the alteration of mi- cas commonly involve the replacement of the interlay- er K by a hydrated cation, generally Mg (Moore and Reynolds, 1989; Nemecz, 1981). In soils, vermiculite is a common constituent and is unquestionably the product of low-temperature weath- ering. There is less certainty about the formation of macroscopic vermiculite which was attributed to either hydrothermal (Morel, 1955) and/or supergene process- es (de la Calle and Suquet, 1988; Zhelyaskova-Pan- ayotova et al., 1992, 1993). According to experimental studies (Roy and Romo, 1957; Komarneni and Roy, 1981), vermiculite is a product of mica alteration by dilute low-alkali solutions. A two-phase history of alteration was found in de- posits in Turkey, that includes an earlier acidic weath- ering causing vermiculite and a later alkaline meta- somatism resulting in secondary Fe-rich phlogopite formation. These processes were recognized in an ophiolitic metagabbro from Central Anatolian Ophiol- ites. In this paper, we report further on the formation of vermiculite resulting from phlogopite transforma- tion process in this rock system. GEOLOGY The assemblage of magmatic and metamorphic rocks, known collectively as the Central Anatolian Crystalline Complex (CACC) (see ‘‘C’’ in Figure 1) consists of three main units: Central Anatolian Meta- morphics (CAM), Central Anatolian Granitoids (CAG), and Central Anatolian Ophiolites (CAO). The metamorphic rocks present in the CACC are over- thrusted by ophiolitic units, and both units are cut by granitoids (Go ¨ncu ¨og ˇlu et al., 1991, 1998; Go ¨ncu ¨og ˇlu and Tu ¨reli, 1993; Yalınız et al., 1996). Unstratified and layered gabbroic rocks occurring as isolated outcrops in the CACC were shown to be parts of supra subduction in Late Cretaceous ophiolite (Yalınız et al., 1996; Yalınız and Go ¨ncu ¨og ˇlu, 1998). The gabbros have variable sizes and compositions ranging from fine grained to pegmatitic, and leuco- to