57 WOGOGOB-2004 CONFERENCE MATERIALS. Edited by O. Hints & L. Ainsaar Kirsimäe, K., Hints, R., Kallaste, T., Kiipli, E. and Kiipli, T. Chloritization of Upper Ordovician Pirgu bentonites – source material or diagenetic environment? Kalle Kirsimäe 1) , Rutt Hints 1,2) , Toivo Kallaste 3) , Enli Kiipli 3) & Tarmo Kiipli 3,4) 1) Institute of Geology, University of Tartu, Estonia <kalle.kirsimae@ut.ee> 2) Estonian Museum of Natural History, Estonia <rutt@loodusmuuseum.ee> 3) Institute of Geology at Tallinn University of Technology, Estonia <kallaste@gi.ee>, <enli.kiipli@egk.ee> 4) Geological Survey of Estonia, Estonia <tarmo.kiipli@egk.ee> Inoue 1995). Smectite-to-chlorite transformation in Mg–Fe-rich rocks/sediments begins usually with the formation of saponite-type Fe-rich smectite, which in progressive diagenesis or hydrothermal alteration transforms into corrensite (Reynolds 1988). The next stage in corrensite-to-chlorite conversion is the growth of chlorite layers in corrensite to form discrete chlorite domains in a corrensite matrix (Beaufort et al. 1997). Corrensite, however, can form directly under hydrothermal conditions at temperatures between about 100 and 200 °C (Inoue & Utada 1991). In alternating pyroclastic sedimentary sequences the formation of dioctahedral smectite and subsequent smectite-to-illite conversion is related to acidic vol- canic materials, whereas the trioctahedral smectite (saponite) and chlorite-to-smectite transformation takes place at the expense of the basic Mg–Fe-rich volcanics (e.g. Son et al. 2001). Chlorite and chlorite- smectite are also found in Ordovician and Silurian K-bentonites in North America, British Isles and rarely in Baltoscandia (e.g., Bergström et al. 1992, 1998). Chloritic minerals in these bentonite beds are suggested to form during alteration under low-grade metamorphic conditions (Krekeler & Huff 1993). However, the origin of chlorite (chlorite-smectite, corrensite) in Pirgu K-bentonites remains unclear. The Palaeozoic sedimentary sequence of the northern Baltic Basin shows no signs of low-grade metamor- phic alteration or even deep diagenesis. Therefore, we suggest that the clay mineral composition of Pirgu bentonites is controlled either by the original pyroclastic material composition or by a specific early diagenetic environment during the volcanic glass devitrification, both of which have provided high Mg needed for saponite-type smectite formation and consequent saponite-to-chlorite transformation. The trace element composition of whole rock and analysis of glass melt inclusions of Ordovician–Silu- rian bentonites suggest the rhyolite–rhyolite-dacite and (rhyo-)dacite–trachyandesite composition of the source magma (Huff et al. 1996; Kiipli & Kallaste 1996). However, the saponite-to-chlorite series have The sedimentary sequence of the Palaeozoic Baltic Basin contains numerous bentonite layers whose composition is dominated by the mineral assem- blage of illite-smectite–K-feldspar–kaolinite. The contents of these minerals may vary between in- dividual bentonite layers as well as laterally from almost pure illite-smectite to K-feldspar and/or kaolinite end-member compositions, but the assem- blage remains principally the same. In this respect the bentonites of the Upper Ordovician Ashgill Pirgu (Regional) Stage are exceptional and unique in the Baltic Basin. The clay mineral composition of these bentonites is characterized by the chlorite- smectite (corrensite) and illite-smectite assemblage. The micritic-bioclastic to argillaceous limestones of Pirgu age in the northern Baltic Basin include up to three (four?) individual bentonite beds, all of which contain chlorite-smectite and/or corrensite miner- als. In this contribution we present preliminary data on the clay mineral composition of these beds. The clay fraction (<2 μm) of 21 samples studied contains random (R0) mixed-layered chlorite-smec- tite and/or R1 ordered corrensite and corrensite-chlo- rite type phases together with illite-smectite. The chloritic phases are the most abundant clay minerals in the majority of samples, but also illite-smectite may dominate in the mixture with minor chlorite- smectite/corrensite. The occurrence of a R1 ordered corrensite (0.5/0.5 interlayered chlorite and smectite mineral) phase is confirmed by the expansion of the superstructure d(001) spacing from 29 Å in air-dried state to 31 Å in EG saturated state. The heating of the corrensite-rich sample at 500 °C for 1 h caused the collapse of the spacing to 24 Å. The proportion of smectite layers in mixed layer chlorite-smectite is according to NEWMOD modelling 0.2–0.4 and probably 0.6–0.7 in R0 and R1 (corrensite-chlorite) ordered minerals, respectively. Chlorite-smectite and corrensite are trioctahedral clay minerals characterizing evaporitic- and vol- canoclastic sedimentary-diagenetic, and hydrother- mal alteration environments (e.g., Reynolds 1988;