The Lower Neogene Unit (LNU) of west Samos Island, Greece, consists of volcanic ash and tuffaceous rocks, claystone, marlstone, and porcelaneous limestone. The initial fresh glass of volcanic rocks has usually transformed to authigenic K‐feldspar, zeolites, opal‐ CT, and smectite. K‐feldspar is the predominant mineral in the central part of the LNU, whereas in adjacent margins, K‐feldspar occurs only in the sandy tuffs. Nowhere is the K‐feldspar associatedw ith fresh glass.Chemical, sem, and xrd analysess how the K‐feldspar to be B‐bearing, with B concentrations up to 2500 ppm. Statistical treatment of chemical data demonstrates a positive correlation of B with K and Rb. This B‐bearing K‐feldspar seems to be analogous to K‐feldspars described in saline-alkaline lake deposits associated with nonmarine evaporates.

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