The paper describes two unusual lithologies that occur in a sequence of Mg-rich lacustrine sediments in northern Greece. The first is a laminated mudstone in which layers composed of magnesian smectite alternate with layers composed of an amorphous clay of similar chemical composition. The second lithology is a thin bed of mudstone in which the only crystalline constituent is hydrotalcite (a magnesium aluminum hydroxide), accompanied by amorphous high-magnesium clay. Hydrotalcite has not previously been recorded from sedimentary rocks except as a product of industrial pollution. The high-magnesium clay is considered to have formed by precipitation from Mg-rich lake water onto volcanic ash particles, whilst the hydrotalcite has filled the molds left by dissolution of volcanic glass within the clay-coated particles.