I am hoping to get some help in identifying these flakes found in a sediment core from Trondheimsfjord, Norway. They are approximately 1mm thick. and 1-2 cm diameter (biggest, but most are broken into smaller flakes). They are solid, but easy to break. They react with HCl on light grey 'top side' but not on dark, rusty (?) 'underside' (see photos). Are they worm burrow linings? They only occur between 26-28 cm down core (although the core is only half a metre long) from about 500 m depth in the fjord. Thanks for your thoughts!